# Rapport quadriennal 2011-2014

Claude Boucher, Pierre Hubert

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Claude Boucher, Pierre Hubert. Rapport quadriennal 2011-2014. Comité national français de géodésie et géophysique. 2015. hal-04787097

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# Rapport quadriennal 2011-2015

## Quadriennal Report 2011-2015

26ᵉ Assemblée générale de l'UGGI, Prague, République tchèque, 22 juin – 2 juillet 2015
26ᵉth IUGG General Assembly, Prague, Czech Republic, June 22 – July 2, 2015
Table des matières / Contents

• Avant-Propos du Président Claude Boucher................................................5

• Le Conseil du CNFGG................................................................................6

• Section 1 Géodésie / Geodesy........................................................................10

o François Barlier

Bureau des longitudes........................................................................11

o Richard Biancale, J.-M. Lemoine, S. Bruisma, J. Laurent-Varin, F. Peronanz

Centre national d'études spatiales (CNES) et Groupe de recherche en géodésie spatiale (GRGS)........................................................................13

o Stéphane Durand

Ecole supérieure des Géomètres et Topographes (ESGT).................................16

o Jean-Paul Boy, Jacques Hinderer, Séverine Rosat, Jean-Daniel Bernard, Frédéric Littel, Frédéric Masson, Patric Ulrich

Ecole et Observatoire des Sciences de la Terre (EOST).................................21

o François l'Ecu, Olivier Jamet

Institut national de l'information géographique et forestière (IGN)..............23

o Pierre Exertier

Observatoire de la Côte d'Azur (OCA)....................................................28

o Christian Bizourd

Observatoire de Paris........................................................................31

o Marie-Françoise Lequentrec-Lalancette

Service hydrographique et océanographique de la Marine (SHOM)........34

o Publications..................................................................................38
• Section 5 Météorologie et sciences de l'atmosphère / Meteorology and Atmospheric Sciences................................................................................45

o Andrea I. Flossmann, Sophie Godin-Beekmann and Nick Hall

• Section 6 Sciences hydrologiques / Hydrological Sciences................................................52

o Charles Perrin, Vazken Andréassian, Patrick Arnaud, Valérie Borrell-Estupina, Isabelle Braud, Christophe Cudennec, Alban de Lavenne, Eric Gaume, Pierre Hubert, Pierre Javelle, Michel Lang, Nicolas Le Moine, Gil Mahé, Roger Moussa, Ludovic Oudin, Jean-Emmanuel Paturel, Olivier Payrastre, Carine Poncelet, Fabrice Rodriguez, Eric Sauquet and Eric Servat

French contribution to the IAHS Prediction in Ungaged Basins (PUB) initiative..................................................................................53

• Section 7 Sciences physiques de l'océan / Physical Sciences of the Oceans............82

o Dewitte B., I. Montes, E. Gutknecht, E. Machu, A. Paulmier, K. Goubanova and V. Garçon

Recent Progresses in biogeochemical regional modelling of the Eastern Boundary Current Systems of the South Hemisphere................................83

o S. Cravatte, F. Marin, W. S. Kessler

Zonal Jets at 1000m in the tropics observed from ARGO floats'drifts....87

o A. Ganachaud, S. Cravatte, G. Eldin, T. Delcroix, L. Gourdeau, F. Durand, F. Marin, C. Maes, J. Fefevre, E. Kestenare

SPICE Southwest Pacific Ocean Circulation................................................92

o S. Cravatte, A. Ganachaud, C. Jeandel, G. Eldin, L. Gourdeau, J. Lefevre, P. Van Beek, F. Lacan, M. Souhaut, C. Pradoux

SOLWARA Solomon Sea circulation and geochemistry................................95

o Christophe Maes

Vertical stratification of the upper ocean: documenting the specific role of the salinity in the Tropics................................................................................98
o J. Boutin, N. Reul, C. Maes, G. Reverdin, T. Delcroix, F. Gaillard

Sea Surface Salinity from SMOS Satellite mission: a synthesis of the main 2010-2015 oceanic results in France..................................................100

o Fabrice Ardhuin

Ocean waves in Earth System sciences: towards a broad-band understanding..................................................................................104

o Y. Morel

EPIGRAM, understanding the ocean dynamics along the french coasts..................108

o Thierry Penduff, Guillaume Sérazin, Sandy Grégorio, Stéphanie Leroux, Laurent Bessières, Jean-Marc Molines, Bernard Barnier, Laurent Terray

Spontaneous emergence of low-frequency variability in the GLOBAL EDDYING Ocean..................................................................................111

o O. Arzel and A.Colin de Verdière

Dansgaard-Oeschger cycles..................................................................................115

o Colin de Verdière, M. Ollitrault

On the Reference Level Problem in Oceanography..................................121

o Sabrina Speich, Silvia Garzoli, Alberto Piola and Edmo Campos

An International initiative: South Atlantic Meridional Overturning Circulation (SAMOC) Years 2011–2014..................................................125

o Christine Provost, Young-Hyang Park, Nathalie Sennéchael, Zoé Koenig and Ramiro Ferrari

The ACC at Drake Passage: Volume transport and meridional heat flux..................................................................................................................130

o Young-Hyang Park and Frédéric Vivier

Direct observations of the ACC Transport across the Kerguelen Plateau..................................................................................................................135
Avant-Propos

Le rapport qui est présenté ici rassemble les contributions de plusieurs sections du CNFGG. Il ne donne malheureusement qu'une vue partielle des activités de la communauté française, faute d'avoir pu obtenir des éléments de l'ensemble des disciplines concernées.

Le CNFGG se réorganise actuellement afin d'assurer à l'avenir des rapports d'activité
raisonnablement complets. Nous avons l'ambition d'être un observatoire de l'activité
nationale dans le domaine de la géodésie et de la géophysique, concrétisé notamment par un
site web rediffusant ce genre d'informations.

En parallèle, le CNFGG s'est engagé pour mobiliser la communauté française en vue
d'une participation active à l'UGGI et ses associations.

Je remercie donc toutes celles et ceux qui ont participé à la rédaction de ce rapport.

Claude Boucher
Président du CNFGG
# Composition du conseil du CNFGG
http://cnfgg.eu

Le Comité National Français de Géodésie et de Géophysique (CNFGG), créé en 1919, a pour mission de représenter, sous l'autorité de l'Académie des sciences, la communauté scientifique française au sein de l'Union Géodésique et Géophysique Internationale (UGGI/IUGG) et de ses Associations disciplinaires.

**Président du CNFGG : M. Claude BOUCHER**

Courriel : claude-boucher@club-internet.fr

**Premier Vice-Président du CNFGG : M. Daniel SCHERTZER**

Courriel : Daniel.Schertzer@enpc.fr

**Deuxième Vice-Président du CNFGG : Poste vacant**

**Secrétaire général : M. Jonathan CHENAL**

Courriel : sgn.cnfgg@ign.fr

**Trésorier du CNFGG : M. Roland SCHLICH**

Courriel : roland.schlich@unistra.fr

## Les sections:

### Section 1 – Géodésie

**Présidente : Mme Françoise DUQUENNE**

Courriel : fh.duquenne@wanadoo.fr

**Président sortant : M. Richard BIANCALE**

Courriel : richard.biancale@cnes.fr

**Vice-Présidente : Mme Marie-Françoise LE QUENTREC-LALANCETTE**

Courriel : mflqc@shom.fr

**Vice-Président : M. Richard BIANCALE**

Courriel : richard.biancale@cnes.fr

**Secrétaire : Mme Muriel LLUBES**

Courriel : muriel.llubes@getobs-mip.fr
## Section 2 - Sismologie et Physique de l'Intérieur de la Terre

Président: M. Tony MONFRET

Courriel: monfret@geoazur.unice.fr

Présidente sortante: Mme Anne DESCHAMPS

Courriel: deschamps@geoazur.unice.fr

Vice-Présidente: Mme Hélène LYON-CAEN

Courriel: helene.lyon-caen@ens.fr

Vice-Président: M. Antoine MOCQUET

Courriel: Antoine.Mocquet@univ-nantes.fr

Secrétaire: Mme Mireille LAIGLE-MARCHAND

Courriel: laigle@geoazur.unice.fr

## Section 3 - Volcanologie et Chimie de l'Intérieur de la Terre

Président: M. Patrick ALLARD

Courriel: pallard@ipgp.fr

Président sortant: Jean François LENAT

Courriel: j.f.lenat@opgc.univ-bpclermont.fr

Secrétaire: M. Pierre BRIOLE

Courriel: briole@ens.fr

## Section 4 - Géomagnétisme et aéronomie

Président: M. Benoit LANGLAIS

Courriel: benoit.langlais@univ-nantes.fr

Présidente sortante: Mme Mioara MANDEA

Courriel: mioara.mandea@cnes.fr

Vice-Président: M. Jean-Baptiste RENARD

Courriel: jbrenard@cnrs-orleans.fr

Secrétaire: M. Erwan THEBAULT

Courriel: ethebault@ipgp.fr
## Section 5 - Météorologie, Physique de l'atmosphère

Président : Mme. Andréa FLOSSMANN

Courriel : a.flossmann@opgc.univ-bpclermont.fr

Président sortant : M. Jean-Louis FELLOUS

Courriel : jean-louis.fellous@esa.int

Vice-président : Mme Sophie GODIN-BEEKMANN

courriel : sophie.godin-beekmann@latmos.ppsl.fr.fr

Secrétaire : M. Nick HALL

courriel : nick.hall@legos.obs-mip.fr

## Section 6 - Sciences Hydrologiques

Président : M. Eric SERVAT

Courriel : servat@msem.univ-montp2.fr

Président sortant : M. Pierrick GIVONE

Courriel : pierrick.givone@irstea.fr

Vice-Président : M. Jean-Marie FRITSCH

Courriel : jean-marie.fritsch@ird.fr

Vice-Président : M. Pierre RIBSTEIN

Courriel : pierre.ribstein@upmc.fr

Vice-Président : Dr Eric SAUQUET

Courriel : eric.sauquet@irstea.fr

Secrétaire : Mme Sandra ARDOIN-BARDIN

Courriel : sbardin@um2.fr

## Section 7 - Sciences Physiques de l'Océan

Président : M. Alain COLIN DE VERDIERE

Courriel : acolindv@univ-brest.fr

Président sortant : M. Michel CREPON

Courriel : crepon@lodyc.jussieu.fr

Secrétaire : Mme Marie-Noëlle HOUSSAIS

Courriel : mnh@lodyc.jussieu.fr
## Section 8 - Sciences de la cryosphère

Président : M. Etienne BERTHIER

Courriel : etienne.berthier@legos.obs-mip.fr

Président sortant : M. Michel FILY

Courriel : michel.fily@lggeobs.ujf-grenoble.fr

Secrétaire : M. Michel FILY

Courriel : michel.fily@lggeobs.ujf-grenoble.fr

## Membres du bureau d'une association de l'UGGI :

**Mme Mioara MANDEA** Secrétaire Général de l'AIGA/IAGA

Courriel : mioara.mandea@cnes.fr

**M. Christophe CUDENNEC** Secrétaire Général de l'AISH/IAHS

Courriel : cudennec@agrocampus-ouest.fr

**Mme Athena COUSTENIS** Présidente de l'AIMSA/IAMAS

Courriel : athena.coustenis@obspm.fr

**M. Pierre HUBERT** Membre du bureau de l'UGGI/IUGG

Courriel : pjy.hubert@free.fr
# Section 1

## Géodésie
Geodesy

<figure><img src="image_2.png" /></figure>
# CNFGG quadrienal report 2011-2015

## Section 1 : Geodesy

Trough its members the section 1 of the CNFGG gathers French competences in terms of research a realization aspects in geodesy. They pertain to national organisms such as BDL, BRGM, CNES, CNRS, EOST, ESGT, IGN, OCA, OMP, OP, SHOM, UPF involved in different geodetic activities.

This quadriennal report of the geodesy section cannot evidently claim being exhaustive. It includes a selection of reports delivered from eighth organisms: Bureau des longitudes, Centre national d'études spatiales, Ecole supérieure des géometres et topographes, Ecole et Observatoire de Strasbourg, Institut national de l'information géographique et forestière, Observatoire de la cote d'azur, Observatoire de Paris, Service hydrographique et océanographique de la marine. It includes moreover in appendix a list of publications from these organisms.

### 1) Bureau des longitudes (François Barlier)

#### Missions

The Bureau des longitudes, established in 1795, devotes its activities to scientific areas which come from its history (astronomy, geodesy, geophysics, etc.), constantly adapting to developments and projects in these areas. It functions as a scientific academy, with one session per month, plus meetings of working groups on specific topics for the preparation of books, documents, or colloquiums.

The Bureau des longitudes has been given the mission of publication and provision of public French astronomical ephemeris (Connaissance des temps, Annuaire du Bureau des longitudes), both having an annual publication. It has the scientific responsibility of these ephemeredes, whose implementation is entrusted, since 1998, to the Institut de mécanique céleste et de calcul des éphémérides (IMCCE), a division of Paris Observatory. The Bureau des longitudes "Commission des éphémérides" has been in charge of defining, in coordination with IMCCE, the content and evolution of these publications, with a special care to comply with the IAU and IUGG resolutions. This was especially the case for the IAU 2006/IUGG 2007 resolutions. As a member of the Research Group of Space Geodesy (GRGS), the Bureau des longitudes has a representative in its Directing Board, its Executive Committee and Scientific Council and gives advice on the GRGS organization, work and projects.

As part of its mission to disseminate scientific information, the Bureau des longitudes organizes each year a series of monthly lectures related to its areas, as well as one scientific colloquium, each year on a specific theme. These lectures and colloquiums have brought together a wide audience during the period 2012-2015. The Bureau des longitudes also participates in the organization and scientific sponsorship of colloquiums at national, European or international level, especially in the field of celestial and space navigation. It gives advice and recommendations on scientific issues related to its areas, in particular as published documents, with the participation of outside experts and in cooperation with other institutions.

#### Organization

The Bureau des longitudes is composed of 13 members, ex-officio representatives and additional corresponding members as described below:

*   **Members** (as of May 2015, provided in chronological order of the year of election): J. Kovalevsky, R. Cayrel, N. Capitaine, J.-L. Le Mouël, S. Débarbat, J.-P. Poirier, F. Barlier, P. Baüer, N. Dimarcq, J. Laskar, C. Boucher, F. Mignard, A. Cazenave. B.Guinot is honorary member.

*   **Ex-officio representatives** (as of May 2015): J. Parent du Châtelet (MétéoFrance), B. Frachon (SHOM, D. Hestroffer (Paris Observatory), D. Priou (IGN), Mioara Mandéa (CNES).
Corresponding members (as of May 2015, provided in chronological order of the year of election): J. Chapront, J.-C.Duplessy, J.-E. Arlot, V. Brumberg, J.-C. Husson, J.-L. Simon, G. Balmino, M. Crépon, P. Willis, S. Ferraz-Mello, E.F. Arias, C. Turon, C. Sotin, J. Achache, Y. Desnoës, F. Rémy, C. Balkovski, V. Déhant, M. Diament, A. Souriau, A. Morbidelli, T. Quinn, P.Charlot, P. Briole, L. Blanchet, M-F Lequentrec-Lalancette, A.Fienga, G.Reverdin.
G. Amat, J-C Pecker, M. Lefebvre are honorary members.

The Bureau is elected and appointed for a 1-year period; its composition for 2012-2015, was:
for 2012-2013 : President: Pierre Baüer, Vice-President: François Barlier, Secretary: Pascal Willis
for 2014-2015: President: François Barlier, Vice-President: Claude Boucher, Secretary: Pascal Willis
for 2014, Marie-Françoise Lequentrec-Lalancette for 2015.

## Meetings 2012-2015

### Monthly meetings and scientific colloquiums

Besides its regular monthly meetings open to a large public (see above), the scientific colloquiums (including 4 to 5 presentations each), organized on a yearly basis at mid-June by the *Bureau des longitudes* have been as follows:

2012 : *De l'Astronomie de J.D Cassini (1625-1712) à la sonde spatiale Cassini* ; 2013: *se repérer dans l'espace et le temps* ; 2014 : *La voie lactée, notre Galaxie redécouverte par Gaia* ; 2015 : *Le système international de référence terrestre - Développement, réalisations et usages*.

### Scientific colloquiums with the scientific sponsorship of the Bureau des longitudes

The *Bureau des longitudes* was involved in the organization of Colloquiums on “Scientific and Fundamental Aspects of the Galileo System”, organized by the European Space Agency, in Toulouse (France) from 1 to 4 October 2007, and in Padova (Italy) from 14 to 16 October 2009, in Copenhagen (Denmark) from 31 August 2011- to 2 September 2011, in Prague (Czech Republic) from 4 - to 6 december 2013, respectively. The Bureau des longitudes was also involved in a scientific day on «GNSS et Sciences» at CNES on 29 January 2015 and in an international workshop in Astronomy and Dynamics organized on the occasion of the 60th birthday of Jacques Laskar by IMCCE/Paris Observatory (April 28-30, 2015). More details as well as past scientific conferences can be found at (http://www.bureau-des-longitudes.fr/conferences.htm).

## Publications 2012-2015

Besides its responsibility of yearly publications of the French astronomical ephemerides (see above), the Bureau des longitudes publishes books, written by its members or corresponding members, with the participation of outside experts and in cooperation with other institutions on selected scientific topics related to its mission. Several « Cahiers thématiques » have been Published in the «Annuaire du Bureau des longitudes/Guide des données astronomiques» :

### Annuaire 2015:

Les systèmes d'observation météorologique par satellite by Jean Pailleux. Mission accomplie pour le programme exoplanètes de CoRot : une population de planètes à l'étonnante diversité by Daniel Rouan.

### Annuaire 2014

Fluctuations hydro-astmosphériques de la rotation terrestre by Christian Bizouard. Les géantes rouges dévoilées par CoRot et Kepler by Benoit Mosser. Météores et bolides by Jérémie Vaubaillon

### Annuaire 2013

Le seigneur des anneaux by André Brahic.

Comètes et astréroïdes, de subtiles analogies by Anny-Chantal Levasseur-Regourd.
Le Soleil : une étoile magnétique et active, source de particules énergétiques by Nicole Vilmer.

A new book will be edited in Summer 2015 by Hermann editor on the references of « Space and Time » coordinated by C. Boucher and P. Willis.

In a coopération with the «Bibliothèque nationale de France» the «Biblothèque de Observatoire de Paris» had put in a numerical form the «Annuaire du Bureau des longitudes» up to 1950, the «Connaissance des temps» up to 1935, the «Annales du Bureau des longitudes» (all the collection).

## 2) Centre national d'études spatiales (CNES) et Groupe de recherche en géodésie spatiale (GRGS)

R. Biancale, J.-M. Lemoine, S. Bruinsma, J. Laurent-Varin, F. Perosanz

The CNES/GRGS Space Geodesy Team has been involved in different projects and studies among others in gravity field determination or precise positioning. For instance main activities of the team have been processing the data of the GRACE and GOCE missions of space gravimetry, the determination of the gravity field of the Churyumov-Gerasimenko comet to determine the Philae lander trajectory in the framework of ESA's Rosetta mission, as well as precise positioning with GNSS and DORIS.

### GRACE

The GRACE mission (Gravity Recovery and Climate Experiment), a NASA-DLR cooperation, was launched on 17 March 2002 and is still operational, despite battery weaknesses that require powering down the accelerometers and the KBR device for a few weeks twice per year, when passing in the shadow of the Earth, in order to reduce the thermal control of the instruments.

CNES/GRGS processes the data regularly with a latency of about six months to generate monthly gravity models, which are also provided as global grids converted to equivalent water height. Moreover, a new release (GRGS-RL03) was recently produced over 12 years allowing improved interpretations of hydrological mass variations. They are publicly available through the GRGS web site. They provide valuable insights into the fields of hydrology, glaciology, ocean dynamics and solid Earth sciences.

<figure><img src="image_3.png" /></figure>
In addition, the high-resolution model EIGEN6-C4 was produced in cooperation with GFZ-Potsdam. It was calculated by combining data from the GRACE, GOCE and LAGEOS satellites with high-resolution terrestrial data from altimetry over oceans (DTU10) and gravimetry over land (EGM2008). This model is expanded up to the spherical harmonic degree and order 2190, equivalent to a resolution of the order of around 9 km and contains as well temporal terms up to degree and order 50.

## GOCE

GOCE (Gravity field and steady-state Ocean Circulation Explorer), ESA's first Earth Explorer, was launched on 17 March 2009 and re-entered the atmosphere on 11 November 2013. The mission objectives are specified at 100 km resolution: Geoid (gravity anomaly) accuracy of 1 to 2 cm (1 mGal). GOCE was equipped with a tri-axial gravity gradiometer, GPS receiver, laser retro-reflector, star sensors and an ion propulsion engine with variable thrust to reach these goals. The High-level Processing Facility (HPF) computed the GOCE level 2 products on behalf of ESA; CNES/GRGS and GFZ Potsdam produced gravity field models computed via the direct numerical approach in this framework. The latest model, DIR-R5, is constructed with LAGEOS-1/2, GRACE and GOCE data in order to reach high accuracy over the entire spectral range of the model.

The cumulated (formal) error of DIR-R5 at degree 200 in terms geoid height (gravity anomaly) is 0.8 cm (0.2 mGal). The calibration factor, estimated using GPS/leveling data over Germany, is 2. The GOCE mission objectives have been reached according to this evaluation.

<figure><img src="image_4.png" /></figure>

The Figure to the left displays the gravity anomaly difference (mGal) to degree/order 200 between DIR-R5 and EGM2008. The large differences over the continents are improvements thanks to GOCE data over regions for which sparse or low quality data were available for EGM2008.

The relative accuracy of the geoid models was assessed through the comparison of geoid-derived and observed (drifters) mean geostrophic currents. The standard deviation (s) of the difference is then calculated. The Figure to the right illustrates this improvement for the Gulf Stream, where DIR-R5 shows better consistency with drifters than every former models. The optimal resolution of GOCE for oceanographic application is estimated to be between 100 and 125 km.

<figure><img src="image_5.png" /></figure>

## ROSETTA

On 12 November 2014, the general public with all the space centres turned their gaze to the landing of Philae from the ROSETTA probe onto the surface of the comet Churyumov-
Gerasimenko, at more than three astronomical units<fnref n="1" /> from the Earth. In addition to the scientific outlook of this mission, this success is a singular technical feat, which required long preparation over many years. The trajectory alone – from the launch pad in French Guiana on 2 March 2004 to the landing on 12 November 2014 – lasted more than ten years. The probe had to follow a complex path through the solar system with multiple gravity assists from Earth and Mars (Earth-Mars-Earth-Earth) to reach the highly eccentric orbit of the comet targeted. During its trip, ROSETTA crossed the asteroid belt twice where it could observe Stein (5 September 2008) and Lutetia (10 July 2010) and thus obtain photos and scientific measurements of remarkable quality.

All systems did not work well, such as the anchor harpoons of Philae, the mission is considered successful and allows for scientific measurements in situ, which is a historic premiere. To accomplish this mission several space agencies, like ESA and CNES, worked coordinated. The French space agency had the responsibility of Philae as well as proposing a consensual "touchdown" site, taking the scientific objectives and operational constraints into account. Several Philae descent trajectory simulations were calculated by the flight dynamic teams, with the objective to reduce the risks.

The CNES/GRGS Space Geodesy Team was involved in modelling the gravity of the comet using knowledge of its shape and the tracking measurements at the approach of the comet. The gravity, which plays a major role in the dynamics of the descent of Philae, was to be known with the greatest possible precision. Two models were developed, in spherical harmonic expansion and as a discrete grid of the space surrounding the comet.

Philae touched down at the predicted location before bouncing twice at least.

## GNSS

The contribution to the International GNSS service has driven our activities in the domain of GNSS processing. Our GPS products are contributing to the combined IGS products since 2010, we became an official Analysis Center for the GLONASS constellation in 2011, and for Galileo in 2012 in the framework of the Multi-GNSS experiment. The motivation of such investment in multi-GNSS processing capability is twofold:

- Hybridizing GNSS signals from multiple constellations will improve earth kinematic observation. Future ITRF realization as well as Earth Orientation Parameters (EOPs) determination will be improved;

- The precise satellite orbit and clock products will be available for high accuracy applications that will benefit from an increased number of satellites in visibility.

Our products are based on an original zero-difference approach. Because they keep the integer property of the satellite clocks, they also include the capability to fix integer phase measurement ambiguities. As a consequence, the users of our orbit and clock products are also able to do PPP positioning with fixed ambiguities. The accuracy is then significantly improved as shown in the following figure.

<footnote n="1">¹
The astronomical unit (au), approximately the Earth-Sun distance is 149,597,870,700 meters exactly.</footnote>
<figure><img src="image_6.png" /></figure>

The position of a fixed point is calculated every 300 seconds during 4 consecutive days in order to evaluate the level of repeatability of the solution. The upper curve represents the PPP solution from daily batches. The middle curve is derived from a single 4 days processing which improves the daily boundaries discontinuities. The down curve corresponds to daily solutions including ambiguities resolution which both guaranties the continuity and improves the repeatability of the solution.

This opened a wide area of applications and has given the opportunity to develop numerous cooperations and to contribute to various scientific projects:

- Antarctic glaciers vibration: GNSS measurements on the Metz Glacier (project CRACICE, B. Legresy, LEGOS) have been processed using the PPP technique as no reference station was available in that area. The existence of proper oscillation modes has been demonstrated by Lydie Lescarmontier in her Ph. D (Lescarmontier et al. 2012);

- Open sea level measurement: the tracking of GNSS buoys at a 5 cm level using the PPP mode has been demonstrated by Francois Fund during his Post Doc at CNES/GRGS (Fund et al. 2013);

- Space altimeter CAL/VAL on lakes: The PPP technique is a new opportunity to track boats and buoys with GNSS measurements in order to evaluate satellite altimeter biases. Several experiments on the Issikkul lake were successful (Crétaux et al. 2013).

These results have demonstrated that the PPP technique can be a powerful alternative to the classical differential GNSS positioning approach. This is the major result of our activity during the period 2011-2014.

## 3) Ecole supérieure des Géomètres et Topographes (ESGT)

(Stéphane Durand)

Le présent rapport ne fait état que de l'activité de recherche de l'équipe L2G dans les domaines liés aux activités de la section 1 du CNFGG, à savoir la géodésie, la gravimétrie et l'interférométrie radar différentielle (DInsar).

Le L2G utilise principalement la géodésie pour mesurer les déformations de l'écorce terrestre, à toutes les échelles, qu'elles soient d'origine naturelle ou anthropique. Différentes techniques sont maîtrisées au laboratoire :

- la topométrie de précision, qui consiste à mesurer des distances, des angles et des dénivelées entre les points d'un réseau ;

- le GNSS différentiel qui fournit des mesures ponctuelles ;
- l'interférométrie radar qui exploite les interférences obtenues entre les images radar acquises par un satellite à deux dates différentes pour mettre en évidence des déformations de surface, et que nous considérons à ce titre comme une technique géodésique ;

- La gravimétrie qui consiste à mesurer les composantes du vecteur gravité.

Cette maîtrise de différents domaines de la géodésie (GNSS, INSAR, gravimétrie, topométrie) est un point fort du laboratoire et lui permet de proposer une approche multi-technique de la mesure de déformation, déclinée en plusieurs sous thèmes :

- L'étude des surcharges et la propagation dans l'atmosphère

- Le développement d'une instrumentation de gravimétrie mobile LiMOG

- La combinaison de techniques

# Surcharges et propagations

## Etude des surcharges

Les effets de surcharge correspondent à des déformations de la croûte terrestre induites par les redistributions des masses atmosphériques, hydrologiques et océaniques. Ces effets sont périodiques (annuels, semi-annuels...) et les déformations qu'ils induisent peuvent atteindre plusieurs centimètres en vertical et ont en général une amplitude horizontale trois fois plus faible. Ces déformations sont aujourd'hui bien observables par les techniques de géodésie spatiale et par le GPS en particulier grâce aux progrès récents dans l'instrumentation et dans le traitement des données. En effet, ces techniques permettent d'atteindre un positionnement millimétrique de points à la surface de la Terre. Ainsi l'étude des effets de surcharge se fait principalement par l'analyse de longues séries temporelles de coordonnées de différentes stations géodésiques. Dans l'idéal plusieurs années de données sont nécessaires afin de déterminer les contributions annuelles et semi-annuelles de chacun des effets de surcharge. Ceci implique donc des temps de traitement souvent longs et coûteux en termes de ressources informatiques.

Au sein du L2G, nous étudions les effets de surcharge depuis de nombreuses années. Les deux études principales menées sont résumées ci-après.

Suite à une première étude issue d'une campagne multi technique, la thèse de François Fund (Nov. 2006 – Nov. 2009) nous a conduit à étudier le phénomène de surcharge océanique maréale dans le grand Ouest de la France. Par rapport aux précédentes études, ce travail apportait une plus forte densité de stations, une période de données plus conséquente et une optimisation du traitement tenant compte des dernières évolutions. Cette étude a été menée en collaboration avec Antoine Mocquet (directeur de thèse, Université de Nantes) et Johannes Boehm (Université de Vienne, Autriche).

En parallèle, une autre étude était centrée sur les autres effets de surcharge qui ne sont pas pris en compte dans le traitement des données de géodésie spatiale, à savoir les surcharges océaniques non maréales, atmosphériques et hydrologiques. Cette étude a permis de déterminer leurs effets sur les résultats de positionnement et en particulier les artefacts induits par ces différents effets de surcharge sur l'estimation des vitesses tectoniques issue de campagnes de mesures GPS espacées dans le temps. Pour cela les résultats GPS ont été comparés aux modèles géodynamiques de déformation. Ceci a été réalisé notamment dans le cadre des campagnes GPS RESPYR pour la mesure des déformations des Pyrénées. Ces travaux ont été menés en collaboration avec des chercheurs de l'Observatoire Midi-Pyrénées (Félix Perosanz, Pascal Gegout, Alexis Rigo, Richard Biancale) et avec Tonie van Dam (Université du Luxembourg) et Zhao Li (Université du Luxembourg) et Jean-Paul Boy (EOST) pour les aspects modélisations.
Une étude plus poussée de ces effets de surcharge a été réalisée dans le cadre de la thèse de Marcell Ferenc (2011-2014) (financement CNES - Région Pays de la Loire), soutenue le 9 décembre 2014.

Nous avons démontré la capacité du positionnement iPPP GPS pour suivre les déformations sub-diurnes induites par la tempête Xynthia (2010) en utilisant les produits REPRO2 du centre d'analyse du GRGS et le logiciel GINS-PC (CNES/GRGS). L'analyse spatiale et temporelle menée a notamment permis de mieux comprendre la réponse de l'océan à une variation brusque et intense de la pression atmosphérique. Ces travaux ont donné lieu à plusieurs communications dans des congrès internationaux et nationaux. D'autre part des outils d'analyse en composante principale des séries temporelles ont été mis en place et validés au cours d'un stage de master de Ayoub Asri. La poursuite de ces travaux à une échelle globale pour un ensemble de sites répartis dans différentes parties du globe avec différentes configurations environnementales contribuera à la validation des modèles des effets de surcharge.

## Propagation troposphérique

Les techniques de géodésie spatiale sont sensibles à la propagation des ondes dans l'atmosphère terrestre. Cet effet fausse les mesures et doit être corrigé lors du traitement des données. La troposphère, couche la plus basse de l'atmosphère, est la couche qui a le plus d'impact sur les mesures. Une bonne correction de la propagation dans la troposphère permet d'atteindre une grande précision dans le positionnement (quelques millimètres), précision requise désormais pour de nombreuses études géodynamiques, et en particulier celle des effets de charge. C'est pourquoi les études des effets de propagation et de charge sont étroitement liées.

Ainsi, afin d'optimiser la précision dans l'étude des surcharges, des travaux sur la propagation des signaux GNSS dans l'atmosphère se sont également poursuivis dans l'équipe au cours de la même période.

Après le doctorat de François Fund portant sur l'évaluation des produits troposphériques issus des données météos dans le cadre du réseau européen GPS qui a donné lieu à plusieurs publications, nous avons poursuivi les travaux sur la propagation troposphérique dans le cadre du travail de fin d'études de Anne-Lise Lavaux en 2011. Le contenu en vapeur d'eau de l'atmosphère a été calculé pour un réseau de stations situé en Espagne. Nous avons étudié la sensibilité de cette estimation aux modèles, aux stratégies de traitement et aux logiciels.

En 2012, nous poursuivons dans ce champ d'investigation par une étude comparative des résultats du contenu en vapeur d'eau sur deux réseaux GPS, l'un en Corse et l'autre en région parisienne. Ces deux réseaux denses de stations permanentes GPS sont situés dans des environnements topographiques et géographiques opposés ce qui permettra d'évaluer la qualité de notre estimation.

Toutes ces études s'inscrivent dans le cadre du projet TOSCA « surcharge et propagation » dans lequel sont impliqués L.Morel (co-proposant), J. Nicolas et M. Ferenc.

## Traitement de données GNSS

Dans l'objectif global du laboratoire qui concerne l'étude des déformations de la croûte terrestre à différentes échelles spatio-temporelles, nous avons poursuivi les études par géodésie spatiale et notamment les traitements de données GNSS. Ces études ont nécessité l'utilisation de la suite logicielle GINS-PC et du logiciel GIPSY sur le serveur de calcul du laboratoire.

Le calcul de la déformation des Pyrénées par GPS, projet financé par l'INSU en 2008, à partir des observations GPS de plusieurs campagnes (1995, 1997, 2008 et 2010) et d'un traitement avec les logiciels GINS-PC et GIPSY a été mené. Il a montré le niveau très faible de déformation de ce massif. Ces résultats s'accordent au premier ordre avec les traitements réalisés par d'autres logiciels mais de futures investigations sont nécessaires pour analyser et comprendre les disparités. Collaboration avec
Alexis Rigo (Observatoire Midi Pyrénées, Toulouse), Félix Perosanz (CNES) et Philippe Vernant (Université Montpellier).

D'autres traitements comme le PPP (Precise Point Positioning) interviennent régulièrement dans le cadre de différents projets (propagation troposphérique, traitements cinématiques à hautes fréquences, trajectographie d'avion pour Lidar) et permettent également l'évolution de l'enseignement en géodésie à l'ESGT et la diffusion des nouvelles méthodes GNSS pour la topographie. De plus, la capacité récente de GINS-PC à faire du PPP avec ambiguïtés entières fixées ouvre de larges perspectives.

Ces traitements se sont poursuivis toujours dans le cadre du projet TOSCA « outils et méthode de traitement GNSS pour les Géosciences » comme par exemple l'étude de l'influence de l'incohérence des valeurs de calibration des antennes GPS (IGS05 et IGS08) avec les orbites IGS (IGS05). Ce projet TOSCA implique J. Nicolas en tant que co-proposant et L. Morel, F. Durand, M. Ferenc en tant que participants.

## Interférométrie radar différentielle (DinSAR)

Contrairement aux techniques GNSS et topométrie terrestre qui ne fournissent des déformations que par dérivation d'une position ponctuelle, la technique DInSAR (interférométrie radar différentielle) fournit directement un déplacement, sous certaines conditions toutefois (notamment une grande stabilité de la surface, qui exclut l'eau et la végétation arborée). La technique classique et la technique des points stables (permanent scatterers) ont été implémentées à partir d'outils gratuits (compilation et écriture de scripts) : DORIS, STAMPS, et testées à partir de données ERS. Des premiers travaux en contexte urbain ont été menés à travers un stage de niveau master 2 en 2008 et un post-doctorat en 2009-2010, avec l'étude des déformations sur la ville de Nantes. Ces travaux s'inscrivaient dans le projet de télédétection urbaine de l'IRSTV.

## Gravimétrie mobile

L'activité de recherche en gravimétrie est ancienne au sein du L2G. Elle est actuellement portée par 3 membres du L2G, José Cali (MCF), Jérôme Verdun (MCF) et Frédéric Durand (IGE).

L'équipe participe à l'action de recherche en gravimétrie mobile du LAREG/IGN qui porte sur le développement de différentes méthodes d'estimation du champ de pesanteur à partir des observations issues du capteur mobile LiMO-g. Une thèse (Li Qi) débutée en 2009 avait pour objectif de développer une méthode intégrale de résolution de l'équation différentielle de navigation. La discrétisation de la solution intégrale permettrait d'estimer par la méthode des moindres carrés implicites la perturbation de pesanteur. L'avantage de cette méthode réside dans le fait que la perturbation de pesanteur peut être considérée comme une variable spatiale ce qui permet l'utilisation de modèles empiriques de variation du champ de pesanteur issus de modèles globaux de champ plus réalistes que ceux utilisés actuellement dans la méthode d'estimation par filtrage temporel de Kalman. Malheureusement, cette thèse n'a pu aboutir.

Néanmoins les compétences acquises sur cette thématique ont permis de mettre en place des collaborations avec l'IUEM de Brest /l'IFREMER qui trouve dans l'équipe du L2G l'expertise qui lui manque pour développer une instrumentation similaire à embarquer à bord d'engins sous-marins (AUV, ROV) pour faire des levés conjoints de bathymétrie/gravimétrie en fond de mer.

Plusieurs projets de recherche ont été déposés pour renforcer cette collaboration :

- Actions de recherche sur projets INSU, sciences de la Terre, appel d'offre 2010 GraviMob (Gravimétrie mobile sous-marine. Le projet n'a pas été retenu en l'état mais un reliquat de financement a été octroyé.
- ANR /ASTRID 2011 : Projet GRAVIMOB : Gradiométrie et Gravimétrie Mobiles Sous-Marines. Projet non retenu.

Au début de l'année 2012, une rencontre à Toulon avec les personnes de l'IFREMER/IUEM et LAREG a eu lieu. Il a été décidé d'embarquer le capteur mobile LiMO-g dans un AUV pour effectuer des tests de faisabilité, prévus en octobre 2012.

Concernant le second point l'effort porte sur le traitement des données issues de la campagne de Sainte-Maxime. Actuellement seules des données semi-synthétiques issues de cette campagne ont été traitées par filtrage de Kalman pour valider la chaîne de traitement. Il s'agit ici de montrer à la communauté scientifique les premiers résultats d'estimation des perturbations de pesanteur le long d'un profil à partir des seules mesures réalisées par le capteur mobile LiMO-g ce qui permettrait de valider le prototype.

Dans ce but, un travail de fin d'études a été proposé, en 2012, à un étudiant de l'ESGT, Nicolas Damenet, pour développer une méthode d'estimation du champ de pesanteur par filtrage de Kalman spatialisé. Les résultats feront prochainement l'objet d'une publication.

Une thèse, intitulée « modélisation à haute résolution du champ de gravité terrestre par combinaison de mesures issues de la gravimétrie satellitaire et la gravimétrie terrestre mobile : application au domaine sous-marin » a débutée en janvier 2014 (Clément Roussel), co-financée par la DGA et la Région Pays de Loire.

## Combinaison de techniques

Le L2G possède des compétences théoriques et pratiques sur plusieurs techniques de mesures de déformation, notamment la topométrie terrestre, le positionnement GNSS et l'interférométrie radar (InSAR). Dès ses origines, le laboratoire a donc naturellement cherché à comparer ces différentes techniques dans ses travaux de mesure de la déformation du sol.

La combinaison des techniques géodésiques de mesures de déformation a été identifiée dans le quadriennal 2007-2010 comme un des axes de recherche du L2G, et l'évaluation de l'AERES en fin de quadriennal a confirmé l'importance de poursuivre cet axe par la suite.

La thèse de Benoit Legru (2007-2011) a permis de mieux cerner les potentialités de combinaison entre les techniques et les différents niveaux de combinaison. Comme illustré par la figure ci-après, la combinaison entre techniques peut en effet être réalisée soit au niveau des coordonnées ou des déformations calculées en utilisant chaque technique séparément, soit au niveau des observations, dans un traitement en bloc ou par cumul d'équations normales.

Ainsi, bien qu'ayant limité la combinaison à deux techniques (GNSS et topométrie), cette thèse a permis d'établir un environnement conceptuel dans lequel les combinaisons d'autres techniques peuvent ensuite être envisagées.

<figure><img src="image_7.png" /></figure>

Au cours du précédent quadriennal, le L2G a suivi deux pistes pour la combinaison des techniques géodésiques :

- la combinaison des observations issues de différentes techniques par cumul d'équations normales ;

- la modélisation de la troposphère par la technique GNSS afin de corriger les interférogrammes en INSAR.

Les deux premières techniques ayant fait l'objet d'une combinaison au sein du L2G sont le GNSS et la topométrie. Le laboratoire a, au cours du précédent quadriennal développé ses
propres outils pour le traitement combiné et la simulation des mesures GNSS et topométriques, appelé CoMeT (Compensation de Mesures Topographiques).

Au cours de la thèse de Benoit Legru, l'intérêt de la combinaison GNSS/topométrie a été montré, d'abord en simulation puis sur des données réelles. Les bénéfices attendus sont l'amélioration de la précision de mesure, mais aussi la possibilité d'accéder à une gamme plus étendue d'échelles spatiales et temporelles.

Le stage ingénieur de Paul Antoine Bacci en 2012 a été l'occasion de s'intéresser à la combinaison entre deux autres techniques : GNSS et INSAR. Il s'agissait d'utiliser des modèles troposphériques estimés par GNSS pour permettre l'amélioration du traitement INSAR. Les résultats obtenus lors de ce travail seront publiés prochainement. Il s'agit là d'une première tentative de combiner les mesures de déformation issues des techniques GNSS et INSAR, qui sont complémentaires à bien des égards :

possibilité de « caler » l'interférogramme sur quelques points GPS pour lever les ambiguïtés de l'interférométrie ; complémentarité des anisotropies de sensibilités (le GPS est plus sensible à la composante horizontale qu'à la composante verticale, et l'InSAR n'est sensible qu'à la projection du déplacement sur l'axe de visée) ; interpolation dans l'espace et dans le temps (utilisation du GPS pour densifier l'InSAR dans le temps et de l'InSAR pour densifier le GPS dans l'espace.

# 4) Ecole et Observatoire des Sciences de la Terre (EOST)

(Jean-Paul Boy, Jacques Hinderer, Séverine Rosat, Jean-Daniel Bernard, Frédéric Littel, Frédéric Masson, Patric Ulrich)

## Introduction

Les activités géodésiques de l'EOST portent sur l'étude des variations temporelles de la gravité et les déformations du sol. Elles reposent entre autre sur les services nationaux d'observatoire géodésie gravimétrie labellisé par le CNRS/INSU.

L'EOST assure la responsabilité de l'Observatoire Gravimétrique de Strasbourg (OGS) comprenant les instruments installés depuis 1987 à station J9 à proximité de Strasbourg et le gravimètre supraconducteur de Djougou (Bénin) depuis juillet 2010, ainsi que d'une dizaine stations GPS permanentes du réseau national RENAG dans le Nord-Est de la France.

En complément, l'EOST a développé un service proposant des modèles des effets induits charges par les charges superficielles (atmosphère, océan et hydrologie continentale) sur les différents observables géodésiques. Ce service est désormais reconnu comme produit officiel du Global Geophysical Fluid Center de l'IERS.

L'EOST a rejoint en 2013 le Groupe de Recherche de Géodésie Spatiale (GRGS) fédérant 12 équipes appartenant à des institutions nationales ayant des activités en géodésie spatiale.

## Infrastructure d'observation

Les mesures des variations temporelles de gravité, à l'aide de gravimètres supraconducteurs, ont débuté en 1987 sur le site de l'OGS (fort J9) ; ce premier instrument a été échangé par un modèle plus récent en 1996, et celui-ci sera prochainement remplacé par un modèle de dernière génération grâce à l'EQUIPEX RESIF.
<figure><img src="image_8.png" /></figure>

Gravimètre supraconducteur GWR CO26 en fonctionnement à l'Observatoire Gravimétrique de Strasbourg et localisation des différents réseaux permanents géophysiques dans le Nord-Est de la France (sismologie, GPS, magnétisme, etc.).

En complément, des mesures répétées à l'aide du gravimètre absolu FG5 #206 depuis 1997 sont effectuées sur le site de l'OGS, permettant d'étalonner le gravimètre supraconducteur et d'investiguer sa dérive temporelle, comme le montre la figure ci-dessous.

<figure><img src="image_9.png" /></figure>

Variations temporelles de gravité mesurées par le gravimètre GWR CO26 (noir) et comparaison avec les mesures absolues (rouge) entre 1997 et 2012 (d'après Rosat et al., 2009).
En parallèle de nombreuses mesures environnementales complémentaires (station météorologique, sondes d'humidité du sol, etc.) ont été installées au cours des dernières années.

En juillet 2010, un autre gravimètre supraconducteur (GWR SG-060) a été installé en zone équatoriale, à proximité de Djougou au Bénin. Cet instrument permet de compléter la couverture géographique du réseau mondial GGP (Global Geodynamics Project) de gravimètres supraconducteurs, à la fois en Afrique (un autre instrument en Afrique du Sud) et dans la zone équatoriale. Malgré des conditions difficiles (climat chaud et humide, logistique), cet instrument fonctionne sans interruption depuis et se caractérise par une qualité et un bruit similaires aux autres instruments du réseau, notamment celui de Strasbourg.

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

The GGP network of Superconducting Gravimeters

<figure><img src="image_12.png" /></figure>

Gravimètre SG-060 installé à Djougou (Bénin) et réseau mondial GGP (Global Geodynamics Project).

## Perspectives

Parmi les projets en cours de l'EOST et de l'Observatoire Gravimétrique figure le développement d'un nouveau service de l'IAG regroupant les activités du Global Geodynamics Project et de l'International Centre for Earth Tides (ICET) : International Geodynamics and Earth Tide Service (IGETS), dont l'EOST sera le bureau central. Ce projet doit être validé par l'IAG lors de la prochaine assemblée générale de l'IUGG à Prague en juin/juillet prochain. Ce service reposera également sur le service des charges développé à l'EOST.

A l'échelle nationale, nous proposerons la labellisation par le CNRS/INSU de la station équatoriale de Djougou, afin de pérenniser son fonctionnement.

## 5) Institut national de l'information géographique et forestière (IGN) (François l'Ecu, Olivier Jamet)

The geodetic activity of IGN is divided in two parts: operational and research, respectively managed by SGN (Service de Géodésie et Nivellement) and LAREG (LAboratoire de Recherche En Géodésie).

### Operational part: SGN

The SGN is in charge of the realization and maintenance of the national geodetic, gravimetric and height references in France, has also a significant activity on international networks and services, and conducts studies and developments in all fields of geodesy.
## National infrastructure

The RGP is the French GNSS permanent network. The constitution of this network is based on a wide partnership between the IGN, public institutions (universities, research laboratories, urban communities, etc...) and private companies, especially RTK network managers. The great increase of the number of stations, from 203 at the beginning of 2011 to 405 at the end of 2014 is mainly due to those private partners. We have to mention, that during this time, the equipment of French overseas territories has been continued by setting up stations in Martinique and in Guadeloupe. The present repartition of stations is IGN (6 %), public institutions (26 %), private companies (68 %). Today, the network is practically full GNSS: the 25 IGN stations are all GPS-GLONASS-GALILEO; 34 % of public institutions stations are GPS-GLONASS and 30 % of them are GPS-GLONASS-GALILEO; 66 % of private societies stations are GPS-GLONASS. SGN collects data, checks data quality, delivers data on the Internet, process hourly, daily and weekly solutions and manages two operational centers. SGN contributes to E-GVAP by sending every hour tropospheric parameters (ZTD) which are introduced by Météo-France in its atmospheric prediction models.

<figure><img src="image_13.png" /></figure>

The maintenance of RBF (Réseau de Base Français), geodetic network of about 1000 points, has been continued. Because the IGN is responsible for the national gravimetric reference since 2011, the RBF has been chosen to set up this physical component on French territory: today, each RBF point obviously has geometrical position (λ, φ, h), an altitude (H); and also at least one point per site has a gravitational value (g). From 2011 to 2014, network geometry and adjustment processes have been significantly improved, thus leading to a global network accuracy of 20 µGals, on metropolitan part, Martinique and Guadeloupe.

Concerning the French Vertical Reference (IGN69), the maintenance process based on "triplets" has been continued. A "triplet" is a group of at least three levelling benchmarks, far
away one another at the most one kilometer. Anywhere in France, somebody is at less than five kilometers far from a “triplet”, thus provides an easy access to the vertical reference. Since 2008, about 1100 “triplets” are observed by year, using precise levelling to control inside stability, and GNSS to provide ellipsoidal height and check the absolute stability with respect to the RGP stations.

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

The first maintenance cycle of then French 13200 triplets will be achieved in 2020. Today, 64% of this step is performed, and 32% has been realized from 2011 to 2014.

The observations of the reference levelling network NIREF (NIvellement de REFérence) have been carried on. This network will be the French component to EVRS.

<figure><img src="image_16.png" /></figure>

During the four last years, the main activity has consisted in a new levelling of the Marseille-Dunkerque line (not finished yet), in order to improve understanding of the famous IGN69 northern-southern bias.

## International involvement

The implication of SGN in different international services has been continued since 2011: IGS (International GNSS Service) as data center and combination center; IDS (International DORIS Service) as data center and analysis center; and EUREF (European Reference Frame) as data center and analysis center.

Concerning DORIS network installation and maintenance, three new stations were set up: Le Lamentin (Martinique), Socorro (Mexico), and Owenga (New-Zealand).

During the period 2011-2014, the new REGINA GNSS network has appeared (REseau Gnss pour l'Igs et la NAVigation), managed by CNES and IGN. Today, REGINA is composed of 30 permanent stations around the world, and six additional are planned from now to the end of 2015. The objectives of REGINA are numerous. Firstly, it aims at improving ITRF: the
stations are collocated on DORIS sites, thus enables accurate ties between the two technics, using GNSS and topometry. Secondly, the idea is to prepare the GALILEO arrival, more specifically by developing real-time PPP multi-constellations tools and services based on this new global network. The SGN is responsible for the ground operational part, and for the implementation of the real-time treatment center CT2R2 (Centre de Traitement Temps Réel Regina).

<figure><img src="image_17.png" /></figure>

On another field, SGN has been strongly involved in SONEL project (Système d'Observation du Niveau des Eaux Littorales). The purpose of SONEL is to acquire, treat, store and distribute data collected from tide gauges and space geodesy technics, so as to study long-period variations of sea level. In this project, SGN is associated with LIENSs (LIttoral ENvironnement et Sociétés, CNRS/Université de La Rochelle), LEGOS (Laboratoire d'Etudes en Géophysique et Océanographique Spatiales, CNRS/Université Paul Sabatier), and SHOM (Service Hydrographique et Océanographique de la Marine), and is responsible for the geodetic ground works : GNSS stations collocation on tide gauges sites in France and abroad (GLOSS and TIGA programs), GNSS and levelling ties, and GNSS data center.

## Studies and developments

About GGSP (Galileo Geodetic Service Provider), collaboration between SGN and LAREG has been increased, the first achieving weekly solutions and combinations, rapid daily solutions; and the second computing GTRF (Galileo Terrestrial Reference Frame) and time series.

On vertical reference aspects, SGN is computing for metropolitan territory a new gravimetric quasi-geoid using RTM process (Residual Terrain Method), using high-density gravimetric data, accurate data terrain model, and EGM08 as global gravity field model. For its adaptation on vertical reference frame, about 6000 ERNIT program GNSS-levelled points will be used. So as to be consistent with the emergence of REGINA network and GALILEO program arrival, SGN has initiated the development of multi-constellations ultra-rapid orbits and satellite clocks. Firstly, for its own present needs: hourly computations of RGP network (Météo-France partnership), RGP observations quality check. Secondly, so as to improve its role of national geodetic reference provider, by contributing in a closed future to a PPP real-time positioning service. Today, operational GPS-GLONASS rapid orbits are computed using Bernese software, and the capabilities of Napeos software are studied in order to deliver GPS-GLONASS-GALILEO products.
Research part: LAREG

The *Laboratoire de Recherche en Géodésie* (LAREG) is a research team of the Research and Training Department of IGN in charge of scientific activities in the fields of reference system theory and realization, gravity field modeling and determination, and more generally space geodesy. The team is part of the french Research Group of Space Geodesy (GRGS) and hosts the IERS International Terrestrial Reference System (ITRS) Center.

The ITRS Center is in charge of the ITRS definition, development, realization and publications of the International Terrestrial Reference Frame (ITRF) solutions.

The members of the ITRS Center, often in cooperation with other scientists, conduct research and developments activities relating to the ITRF in particular and reference frames in general. R&D activities include ITRF accuracy evaluation, mean sea level, loading effects, combination strategies, and maintenance and update of CATREF (Combination and analysis of terrestrial reference frames) software developed by IGN for the ITRF computations. The main scientific ITRF results, as well as specific data analysis and combination are published in peer-reviewed journals, but also presented at international scientific conferences.

The main highlights of the ITRS Center activities during the period 2011-2015 are reflected by the list of selected publications as summarized in the references' section. Among other highlights, we mention here the ITRF2008 publication and its corresponding plate motion model (ITRF2008-PMM). The following figure illustrates the main results of this study, showing the magnitude of the velocity differences between ITRF2008-PMM and the two known geophysical models (NNR-NUVEL-1A and NNR-MORVEL56). We observed in particular a large angular residual velocity of the order of 4 mm/yr of the Australian plate between ITRF2008-PMM and NNR-MORVEL56, as illustrated by figure 1. This bias is not observed when comparing ITRF2008-PMM to NNR-NUVEL-1A, suggesting that it is originated from the new model NNR-MORVEL56.

<figure><img src="image_18.png" /></figure>

Velocity differences after rotational transformation between ITRF2008 and NNR-NUVEL-1A (left) and NNR-MORVEL56 (right). In mm/yr: green: less than 2; blue: between 2 and 3; orange: between 3 and 4; red: between 4 and 5; black: larger than 5.

Besides the activities of the ITRS Center, LAREG is involved in several international activities, either in the context of scientific international services (in particular the IERS Combination at the observation level working group, the ILRS GRGS analysis center, and several IAG commission activities) or in the context of research contracts (LAREG contributed in particular to the HYMEX – hydrological cycle in the Mediterranean experiment – international project).
# 6) Observatoire de la Côte d'Azur (OCA) (Pierre Exertier)

## Equipe « astrogeo » de l'UMR 7329 GEOAZUR

### Introduction

Geodesy is directly involved in the measurements of key parameters, and questions as: what is the current speed of the rise of sea level in different regions of the globe? How the Earth deforms it under the influence of continental drift, seismic events or melting ice caps? These questions are asked now to scientists, but first and foremost, Fundamental Geodetic Observatories should allow to build and maintain a common reference system very accurate, which should provide the measurement of 3D displacements of about 0.1 mm / year in recent years. Fundamental Geodetic Observatories must define a primary global reference frame by maintaining the accuracy to which belong the secondary frames, regional or local, for the development of the society.

### Astrogeo team, and Grasse geodetic facilities

The team “astrogeo” (Geodesy and Metrology of the Nearby Universe) works on topics first based on fundamental Astronomy (Space mechanics, geodynamics and space&time reference systems and frames, Relativity), and on topics on the edge of geophysical and geodesy. Its research activities are organized into some major themes, as:

- Combined methodological approach of gravitational systems (trajectories of natural bodies and space vehicles), physics of the planets/satellites environment (internal structure, gravity, rotation) and fundamental physics (G dot, PPN parameters, dynamics and geometry),

- Equivalence Principle (EP) of Einstein theory, and the development of the future Microscope (CNES, ESA, for 2016) space satellite,

- Study of the constancy of the terrestrial reference by space techniques (GPS, laser ranging, and radar altimetry of ocean surface: marine geoid, with Jason missions).

- Time transfer techniques comparison (optics based on laser and GNSS), and the exploitation/ operation of the T2L2 space mission on Jason-2 satellite: for time transfer by space laser studies and operations (ground/ground and ground/space) and fundamental physics.

Potential participation in the ACES/Pharao space project (on ISS), in terms of time&frequency transfer studies, and data processing with the ELT laser instrument (very close to T2L2).

In addition, « astrogeo » team is also developing the Satellite/Lunar Laser Ranging (SLR/LLR) station, MeO - CERGA, which is located on the « plateau de Calern », near Grasse, France.

The MeO telescope with its electronics and opto-mechanical parts (it is able to proceed Time Transfer, SLR and LLR techniques), was completely renewed during the 2005-2009 period. From 2010 up to now, the laser ranging operations re-started, essentially for LLR on the Moon and dedicated spacecraft like LRO, Radio-Astron, etc., and satellite missions as Jason-2, GNSS and LAGEOS.

In synergy with « Observatoire de Paris », the team is also involved in the International Laser Ranging Service (ILRS) as Analysis Center for Earth rotation and polar motion, station geocentric coordinates, and ephemeris (via the INPOP planetary solution). As an example of data acquisition on the Moon targets (Apollo and Lunakhod reflectors), the figure above shows the 5 year history of ranging Normal Points, after 2009.
<figure><img src="image_19.png" /><figcaption>Laser Ranging on the Moon (LLR) from Grasse, France.</figcaption></figure>

## SLR-NG for Tahiti, a future Geodetic Observatory

The team « astrogeo » was positioned in 2011 for participating in the development of a Fundamental Geodetic Observatory in Tahiti, including the 4 space techniques (GPS, DORIS, Laser and VLBI). Tahiti, which is one of the MOBLAS laser site of NASA, must be developed (with VLBI2010 antenna and New Generation Laser Ranging) for reasons of global Geodesy, satellite tracking (orbitography), etc. Above all, the objective is to establish a new form of cooperation between several French organizations, the University of French Polynesia (UPF), the authorities of the territory, and NASA.

A summary document and recommendations of the Fundamental Geodetic Observatory has been drafted and was presented to authorities in March 2012; it tooks particular objectives of GGOS. This is currently underway, with the following objectives:

- How the state of the Tahiti instruments (laser station, tracking instruments (GPS / DORIS, tide gauges, VLBI))

- Establish a budget for the new Geodetic Observatory - assessment of current support (UPF, CNES, others)

- Propose a labeling (GGOS oriented)

- Organize the administrative structure

- List the production constraints (land, access, infrastructure, tools, staff ...)

- Consolidate the funding opportunities for new instruments and infrastructure

- A schedule

An « EQUIPEX » project was prepared and submitted in September 2011, involving CNES, IGN and several laboratories led by INSU, in the sense of developing new technologies for the future geodetic observatory, essentially a New Generation Laser Ranging system. Several studies and research activities were conducted with new ingeneers, leading to a novel concept : a transportable system of 500 kg, a telescope of 45cm, and a kilo-Hertz laser. The
complete budget has been estimated at 1.3 MEuros ; several companies were approached to
help in defining the concept and costs.

Time Transfer by Laser Link (T2L2) space experiment

The Time Transfer by Laser Link (T2L2) instrument is passenger of the oceanographic space mission Jason2, that was launched in June 2008 at an altitude of 1335 Km. T2L2 consists of an optical system (detection) and an electronic device for the timing. The principle of the experiment is based on the SLR technique; thanks to the laser reflector array located on the satellite in support of the precise orbitography, T2L2 benefits from the 2-way ranging, whose repeatability error and accuracy reach a few millimeters and less than 1 centimeter, respectively. At ground level, T2L2 relies on the International Laser Ranging Service (ILRS) network, whose stations are tracking the satellite for 5 to 6 times per day maximum of 10 to 15 minutes each.

The a priori potential of the T2L2 experiment is important in terms of time&frequency metrology, space geodesy and fundamental physics. The satellite altitude and the field of view of the optics of T2L2 being of ±55°, Common View (CV) passes are possible between remote ground stations at 6000 km. Great efforts have been made on the ground (OCA, Grasse geodetic site) in order to: i) improve the measurement of the time delays in the cables (principle, methods and hardware), ii) determine the best point of reference of the time and frequency laboratory (PPS and its distribution to devices: GPS, laser, etc.), iii) calibrate each time transfer technique, optical and microwaves.

Campaigns

Several calibration time transfer campaigns have been conducted since 2009, notably in the « Observatoire de Paris », Grasse (F), Herstmonceux (UK), and Wettzell (G) with different configurations in order to: i) properly measure the performances of the time transfer by laser link (for short and long term stability), ii) progress in the measurement (method and hardware) of the ground links (delay, cables, etc.) including the time distribution of pulses between equipment such as clock, SLR system, and GPS antennas, iii) compare the T2L2 time transfer to existing microwave techniques such as GPS. In addition, we used the French Transportable Laser Ranging Station (FTLRS, Nicolas et al., 2000) to conduct campaigns from the Observatoire de Paris in addition to European SLR's and the Grasse geodetic observatorie (Guillemot et al., 2012; Samain et al., 2011; Exertier et al., 2014).

<figure><img src="image_20.png" /></figure>
## Analyses and performances

Given the very high reliability of T2L2 over time (7 years in space), we developed a data reduction scheme that operates on a daily basis. In parallel, we have developed an interactive website (https://t2l2.oca.eu/) to describe the data over time: ground and board data and statistics. For the web user, it's possible to compute a given ground to space time transfer (from a selected set of SLR stations) or to obtain a history over several days. Actually, the website serves primarily to calculate the ground to ground time transfer ($\Delta t_{B-S}$ - $\Delta t_{B-S'}$) between the two reference clock systems of two selected SLR stations (S) and (S').

T2L2 is able to record an average of 100,000 optical events ($t_B$ epochs) per day, including noise and true laser shots. From 5 to 6 SLR participating stations in 2008, T2L2 implies today 22-24 stations (see Figure 2). They send their data ($t_e$ epochs in UTC and 2-way distances $dt_{2w}$) regularly to the European Data Center (EDC, located in Muenchen, Germany) and to the Crustal Dynamics Data Information System (CDDIS, in the US). Given the wide variety of clock systems and laser stations, we built a data reduction scheme based on the robust estimation (mathematical and numerical meaning). Statistically, we can say that the estimated noise at 1 s ranges from 45 to 65 ps (Exertier et al., 2010). The average stability achieved in

today's data processing is 4-8 (average of the standard obtained from passages of space time for 2 laser

<figure><img src="image_21.png" /></figure>

ps at 75 s time deviation many ground to transfer stations

equipped with H-maser). The ground-to-ground time transfer (in CV) is computed from both time synchronization ($\Delta t_{B-S1}$ and $\Delta t_{B-S2}$) between station (S1 and S2) clocks (Exertier et al., 2014). We made the demonstration of a repeatability of 35 ps over 1 day and more, and an accuracy of 140 ps after the time calibration of the involved laser telescopes.

# 7) Observatoire de Paris (Christian Bizouard)

## ACTIVITES DE RECHERCHES

### Astronomie fondamentale : conceptualisation et réalisation des systèmes de référence spatiaux-temporels

L'Observatoire de Paris poursuit la longue tradition de l'Astronomie fondamentale développée depuis sa fondation en 1667. Tout en veillant à la définition conceptuelle des divers systèmes de référence spatio-temporels, nous participons à leur réalisation. Un aspect majeur est la maintenance du repère de référence céleste par VLBI [Lambert 2014, Lambert 2013] et l'étude astrométrique des quasars le constituant [Titov et Lambert 2013, Liu et al 2012, Taris et al 2011, Taris et al 2013, Moor et al 2011], ce qui permet de sonder les paramètres de l'espace-temps au sens de la relativité générale [Lambert S. et Le Poncin-Lafitte, 2011]. En parallèle nous raffinons la paramétrisation de la transformation de rotation du système de référence céleste au système de référence, c'est-à-dire la rotation terrestre elle-même. Sous l'effet des forces de marées luni-solaires, ces paramètres varient d'une façon qui est en partie prédictible et donc sujette à des modèles analytiques, que nous nous attachons à façonner. Ces activités reçoivent leur consécration dans les travaux de l'Union Astronomique Internationale au sein des groupes chargés de préparer les résolutions UAI, la terminologie
scientifique, les modèles et les standards propres à cette thématique, [Capitaine 2012, Vondrak et al 2011].

## Détermination des variations de la rotation terrestre par géodésie spatiale

Non seulement la vitesse de rotation de la Terre et la direction de son axe varient légèrement, mais ces variations sont en partie irrégulières. Faute de pouvoir les modéliser convenablement et donc les prédire à long terme, on doit les déterminer de façon routinière en vue de multiples applications allant de la géolocalisation à la géophysique. A cette fin, on traite les observations obtenues par les techniques de géodésie spatiale: systèmes de positionnement GNSS et DORIS, télémétrie laser sur les satellites artificiels (SLR) et sur la Lune (LLR) [Bourgoin et al, 2014, Bouquillon et al 2013] et interférométrie à très longue base sur les quasars extra-galactiques (VLBI).

Ainsi, déterminer les variations de la rotation terrestre est l'une de nos activités phares, s'effectuant en grande partie pour le compte du Service de la Rotation de la Terre, qui est l'un des membres moteurs de l'International Earth Rotation and Reference System Service (IERS). D'une part nous participons, dans le cadre de nos services (voir ci-dessous), à la réduction intra-technique des observations (Solution VLBI – OPA de Sébastien Lambert, reconnue par l'International VLBI Service), solution LLR de Sébastien Bouquillon avalisée par l'International Laser Ranging Service) ou combinons leurs résultats finaux (solution C04, Bulletin B). D'autre part nous développons une solution multi-technique fondée sur la combinaison des équations normales de l'ensemble des observations GNSS, VLBI, SLR et DORIS (projet DYNAMO effectué en partenariat avec les équipes du GRGS). Cette méthode présente deux intérêts : i) en unifiant le traitement, elle élimine les incohérences entre les techniques ii) elle donne un ajustement cohérent des paramètres de rotation de la Terre et des coordonnées des stations géodésiques les sous-tendant (le repère terrestre) iii) elle permet en principe d'augmenter la résolution temporelle des paramètres géodésiques globaux, dont les paramètres de rotation de la Terre comme la nutation et UT1, mais cet objectif n'est pas encore atteint [Biancale et al, 2013].

## Effets géophysiques

Notre domaine devient de plus en plus important pour les géosciences. En effet, grâce à la précision croissante des techniques de géodésie spatiale, les variations de la rotation de la Terre dévoilent toute une gamme d'irrégularités traduisant les redistributions de masse se déroulant à la surface de la Terre (atmosphère+océans+eaux continentales ou hydro-atmosphère) ou en son sein [Bizouard 2014]. Nous les modélisons à la lumière des données concernant ces processus [Bizouard et al 2014, Sidorenkov et al 2014]. Quelques progrès substantiels ont faits dans la compréhension du terme de Chandler [Bizouard et al 2011, Zotov et Bizouard 2012, Bizouard 2014, Zotov et Bizouard 2015]. En retour, les données de rotation terrestre offrent un verdict sur la qualité des modèles de circulation globale dans l'hydro-atmosphère ou certaines hypothèses géophysiques [Zotov et al, 2015, Seaone et al 2011]; dans certains cas elles permettent même de préciser les propriétés rhéologiques de la Terre (élasticité, visco-élasticité), voire sa structure interne (aplatissement du noyau fluide par exemple). Cette jonction entre géodésie spatiale et processus géophysiques globaux est en constant renouvellement, et d'année en année offre un éclairage de plus en plus précis sur les couplages existant entre la Terre solide et ses parties fluides, qu'elles soient internes ou externes. L'objet géophysique de ces études est parfois élargi aux planètes [Karatekin et al 2011], et est appliqué à la prédiction des PRT [Gambis et al 2011], nécessaire à la mise en œuvre des techniques de géodésie spatiale.
## Théorie de la rotation de la Terre

La modélisation géophysique doit s'appuyer sur une théorie solide d'un corps en rotation. La précision des observations augmentant, nous avons abordé des effets négligés communément: Modélisation des effets asymétriques [Bizouard et Zotov, 2013]

## Une recherche interdisciplinaire au sein du département.

Nos activités évoluent dans un cadre interdisciplinaire dépassant celui de l'astronomie et des géosciences, touchant à la fois aux capteurs inertiels à ondes de matière comme alternative éventuelle à la géodésie spatiale pour mesurer la rotation terrestre (projet MIGA), aux liens optiques fibrés (interféromètre Sagnac géant), à la réalisation du temps légal, et à l'histoire de l'astronomie (projet Observations, Mesures, INcertitudes).

## ACTIVITES DE SERVICES

Plusieurs composantes de services internationaux, à savoir centres de produits IERS, centres

### Service international de la rotation de la Terre et des systèmes de référence (IERS)

Le «Earth Orientation Parameter Product Center» (EOP-PC) est chargé de produire et de diffuser en tant que référence internationale les variations irrégulières de la rotation terrestre sous la forme des Paramètres de Rotation de la Terre (PRT) ou Earth Orientation Parameters (EOP). Les séries temporelles correspondantes résultent d'une combinaison optimale des séries issues d'une technique astro-géodésique donnée (GNSS, VLBI, SLR, DORIS. Leur utilisation travaillant dans les domaines liés à la navigation, l'astronomie, la géodésie et les sciences spatiales, la géophysique, et le temps. Une base de données sous ORACLE contient l'ensemble des informations historiques de référence concernant les variations de la rotation de la Terre.

Les principales « innovations » pour la période incriminée sont :

- la mise en place d'un service WEB permettant la diffusion de UT1-UTC par NTP

- la création et la maintenance d'une série à long terme de la durée du jour et de UT1 de 1830 jusqu'à aujourd'hui (pour les études des géophysiciens).

- la restructuration de la page WEB du service : hpiers.obspm.fr/eop-pc

### Le Centre de produits des Systèmes de référence célestes (ICRS-PC).

Il a été chargé par l'UAI du suivi du Repère International de Référence Céleste (ICRF) et d'en estimer les rattachements avec d'autres repères célestes (optique, GAIA,...). Il poursuit en parallèle plusieurs thématiques de recherche associées à la géodésie spatiale et aux systèmes de référence célestes : il s'agit du suivi au sol de satellites géostationnaires, de la construction d'une base de données des quasars par le biais du catalogue LQAC et de l'astrométrie grand champ.

### Le Groupe de travail sur les combinaisons au niveau des observations

Actuellement les séries de variations de la rotation terrestre sont calculées séparément des systèmes de référence terrestre et céleste. Cependant la cohérence entre les produits dérivés par chaque technique n'est pas pleinement assurée. L'approche de combinaison inter-technique globale devrait résoudre cette problématique. La tâche principale de ce groupe crée en 2009 est d'étudier les avantages de ces méthodes de combinaison par rapport aux méthodes courantes de combinaison de séries temporelles.

Centres d'analyse et de données du Service international VLBI pour la géodésie et l'astrométrie (IVS/OPAR). Les développements récents du centre d'analyse concernent la
mise en place des solutions opérationnelles trimestrielles et rapides, ces dernières devant être lancées deux à trois fois par semaine. La solution trimestrielle donne des séries temporelles d'EOP et des catalogues de positions/vitesses des stations d'observation et de coordonnées de radiosources extragalactiques. Les EOP sont disséminés à l'IVS via les centres de données primaires de ce service et au centre de produits de l'IERS (IERS/EOP-PC). La solution rapide, générée dans les 48 heures après la corrélation des observations, donne uniquement les paramètres de rotation de la Terre (EOP) et les positions des stations d'observation. Les produits du centre d'analyse sont compatibles avec le format de l'Observatoire Virtuel (OV).

## Centre d'analyse des données Laser Lune

POLAC est un centre d'analyse des observations LLR (Lunar Laser Ranging) au sein du réseau ILRS (International Laser Ranging Service). Il appartient au service INSU labellisé S01. Il travaille en étroite collaboration avec l'équipe de télémétrie laser de l'Observatoire de la côte d'Azur (ûquipe ASTROGéo) et avec les services EOP-PC et ICRS-PC de l'IERS implantés à l'Observatoire de Paris (SYRTE).

Les responsabilités du service POLAC sont les suivantes :

- La collecte, la validation, l'archivage et la mise à disposition des observations de télémétrie LLR.

- Le développement, la maintenance et l'amélioration des logiciels de réduction des observations LLR (programme CAROLL).

- La réduction des observations LLR et l'estimation des paramètres qui ont une signature sensible sur les résidus LLR (paramètres caractéristiques de la dynamique du système terre-lune, raccordement du repère céleste dynamique à d'autres repères célestes et terrestres, etc...).

- la mise à disposition de ces analyses et résultats à l'ensemble de la communauté scientifique (site web POLAC: polac.obspm.fr).

- Mise en place depuis 2012 d'un service de prédiction et de validation des observations LLR destiné aux observateurs des stations Laser Lune (web service et service mail automatique). (à noter que Lunokhod 1 a été observé pour la première fois avec MeO par Jean-Marie et Dominique Feraudy fin Mars 2013 à l'aide de nos prédictions qui sont depuis utiliser pour toutes leurs observations LLR)

Les principaux scientifiques collaborant avec le service POLAC sont Jean-Marie Torre, Dominique Feraudy et Etienne Samain (Nice, France), Hervé Manche (Paris, France), Jürgen Müller (Hannover, Allemagne), Ulrich Schreiber (Wettzell, Allemagne), James Williams (Pasadena, USA) et Randall Ricklefs (Austin, USA). Les deux télescopes qui fournissent actuellement la majorité des observations LLR sont la station laser MeO de Grasse en France, la station laser de l'observatoire d'Apache Point au Nouveau-Mexique.

## 8) Service Hydrographique et Océanographique de la Marine
### (SHOM) Marie-Françoise Lequentrec-Lalancette

Besides the classical geodetic activity made for the benefit of hydrography, SHOM activities during these four years, have been developed on observation and modeling of marine reference surfaces and the geoid. As French referent institute for the ocean tide, SHOM is involved in the sea level observatory networks and in most of the French tides gauges (REFMAR, RONIM projects). Shipborne gravimetry continued on the sea surface (vessels : BHO Beautemps-Beaupré and N/O Pourquoi-Pas?). On the ocean bottom some experiments have been done with the Nautile submersible (DEMANE survey). The contribution of satellite gravimetric and altimetric data has not been overlooked thanks to gravimetric satellites
(CHAMP, GRACE, GOCE) and to retracking altimetric profiles (transition from 1Hz to 10Hz recording). Regarding the observation of sea surface heights, implementation of a GPS buoy was performed. SHOM has been also involved in national collaborations and projects (ANR Gremlit for example). He is involved in GRGS (Groupe de Recherche en Géodésie Spatiale), in the Solid Earth thematic cluster "Form @ Ter", in the CNES or INSU prospective meetings. As This report summarizes the key points and lists the conference presentations or publications.

## BATHYELLI Project

The purpose of this project is the definition of the ocean height references to realize bathymetric surveys directly compared to a reference ellipsoid rather than to the mean sea level. In the oceans, the creation of vertical reference surfaces, related to a reference ellipsoid is a necessary step to enable the use of GPS (Global Positioning System) in bathymetric soundings. This is a recommendation of the International Hydrographic Organization which should lead in the longer term to homogenous references between countries. Now, the vertical reference on the oceans has to be referred to a Chart Datum related to the ITRS (International Terrestrial Reference System). This is to provide a stable and precise Datum in coastal and deep water oceans. Indeed the relationship between the Mean Sea level and the Chart datum are well known. The problem is then to compute the mean sea level related to the ITRS. The surface is then computed using satellite altimetry, GNSS observations and tide gauge records. This work is still in progress but already a first reference series as the limit of the lowest astronomical seas was posted on the gate of SHOM: http://data.shom.fr

<figure><img src="image_22.png" /><figcaption>Bouée GPS SHOM
(From André G.et al., . XYZ, 140, 51-56)</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>Principe du levé sur l'ellipsoïde
(From Tanguy et al., XYZ, 140, 43-50)</figcaption></figure>

<figure><img src="image_24.png" /></figure>

## GEOMED Project

This project aims to achieve a global geoid the Mediterranean Sea. It is an international collaboration that includes the International Gravimetric Bureau, CNES, the Observatoire de Paris, SHOM, the universities of Milan, Thessaloniki, Jaén and Zagreb as well as Turkey Mapping Institute and the DTU space from Denmark.

For this project, SHOM has the responsibility for validation and processing the marine gravimetric data that represent more than two millions marine gravity measurements to analyze. Surveys extend over several decades with various accuracies in gravity measurements or in geographical positioning. The methodology and software used for SHOM processing have been applied to the Mediterranean data.
<figure><img src="image_25.png" /><figcaption>Public marine gravity data</figcaption></figure>

(from MF Lequentrec-Lalancette et al., EGU General Assembly 2014, Geophysical Research Abstracts, Vol. 16, EGU2014-5961)

## Swotter Project

During this four-year work SHOM supported by CNE has been estimated the contribution of the acquisition of satellite SWOT on the knowledge of the oceanic geoid and the free air anomaly. This satellite is a wide swath sensor, and represents a real technological gap in the field of the nadir altimetry. The SHOM contributes to the SWOT scientific definition team.

## Geoid computation – absolute gravimetry – stability references

SHOM has developed computational tools to determine gravimetric quantities at sea: the free air anomaly, the geoid undulations and the vertical deviation. They are based on the objective method of collocation and the resolution by the equation Stokes and Vening Menez. Studies have been made in the Atlantic Ocean and especially in the Bay of Biscay. It has been shown that shipborne gravity data improve the knowledge of the geoid to ten centimeters compared to satellite altimetry.

<figure><img src="image_26.png" /><figcaption>Geoid improved by marine gravity data in the Bay of Biscay
(from Lequentrec-Lalancette et al., 2011, Proceedings of ESA, Bergen)</figcaption></figure>

The geodetic observations and long-term complementary gravimetric tide on outstanding issues such as Brest is recommended by GLOSS group. In this context SHOM supports absolute gravimetric campaigns. Since 1998 many measures have been acquired in order to have an independent observation of GPS of the stability of the basement near the Brest tide
gauge. Annual measurements of FG5 absolute gravimeter have been conducted by the University of Montpellier and Strasbourg EOST for fifteen years. Furthermore, SHOM acquired an absolute field gravimeter A10 to allow the connection in the harbors between off shore relative gravimetric data and absolute gravity values. Between sea campaigns, the A10 will be used to supplement the FG5 measures depending on its observed accuracy. This is an ongoing work.

Brest SHOM - absolute gravity at 0 cm

<figure><img src="image_27.png" /></figure>

(From M.F. LeQuentrec-Lalancette et al., IAG, Postdam, 2013)
PUBLICATIONS 2011-2015

2015

Roland, J., Britzen, S., Kun, E., Henri, G., Lambert S. et Zensus, A. (2014), *Structure of the nucleus of 1928+738*, Astron. Astrophys., à paraître

Samain, E., Exertier, P., Courde, C., Fridelance, P., Guillemot, Ph., Laas-Bourez, M., Torre, J.-M., *Time Transfer by Laser Link- A complete analysis of the error budget*, Metrologia, 52, 2015.

Zotov L., Bizouard C. (2015), *Reconstruction of prograde and retrograde Chandler excitation*, Journal of Inverse and Ill-posed problems, à paraître

Zotov L., C. Bizouard (2015), *Regional atmospheric influence on the Chandler wobble, Advances in Space Research*: 1300-1306

2014

André G., Ballu V., Martín Míguez B., Testut L., Wöppelmann G., Tiphaneau P., 2014. La mesure du niveau de la mer par bouées GPS : l'expérience multi-capteurs de l'île d'Aix. XYZ, 140, 51-56.

Arnoso, J., U. Riccardi, J. Hinderer, B. Córdoba & F. G. Montesinos, 2014. *Analysis of co-located measurements made with a LaCoste&Romberg Graviton-EG gravimeter and two superconducting gravimeters at Strasbourg (France) and Yebes (Spain)*, Acta. Geod. Geophys., DOI 10.1007/s40328-014-0043-y

Bizouard C. (2014), *Le mouvement du pôle de l'heure au siècle*, 284 pages, Presses Académiques Francophones.

Bizouard C., Zotov L., Sidorenkov N. (2014), *Lunar influence on equatorial atmospheric angular momentum*, Journal of Geophysical Research - Atmosphere, Volume 119, Issue 21, 11920-11931.

Bock, O., P. Willis, J. Wang, C. Mears (2014) *A high-quality, homogenized, global, long-term (1993-2008) DORIS precipitable water dataset for climate monitoring and model verification*, J. Geophys. Res., Atmospheres, 119(12): 7209-7230, doi: 10.1002/2013JD021124.

Bourgoin A., Le Poncin-Lafitte C., Bouquillon S., Francou G., Angonin M.-C. (2014), New dynamical relativistic modeling of the Moon orbital and rotational motion developed by POLAC, Proceeding of the 19th International Workshop on Laser Ranging, Oct 2014, Annapolis, USA*.

Bruinsma, S., C. Foerste, O. Abrikosov, J.-M. Lemoine, J.-C. Marty, S. Mulet, M.H. Rio, S. Bonvalot, ESA's satellite-only gravity field model via the direct approach based on all GOCE data, Geophys. Res. Lett., doi:10.1002/2014GL062045, 2014.

Calvo, M., Hinderer, J., Rosat, S., Legros, H., Boy, J.-P., Ducarme, B., W. Zürn, 2014. *Time stability of spring and superconducting gravimeters through the analysis of very long gravity records*, J. of Geodyn., doi:10.1016/j.jog.2014.04.009.

Collilieux, X., Z. Altamimi, D.F. Argus, C. Boucher, A. Dermanis, B.J. Haines, T.A. Herring, C. Kreemer, F.G. Lemoine, C. Ma, D.S. MacMillan, J. Makinen, L. Métivier, J.C. Ries, F.N. Teferle and X. Wu (2014) *External evaluation of the Terrestrial Reference Frame: report of the task force of the IAG sub-commission 1.2, Proceedings of the XXV IUGG General Assembly, International Association of Geodesy Symposia*, Vol. 139, doi:10.1007/978-3-642-37222-3_25

Crossley, D., J.-P. Boy, J. Hinderer, T. Jahr, A. Weise, H. Wziontek, M., Abe, and C. Foerste, 2014. *Comment on: 'The quest for a consistent signal in ground and GRACE gravity time series', by Michel Van Camp et al. (2014)*, Geophysical Journal International, accepted

Cui, X., Sun, H.-P., Rosat, S., Xu, J.-Q., Zhou, J.-C., Ducarme, B., 2014. *Investigation of the time variability of diurnal tides and resonant FCN period*, J. of Geodyn., 79, 30-38, doi:10.1016/j.jog.2014.05.003

Douch, K., Christophe, B., Foulon, B., Panet, I., Pajot-M'étivier, G. and Diament, M. (2014). *Ultra-sensitive electrostatic planar acceleration gradiometer for airborne geophysical surveys*, Measurement Science and Technology, 25(10):105902

Ferenc M., Nicolas J., van Dam T., Polidori L., *An estimate of the influence of loading effects on tectonic velocities in the Pyrenees*, Studia Geophysica et Geodaetica, 58, 56-75, doi: 10.1007/s11200-012-0458-2, 2014.

Garcia, R., E. Doornbos, S. Bruinsma, H. Hebert, *Atmopsheric gravity waves due to the Tohoku-Oki tsunami observed in the thermosphere by GOCE*, J. Geophys. Res., doi:10.1002/2013JD021120, 2014.

Habib B. Dieng, A. Cazenave, B. Meyssignac, O. Henry, K. von Schuckmann, H. Palanisamy, J.-M. Lemoine, *Effect of La Niña on The Global Mean Sea Level And North Pacific Ocean Mass Over 2005-2011*, Journal of Geodetic Science. Volume 4, Issue 1, ISSN, 2081-9943, DOI: 10.2478/jogs-2014-003, 2014.
Hector, B., Hinderer J., Séguis L., Boy J.-P., Calvo, M., Descloitres M., Rosat, S., Galle, S., & Riccardi, U., 2014. *Hydro-gravimetry in West-Africa: First results from the Djougou (Benin) superconducting gravimeter*, Journal of Geodynamics, 2014, http://dx.doi.org/10.1016/j.jog.2014.04.003

Hinderer, J., Hector, B., Boy, J.-P., Riccardi, U., Rosat, S., Calvo, M., F. Littel, 2014. *A search for atmospheric effects on gravity at different time and space scales*, J. of Geodyn., doi: 10.1016/j.jog.2014.02.001

Lambert S. (2014), Comparison of VLBI radio source catalogs, Astron. Astrophys., 570, 108

Lequentrec-Lalancette M.F., Salaün C., Moysan Y., Rouxel D., and Bonvalot S., *Validation of global gravity models by marine data in the West Mediterranean Sea* ; EGU General Assembly 2014, Geophysical Research Abstracts, Vol. 16, EGU2014-5961, 2014

Mémin, A., G. Spada, J.-P. Boy, Y. Rogister, J. Hinderer, 2014. *Decadal geodetic variations in Ny-Alesund (Svalbard): role of past and present ice-mass changes*, Geophysical Journal International, Geophysical Journal International 01/2014; 198:285-297. DOI: 10.1093/gji/ggu134

Métivier, L., X. Collilieux, D. Lercier, Z. Altamimi, and F. Beauducel (2014), *Global coseismic deformations, GNSS time series analysis, and earthquake scaling laws*, *J. Geophys. Res. Solid Earth*, 119, doi:10.1002/2014JB011280.

Meyssignac B., Lemoine J.-M., Cheng M., Cazenave A., Gégout P., Maisongrande P., *Variations in degree 2 spherical harmonics of the gravity field reveal large-scale mass transfers of climatic origin in the ocean and over land*, Geophysical Research Letter, doi:10.1002/grl.50772, 2014.

Nicolas J., M. Ferenc, Z. Li, T. van Dam, and L. Polidori, *Ocean dynamics during the passage of Xynthia storm recorded by GPS*, poster, session G1.3, Geophysical Research Abstracts, Vol. 16, EGU2014-11356, EGU General Assembly, Vienna, Austria, 27 April - 02 May, 2014

Panet, I., Pajot-Métivier, G., Greff-Lefftz, M., Métivier, L., Diament, M. and Mandea, M. (2014). *Mapping the mass distribution of Earth's mantle using satellite-derived gravity gradients*, Nature Geoscience, doi:10.1038/ngeo2063

Rebischung, P., Z. Altamimi and T. Springer (2014) *A collinearity diagnosis of the GNSS geocenter determination*, Journal of Geodesy, 88(1), p. 65-85, doi:10.1007/s00190-013-0669-5

Rosat S., J.-P. Boy, Y. Rogister, 2014. *Surface atmospheric pressure excitation of the translational mode of the inner core*, Phys. Earth Planet. Int., 227, 55-60, doi:10.1016/j.pepi.2013.12.005

Rosat, S., Calvo, M., Hinderer, J., Riccardi, U., Arnoso, J. and W. Zürn, 2014. *Comparison of the performances of different Spring and Superconducting Gravimeters and a STS-2 Seismometer at the Gravimetric Observatory of Strasbourg, France*, Stud. Geophys. Geod., accepted.

Roussel N., Frappart F., Ramillien G., Darrozes J., Desjardins C., Gegout P., Pérosanz F., Biancale R., *Simulations of direct and reflected waves trajectories for in situ GNSS-R experiments*, Geoscientific Model Development, 7(5), 2261-2279, doi:10.5194/gmd-7-1-2014.

Sakumura, C., S. Bettadpur, S. Bruinsma, *Ensemble prediction and intercomparison analysis of GRACE time-variable gravity field models*, Geophys. Res. Lett., doi:10.1002/GL058632, 2014.

Sidorenkov N., Bizouard C., Zotov L., Salstein D. (2014), *Moment impulsa atmosferi (Le moment cinétique de l'atmosphère)*, Priroda (revue scientifique de l'Académie des Sciences de la Fédération de Russie) Vol. 4, p. 22-28.

Tanguy Y.-M., Jan G., Pastol Y., 2014. *Les références verticales maritimes en France : méthodologie de création des surfaces BATHYELLI*. XYZ, 140, 43-50.

2013

Altamimi, Z. and X. Collilieux (Editors) (2013) *Reference Frames for Applications in Geosciences*, Proceedings of IAG Symposium REFAG2010 (Marne la Vallée, France, October 4-8, 2010), International Association of Geodesy Symposia, Vol. 138, Springer Berlin Heidelberg, doi:10.1007/978-3-642-32998-2 (ISBN: 978-3-642-32997-5

Altamimi, Z., X. Collilieux and L. Métivier (2013) *ITRF combination: theoretical and practical considerations and lessons from ITRF2008*. In: Z. Altamimi and X. Collilieux (eds) Reference Frames for Applications in Geosciences, Springer Berlin Heidelberg, International Association of Geodesy Symposia, Vol. 138, p. 7-12, doi: 10.1007/978-3-642-32998-2_11

André G., Martín Míguez B., Ballu V., Testut L., Wöppelmann G., 2013. *Measuring sea-level with GPS-equipped buoys: A multi-instruments experiment at Aix Island*. International Hydrographic Review, 27-38.

Biancale R., Gambis D., Richard J.Y., Bizouard C. (2013), *Why combining at the Observation Level? IAG series*. IAG Symposium : Reference Frame for Application in Geodesy.

Bizouard C., Zotov L. (2013), *Asymmetric effect on the Earth's polar motion, Celestial Mechanics and Dynamical Astronomy, Volume 116, Issue 2 : 195-212*
Bock, O., Bosser, P., Bourcy, T., David, L., Goutail, F., Hoareau, C., Keckhut, P., Legain, D., Pazmino, A., Pelon, J., Pipis, K., Poujol, G., Sarkissian, A., Thom, C., Tournois, G., and Tzanos, D.: Accuracy assessment of water vapour measurements from in-situ and remote sensing techniques during the DEMEVAP 2011 campaign at OHP, Atmos. Meas. Tech., 6, 2777-2802, 2013, doi:10.5194/amt-6-2777-2013

Bouquillon S., Francou G., Manche H., Torre J.M., Le Poncin-Lafitte C., Lhotka C. (2013), *Lunar Laser Ranging: Recent activities of Paris Observatory Lunar Analysis Center*, Proceeding of the 18th International Workshop on Laser Ranging, Nov 2013, Fujiyoshida, Japan*.

Bruinsma, S., C. Foerste, O. Abrikosov, J.M. Marty, M.-H. Rio, S. Mulet, S. Bonvalot, *The new ESA satellite-only gravity field model via the direct approach*, Geophys. Res. Lett., 40, 1-6, doi:10.1002/grl.50716, 2013.

Calvo, M., S. Rosat, J. Hinderer, H. Legros and J.-P. Boy, 2013. *Study of the time stability of tides using a long term (1973-2011) gravity record at Strasbourg, France, Earth on the Edge: Science for a Sustainable Planet*, Proceedings of the IAG General Assembly, Melbourne, Australia, June 28 - July 2, 2011, Series: International Association of Geodesy Symposia, Vol. 139, Rizos, Chris; Willis, Pascal (Eds.), 377-382.

Collilieux, X. and Z. Altamimi (2013) *External Evaluation of the Origin and the Scale of the International Terrestrial Reference Frame*, Proceedings of the IAG Symposium. REFAG2010. Marne-La-Vallée, Z. altamimi and X. Collilieux Ed., International Association of Geodesy Symposia, Vol.138, p. 27-31, Springer, doi:10.1007/978-3-642-32998-2_5

Crétaux, J.-F., Bergé-Nguyen, M., Calmant, S., Romanovski, V., Meyssignac, B., Pérosanz, F., Tashbaeva, S., Arsen, A., Fund, F., Martignano, N., Bonnefond, P., Laurain, O., Maisongrande, P. (2013) *Calibration of Envisat radar altimeter over the Lake Issykkul*, Advances in Space Research 51(8), 1523-1541, 2013.

Crossley, D., Hinderer, J., & Riccardi, U., 2013. *The measurement of surface gravity*, Reports on Progress in Physics 76, 10.1088/0034-4885/76/4/046101.

Ducrocq, V., I. Braud, S. Davolio, R. Ferretti, C. Flamant, A. Jansa, N. Kalthoff, E. Richard, I. Taupier-Letage, P.-A. Ayral, S. Belamari, A. Berne, M. Borga, B. Boudevillain, O. Bock, J.-L. Boichard, M.-N. Bouin, O. Bousquet, C. Bouvier, J. Chiggiato, D. Cimini, U. Corsmeier, L. Coppola, P. Cocquerez, E. Defer, J. Delanoë, P. Di Girolamo, A. Doerenbecher, P. Drobinski, Y. Dufournet, N. Fourrié, J. Gourley, L. Labatut, D. Lambert, J. Le Coz, F. S. Marzano, G. Molinié, A. Montani, G. Nord, M. Nuret, K. Ramage, B. Rison, O. Roussot, F. Said, A. Schwarzenboeck, P. Testor, J. Van Baelen, B. Vincendon, M. Aran, J. Tamayo (2013) *HyMeX-SOP1*, the field campaign dedicated to heavy precipitation and flash flooding in the northwestern Mediterranean, Bulletin of the American Meteorological Society, http://dx.doi.org/10.1175/BAMS-D-12-00244.1

Exertier, P., E. Samain, N. Martin, C. Courde, M. Laas-Bourez, C. Foussard, Ph. Guillemot, *Time Transfer by Laser Link: Data analysis and validation to the ps level*, Adv. Space Res. 54(11), 2371-2385, 2014.

Ferenc M., Z. Li, J. Nicolas, T. Van Dam, *GPS observation of subdaily loading deformation in France associated with a violent storm surge*, oral presentation, session 3.5, IAG Scientific Assembly, Potsdam, September 1-6, 2013

Ferenc M., Nicolas J. van Dam T., *Application of PPP for subdaily loading deformation observation*, Colloque G2, 13 - 15 novembre 2013, Rennes, oral

Ferenc M., Z. Li, J. Nicolas, T. Van Dam, *Study of seasonal and sub-daily loading effects using IPPP GPS time series*, poster, session S.1.6, IAG Scientific Assembly, Potsdam, September 1-6, 2013

Fund, F., F. Pérosanz, L. Testut, S. Loyer, *An Integer Precise Point Positioning Technique for Sea Surface Observations Using a GPS Buoy*, J. Adv. Space Res., DOI: 10.1016/j.asr.2012.09.028, 2013.

Garcia, R., S. Bruinsma, P. Lognonné, E. Doornbos, F. Cachoux, *GOCE satellite: the first seismometer in orbit around the Earth*, Geophys. Res. Lett., doi:10.1002/grl.50205, 2013.

Hector B., Séguis L., Hinderer J., Descloitres M., Vouillamoz J.M., Wubda M., Boy J.-P., Luck B., Le Moigne N., 2013. *Gravity effect of water storage changes in a weathered hard-rock aquifer in West Africa: results from joint absolute gravity, hydrological monitoring and geophysical prospection*, Geophysical Journal International (2013) 10.1093/gji/ggt146

Hinderer, J., S. Rosat, B. Hector, M. Calvo, J.-P. Boy et al., 2013. *Preliminary results from the superconducting gravimeter OSG-60 installed in West Africa (Djougou, Benin), in Earth on the Edge: Science for a Sustainable Planet*, Proceedings of the IAG General Assembly, Melbourne, Australia, June 28 July 2, 2011, Series: International Association of Geodesy Symposia, Vol. 139, Rizos, Chris; Willis, Pascal (Eds.), 413-420.

Laas-Bourez, M., Samain, E., Courde, C., Oneto, J.L., Exertier, P., Rovera, D., Abgrall, M., Fridelance, P., Guillemot, Ph. *Time and Frequency distribution improvement in Calern/Geoazur laboratory for T2L2 campaigns*. Proceedings of the EFTF & International Frequency Control Symposium (EFTF/IFC) 2013, Prague, Republik Tcheque, July 21-25, pp 869-872, 2013

Lambert S. (2013), *Time stability of the ICRF2 axes*, Astron. Astrophys., 553, 122.
LeQuentrec-Lalancette M.F., Woppelman G., Rouxel D., Le Moigne N., Bayer R., Lucas S., Hinderer J., Boy J.P., Bernard J.-D., *Absolute gravity at tide gauge stations : the french experiment*, IAG Scientific Assembly, Postdam, 1-6 sept, 2013

Masiello, G., C. Serio, Deleporte, T., H., Herbin, H., P. Di Girolamo, C. Champollion, A. Behrendt, P. Bosser, O. Bock, V. Wulfmeyer, M. Pommier, and C. Flamant (2013) *Comparison of IASI water vapour products over complex terrain to COPS campaign data*, Meteorologische Zeitschrift, 22, 471-487, http://dx.doi.org/10.1127/0941-2948/2013/0430.

Meyssignac B., Lemoine J.-M., Cheng M., Cazenave A., Gegout P. and Maisongrande P., *Interannual variations in degree-two Earth's gravity coefficients C2,0, C2,2 and S2,2 reveal large-scale mass transfers of climatic origin*, Geophys. Res. Lett., 40, 1-6, doi:10.1002/grl.50772, 2013.

Panet I., Pajot-Métivier G., Holschneider M., Jamet O., Lalancette M.F., *Multi-scale modelling of the Earth's gravity field from GOCE*, AGU, San-Franciso, 2013

Pfeffer, J., Champollion, C., Favreau, G., Cappelaere, B., Hinderer, J., Boucher, M., Nazoumou, Y., Oï, M., Mouyen, M., Henri, C., Le Moigne, N., Deroussi, S., Demarty, J., Boulain, N., Benarrosh, N., and Robert, O., 2013. *Evaluating surface and subsurface water storage variations at small time and space scales from relative gravity measurements in semiarid Niger*, Water Resour. Res., 49, doi:10.1002/wrcr.20235.

Ray, J., J. Griffiths, X. Collilieux and P. Rebischung (2013) *Subseasonal GNSS Positioning Errors*, Geophysical Research Letters, 40(22), p. 5854-5860, doi:10.1002/2013GL058160

Seoane L., G. Ramillien, F. Frappart, M. Leblanc, *Regional GRACE-based estimates of water mass variations over Australia: validation and interpretation*, Hydrol. Earth Syst. Sci. Discuss., 10, 5355-5395, 2013, doi: 10.5194/hessd-10-5355-2013.

Taris, F., Andrei, A. H., Klotz, A., Vachier, F., Côte, R., Bouquillon, S., Souchay, J., Lambert, S., Anton, S., Bourda, G., & Coward, D. (2013), *Optical monitoring of extragalactic sources for the link between the ICRF and the future GAIA celestial reference frame*, Astron. Astrophys., 552, 98.

Titov O., Lambert S. (2013), Improved VLBI measurement of the solar system acceleration, Astron. Astrophys., 559, 95

Valty, P., de Viron, O., Panet, I. and Van Camp, M. (2013). *Assessing the precision in loading estimates by geodetic techniques in Southern Europe*, Geophys. J. Int., 194, 1441-1454, doi : 10.1093/gji/ggt173

Willis P., S. Mertikas, D. F. Argus, and O. Bock (2013) *DORIS and GPS monitoring of the Gavdos calibration site in Crete*, Adv. Space Res, 51, 1438-1447.

2012

Altamimi, Z., L. Métivier and X. Collilieux (2012), *ITRF2008 plate motion model*, J. Geophys. Res., 117, B07402, doi:10.1029/2011JB008930.

Altamimi, Z., X. Collilieux and L. Métivier (2012), *Analysis and Results of ITRF2008*, IERS Technical Note 37, Verlag des Bundesamts für Kartographie und Geodäsie, Frankfurt am Main, 2012.

Balmino, G., Valès, N., Bonvalot, S., Briais, A., *Spherical harmonic modelling to ultra-high degree of Bouguer and isostatic anomalies*, Journal of Geodesy, Vol. 86 (7), pp 499-520, DOI: 10.1007/s00190-011-0533-4, 2012.

Boerez, J., Hinderer, J., Rivera, L., & Jones, M., 2012. *Analysis and modeling of the effect of tides on the hydrostatic leveling system at CERN*, Survey Reviews, Volume 44, Number 327, pp. 256-264.

Boy, J.-P., Hinderer, J., & de Linage, C., 2012. *Retrieval of large scale hydrological signal in Africa from GRACE time-variable gravity fields*, Pure and Applied Geophysics, Volume 169, Number 8, 1373-1390, DOI: 10.1007/s00024-011-0416-x

Capitaine N. (2012), *Micro-arcsecond Celestial Reference Frames: definition and realization - Impact of the recent IAU Resolutions*, Res. Astron. Astrophys. 12, 8, 1162-1184; DOI:10.1088/1674-4527/12/8/013.

Crossley, D., de Linage, C., Hinderer, J., Boy, J.-P. & Famiglietti, J., 2012. *A comparison of the gravity field over Central Europe from superconducting gravimeters, GRACE and global hydrological models, using EOF analysis*. Geophysical Journal International, 189: 877-897. doi: 10.1111/j.1365-246X.2012.05404.x

Durand S. et c. Guérin. *Validation du logiciel CoMeT d'ajustement de mesures topographiques*, Revue XYZ de l'Association Française de Topographie, n°132, 3ème trimestre 2012, pp32-40, 2012

Exertier, P. R. Biancale and GRGS Science Comittee, « Observatoires Géodésiques Fondamentaux – recommandations », 2012, Ed. GRGS/CNES, 42pp

Ferenc M., Nicolas J., T. van Dam, P. Gegout, *Can environmental loading effects be an artifact in tectonic velocity obtained from GPS measurements?*, EGU General Assembly 2012 session G5.3, Vienna, Austria, 22 – 27 April 2012, EGU2012-8184, 2012

Fund, F., F. Pérosanz, L. Testut, S. Loyer, *An Integer Precise Point Positioning Technique for Sea Surface Observations Using a GPS Buoy*, J. Adv. Space Res., DOI: 10.1016/j.asr.2012.09.028, 2012
Fund F., L. Morel, J. Boehm, A. Mocquet. *Tropospheric Models and Multipath Effects for Ocean Tide Loading Displacements in the West of France*. Proceeding of the Geodesy for Planet Earth, IAG Scientific Assembly 2009, Buenos Aires, Argentine, 2012.

Gegout P. et al., *Surcharges et propagation*, poster, journées TOSCA, CNES, Paris, 21-22 mars 2012

Guillemot, Ph. et al., *Improvement of comparisons between T2L2 & RF Links*, Proceedings of the EFTF 2012, Gothenburg, Sweden, April 23-27 2012, pp 345-348.

Hayn, M., Panet, I., Diament, M., Holschneider, M., Mandea, M. and Davaille, A. (2012). *Wavelet-based directional analysis of the gravity field: evidence for large-scale undulations*, Geophys. J. Int., 189, 1430-1456, doi: 10.1111/j.1365-246X.2012.05455.x

Hinderer, J., J. Pfeffer, M. Boucher, S. Nahmani, C. De Linage, J.-P. Boy, P. Genthon, L. Seguis, G. Favreau, O. Bock, et al., 2012. *Land water storage changes from ground and space geodesy: first results from the GHYRAF (Gravity and Hydrology in Africa) experiment*, Pure and Applied Geophysics, Volume 169, Number 8, 1391-1410, DOI: 10.1007/s00024-011-0417-9

Jones M., D. Missiaen, M. Crespi, G. Colosimo, A. Mazzoni, S. Durand. *The distance from CERN to LNGS, International Workshops on Accelerator Alignment (IWAA)*, 10-14 September 2012 6pages, 2012

Koulali Idrissi, A., D. Ouazar, O. Bock and A. Fadil, *Study of seasonal-scale atmospheric water cycle with ground-based GPS receivers*, radiosondes and NWP models over Morocco (2012) Atmos. Res., 104-105, February 2012, Pages 273-291

Laas-Bourez Myrtille, Sébastien Wailliez, Florent Deleflie, Alain Klotz, Dominique Albanese, Nathalie Saba, *First astrometric observations of space debris with the MéO telescope*, Adv. in Space Res. 603-611, 2011-2012

Legros R., L. Morel, F. Viguier et F. Birot. *Méthodes de travail dans les réseaux GNSS. Le positionnement statique temps réel par « filtrage et moyenne de positions NRTK »*, Revue XYZ de l'Association Française de Topographie, n°132, pp. 53-59, 3ème trimestre 2012

Lescarmontier, L., Legresy, B., Coleman, R., Pérosanz, F., Mayet, C. and Testut, L. *Vibrations of Mertz Glacier ice tongue, East Antarctica*, Journal of Glaciology 58(210), 665-676, 2012.

Liu J.-C., Capitaine N., Lambert S., Malkin Z. & Zhu, Z. (2012), *Systematic effect of the Galactic aberration on the ICRS realization and the Earth orientation parameters*, Astron. Astrophys., 548, 50.

Loyer S., Pérosanz F., Mercier F., Capdeville H., Marty J.-C., *Zero-difference GPS ambiguity resolution at CNES-CLS IGS Analysis Center*, Journal of Geodesy, Vol. 86 (11), pp 991-1003, DOI:10.1007/s00190-012-0559-2, 2012.

Mandea, M., Panet, I., Lesur, V., de Viron, O., Diament, M. and Le Mouel, J-L. (2012). *Recent changes of the Earths core derived from satellite observations of magnetic and gravity fields*, PNAS, 109(47), 19129-19133

Mémin, A., Hinderer, J., & Rogister, Y., 2012. *Separation of the geodetic consequences of past and present ice-mass change: Influence of Topography with Application to Svalbard (Norway)*, Pure and Applied Geophysics, Volume 169, Number 8, 1357-1372, doi: 10.1007/s00024-011-0399-7.

Métivier, L., X. Collilieux, and Z. Altamimi (2012), *ITRF2008 contribution to glacial isostatic adjustment and recent ice melting assessment*, Geophys. Res. Lett., 39(L01309), doi:10.1029/2011GL049942.

Morel L., P.S. De Oliveira, F. Fund, F. Durand. *Etude de sensibilité de l'estimation de la troposphère par GNSS*. Colloque CNFGG, 10-12 Octobre, Clermont Ferrand, 2012

Nahmani, S., O., Bock, M.-N., Bouin, A. Santamaría-Gómez, J.-P. Boy, X. Collilieux, L. Métivier, I. Panet, P. Genthon, C. de Linage, G. Wöppelmann (2012) *Hydrological deformation induced by the West African Monsoon: a comparison of GPS, GRACE and loading models*, Journal of Geophysical Research – Solid Earth, 117, B05409, doi:10.1029/2011JB009102

Nicolas, J., F. Perosanz, A. Rigo, G. Le Bliguet, L. Morel, F. Fund, *Impact of Loading Phenomena on Velocity Field Computation from GPS Campaigns: Application to the ResPyr GPS Campaign in the Pyrenees, Geodesy for Planet Earth*, Proceeding of the 2009 IAG Symposium, Buenos Aires, Argentina, 31 August 31 - 4 September 2009, Series: International Association of Geodesy Symposia, Springer, Vol. 136, Part 3, pp. 643-649, doi: 10.1007/978-3-642-20338-1_79, 2012

Nicolas J., S. Durand, M. Vergnolle, L. Morel, S. Melachroinos, M.-N. Bouin and F. Fund, *Ocean Loading in Brittany, Northwest France: Impact of the GPS Analysis Strategy*, Proceedings of the VII Hotine-Marussi Symposium on Mathematical Geodesy in Rome, 6-10 June, 2009, International Association of Geodesy Symposia, Springer, Vol. 137, Part 9, pp. 367-372, DOI: 10.1007/978-3-642-22078-4_55, 2012

Panet I., J. Flury, R. Biancale, T. Gruber, J. Johannessen, M. Van den Broeke, T. van Dam, P. Gégout, C.W. Hughes, G. Ramillien, I. Sasgen, L. Seoane, M. Thomas, *Earth system mass transport mission (e.motion): A concept for future Earth gravity field measurements from Space*, Surv. Geophys., Springer Verlag, DOI 10.1007/s10712-012-9209-8, 2012.
Riccardi, U., Rosat, S., & Hinderer, J., 2012. *On the accuracy of the calibration of superconducting gravimeters using absolute and spring sensors: A critical comparison, Pure and Applied Geophysics*, Volume 169, Number 8, 1343-1356 DOI: 10.1007/s00024-011-0398-8

Roques C, Aquilina L., Bour O., Longuevergne L., Dewandel B., Schroetter JM, Le Borgne T., Labasque T., Lavenant N., Hochreutener R., Durand S., Ferré G., Dauteuil O., Vergnaud V., Mougin B., Palvadeau E., Lacassou F., *Hydrodynamic and geochemical characterization of deep hard rock aquifer (Saint-Brice en Coglès, French Brittany)*, International Association of Hydrogeologists, Pragues, mai 2012

Rosat, S., T. Sato, Y. Imanishi, J. Hinderer, Y. Tamura, H. McQueen, and M. Ohashi, 2012, *Correction to “High-resolution analysis of the gravest seismic normal modes after the 2004 Mw = 9 Sumatra earthquake using superconducting gravimeter data,”* Geophys. Res. Lett., 39, L22601, doi:10.1029/2012GL054248.

Rosat S. et Y. Rogister, 2012. *Excitation of the Slichter mode by collision with a meteoroid or pressure variations at the surface and core boundaries*, Phys. Earth Planet. Int., vol. 190-191, 25-33, doi:10.1016/j.pepi.2011.10.007.

Samain, E. et al., T2L2: *Ground to ground Time Transfer*, Proc. 26th European Frequency and Time Forum, Gothenburg, Sweden, May 23-27 2012, pp. 36-40.

Simonetto E., J.M. Follin, *An overview on interferometric SAR software and a comparison between DORIS and SARSCAPE processing*, Springer series: Lecture Notes in Geoinformation and Cartography (LNG&C) series. Geospatial free and open source software in the 21st century, Part 2, pp. 107-122, 2012

Soheili Majd M., E. Simonetto, L. Polidori, *Maximum Likelihood Classification of Single High-resolution Polarimetric SAR Images in Urban Areas*, PFG (Photogrammetrie, Fernerkundung, Geoinformation), Vol. 2012, Number 4, August 2012, pp. 395-407 (13).

Van Dam, T., X. Collilieux, J. Wuite, Z. Altamimi and J. Ray (2012) *Nontidal ocean loading effects in GPS height time series*, Journal of Geodesy, DOI 10.1007/s00190-012-0564-5

Zotov L., Bizouard C. (2012), *On modulations of the Chandler wobble excitation*, Journal of Geodynamics doi 10.1016/j.jog.2012.03.010

## 2011

Altamimi, Z., X. Collilieux, and L. Métivier (2011), ITRF2008: *an improved solution of the International Terrestrial Reference Frame*, Journal of Geodesy, doi:10.1007/s00190-011-0444-4

Bizouard C., Rémus F., Lambert S., Seoane L., Gambis D. (2011), *The Earth's variable Chandler wobble, A&A 526(A106)*.

Collilieux X., van Dam T., Ray J., Coulot D., Métivier L., Altamimi Z., 2011, *Strategies to mitigate aliasing of loading signals while estimating GPS frame parameters*, in Journal of Geodesy, published online, doi: 10.1007/s00190-011-0487-6.

Collilieux, X., L. Métivier, Z. Altamimi, T. van Dam and J. Ray (2011) *Quality assessment of GPS reprocessed Terrestrial Reference Frame, GPS Solutions*, vol. 15, n. 3, pp. 219-231, doi:10.1007/s10291-010-0184-6

Fadil, A., L. Sichoix, J.-P. Barriot, P. Ortéga, P. Willis, *Evidence for a slow subsidence of the Tahiti Island from GPS, DORIS, and combined satellite altimetry and tide gauge sea level records*, Compte-rendus Geoscience, Volume 343, Issue 5, pp 331-341, May 2011.

Fadil, A., J. Serafini, J.-P. Barriot, P. Ortega, *Correlation between Lightning activity and Integrated Water Vapor from GPS signal in Tahiti*, Proceedings of the Int. Conf. on Atmospheric Electricity, Rio 2011.

Ferenc M., Nicolas J., van Dam T., *Impact of loading phenomena on GPS velocities in the Pyrenees*, poster, Colloque G2, Marne-La-Vallée, 7-9 novembre 2011

Fund F., L. Morel, J. Boehm, A. Mocquet. *A discussion of Height reductions for Zenith Hydrostatic Delays derived for weather models*, Journal of Applied Geodesy, pp. 71-80, doi: 10.1515/JAG.2011.006, 2011

Gambis D., Salstein D. A. et Lambert, S. (2011), *Use of atmospheric angular momentum forecasts for UT1 predictions: analyses over CONT08*, J. Geod., 85, 435

Gégout, P., R. Biancale, and L. Soudarin, *Adaptive mapping functions to the azimuthal anisotropy of the neutral atmosphere*, Journal of Geodesy, Vol. 85 (10), pp 661-677, 2011.

Jiang, Z. et al. (including Hinderer J.), 2011. *Final report on the Seventh International Comparison of Absolute Gravimeters (ICAG 2005)*, Metrologia, 48, 246-260.

Karatekin O., de Viron, O., Lambert S., Dehant V., Rosenblatt P., Van Hoolst T., et Le Maistre S. (2011), *Atmospheric angular momentum variations of Earth, Mars and Venus at seasonal time scales*, Plan. Space Sci., 59, 923.

Lambert S. et Le Poncin-Lafitte, C. (2011), *Improved determination of γ by VLBI*, Astron. Astrophys., 529, 70.
Legros R., L. Morel, F. Viguier et F. Birot. *Méthodes de travail dans les réseaux GNSS*. Revue XYZ de l'Association Française de Topographie, n°129, pp. 39-42, 4ème trimestre 2011

Lequentrec-Lalancette MF, Rouxel D, *Comparison of a marine gravimetric geoid and global satellite model in the Atlantic ocean*, contribution 184-D4, Proceedings of the ESA Living Planet Symposium, 28 June-2 July, Bergen, 2011

Lequentrec-Lalancette M.F., Pineau-Guillou L., Rouxel D., 2011, *Oceanic height references: How to meet the needs?* In session: "Towards a Unified World Height System". IUGG, June 27-July 9, in Melbourne, Australia

Mémin, A., Rogister, Y., Hinderer, J. O.C. Omang & B. Luck, 2011. *Secular gravity variations at Svalbard (Norway) from ground observations and GRACE satellite data*, Geophys. J. Int. (2011) 184, 1119–1130 doi: 10.1111/j.1365-246X.2010.04922.x

Moór, A., Frey, S., Lambert S., Titov, O. et al. Bakos, J. (2011), *On the connection of the apparent proper motion and the VLBI structure of compact radio sources*, Astron. J., 141, 178.

Morel L., A.-L. Lavaux, F. Fund, F. Durand. *Impact de la configuration de traitement sur l'estimation troposphérique par GNSS*, Colloque G2. 7-9 Novembre, Marne La Vallée, 2011

Pail R., S. Bruinsma, F. Migliaccio, C. Foerste, H. Goiginger, W.-D. Schuh, E. Hoeck, M. Reguzzoni, J.M. Brockmann, O. Abrikosov, M. Veichert, T. Fecher, R. Mayrhofer, I. Krasbutter, F. Sanso and C.C.Tscherning, *First GOCE gravity field models derived by three different approaches*, Journal of Geodesy, Vol. 85 (11), pp 819-843, 2011

Pfeffer, J., Boucher, M., Hinderer, J., Favreau, G., Boy, J.-P., de Linage, C., Luck, B., Oi, M., & Le Moigne, N., 2011. *Hydrological contribution to time - variable gravity: influence of the West African Monsoon in Southwest Niger*, Geophysical Journal International, Vol. 184, Issue 2, pages 661–672.

Ramillien, G., R. Biancale, S. Gratton, X. Vasseur, and S. Bourgogne, *GRACE-derived surface water mass anomalies by energy integral approach: application to continental hydrology*, Journal of Geodesy, Vol. 85 (6), pp 313-328, 2011.

Riccardi, U., Rosat, S., & Hinderer, J., 2011. *A 300-day parallel gravity record with the gPhone-054 spring gravimeter and the GWR-C026 superconducting gravimeter in Strasbourg (France): first comparative study*, Metrologia, 48, 1, 28-39.

Rosat, S., & Hinderer, J., 2011. *Seismic noise levels of superconducting gravimeters: updated comparison and time-stability*, BSSA, BSSA, Vol. 101, No. 3, pp. 1233-1241.

Samain, E., et al., *Time Transfer by Laser Link - T2L2: Current Status and Future Experiments*", Proc. of the European Frequency and Time Forum, San Francisco, California USA, May 1-5 2011, pp. 378-383.

Seoane L., Nastula J., Bizouard C., Gambis D. (2011), *Hydrological excitation of polar motion derived from GRACE gravity field solutions*, International Journal of Geophysics ID 174396, doi:10.1155/2011/174396.

Soheili Majd M., E. Simonetto, L. Polidori, *Maximum Likelihood classification of high-resolution polarimetric SAR images in urban area*, ISPRS Hannover Workshop 2011, High-resolution earth imaging for Geospatial information, Hannover, Germany, 14-17 juin 2011

Taris F., Souchay J., Andrei, A. H., Bernard M., Salabert M., Bouquillon S., Anton S., Lambert S., Gontier A.-M., & Barache, C. (2011), *Astrophotometric variability of CFHT-LS Deep 2 QSOs*, Astron. Astrophys., 526, 25.

Titov, O., Lambert, S., & Gontier, A.-M. (2011), *VLBI measurement of the secular aberration drift*, Astron. Astrophys., 529, 91.

Tiwari, V.M., & Hinderer, J., 2011. *Gravity field: Time variations from surface measurements*, Encyclopedia of Solid Earth Geophysics, Part 7, H. Gupta (ed.), Springer, 489-494.

Vondrák, J., Capitaine, N., Wallace P.T. (2011), *New precession expressions, valid for long time intervals*, Astron. Astrophys 534, A22, doi: 10.1051/0004-6361/201117274.

Wang, X., Bonnefond, P. Extertier, et al., *Laser ranging data analysis for a colocation campaign of French Transportable Laser Ranging System (FTLRS) in Tahiti*, Journ. of Geodesy, in press, DOI: 10.1007/00190-014-0755-3, 2011

Wu, X., X. Collilieux, Z. Altamimi, B. Vermeersen, R.S. Gross and I. Fukumori (2011) *Accuracy of the International Terrestrial Reference Frame origin and Earth expansion*, Geophysical Research Letters, 38(L13304), doi:10.1029/2011GL047450
Section 5

Sciences de l'atmosphère
Atmosphéric Sciences

<figure><img src="image_28.png" /></figure>
# Report of “section 5” of the CNFGG (IAMAS-France) to IUGG
by Andrea I. Flossmann, Sophie Godin-Beekmann and Nick Hall

## Abstract :

Section 5 of CNFGG represents the French meteorology and atmospheric physics community at IUGG/IAMAS. Even though section 5 is the smallest section of CNFGG, the community it represents has contributed major achievements towards advancing our understanding of the functioning of the atmosphere. Some of these contributions, where initiatives originating in France have organized the international community (AMMA, MISTRALS) and where the French input to understanding climate change (modelling, satellite observation, in-situ monitoring) was particularly visible are detailed.

## Introduction :

The section 5 (Meteorology and Atmospheric Physics) of the CNFGG was without activity for quite some time. Only in 2013, it reconstructed itself with a bureau that is composed of a president (A.I. Flossmann), a vice-president (S. Godin-Beekmann) and a secretary (N. Hall). Even though section 5 is assuming now its task of representing the community at IAMAS (IUGG), it is still the smallest section of the CNFGG. Furthermore, due to its long absence, it suffers from an on-going recruiting problem. Efforts have been made to increase its visibility, e.g. through a CNFGG conference on one of its themes (Colloque « Aérosol » in Orléans, 5-7 Nov 2014) and a dedicated Facebook page (https://www.facebook.com/pages/IAMAS-France/661069707243119?ref=hl). However, it remains difficult to explain to our otherwise quite active community the interest to adhere (and pay) to the CNFGG.

Despite the long absence of section 5, the atmospheric science community in France is currently quite well organized. Institutional scientific structures are provided by the French INSU (Institut national des sciences de l'Univers)/CNRS), the CNES (Centre national d'études spatiales), Météo-France (the French weather service) and IRD (Institut de recherche pour le développement) and their different committees, among others. For the contact with the larger public different associations (e.g. “Météo et Climat”) and individual initiatives (e.g. “Le climat en question”) complete the landscape. The scientific representation on the international level is mostly based on individual initiatives, including representation in IAMAS or other IUGG, EGU or AGU committees. French scientists have also, e.g., contributed to the elaboration of the recent IPCC and WMO/UNEP stratospheric ozone reports.

In the framework of this reporting exercise, a report of the atmospheric science in France based on section 5 alone is obviously without interest. A report of the entire achievements of the atmospheric science in France since 2012 is equally not feasible.

Thus, below some highlights have been selected. The material presented here is based on the institutional reports of INSU, CNES, Météo-France and some scientific publications. **The four subjects are chosen as they pertain to areas where France has made a unique contribution to international science through its capacity of structuring its community on crucial questions where the available competence could make a significant impact.**

The first subject is the study of the African Monsoon. France has historically numerous ties to and responsibilities in Africa and successfully applied its competences in the project AMMA (“Analyse Multidisciplinaire de la Mousson Africaine”, http://www.amma-international.org/). Even though the core of the AMMA experiments were performed prior to the reporting
period, it should nevertheless appear here for two reasons: first, it was not reported in the last
period as section 5 was inexistent and, thus, did not furnish a report; second: the work is still
on-going and important parts of the exploitation and analysis of the data were done in the
present reporting period. The two leading French scientists (JL Redelsperger and T. Lebel)
were awarded the CNRS silver medal for this exceptional effort.

A second subject is centred on the study of the Mediterranean, which has been identified as a
region particularly vulnerable to climate change. Here, France has contributed its
competences by organizing the HYMEX (HYdrological cycle in the Mediterranean
EXperiment) and CHARMEX (the Chemistry - Aerosol Mediterranean Experiment) projects
as part of the MISTRALS (Mediterranean Integrated STudies at the Regional and Local
Scales) initiative.

A third paragraph is dedicated to the French space initiatives in the domain of atmospheric
remote sensing. Here, the role of the CNES-associated satellites PARASOL, CALIPSO,
Mega-Tropiques and IASI will be highlighted.

The last subject touches the contribution of France to the improvement of our understanding
of climate change, in the frame of the 5th IPCC report. It describes in particular the effort to
organize the acquisition, exploitation and long-term storage of in-situ data to monitor and
document the change in the composition of the atmosphere.

Understanding of the African Monsoon:

African Monsoon Multidisciplinary Analysis (AMMA) is an international project to improve
the knowledge and understanding of the West African monsoon, as well as the environmental
and socio-economic impacts of its variability. The following information was taken mainly
from Lebel et al (2009). AMMA was launched by scientists of the French community, based
on the privileged ties of France in West-Africa. AMMA relies on a field programme, which
was the largest, and most extensive ever attempted in Africa. This field programme was
organised in nested time-scales: The long-term monitoring programme (LOP, 2001 – 2009)
was based on existing infrastructure, some of which has been active for many years (including
operational networks and specific research projects) and some of which was reinforced thanks
to AMMA. In 2005, a widespread intensification and co-ordination of these networks was set
up for the duration of the so-called Enhanced Observing Period (EOP, 2005 – 2007), which
also saw the implementation of specific land-based and sea-based instruments. At the core of
the EOP, a one-year series of Special Observing Periods (SOPs) was organized in 2006,
during which intensive measurements from the surface (continent-based and ocean-based) and
from the air (research aircraft and balloons) took place.

Principal findings from the AMMA program related specifically to atmospheric science can
be listed as the following:

- Detection of regional modes of intra-seasonal variability and their relation to other tropical disturbances,

- Advances in the theory relating to growth and initiation of African easterly waves

- Comprehensive documentation of multiple aspects of the monsoons system: from the Saharan heat low to the intertropical front, the population of different classes of convective system, the diurnal cycle, the monsoon flux and the temporal character of the monsoon onset.
For further details see Janicot et al (2011) and Lafore et al (2011).

The scientific diversity of measurements embraced oceanic, hydrological and atmospheric
studies, in physical, chemical and socio-economic disciplines, from the global scale to the
'local' scale of specific instrument sites. Several hundred scientists, working in a difficult
environment, were, and are still, involved in the AMMA programme. In spite of the
difficulties, this programme has been very successful and has generated a multidisciplinary
observational database, whose exploitation is still ongoing. In addition, the importance of the
AMMA project for structuring and educating the scientific community in West-Africa needs
to be highlighted.

The Mediterranean basin:

The climate change reports (IPCC, 2013) have identified the Mediterranean as a particularly
vulnerable region in the future due to the predicted pole-ward shift of the northern end of the
Hadley cell and the storm tracks. As a consequence, MISTRALS (Mediterranean Integrated
STudies at Regional And Local Scales; http://www.mistrals-home.org/spip/?lang=en) has
been set up. In particular due to the initiative of Météo-France and INSU, a decennial
program for systematic observations and research dedicated to the understanding of the
Mediterranean Basin environmental process under the planet global change was conceived.

MISTRALS aims to coordinate, across the Mediterranean Basin, interdisciplinary research on
atmosphere, hydrosphere, lithosphere and paleo-climate, including environmental ecology
and social sciences. The objective is to achieve a better understanding of the mechanisms
shaping and influencing landscape, environment and human impact of this eco-region.

Thus, its aim is to anticipate the behaviour of this system over a century, from an
interdisciplinary analysis conducted over the 2010-2020 decade. The ultimate goal of this
program is to predict the evolution of habitable conditions in this large ecosystem, to meet the
public policies concerning resources and environment, to anticipate evolution of the societies
and to propose policies and adaptive measures that would optimize them. 7 programs
coordinate the research concerning different aspects, where two of these projects interest the
section 5 in particular: ChArMEx and HyMeX.

ChArMEx (Chemistry-Aerosol Mediterranean Experiment) aims to evaluate the current
situation of the Mediterranean basin atmospheric environment and to anticipate its future
development and its impact on regional climate, air quality, marine and continental
biogeochemistry. The main issues addressed within ChArMEx are the evaluation of the
regional budgets of tropospheric ozone and secondary organic aerosols, and the soluble input
of key elements possibly limiting (P, N, Fe) or contaminating (e.g. Hg) the marine trophic
chain.

HyMeX (Hydrological cycle in the Mediterranean Experiment; Drobinski et al, 2014) aims at
a better understanding, quantification and modelling of the hydrological cycle in the
Mediterranean, with emphasis on the predictability and evolution of extreme weather events,
inter-annual to decadal variability of the Mediterranean coupled system, and associated trends
in the context of global change. The multidisciplinary research and database developed within
HyMeX aim to improve observational and modelling systems, better predict extreme events
(in particular flooding events in Southern France), simulate the long-term water-cycle, and
provide guidelines for adaptation measures. Launched by the French scientific community,
the definition of HyMeX has started with the drafting of a White Book released in 2007
identifying the main scientific challenges for HyMeX regarding the Mediterranean water
cycle, and making suggestions on how to address them. HyMeX has since extended to the
international community.

Based on the example of AMMA, the MISTRALS projects were structured in LOPs, EOPs
and SOPs. The special observation periods (SOPs) of MISTRALS took mainly place in 2012
and 2013, while the long-time observation is ongoing up to 2020. However, the analysis,
compilation and interpretation of the findings have already started to appear in the peer-
reviewed literature and numerous additional papers will be published until the end of the
program.
**Innovation from satellite:**

Atmospheric sciences are increasingly benefitting from space observations, as they provide a unique overview of the entire globe or at least large fraction of it at almost the same time. The French national centre of space studies (CNES) is closely linked to the European and other worldwide space agencies. The following information was extracted mainly from the CNES report to COSPAR (2014) in Moscow.

CNES developed the infrared atmospheric sounding interferometer (IASI), launched by EUMETSAT on-board of the METOP satellite series. It provides essential input since 2006 to weather forecast and monitors key parameters (CO₂, CH₄, CO, dust, ..) for climate and atmospheric chemistry in quasi real time, as a contribution to the European GMES/Copernicus initiative.

CNES contributed to the A-train via the space missions PARASOL (CNES) and CALIPSO (NASA-CNES) to better understand the role of aerosol particles and clouds in climate change. PARASOL was launched in 2004 and provided global measurements with the Polder-type radiometer measuring polarised and directional reflectances from the Earth and the atmosphere. 2009 PARASOL left the A-train, but continued its observations on a lower orbit. CALIPSO was launched in 2006 and is providing global measurements of clouds and aerosol vertical profiles through its lidar. The thematic consortium ICARE is managing the production and distribution of the scientific outputs of PARASOL and CALIPSO through its Data Centre in Lille. In the case of CALIPSO, it acts as a mirror site of NASA.

The French-Indian space mission MEGHA-TROPIQUES (CNES-ISRO) was launched in 2011 and is dedicated to the study of the water cycle, energy exchanges and the evolution of the climate in the tropics. It carries e.g. the microwave imager MADRAS (CNES-ISRO, out of service since 2013), the microwave sounder SAPHIR (CNES) and the Earth radiation budget sensor SCARAB (CNES).

The French scientific community contributed to the development of the sensors as well as their data acquisition and processing. Furthermore, it organised and performed numerous campaigns for in-situ (ground-based and air-borne) data validation. The work resulted in an important amount of peer-reviewed publications from the French as well as the international scientific community.

**Improving our understanding of climate change:**

France has developed two climate models, one by Météo-France and CERFACS, and the other by IPSL, which mainly differ in their atmospheric components. Numerous improvements have been made over the reporting period to all the components in these climate models: atmosphere (representation of convection, clouds, aerosols and orography), oceans (free surface formulation), sea ice (rheology) and land (land use). Model resolution, assimilation procedures of data and coupling between components have also been improved. Finally, several studies have focused on coupling these climate models with models of chemistry, aerosols, biogeochemical cycles and vegetation dynamics. Both models have contributed to the AR5 exercise (IPCC, 2013).

The model performance is tested against past-climate parameters, but also against present time observations. Here, satellite observations (see previous paragraph), as well as in-situ long-time monitoring data are used. In France, several such monitoring activities are performed that have developed into or merged with global monitoring networks, such as the PHOTON/AERONET network (http://aeronet.gsfc.nasa.gov/; measuring aerosol optical properties), the ICOS-RAMCES network (http://www.icos-infrastructure.eu/; measuring
greenhouse gases), the MOZAIC-IAGOS network (http://www.iagos.fr/web/; atmospheric composition, aerosol and cloud particles on a global scale from commercial aircraft) and the Network for the Detection of Atmospheric Composition Changes (NDACC http://www.ndsc.ncep.noaa.gov/; monitoring the atmospheric composition of the free troposphere and the stratosphere). In addition multi-instrumented dedicated observation stations in the Paris Region (SIRTA ; http://sirta.ipsl.fr/), in the Massif Central (CO-PDD ; http://wwwobs.univ-bpclermont.fr/SO/mesures/instru.php) and in La Réunion Island (http://osur.univ-reunion.fr/) contribute to the monitoring of the atmosphere, and are part of the EU « Aerosols, Clouds, and Trace gases Research InfraStructure Network » ACTRIS (http://www.actris.net/).

**Conclusion and outlook:**

Obviously, selecting just four aspects of the French atmospheric science landscape is not
doing justice to the overall rich and active French scientific community. In addition to West-
Africa and the Mediterranean also other regions were studied (e.g. around the poles and in
Asia or South America) and a variety of different thematic fields, also in collaboration with
the other sections, were successfully treated, resulting in an enormous amount of peer
reviewed international publications.

The examples presented here were mainly selected to document the benefits of an organized
scientific approach to science where the harvest outweighs heavily the impact isolated
individual efforts can have. This organising capacity of the French community on selected
identified topics is currently endangered due to a change in French science funding
philosophy. With the increasing importance of the French National Research Agency ANR
which encourages individual competitiveness and the severe decrease in governmental
funding of its public scientific entities, community-encompassing initiatives like AMMA or
MISTRALS are not likely to occur again in the near future.

In an effort to help counteracting the occurring de-structuring of the French community,
section 5 will increase its activities. Currently, it is constructing together with the French
meteorological society (SMF: Météo et Climat) a web-based discussion platform, to increase
the exchange of information and ideas in the French atmospheric science community. Other
efforts will have to be added to increase its size and convince the community of the interest to
adhere an academic society.

**References:**

CNES, 2014: Report to COSPAR 2014, Moscow; pp. 101

Drobinski, P., V. Ducrocq, P. Alpert, E. Anagnostou, K. Béranger, M. Borga, I. Braud, A. Chanzy, S. Davolio, G. Delrieu, C. Estournel, N. Filali Boubrahmi, J. Font, V. Grubisic, S. Gualdi, V. Homar, B. Ivancic, Picek, C. Kottmeier, V. Kotroni, K. Lagouvardos, P. Lionello, M. C. Llasat, W. Ludwig, C. Lutoff, A. Mariotti, E. Richard, R. Romero, R. Rotunno, O. Roussot, I. Ruin, S. Somot, I. Taupier-Letage, J. Tintoré, R. Uijlenhoet, H. Wernli, 2014 : HyMeX, a 10- year multidisciplinary program on the Mediterranean water cycle, Bulletin of the American Meteorological Society, doi : 10.1175/BAMS-D-12-00242.1

IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp

Janicot, S., G. Caniaux, F. Chauvin, G. de Coe □tlogon, B. Fontaine, N. Hall, G. Kiladis, J.-P. Lafore, C. Lavaysse, S. L. Lavender, S. Leroux, R. Marteau, F. Mounier, N. Philippon, R. Roehrig, B. Sultan and C. M. Taylor, 2011: Intraseasonal variability of the West African monsoon. Atmos. Sci. Letters, 12, 58-66.

Lafore, J.-P., C. Flamant, F. Guichard, D. J. Parker, D. Bouniol, A. H. Fink, V. Giraud, M. Gosset, N. Hall, H. Ho □ller, S. C. Jones, A. Protat, R. Roca, F. Roux, F. Sa □id and C. Thorncroft, 2011: Progress in understanding of weather systems in West Africa, Atmos. Sci. Letters, 12, 7-12.
Lebel, T., D. J. Parker, C. Flamant, B. Bourle's, B. Marticorena, E. Mougin, C. Peugeot, A. Diedhiou, J. M. Haywood, J. B. Ngamini, J. Polcher, J.-L. Redelsperger and C. D. Thorncroft, 2009: The AMMA field campaigns: Multiscale and multidisciplinary observations in the West African region; Q. J. R. Meteorol. Soc.; DOI: 10.1002/qj.486

Prospectives INSU Océan Atmosphère, 2011-2016; pp 80.

World Meteorological Organisation-Global Ozone Research and Monitoring Project, Scientific Assessment of Ozone Depletion WMO, Report 55, 2014
Section 6

Sciences hydrologiques
Hydrological Sciences

<figure><img src="image_29.png" /></figure>
# French contribution to the IAHS
## Prediction in Ungauged Basins (PUB) initiative

Charles Perrin (1), Vazken Andréassian (1), Patrick Arnaud (2), Valérie Borrell-Estupina (3), Isabelle Braud (4), Christophe Cudennec (5), Alban de Lavenne (6), Eric Gaume (7), Pierre Hubert (8), Pierre Javelle (2), Michel Lang (4), Nicolas Le Moine (8), Gil Mahé (3), Roger Moussa (9), Ludovic Oudin (8), Jean-Emmanuel Paturel (10,3), Olivier Payrastre (7), Carine Poncelet (1), Fabrice Rodriguez (7), Eric Sauquet (4), Eric Servat (3)

(1) Irstea, Antony, France

(2) Irstea, Le Tholonnet, France

(3) HydroSciences, Montpellier, France

(4) Irstea, Lyon, France

(5) Agrocampus Ouest, UMR SAS, Rennes, France

(6) Université François Rabelais, Tours, France

(7) IFSTTAR, Bouguenais, France

(8) UPMC, Paris, France

(9) LISAH, Montpellier, France

(10) IRD, Abidjan, Ivory Coast

vazken.andreassian@irstea.fr; charles.perrin@irstea.fr; carine.poncelet@irstea.fr

patrick.arnaud@irstea.fr; pierre.javelle@irstea.fr

borrell@msem.univ-montp2.fr; mahe@msem.univ-montp2.fr; jean-

emmanuel.paturel@ird.fr; eric.servat@msem.univ-montp2.fr

isabelle.braud@irstea.fr; michel.lang@irstea.fr; eric.sauquet@irstea.fr

cudennec@agrocampus-ouest.fr

a.lavenne@gmail.com

eric.gaume@ifsttar.fr; olivier.payrastre@ifsttar.fr; fabrice.rodriguez@ifsttar.fr

pierre.hubert@keriel.bzh; nicolas.le_moine@upmc.fr; ludovic.oudin@upmc.fr;

moussa@supagro.inra.fr

jean-emmanuel.paturel@ird.fr

## Abstract

This article reviews the main contributions of the French community to the field of quantitative hydrology within a perspective of prediction in ungauged basins (PUB). A decade launched by the International Association of Hydrological Sciences (IAHS) in 2003 fostered the research on PUB, resulting in a tremendous effort worldwide to improve our understanding of hydrological processes and hydrological behaviours at the regional scale. In this respect, France offers a diversified playground to develop and test methodologies and modelling tools, given its great hydroclimatic variability and the wealth of information available through relatively dense measurement networks. We detail the main outcomes on various PUB-related objectives, including the construction of large data sets, the improved use of data, the characterization of hydrological regimes and extremes, the application of hydrological models to ungauged basins, results on specific types of basins and the outcomes of investigations conducted in other countries through international collaborations. The remaining PUB challenges as well as perspectives related to the new Panta Rhei IAHS decade on change in hydrology and society are discussed.

## Keywords

Hydrology; Ungauged basins; Modelling; France; Prediction; PUB decade; IAHS

## 1 Introduction

Hydrology is a data-based science. In all their investigations, hydrologists need observations of the complex systems they study, for process understanding, conceptualization, model parameter estimation and model validation. Such data are produced by observational networks, which measure a variety of variables such as river flow, sediment and pollutant loads, precipitation, temperature and other meteorological variables, and by geographic surveys of natural and anthropogenic features. However, the hydrological and meteorological networks are unevenly distributed in space, and measurements often unevenly made in time. One consequence is that whole geographic areas and/or periods of time are not monitored, often referred to as ungauged situations. Compared to gauged conditions, such cases raise
difficult methodological aspects when addressing hydrological questions over a range of
epistemic to engineering perspectives.

To address this problem, the International Association of Hydrological Sciences (IAHS) launched the Prediction in Ungauged Basins (PUB) decade (2003–2012). The main objective of this initiative was to foster collaborative research in the scientific community in order to elaborate “new, innovative methods for hydrological predictions in ungauged basins in different parts of the world, combined with significant reductions of predictive uncertainty” and “to demonstrate the value of data, as well as to provide the information needed to make predictions in ungauged basins, and assist in capacity building in the use of new technologies” (Sivapalan et al., 2003). A science work plan was designed, starting from a set of six key scientific questions:

1. What are the key gaps in our knowledge that limit our capacity to generate reliable predictions in ungauged catchments?

2. What are the information requirements to reduce predictive uncertainty in the future?

3. What experimentation is needed to underpin the new knowledge required?

4. How can we employ new observational technologies in improved predictive methods?

5. How can we improve the hydrological process descriptions that address key knowledge elements that can reduce uncertainty?

6. How can we maximize the scientific value of available data in generating improved predictions?

To answer these agenda-setting questions, various working groups were set up and many
capitalization and dissemination activities (conferences, special issues in scientific journals,
etc.; see www.iahs.info) were initiated. This resulted in a huge amount of scientific
production and publications. Extended reviews of these studies conducted worldwide were
provided by Blöschl et al. (2013), Hrachowitz et al. (2013) and Pomeroy et al. (2013).
Summaries were also provided on more specific themes by He et al. (2011), Razavi and
Coulibaly (2013), Parajka et al. (2013) and Salinas et al. (2013), and with national
perspectives, e.g. in Japan (Ishidaira et al., 2012), Canada (Spence et al., 2013a; Spence et al.,
2013b) and China (Liu et al., 2014).

The French scientific community was actively involved in the PUB decade and made original
contributions related to observations, process understanding and modelling. France displays a
variety of hydroclimatic conditions on a relatively limited territory (550,000 km²): in the
Köppen classification, oceanic, Mediterranean and mountainous conditions as well as
continental influences exist in metropolitan France. Fortunately also, France has dense
meteorological and hydrological measurement networks, which provide a wealth of
information to hydrologists. This makes France a valuable playground to develop and test
methods to answer the questions raised by the PUB decade. Furthermore, it is also the hub of
several international projects and participates actively in international networking.

The main objective of this article is to provide an overview of the key findings and major
achievements made in research on ungauged basins over the PUB decade, essentially in
France, but reaching out to foreign contexts for the sake of acknowledging genericity,
transposition, comparison and cooperation. We also give research perspectives within the
context of the new IAHS decade, called Panta Rhei, launched in 2013 to address the issues of
hydrology under change (Montanari et al., 2013).

The article is organized as follows. Section 2 provides an overview of the hydroclimatic and
physical contexts where the research reviewed here was conducted. Section 3 presents the
main achievements for various hydrological objectives. Section 4 discusses the main lessons
from this research and presents a few of the main challenges for the future.
## 2 Data

The studies reviewed in this article were carried out mainly in metropolitan France but also in other parts of the world, such as central and western Africa. Many studies were based on large datasets, which have various advantages from a PUB perspective. Here we mainly detail the French context and the main data sources used for these studies, since they supported the core of the French developments.

### 2.1 On the need for large datasets for PUB

Designing methods applicable to ungauged basins requires working on gauged basins. Indeed, the efficiency and reliability of such methods must be tested against observations, using basins as if they were ungauged. It also requires quite large datasets, since there is a need to define a *priori* or regional information using neighbouring gauged catchments, which will be transferred to ungauged catchments.

Aside from detailed analysis of processes on case studies, a great deal of research focusing on PUB is based on the extensive use of data, with large catchment sets. One key motivation in using large datasets is to obtain general results and conclusions, as underlined by several authors (Andréassian et al., 2006; Andréassian et al., 2009; Gupta et al., 2014). In the context of PUB, another motivation is to describe a wide range of catchment characteristics, the variability between catchments being a prerequisite to find robust explanatory relationships between hydrological target variables and catchment descriptors. A large number of issues can be addressed on such datasets, e.g. mapping hydrological characteristics (Cipriani et al., 2012), flood frequency analysis (Aubert et al., 2014; Kochanek et al., 2014) and low flow frequency analysis (Folton and Lavabre, 2007; Nicolle et al., 2013).

In France, hydrologists enjoy quite dense observational networks managed by regional and national institutions as well as databases which provide long records. Hence it is possible to build sets of catchments gathering several dozen to more than 1,000 catchments with records covering several decades. Such large datasets require specific processing approaches (e.g. to automatically check data quality), selection options (Boldetti et al., 2010) and assessment methodologies (Mathevet et al., 2006).

### 2.2 Hydroclimatic conditions in France

Hydrologically, metropolitan France is drained by a few large basins – the Loire, Seine, Rhône, Garonne and Rhine rivers – and a large number of small coastal basins that flow into the North Sea, the English Channel, the Atlantic Ocean and the Mediterranean Sea, from north to south, or to surrounding countries. These basins are formed by a marked topography, with high mountains in the Alps and the Pyrenees, a more moderate relief in the Massif Central and the Vosges, and sedimentary basins in several parts of the country. They are subjected to various meteorological influences of neighbouring oceanic features. The range of climate conditions can be illustrated for example by the spatial variability in mean annual precipitation, which ranges from 300 to 2000 mm (amounts on mountains may be locally larger), or in mean annual temperature, which ranges from about 0 to 16°C. A wide variety of processes can be observed regionally, such as snowmelt and sediment transport processes in mountainous basins (Alps and Pyrenees); flash floods producing intense rainfall events in Mediterranean basins; long summer drought periods that can occur at regional and national
scales; slow routing processes in large lowland basins; substantial exchanges between surface
and groundwater in the sedimentary areas and the karstic basins; and artificialized
hydrological conditions due to urbanization, irrigation and water infrastructures such as dams.
There are even tropical basins subjected to heavy rainfall and cyclones in overseas
departments (e.g. Réunion Island, Guadeloupe, Martinique, French Guiana). This results in
various regime types, ranging from rain-fed, snow and glacial regimes to Mediterranean and
groundwater-fed regimes (Sauquet, 2006; Snelder et al., 2009). This variety of conditions,
processes and regimes provides many opportunities to address the PUB issues.

**2.3 National observational networks and data**

In terms of flow observation, there are more than 3,000 gauging stations spread over the
hydrographic network for various purposes. Many of these stations were installed in the
1960s, when water agencies were created to monitor water resources and improve water
quality. This means that today a large part of the corresponding flow records are several
decades long, providing a wealth of data for long-term analyses. Flow data are available in the
national HYDRO archive with free access (www.hydro.eaufrance.fr) (Leleu et al., 2014).

Many studies have also made substantial use of meteorological and climatic inputs. Historically, the national meteorological institute in France, Météo-France, is the main data producer and manages a dense network of stations throughout the country (Champeaux et al., 2009). Other institutions also manage their own complementary networks for specific purposes, such as the French electricity producer EDF for reservoir management (Perret et al., 2012) and the AgroClim service at the French National Institute for Agricultural Research (INRA) for agronomic applications (https://www6.paca.inra.fr/agroclim). For precipitation, which is one of the main drivers for hydrologists, the operational national network has more than 4,000 daily stations and nearly 1,000 sub-daily stations. Therefore the average density is good at a daily resolution (one station for about 140 km²), and moderate at a sub-daily resolution (one station for about 550 km²). Small catchments are often poorly monitored, as are the upper parts of mountainous catchments (no raingauge above 2250 m and very few between 1750 and 2250 m). This ground information is now supplemented by information provided by weather radars, which have covered most of the national territory since 2004 (Tabary, 2007). Statistical reanalyses are also more and more commonly produced. Key products include the SAFRAN reanalysis produced by Météo-France, which has provided 8×8-km gridded data since 1958 (Vidal et al., 2010), the kilometric SPAZM reanalysis produced by EDF over French mountain areas (Gottardi et al., 2012) and the COMEPHORE reanalysis combining raingauges and weather radar data (Delrieu et al., 2014; Tabary et al., 2012). Météo-France also produces various deterministic or ensemble weather forecast products that can be used as input to modelling chains for hydrological forecasting (Vincendon and Carriere, 2009). A wide range of observational data can be found on the Météo-France website (http://publitheque.meteo.fr/). In French Guiana, a climatology of this overseas department has recently been produced by adjusting the TRMM satellite product based on ground measurements (Brochart et al., 2015).

Based on these datasets, large hydroclimatic datasets were assembled, e.g. for hydrological
modelling purposes (Le Moine, 2008; Lobligeois, 2014).
## 2.4 Research observatories

Datasets documenting long hydrological processes are highly valuable for understanding processers, hydrological modelling and trend studies. For this purpose, a number of research observatories (www.allenvi.fr/groupes-transversaux/infrastructures-de-recherche/les-soere2) have been set up in the past two decades, some covering over 50 years (Orgeval and Réal Collobrier Observatories, http://bdoh.irstea.fr). Several of these are standardized and promoted through the national network of experimental research basins (RBV; see http://rnbv.ipgp.fr) on critical zones, offering a wide range of hydrological contexts. Several observatories (e.g. ORACLE, OMERE, and AGHRYS) focus on the relationship between agriculture and hydrology/hydrochemistry. For example, the OMERE observatory (Voltz and Albergel, 2002) was founded to investigate the impact of human-made hydrological discontinuities in farmed Mediterranean catchments on runoff genesis (Chahinian, 2004; Hallema, 2011; Moussa et al., 2002), surface–subsurface interaction (Dages et al., 2009), contaminant transport (Lagacherie et al., 2006; Louchart et al., 2001) and erosion (Raclot et al., 2009). The Strengbach, Draix, Auradé and other observatories are dedicated to hydrogeochemical issues subjected to lower anthropogenic pressure. The observatory on the Cévennes-Vivarais region close to the Mediterranean (OHM-CV, www.ohmcv.fr, Boudevillain et al., 2011) is more specifically dedicated to the collection of rainfall and discharge data in a flash-flood-prone area, with field experiments dedicated to process understanding and modelling (see Braud et al., 2014). Urban and periurban observatories, especially in Lyon, Nantes and Paris (www.urbis-soere.org), have also been set up to investigate the impact of urban environments on the hydrological cycle. These observations have contributed to the advance of distributed hydrological modelling in periurban catchments (Braud et al., 2010a), as well as in farmed basins (Hallema et al., 2013; Moussa et al., 2002).

## 2.5 Complementary data for PUB studies

To set up hydrological models, information on elevation, geology, land use, pedology as well as urban infrastructure is needed and can be obtained from French institutes:

- digital terrain models and information on roads, buildings, etc. from the portal of the national geographic institute (IGN Geoportail, www.geoportail.gouv.fr);

- geological information from the national geological survey (BRGM, http://infoterre.brgm.fr);

- land use data from the Corine Land cover site (www.statistiques.developpement-durable.gouv.fr/donnees-ligne/li/1825.html);

- pedology from the soil conservation inventory (IGCS, www.gissol.fr/programme/igcs/igcs.php).

In top-down PUB studies, catchment descriptors are needed to evaluate explanatory relationships between hydrological variables and catchment descriptors at gauged sites. The catchment descriptors are computed on the basis of the information also used for distributed hydrological models. Given the empiricism of these relationships, these approaches require catchment sets as large and diverse (in terms of physical characteristics) as possible. Thus, a major issue in the use of these datasets from varied catchment sites is the availability/calculation of homogeneous catchment properties for several catchments. Global or European datasets are available for elevation data (SRTM database), land use (Corine Land cover) and soil (European Soil Database, Panagos et al., 2012). The subdivision of the
national territory into “hydro-eco-regions” (Wasson et al., 2002) has also been used for regionalization purposes.

The French hydrological community is also actively involved in studies outside France. Historically, the ORSTOM institute (now IRD, Institut de Recherche pour le Développement) has contributed to developing hydrological networks in many countries around the world, more specifically in Africa, since the 1950s. The collected rainfall and runoff data, mostly from central and western Africa, are stored in the SIEREM information system, developed by Boyer et al. (2006) (www.hydrosciences.fr/sierem/), which also includes gridded rainfall datasets at the continental (Mahé et al., 2012) and regional (Paturel et al., 2010b) scales (at 0.5×0.5° and monthly resolutions). Time series for hydrological modelling are available for more than 300 catchments. The database also includes soil and vegetation data. It supported a number of studies dedicated to tropical and equatorial environments, where modelling tools were tested in various inter-tropical climatic conditions.

# 3 Advances in PUB

The methods and models developed within a PUB perspective cover a wide spectrum of domains, objectives, conditions, etc. The following sections give an overview of the main aspects investigated by the French community during the PUB decade.

## 3.1 Enhanced acquisition and use of data

### 3.1.1 Use of meteorological data

Meteorological data are considered essential for predicting runoff, especially precipitation (rain and snow) and temperature data. They are largely responsible for the space-time variability of flows and for their random nature. Temperature partly controls snowmelt and evapotranspiration processes (Oudin et al., 2005). Therefore good knowledge of these variables is essential to explain the variability of flows, for example through hydrological modelling. The sensitivity of hydrological models to available rainfall data has been largely investigated and demonstrated by several authors (Andréassian et al., 2004a; Andréassian et al., 2004b; Arnaud et al., 2011).

When only partial knowledge of this information is available, interpolation methods are required to estimate variables in time (Oudin et al., 2010b) and space (Gottardi et al., 2012). The new climatic products presented above (statistical reanalyses and radar-raingauge merging) make it possible to better take into account the spatial variability of rainfall. These data are increasingly usable for prediction in ungauged basins over French territory (Javelle et al., 2010; Randrianasolo et al., 2014; Thirel et al., 2010). The added value of radar information was also investigated in various works on gauged and ungauged catchments for flood simulation, forecast and alert (Harader et al., 2012; Javelle et al., 2014; Lobligeois et al., 2014).

### 3.1.2 Use of sparse flow data

Between the well-gauged and fully ungauged situations, there are many cases where only sparse data are available (in space and/or time), thus corresponding to poorly gauged or sparsely gauged situations. In these cases, the available data are often too limited for applying classical hydrological estimation methods, but they do nonetheless provide valuable local information, compared to less detailed regional information. Several approaches have
attempted to find the best way to combine local and regional sources: Rojas-Serna et al. (2006) for rainfall-runoff models (see section 3.5.3), Ribatet et al. (2007) for estimating flood quantiles and Chopart and Sauquet (2008) for estimating low-flow characteristics (Chopart and Sauquet, 2008). Ribatet et al. (2007) further developed an index flood model within a Bayesian framework for sites with a short record, incorporating regional information in the prior distribution. Chopart and Sauquet (2008) used estimates of low-flow characteristics derived from spot gauging data (Catalogne et al., 2014) and quantified their uncertainties in a kriging system of equations.

### 3.1.3 Use of historical flood and paleoflood data

Various sources of information can be combined to estimate extreme floods. Indeed in many cases, discharge data are available on a few years only, which can be considered as ungauged situations from a statistical point of view. Hence hydrologists have turned their attention to historical flood information and how to best incorporate documentary evidence of such historical floods into flood frequency estimation. In the past, several French authors collected historical data on floods at a large scale. Champion (1858-1864) provided a national inventory on French floods since the 6th century, Pardé (1961) did numerous studies of large floods over the world (see also https://iga-map.ujf-grenoble.fr/parde/), and Rodier and Roche (1984) produced a world catalogue of maximum observed floods. Table 1 summarizes the available French historical flood series collected during the PUB decade.

<table><caption>Table 1: Available French historical flood series collected during the PUB decade (2003-2012)</caption><thead><tr><th>Basin</th><th>Stations</th><th>Systematic records</th><th>Historical period (largest floods)</th><th>References/Research project</th></tr></thead><tbody><tr><td>Isère</td><td>Grenoble</td><td>1968–1998</td><td>1601–1967</td><td>Auffray et al. (2011), Coeur (2008)<br>Historisque-Isère project (1998–1999) <sup>(1)</sup></td></tr><tr><td>Ardèche</td><td>Vallon and St Martin</td><td>1980–2001</td><td>1644–1979</td><td>Naulet (2002), Naulet et al. (2005)<br>Historisque-Ardèche project (1998–1999) <sup>(2)</sup><br>Sphere project (2000–2002) <sup>(3)</sup></td></tr><tr><td rowspan="4">Gard</td><td>Alès</td><td>1893–1980</td><td>1604–1891</td><td rowspan="4">InondHis project (2005–2007) <sup>(4)</sup><br>ExtraFlo project (2009–2013) <sup>(5)</sup><br>Dezileau et al. (2014)</td></tr><tr><td>Anduze</td><td>1892–2005</td><td>1741–1891</td></tr><tr><td>Mialet</td><td>1892–2005</td><td>1741–1891</td></tr><tr><td>St Jean</td><td>1892–2005</td><td>1841–1891</td></tr><tr><td rowspan="2">Hérault</td><td>Ganges</td><td>1970–2005</td><td>1795–1969</td><td rowspan="2">InondHis project (2005–2007) <sup>(4)</sup></td></tr><tr><td>Gignac</td><td>1989–2005</td><td>1812–1988</td></tr><tr><td rowspan="5">Aude</td><td>Orbiel</td><td>1968–2002</td><td>1788–1967</td><td rowspan="5">Payrastre (2005)<br>InondHis project (2005–2007) <sup>(4)</sup><br>HYDRATE project (2006–2009) <sup>(6)</sup></td></tr><tr><td>Clamoux</td><td>1964–1991</td><td>1844–1963</td></tr><tr><td>Saltz</td><td>1968–2003</td><td>1820–1967</td></tr><tr><td>Lauquet</td><td>1968–1996</td><td>1820–1967</td></tr><tr><td></td><td></td><td></td><td></td></tr></tbody></table>

Three recent European projects covered historical and paleofloods. Benito et al. (2004) reviewed the historical and paleoflood data for flood risk estimation within the SPHERE project (2000–2003). Sheffer et al. (2003) concluded from paleoflood analysis that the 19th century floods on the Ardèche River were the largest of the millennium. Gaume et al. (2009) provided an extensive compilation of over 550 peak discharges of flash floods at ungauged sites over seven countries in Europe within the HYDRATE project (2006–2009). Last,

<footnote>(1) http://www.risknat.org/baseprojets/resultats.php?num=10108&name</footnote>

<footnote>(2) http://www.risknat.org/projets/cper/projets_1994-1999/02-Historisque-Ardeche.pdf</footnote>

<footnote>(3) http://cordis.europa.eu/project/rcn/52072_en.html</footnote>

<footnote>(4) http://carteau.onema.fr/content/projet/inondhis_lr</footnote>

<footnote>(5) http://extraflo.irstea.fr</footnote>

<footnote>(6) http://www.hydrate.tesaf.unipd.it/</footnote>
Kjeldsen et al. (2014) reviewed flood frequency estimation guidelines in Europe in the FLOODFREQ Cost Action (2009–2013).

A national historical database on floods in France (http://bdhi.fr/) has been available online since 2015, based on an inventory of major floods produced in 2011 within the framework of the EU Flood Directive implementation (Lang and Coeur, 2014; Lang et al., 2012). It contains a description of approximately 175 flood events on French rivers from 1770 to 2011.

### 3.1.4 Data collection on extreme flood events in ungauged basins

Better understanding the processes involved in recent major flood events is also one of the PUB objectives. Several studies were conducted in France, especially around the Cévennes-Vivarais observatory in southern France (see section 2.4), which is prone to massive precipitations at the end of the summer and in autumn, potentially causing disastrous floods (Braud et al., 2014). Post-event studies were initiated (Abbott et al., 1986; Bonnifait et al., 2009; Delrieu et al., 2005; Gaume et al., 2004; Manus et al., 2009) and progressively extended to all of Europe during the Floodsit, HYDRATE, and HYMEX projects. In the course of these projects, methods for conducting post-flood investigations in ungauged basins after flash floods have been significantly improved (Borga et al., 2008; Gaume and Borga, 2008; Ruin et al., 2013), including methods for the estimation of peak discharges (Lumbroso and Gaume, 2012). The database of well-documented extreme floods that have affected ungauged basins now includes the most significant events recently observed in France (Aude département, 1999; Gard, 2002 and 2008; Var, 2010; Pyrenees, 2013; Hérault and Gard, 2014) and some of the major events which occurred in Europe (Marchi et al., 2010; Marchi et al., 2009; Ruiz-Villanueva et al., 2012; Zanon et al., 2010). The 2002 flood event in the Gard region was one of the largest observed and was extensively analysed through data collection, reanalyses and modelling (Anquetin et al., 2010; Braud et al., 2010b; Delrieu et al., 2005; Gaume and Bouvier, 2004).

### 3.1.5 Design and sensitivity to observational networks

Given the importance of acquiring data for hydrological applications, the issue of observational network design is of crucial importance in a PUB perspective. Indeed, hydrological estimations at ungauged locations often rely on regional information and/or neighbouring gauged sites. Therefore the configuration (density, spatial distribution) of measurement networks can have a direct impact on the reliability of these hydrological estimates. This issue has long been investigated in many places around the world (see the review by Lebecherel, 2015). Within the PUB decade, several studies attempted to provide a new perspective on this issue, using various approaches, tools and models.

Several studies investigated the sensitivity of hydrological modelling to the quality of information used to calculate inputs at the catchment scale, especially rainfall and potential evapotranspiration (Anctil et al., 2006; Andréassian et al., 2004a; Andréassian et al., 2004b; Andréassian et al., 2001; Arnaud et al., 2011; Dezetter et al., 2008; Oudin et al., 2010b; Oudin et al., 2006b; Paturel et al., 2010a; Paturel et al., 1995). Techniques were developed for testing the sensitivity of hydrological calculations to network configuration, namely the random decrease in density and the hydrological desert by removing neighbouring stations (Lebecherel et al., 2014).
**3.2 Regimes and mean flow characterization**

**3.2.1 Catchment classification and regimes**

Classification in hydrology aims at grouping basins with similar runoff characteristics and/or similar catchment behaviour. There are numerous ways to define catchment classes due to the various hydrologic metrics available to characterize flow regimes. Many classifications are based on the seasonal variations of flow, to identify the dominant drivers of runoff generation and the occurrence of high and low flows within the year, following the pioneering work of Pardé (1933).

Sauquet et al. (2008) developed an interpolation framework adapted for mapping monthly runoff patterns along the river network, and the results are presented using a classification adapted to the French hydro-climatic context. A hierarchical cluster analysis applied to the 12 Pardé coefficients from 872 gauging stations was adopted in their study. Deviations from the natural flow regime can be introduced and propagated along the river network to account for human influences at the monthly time scale. Snelder et al. (2009) developed a classification of river flow regimes for France from cluster analysis of 157 of the 171 hydrological indices suggested by Olden and Poff (2003). The hydrological characterization of each basin includes monthly flows, flood and low-flow descriptors estimated from daily data, indices related to interannual variability, etc. The characterization of flow variability can also be an indicator of catchment behaviour and can be made using the flow duration curve (FDC). Sauquet and Catalogne (2011) proposed a methodology to estimate FDC on ungauged catchments using a large set of 1080 French catchments.

In the field of rainfall-runoff modelling, catchment classification is also widely used for parameter transfer (between gauged *donor* catchments and ungauged *receiver* catchments). While comparing catchments identified as *physically* similar with catchments recognized as *hydrologically* similar, Oudin et al. (2010a) underlined that the two classifications were not equivalent: it is difficult to infer efficient hydrological classes based only on physical similarity classes. Andréassian et al. (2014) identified 27 classes of hydrological behaviour, based on the capacity of French catchments to be represented by a limited list of model parameters.

**3.2.2 Mean flow estimation and mapping**

The estimation of reference flows such as mean flow or variables characterizing low or high flows are essential for various water management applications. In France, mean flows and minimum monthly flows are typically used for withdrawal authorizations. Over the PUB decade, several studies at the national scale aimed at estimating these flow values. Sauquet (2006) developed a geostatistical approach to map mean flows on 900 catchments. Folton and Lavabre (2006) proposed a regionalized rainfall-runoff model to simulate monthly flow series and estimated mean flow characteristics. Lebecherel et al. (2013) regionalized the Turc-Mezentsev formula to estimate long-term water balance, using a large set of 609 French catchments. Yan et al. (2012) proposed a geostatistical method to map interannual water balance components, taking into account uncertainties and the water balance closure. The approach was extended to other streamflow characteristics by Gottschalk et al. (2013).
## 3.3 Hydrographic network characterization and hydrogeomorphological approaches

The channel network controls the spatial pattern of hydrological processes within a catchment. Moreover, the topology of the channel network and the geometric properties of reaches are the main factors controlling the routing function of hydrologic models.

Therefore, the classification of channel networks can be considered as a first step in the process of analysing and synthesizing catchments’ morphometric properties. The identification of key hydrological features characterizing the channel network can contribute to the efforts to produce a rational classification of catchments for PUB regional hydrological studies and predictions. The morphometric properties of the channel network are derived from digital elevation models to determine geometric descriptors of the channel network (Le Moine, 2008; Moussa, 2009; Moussa and Bocquillon, 1993; Moussa and Bocquillon, 1996), the shape of subbasins (Moussa, 2003) and hypsometric properties (Gsell et al., 2015) and to identify the invariant properties of channel networks (Gsell et al., 2015; Moussa et al., 2011).

These easily accessible descriptors control the shape of the width function so they can be used to either derive the essential features of the catchment Geomorphologic Instantaneous Unit Hydrograph (GIUH) from topological analysis or directly estimate the Width Function Instantaneous Unit Hydrograph (WFIUF) from morphometric analysis (Cudennec and de Lavenne, 2015; Cudennec et al., 2004a; Moussa, 2008). Specific algorithms were developed to detect and describe the hydrographic network from various databases and in various hydrological contexts (Aouissi et al., 2013; Gironás et al., 2010; Lagacherie et al., 2010; Moussa and Bocquillon, 1994; Rapinel et al., 2015; Rodriguez et al., 2013). A geomorphology-based IUH approach was then used as a transfer function in both lumped and spatially distributed models for applications on natural basins (Cudennec et al., 2009; Cudennec et al., 2005; Moussa, 1997), urbanized basins (Gironás et al., 2009; Rodriguez et al., 2013; Rodriguez et al., 2005) and farmed basins (Hallema and Moussa, 2014). Application studies were also conducted to examine the design and impact of dams (Cudennec et al., 2004b; Nasri et al., 2004), the sensitivity of the descriptor values to spatial discretization (Gironás et al., 2010; Rodriguez et al., 2013), the role of rainfall space-time distribution (Cudennec et al., 2005) and for regionalization and transposition studies (Boudhraâ et al., 2009; de Lavenne et al., 2015; de Lavenne and Cudennec, 2014).

## 3.4 Prediction of extremes in ungauged basins

Predicting extreme hydrological events is highly valuable for various purposes (regulation, design, alert, etc.). Note that prediction can be understood as the statistical estimation of events in the frequency domain or their forecast in the time domain.

Extreme low-flow or high-flow estimation and forecast is necessary for design purposes, to improve risk anticipation and real-time management of water infrastructures. Extreme value assessment in hydrology is a difficult task because it is an extrapolation exercise, i.e. by definition it refers to hydrological events which are beyond the range of usual observations. Hydrological or statistical models will then be out of their domain of calibration, and the dominant hydrological processes may be very different from ordinary events.

Hereafter we distinguish low-flow and flood estimation studies and flood forecasting studies conducted in a PUB perspective (there are no reports on low-flow forecasting in ungauged basins in France to our knowledge).
**3.4.1 Low-flow estimation**

Catalogne (2012) developed a general framework for estimating the 5-year annual minimum monthly flow (noted QMNA5) used in water allocations in France. The framework suggests different strategies depending on data availability in the neighbourhood of the target site or at site (short records or only a few discharge measurements). Nicolle et al. (2013) had a similar objective and proposed a methodology to robustly estimate the QMNA5 when only short time series are available, applied to 527 catchments in France. Snelder et al. (2013) pointed out the difficulty of predicting intermittency due to the possible influence of small-scale processes.

Methodologies based on regionalized rainfall-runoff modelling were also developed to estimate QMNA5. A regionalized version of the GR2M model was implemented in the LOIEAU tool for application on ungauged basins (Folton and Lavabre, 2006).

**3.4.2 Flood frequency estimation**

As pointed out by Merz and Blöschl (2008), flood frequency analysis can be improved by expanding the information beyond a simple set of observations at the target site. Three types of information can be exploited: temporal (using past information from documentary sources or proxy data), spatial (gathering information from neighbouring sites or within a homogeneous region) and causal (using for example rainfall information and a rainfall-runoff model).

An additional level of difficulty is encountered when no data are directly available at the site of interest. Several approaches have been proposed in the past few years to handle this situation. Gaume et al. (2010) and Nguyen et al. (2014) presented a regional approach pooling estimated extreme discharges at ungauged sites. The new proposed inference procedure, based on Bayesian Monte Carlo Markov Chain algorithms, was included in the nsRFA package (R software). The added value of historical extremes at poorly gauged sites was also the focus of several studies with particular attention given to the estimation of uncertainties on extreme historical discharge (Neppel et al., 2010; Payrastre et al., 2011). Cipriani et al. (2012) compared different regionalization approaches applied to estimate the instantaneous peak discharges with a 10-year return period and a flood duration derived from a selection of hydrographs. To make advanced use of rainfall information, Organde et al. (2013) developed a regionalized version of a continuous simulation approach, SHYREG, coupling a regionalized rainfall generator (Arnaud et al., 2008) with a rainfall-runoff model to estimate flood quantiles at ungauged sites throughout France (Aubert et al., 2014). Similarly, the SCHADEX method (Paquet et al., 2013), developed at EDF for extreme flood assessment, was tested in ungauged conditions by Penot (2014), using a regionalized SCS model by grouping catchments based on the hydrological regimes.

These methods and many others classically used in France for flood estimation were compared in the Extraflo national research project (http://extraflo.irstea.fr/) (Lang et al., 2014). Kochanek et al. (2014) presented the results of this national comparison of flood frequency methods, showing that the SHYREG method provides better results at ungauged sites than a standard method based on the regionalization of the parameters of a GEV distribution. Since 2014, a national database has been available on line (http://shyreg.irstea.fr), providing an estimation of SHYREG flood quantiles for 130,000 French catchments varying in size between 5 and 5,000 km².

**3.4.3 Flood forecasting**

Flood forecasting at ungauged sites is a challenge. It requires that rainfall-runoff models be implemented at ungauged locations, with parameters estimated by regionalization approaches, and that these models can be run in real-time with adequate inputs. Outside France, examples
of such systems include the Grid-to-Grid model in the United Kingdom (Cole and Moore, 2009) and the European Flood Alert System (EFAS) running at the European scale for large catchments (Bartholmes et al., 2009; Thielen et al., 2009). Due to the difficulty of providing accurate ‘absolute’ forecasts, results are expressed in a ‘relative’ way, in comparison with thresholds (quantiles) derived from the climatology of the model. In France, Météo-France developed the SAFRAN-ISBA-MODCOU (SIM) model, which also operationally provides discharge forecasts for the whole country at a daily time step, without at-site calibration (Habets et al., 2008). Focusing on smaller spatial and temporal scales, the AIGA method, jointly developed by Météo-France and Irstea, aims at providing flash-flood alerts at ungauged catchments, using radar information and a simple regionalized gridded model running at the hourly time step (Javelle et al., 2014). The method will be used in real time covering all of France to provide alerts on ungauged basins. The PreDiflood project (2009–2012) enabled the development of a flash-flood forecasting prototype devoted to the identification of road inundation risks at the scale of the entire Gard region in France (5000 km²), with more than 3000 road sections exposed to flooding, mostly downstream of small and ungauged basins (Naulin et al., 2013; Versini et al., 2010a).

To better quantify uncertainties associated with future precipitation, Randrianasolo et al. (2011) developed an approach for flood forecasting on ungauged sites using ensemble meteorological forecasts. Given that the assimilation of flow observations often has a strong impact on the efficiency of forecasting models, they also proposed a specific updating procedure based on observed flows from neighbouring gauged catchments.

A case study application of artificial neural networks for flood forecasting was conducted on a small catchment in southern France subject to flash floods by Artigue et al. (2012). Given the strong variability of rainfall on this catchment, it can be considered as poorly gauged by the existing network. The authors show the importance of the appropriate selection of inputs and model complexity in this case. Furthermore, Artigue (2012) demonstrated that the neural network forecast towards ungauged or proxy basins could be generalized using the area-discharge relationship. Last, the HYDRATE EU project also investigated flash-flood forecasting at ungauged locations, advising the use of distributed models and demonstrating the good skill of procedures based on combining the Flash Flood Guidance approach and a method of model-based threshold runoff computation (FFDI – Flash Flood Diagnostic Index), with tests on catchments in Italy and central France (Borga et al., 2011; Norbiato et al., 2008).

## 3.5 Use of hydrological models to predict runoff hydrographs in ungauged basins

When the objective is to produce whole flow series, continuous rainfall-runoff models are often used and their regionalization consists in transferring hydrological information (model parameters, hydrological signatures or relationships between model parameter and physiographic descriptors) from gauged (donor) catchments to ungauged catchments. Regionalization methods are usually grouped into four classes: regression-based methods, physical similarity (catchment grouping), spatial proximity and calibration on regionalized hydrological signatures. But alternative approaches are also being developed.

### 3.5.1 Performance of classical regionalization approaches in France

Regression-based methods have been implemented with limited performance for two daily lumped models (GR4J and TOPMO) (Oudin et al., 2006a; Rojas-Serna et al., 2006) and with more success for a simplified version of the GR3H (Aubert, 2012) and the GR2M (Folton and
Lavabre, 2006) models. Another opposite but complementary approach consists in using
models that are more explicitly related to physical properties, as tested by Drogue et al.
(2002) in the Alzette basin. Whatever the approach followed, deriving the relevant physical
descriptors used in regression equations is probably the key for future progress of regression-
based approaches (Boldetti, 2012).

Physical similarity has been used successfully by Garambois (2012) to transfer the parameters of the distributed MARINE model. On a larger set of catchments, physical similarity and spatial proximity approaches were compared by Oudin et al. (2008). The study showed that the best-performing methods in France are based on spatial proximity, followed by physical similarity and regression. The good performance of spatial proximity was attributed to the dense gauging network available in France (one gauging station per 100–200 km² compared with one per 1000 km² in the United States, for example). These comparative studies raised the question of the relation between hydrological similarity and the similarity as it can be assessed by physical descriptors or through spatial proximity (Oudin et al., 2010a).

Boldetti (2012) used hydrological signatures (such as flow quantiles or baseflow indices) to constrain the calibration of a hydrological model on ungauged catchments. His study showed that the success of this kind of approach depends to a large extent on the performance of hydrological signature regionalization. Overall, he was not able to surpass the proximity-based approach with the regionalized signature-based approach.

3.5.2 Alternative approaches

All the previous methods make the assumption that a single model structure can reproduce all catchment behaviours, the variability between the catchments being handled by the parameter set. A complementary approach is to develop tailor-made models, i.e. adjust the model structure to the dominant processes of the catchments. A single parameter set can then be used for catchments sharing the same dominant processes. The study by Crabit (2010) (see also Crabit et al., 2012; Vannier, 2013) used this approach. To avoid model calibration issues, an alternative approach where distributed models are used to test functioning hypotheses (Clark et al., 2011) was also implemented. Models tailored to the specificities of the catchments are built, generally using modelling platforms (Branger et al., 2010). Parameters are not calibrated but assigned based on existing databases and published values. By comparing simulations with observations, it is possible to determine which functioning hypotheses / parameter sets present the best agreement between simulation and observation. Vannier (2013) and Adamovic (2014) provide examples of such applications for Mediterranean regions and Jankowfsky et al. (2014) for periurban catchments.

To cope with calibration uncertainties, a promising path is to look for more robust parameter
values. Several authors have indeed shown that a finite number of parameter sets is sufficient
to accurately reproduce streamflow in various catchments types (Andréassian et al., 2014;
Perrin et al., 2008). It is expected that those generic, robust parameter sets will be easier to
regionalize, and that they can serve as a basis for hydrological classification.

Last, distributed or geomorphology-based models can also be useful tools to consider on
ungauged basins. The simultaneous calibration of a model on a set of catchments (also called
regional calibration) can be an efficient option, as shown for example by Engeland et al.
(2006) with the distributed Ecomag model on the Saone basin. On a set of 23 catchments in
the Cévennes region, Vannier et al. (2014) also showed that soil properties could be estimated
by considering recession curves at a regional scale. Another approach consists in
deconvoluting the discharge signal at a gauged outlet and using it to simulate the discharge at
another ungauged outlet, based on assumptions on the similarity of the corresponding basins
(possibly nested). This has been precisely developed with geomorphology-based transfer
functions (Cudennec and de Lavenne, 2015; de Lavenne et al., 2015).
### 3.5.3 Poorly gauged catchments

In poorly gauged catchments, where only a few flow measurements or very short flow series are available, specific methodologies have been proposed for model parameter estimation, based on a combined use of regional and local information. Perrin et al. (2007) and Rojas-Serna et al. (2006) investigated the required number of streamflow measurements needed to calibrate a model. They showed that quite limited information can produce reliable parameter sets. Starting from similar ideas, combination approaches were developed by Rojas-Serna et al. (2006) and Lebecherel (2015), in which the parameter estimation is made by weighting regional information and local information. These techniques were applied by Drogue and Plasse (2014) and Plasse et al. (2014) to the French parts of the Meuse, Moselle and Rhine basins, with efficient results compared to classical regionalization approaches. Paturel et al. (2006) also applied such combination approaches on a set of catchments in Western Africa, with promising results. Also acknowledging the value of point flow measurements, Crabit et al. (2011) proposed an easily implementable method to acquire streamflow measurements. These studies stressed that a few flow measurements can be very useful to constrain model parameter estimation and that going in the field to collect these data is a viable alternative to heavy statistical methods. Post-flood event data can also provide valuable information for assessing the reliability of regional hydrological models dedicated to flash-flood simulation (Anquetin et al., 2010; Vannier, 2013).

### 3.5.4 Uncertainty quantification in ungauged basins

Running hydrological models on ungauged basins statistically increases the level of model error compared to the case where models can be calibrated in gauged conditions. This is clearly shown for example by Oudin et al. (2008) who compared calibration and regionalization approaches, showing the lower efficiency of the latter. The determination of model parameters is indeed a key step which introduces additional errors and uncertainty in the modelling process. Quantifying uncertainty on model simulation outputs in these conditions is, however, a difficult task, since by definition model errors cannot be quantified in ungauged catchments.

To answer this problem, Bourgin et al. (2014) proposed a two-step approach, in which the uncertainty estimated on gauged donor catchments is transferred to the target ungauged catchment, after considering the donors as ungauged themselves. The approach, applied to a large set of French catchments, provided quite sharp and reliable flow confidence intervals.

Another approach was proposed by Sellami et al. (2014), but more specifically focusing on parametric uncertainty, using ensembles of parameter sets with the SWAT model.

Another completely different avenue has also been opened: the use of indirect information on the magnitude of floods (observed damage or traffic disruptions) for the evaluation of models in ungauged basins has been tested with very promising results in the ANR PreDiflood (2009–2012) and CPER Rythmme (2008–2013) projects (Javelle et al., 2014; Naulin et al., 2013; Versini et al., 2010b).

## 3.6 Regional outliers and hydrological singularities in a PUB perspective

A key hypothesis in most PUB methodologies (Blöschl et al., 2013; Hrachowitz et al., 2013) is that at least a modest amount of information is transferable to the target ungauged basin (through knowledge of dominant processes, a spatial proximity with gauged basins, regional
calibration, etc.). Despite the progress made in understanding which information is relevant and how to transfer it, there will always be cases where this transfer will fail inevitably, making these cases very interesting to push modelling approaches to their limits (Andréassian et al., 2010). This kind of hydrological singularity mainly occurs when the system under study and/or its forcings cannot be at all defined *a priori*, and hence even the most basic equation of mass balance closure cannot be written. This typically includes karstic catchments, urban catchments and, to a lesser extent, farmed and mountainous catchments. In these cases, the problem is more often to characterize the physiography of the catchment knowing its discharge, rather than the reverse. Here we review the work done in France over the past decade on these types of catchments.

### 3.6.1 Karstic basins

Karstic aquifers are characterized by complex internal organizations with heterogeneous properties and internal structures and actual extent that are frequently poorly known. Indeed, the actual area drained by a karstic river does not coincide with topographic catchment boundaries, with often a highly non-linear response (Taver, 2014). Hence the water balance is difficult to establish (Bakalowicz, 2005) and the contribution to floods is highly variable (Bailly-Comte et al., 2009; Bonacci et al., 2006). For these reasons, karstic basins can be considered poorly gauged, even when surface flow measurements exist, and hydrological methodologies must often be customized for them.

In terms of hydrological modelling, various approaches may be considered to model karst or to simulate spring hydrographs, ranging from lumped, semi-distributed conceptual models to systemic approaches including neural network models (Bailly-Comte et al., 2012; Coustau et al., 2012; Darras et al., 2015). For example, Le Moine et al. (2008) proposed a specific hydrological modelling approach of the Touvre karstic system using parsimonious conceptual models. The results showed that proper conceptualization of the system provides a satisfactory closure of the water balance of the whole system. Coustau et al. (2012) proposed an event-based parsimonious model and successfully applied it to the karst flash floods of the Lez and the Cesse rivers (Southern France) (Raynaud et al., 2015a).

In a forecasting perspective, Coustau et al. (2013) showed that a data assimilation loop, built on top of the conceptual model, can improve the discharge simulation on the Lez basin by estimating more suitable initial conditions. A graphical operational tool aimed at assisting decision-making in a context of limited data was built to roughly estimate the flood peak discharge (Borrell Estupina et al., 2014; Borrell Estupina et al., 2015). Hydrological modelling was also used to produce an abacus for flood management in the city of Nîmes located downstream of a karstic system (Fleury et al., 2013). Other approaches were proposed to better estimate flow components in karstic systems, such as the study by Kong-A-Siou et al. (2013) based on a neural network model designed to estimate the various contributions to the catchment discharge and the approach proposed by Raynaud et al. (2015b) based on hydrogeomorphological observations, chemical analyses and hydrodynamic modelling to estimate the karst contribution to surface discharge.

### 3.6.2 Urbanizing basins

Like karstic basins, catchments on which urbanization plays a significant role require specific attention: dedicated observatories have focused on these basins in the past few years in France (see section 2.4). In urban and periurban catchments, surface and subsurface runoff is constrained by sewer and rainwater networks. In periurban catchments, where land use is composed of a patchwork of natural and urbanized areas, analyses of long-term time series from the observatories showed contrasted behaviours, depending on the season and soil
moisture, with a hydrological response mainly governed by urban areas in dry conditions and a significant contribution of rural areas during wet seasons (Braud et al., 2013; Furusho et al., 2014). In such basins, water pathways and catchment boundaries are modified by the various sewer networks (Jankowfsky et al., 2013). As in natural catchment hydrology, hydrologists dealing with urban catchments used the morphological features of these catchments to represent their hydrological response. The GIS maps that cities developed considerably helped in determining these features (Rodriguez et al., 2003). It was shown that the main descriptors used to characterize the response of ungauged urban catchments (physical descriptors, hydrographic network) can be sensitive to the type of data used and the segmentation options (Gironás et al., 2010; Rodriguez et al., 2013). The hydrological response determined using the drainage network pattern of the city of Nantes, in western France, is also sensitive to the spatial distribution of impermeable areas, which is essential to estimate urban surface runoff (Gironás et al., 2009; Rodriguez et al., 2005).

To better understand the role of the spatial variability of periurban land use, Jankowfsky (2011) and Jankowfsky et al. (2014) developed the object-oriented PUMMA model to simulate and understand water pathways and water balance components in small French periurban catchments. The approach was found to be efficient in reproducing the main components of the water cycle, in particular contrasted hydrological responses between wet and dry seasons. At larger scales, existing models were enhanced to better account for periurban catchment specificities (Furusho et al., 2013; Labbas, 2015). Both studies highlight the role of sewer networks in modifying the hydrological cycles in these catchments. Labbas (2015) also used the distributed model to compare different functioning hypotheses, highlighting the role of the soil and shallow groundwater to better simulate the whole hydrological cycle.

### 3.6.3 Farmed basins

Management of land and water resources of agricultural catchments has emerged in the last two decades as an environmental priority due to the effects of land use on runoff, erosion and pollutant transport. In agricultural catchments, the division of the landscape into fields and the ditch network introduce significant hydrologic discontinuities which partially control runoff. Human-made structures modify the natural pathways of storm water flow. Agricultural operations such as tillage have a significant influence on local surface runoff infiltration as well as surface storage by altering soil hydrologic properties and soil surface roughness (Chahinian, 2004). Also, the networks of ditches and embankments influence the water transfer from the fields to the catchment outlet (Hallema, 2011) and the flow exchange between the surface and groundwater (Dages et al., 2009). The MHYDAS model (Moussa et al., 2002) was specifically developed to account for the hydrological specificities of agricultural catchments. Various modelling applications were conducted in order to simulate runoff genesis at the plot scale, taking into account crusts (Chahinian et al., 2005; Chahinian et al., 2006b) and tillage (Chahinian et al., 2006a) when modelling overbank flow during extreme flood events (Moussa and Bocquillon, 2008); to couple hydrologic and erosion models (Gumiere et al., 2011); to develop specific parameterization strategies (Hallema et al., 2013) and sensitivity analysis procedures (Cheviron et al., 2010); and finally to use the model to quantify the impact of the spatial arrangement of land management practices on surface runoff (Colin et al., 2012).

### 3.6.4 Mountainous basins

From a PUB perspective, mountain catchments present two specificities. First, they are not only ungauged from the streamflow point of view, but they are also widely ungauged from the