Vol. 31, No. 2 | Quarter 2 2021
GEWEX is a Core Project of the World Climate Research Programme on Global Energy and Water Exchanges

SoilWat: Improving Soil Representation in Land Models
Bottom-up approach

Upscaled 1D root water uptake model

Top-down approach
Parallel root system

Transpiration = ∑" 𝑄𝑄"

Hsoil Qup

Big root system

HcollarQup,1 Hsoil,1

ing
cal
ups

Kx,up,jQup,jKr,up,jHsoil,jHcollarrj
Laye

Qup,j Hsoil,j

𝑄𝑄$%,' = ) 𝑄𝑄""∈'

𝐻𝐻,-".,"≈ 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑖𝑖𝑖𝑖 𝑗𝑗
Qi
Hsoil,i
Kr,i

Kx,i
Hx,i

Estimation of root conductances in soil layers fromroot segment conductances

3D root system architecture

𝐻𝐻,-".," is the hydraulic head of the soil water that isin contact with node i.

𝐻𝐻:," is the hydraulic head of the water in the xylem.

𝐾𝐾: is the hydraulic conductance in the xylem.

𝐐𝐐𝐮𝐮𝐮𝐮 = 𝐂𝐂𝐂𝐂 𝐇𝐇𝐬𝐬𝐬𝐬𝐬𝐬𝐬𝐬 + 𝐻𝐻𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝐂𝐂𝟓𝟓
C4 and C5 depend root segment conductances Kr and Kx andthe connection of the segments in the 3D root architecture

𝐾𝐾9 is for radial flow from the soil to the xylem.

𝐾𝐾9,$%,' = ) 𝐾𝐾9,$%,'"∈'

𝐾𝐾:,$%,' = ∞𝐾𝐾9,$%,' = ) 𝐾𝐾9,$%,'"∈'

𝐾𝐾:,$%,' = 𝑓𝑓 𝐾𝐾:," "∈'C4 and C5 depend on upscaled root segment
conductances Kr,up and Kx,up and are derived based on
an a-priori concept of the root architecture and theconnection of the segments

Figure 3 compares a bottom-up approach versus top-down approaches for a parallel and a big root system model to derive andparameterize an upscaled one-dimensional root water uptake model. (Adapted from https://doi.org/10.5194/hess-2021-14.) See
Zeng et al. on page 4.

InsideThis
EditionCommentary and News

GeneralMeeting Reports

Commentary: It's time for
GEWEX to tackle irrigation [p. 2]

Young Earth System Scientists
webinars and new Science
Interview series [p. 3]

The 2nd Determining
Evapotranspiration Workshop
evaluates the state-of-the-art andfuture challenges in ET [p. 11]

Xubin Zeng is the new Scientific
Steering Group Co-Chair [p. 3]
Sandrine Bony joins the GEWEX
Atmospheric System Studies
Panel as Co-Chair [p. 3]

New members and a databasepromoting diversity, equity, andinclusion for the Hydrology Section
Student Subcommittee [p. 4]
SoilWat initiative makes progress in
improving representation of soil inclimate models [p. 4]

The Gobal Land/Atmosphere
System Study (GLASS)
reviews its process-oriented,
benchmarking, and modelintercomparison activities at its
annual meeting [p. 14]

Commentary:
Irrigation, an Anthropogenic Perturbation
to the Continental Water Cycle GEWEX
Should Address
Jan Polcher
GEWEX Scientific Steering Group (SSG) Co-Chair

Irrigation is arguably the largest water usage by mankind andcertainly the most important consumptive abstraction fromthe natural water cycle. If GEWEX aims to understand thechanges occurring in the continental branch of the water cycle, we need to tackle this process. Irrigation is an intervention
in the water cycle which is not purely geophysical, as it also involves socio-economical choices such as water rights, culturaland historical considerations, the cost of water, and engineering issues linked to water management and transport. But allthese processes are framed by the geophysical constraints ofwater demands by the atmosphere and water availability. These
two mechanisms are squarely within the core competencies of
GEWEX. As both are expected to change in a warmer climate,
we have a responsibility to advance our knowledge in order tocontribute to the discussions on adaptation of water usage toclimate change occurring in many regions of the world. For
our colleagues within WCRP, we need to evaluate the impactof irrigation on continental fresh water discharge and place itin an Earth system perspective.
A number of activities are already underway within the
GEWEX community that are relevant to the topic of irrigation. For instance, we can cite the Land surface Interactions
with the Atmosphere over the Iberian Semi-arid Environment(LIAISE) field campaign, which will be carried out in July
2021 within the Hydrological Cycle in the Mediterranean Experiment (HyMeX) Regional Hydroclimate Project. It aims
to create a better understanding of how irrigation modifies
regional evaporation and the structure of the atmosphericboundary layer. The evaporation cross-cut spearheaded by the
GEWEX Hydroclimatology Panel (GHP) will improve ourunderstanding of one of the main drivers behind the irrigation of crops. The European Space Agency (ESA) has funded a
project to try and quantify water use for irrigation through theinversion of remote sensing observations.
A greater challenge will be acquiring the ability to simulate
and predict water uptake by irrigation and its impact on thecontinental water cycle. A number of land surface models
already include some sort of representation of irrigation, but
often with strong assumptions. The first step in that modelingchain is to predict the water stress of crops, as it is the driving
force behind the need to irrigate. But the amount of water applied will depend on regulations, the cost of water, and finallythe farmers’ choices. Once that irrigation demand is predicted,
it needs to be determined where in the water cycle it will be
extracted and how it will be applied in order to simulate its
impact on climatic processes. All this obviously while respecting the water conservation principles central to our land surface models. As can be seen, modeling all these aspects will bechallenging and will require expertise well beyond GEWEX.
Nevertheless, we must start that process and interact with theother disciplines developing knowledge on irrigation. In the
coming decade, our Earth system models will need to represent
this important human intervention and enable us to predict
the actual water cycle and its sensitivity to climate fluctuations.
GEWEX needs to identify a strategy to ensure progress in our
knowledge of the role of irrigation in the continental water
cycle. The observational capabilities that can contribute toadvancing our understanding need to be identified and nurtured. We need to support a trans-disciplinary effort to monitor “Essential Water Variables” parallel to the one for essential
climate variables. Development of land surface models needs
to be encouraged so that various approaches can be comparedand their qualities well-understood. Only once the GEWEX
community recognizes what it can contribute to the discussion can a fruitful collaboration with agronomers, hydroeconomists, and the other specialists be had. This trans-disciplinary conversation will allow us to better understand how
climate change will affect water scarcity and move beyond theprediction of natural water resources we are performing today.

Table of Contents

2

Commentary and News

General (Cont'd)Commentary: Irrigation, an Anthropogenic Perturbation to
the Continental Water Cycle GEWEX Should Address 2
New SSG Co-Chair
3
New GASS Co-Chair
3GEWEX-ISMC SoilWat Project: Taking Stock andLooking Ahead

4

WCRP’s New Lighthouse Activities

10

GeneralMeeting and Workshop Reports

YESS Engagement in the International Agenda anda Glimpse of our Latest Activities

3

H3S Welcomes New Members and Introduces New
Initiatives in 2021

4

The Second Determining Evapotranspiration Workshop 11
Virtual 2020 GLASS Panel Meeting

14

Quarter 2 2021

New SSG Co-Chair
Dr. Xubin Zeng, most recently
Co-Chair of the GEWEX Global
Atmospheric System Study (GASS)
Panel, is the new Co-Chair of the
GEWEX Scientific Steering Group(SSG), joining Co-Chair Jan Polcher.
Dr. Zeng replaces Graeme Stephens,
who served as SSG Co-Chair from2014 to 2021. Dr. Zeng is the Agnese N. Haury Chair inEnvironment, Professor of Atmospheric Sciences, and Directorof the Climate Dynamics and Hydrometeorology Center at the
University of Arizona. He is also an affiliated professor of the
Applied Mathematics, Global Change, and Remote Sensing
and Spatial Analysis Interdisciplinary Programs. Through202 peer-reviewed papers, Dr. Zeng's research has focused onland-atmosphere-ocean interface processes in the Earth system,
weather and climate modeling, hydrometeorology, remotesensing, nonlinear dynamics, and big data analytics.
He is a fellow of the American Meteorological Society (AMS),
American Association for the Advancement of Science (AAAS),
and University of Arizona Academic Leadership Institute. He
just accepted the AMS 2021 Charles Franklin Brooks Award
for Outstanding Service to Society, and received the Special
Creativity Award from the National Science Foundation. He
received his Ph.D. from Colorado State University in 1992 andwas given its Atmospheric Science Outstanding Alumni Award.

New GASS Co-Chair
Sandrine Bony joins the GEWEX
Atmospheric System Studies (GASS)
Panel as its new Co-Chair. Currently,
Dr. Bony is a director of research at theLaboratory of Dynamical Meteorology(LMD) in Paris. After her Ph.D. from
Sorbonne University, she worked at theMassachusetts Institute of Technology
and joined the French National Center for Scientific Research(CNRS). Sandrine was a lead author for the 4th Assessment
Report of the Intergovernmental Panel on Climate Change, andserved as an editor for the Journal of Climate and the Bulletin of theAmerican Meteorological Society. She has been active in WCRP asa co-chair of the Cloud Feedback Model Intercomparison Project(CFMIP), as a co-chair of the Working Group on Coupled Models(WGCM), and as a co-lead coordinator of the WCRP Grand
Challenge on Clouds, Circulation, and Climate Sensitivity.
Recently, she co-led the international Elucidating the Role ofCloud-Circulation Couplings in Climate (EUREC4A) fieldstudy over the western tropical Atlantic. Through the combined
use of observations and models, she has developed extensive
expertise in cloud processes and their role in climate, including
in cloud feedbacks and in convective organization. By joining the
GASS Panel as a Co-Chair, she wishes to promote observationaland modeling studies of atmospheric processes that can bothcontribute to improved models and to address challenging
science questions that are at the core of climate research.
3

YESS Engagement in the International
Agenda and a Glimpse of Our
Latest Activities
Carla Gulizia1, Faten Attig Bahar2, Valentina Rabanal3,
and the YESS Executive Committee
Centro de Investigaciones del Mar y la Atmósfera (CIMA/
CONICET-UBA), Buenos Aires, Argentina; 2Applied
Mechanics and Systems Research Laboratory, Tunisia
Polytechnic School, University of Carthage, Al Marsa,
Tunis, Tunisia; 3Servicio Meteorológico Nacional (SMN),
Buenos Aires, Argentina
1

As part of the objectives elaborated in the Strategic Action
Plan, the Young Earth System Scientists (YESS) community
has been very active in fostering new collaborations and opportunities for Early Career Researchers (ECRs) at international panels and meetings. Several YESS members (Pablo
Borges de Amorim, Feba Francis, Leandro Diaz, and Gaby
Langendijk) were selected to represent YESS in the World
Climate Research Programme (WCRP)’s new Lighthouse
Activities. In addition, our YESS members Faten Attig Bahar
and Jorge Saturno were invited to represent YESS at the 25th
German Committee Future Earth (DKN) meeting held in
February 2021. YESS will also host a virtual side event during the next DKN Summit (https://www.dkn-future-earth.org/
events/087592/index.php.en), to be held in July of 2021.
The YESS Online Events Working Group (WG), in collaboration with the Pan African University Institute of Water
and Energy Sciences (PAUWES) and the contribution of theUnited Nations University and Future Earth Nexus Knowledge-Action-Network, is planning the third edition of the
YESS-PAUWES webinars. The focus will be on sustainable
development in Africa post-COVID. The first edition of the
Global Atmosphere Watch (GAW) Training and Education
Centre (GAWTEC) webinars series, a joint activity between
the YESS community and World Meteorological Organization (WMO)’s GAW program, took place between December
2020 and January 2021, and focused on greenhouse gasses
and atmospheric composition measurements. All the webinars
can be accessed here: https://www.yess-community.org/gawtecwebinar-series/.
Finally, a new initiative, coordinated by the YESS Science WG,
is the Science Interview series. The aim is to provide insights
into ongoing Earth System science research projects and thecurrent or potential opportunities for ECR engagement. The
first article addresses the role of another core WCRP program,
Stratosphere-troposphere Processes And their Role in Climate(SPARC), in fostering the new generation of Earth-Systemscientists. All the information on this initiative can be foundat https://www.yess-community.org/yess-science-interviews/.
We invite international scientific community members tonominate any project you are aware of that offers opportunities
for ECRs to be considered for our series. For this, please get intouch with us at interviews@yess-community.org.
Quarter 2 2021

H3S Welcomes New Members andIntroduces New Initiatives in 2021

GEWEX-ISMC SoilWat Project:
Taking Stock and Looking Ahead

Julia Guimond
National Science Foundation (NSF) Postdoctoral Fellow,
Dalhousie University

Yijian Zeng1, Anne Verhoef2, Dani Or3,4, Matthias Cuntz5,6,
Lukas Gudmundsson7, Lutz Weihermueller8, Stefan
Kollet8, Jan Vanderborght8, and Harry Vereecken8

On January 28th, the American Geophysical Union (AGU)
Hydrology Section Student Subcommittee (H3S) welcomed 15
new members to the committee, ranging in career stage fromundergraduate student to postdoctoral researcher. Since the firstmeeting, H3S members and subcommittees have been busy
brainstorming new initiatives, resources, and opportunities forstudents and early career researchers. On Friday, March 26th, H3S
hosted the first virtual networking event of the year. This event
was in the form of a speed networking social with approximately
30 one-minute blocks for early career researchers to meet andshare interests. Similar events will be held virtually every month
this spring with the hope of strengthening the early career
community and building students’ and early career researchers’
networks. Events with similar goals are also in the planning stage,
to be held this summer as well as at the 2021 AGU Fall Meeting.
One year ago, H3S launched its website (agu-h3s.org) to provide
resources and highlight the research of early career scientists.
This spring, H3S members are working hard to expand thewebsite, including absorbing the WaterPOC database, createdby students from the University of Waterloo. This database is atool to support efforts of diversity, equity, and inclusion in thefield of water research, including:•

Increasing the visibility of these researchers to their peers(both minoritized and not) up and down the career ladder•

Increasing recognition of the excellence of research
conducted by these individuals•

Facilitating nominations and recruitment of these individuals
for awards, leadership positions, and paid positions

This database also highlights the intersections of identities withinwater researchers, including race, gender, and career stage. Thereare currently 180+ entries in the database and it continues togrow. The WaterPOC database will soon be accessible throughthe H3S website for long-term access and maintenance.
Stay tuned for more event announcements in the coming months!
Interested in learning more about H3S activities? Follow us on
Twitter (@AGU_H3S) or Instagram (agu_hydrology).

Submit an Article to
Share your GEWEX experiences and activities, including scientificresearch results and other information associated with global
water and energy cycle studies. Articles should be 800–2400
words (1–3 pages) and feature 1–2 figures. If you have an idea fora piece, please contact us at gewex@gewex.org.

4

ITC Faculty of Geo-Information and Earth Observation,
University of Twente, Enschede, the Netherlands; 2Departmentof Geography and Environmental Science, University ofReading, Reading, UK; 3Department of Environmental
Systems Science, ETH Zürich, Zürich, Switzerland; 4Divisionof Hydrologic Sciences (DHS)–Desert Research Institute,Reno, NV, USA; 5INRA, Université de Lorraine, Ecologie
et Ecophysiologie Forestiéres, Champenoux, France;
6
Department of Computational Hydrosystems, UFZ–
Helmholtz Centre for Environmental Research, Leipzig,
Germany; 7Institute for Atmospheric and Climate Science,
Department of Environmental Systems Science, ETH
Zürich, Zürich, Switzerland; 8Agrosphere Institute IBG-3,
Forschungszentrum Jülich GmbH, Jülich, Germany
1

The Soil and Water (SoilWat) initiative is a joint activity
between the Global Energy and Water Exchanges (GEWEX)
project and the International Soil Modelling Consortium(ISMC). SoilWat aims to bring together two researchcommunities to improve the representation of soil andsubsurface processes in climate models. The soil modelingcommunity (represented by ISMC) and the climate
modeling community (represented by GEWEX) are working
together to identify the most pressing challenges and topics
related to this effort, in terms of understanding the role ofsoil properties and parameters, soil physical processes (e.g.,
infiltration, surface evaporation, water and heat flow), root
hydraulics, groundwater dynamics, and their interactions
with the vegetation and biogeochemical cycles, deploying
a combination of Earth observations (in situ and remotesensing), modeling, and data assimilation.
Soil Properties and Pedotransfer Functions

There is a longstanding tradition in soil science of developing
and applying pedotransfer functions (PTFs) for the calculationof hydraulic properties (Clapp and Hornberger, 1978; Cosby etal., 1984; Pachepsky and Rawls, 2004; Vereecken et al., 2010;
Van Looy et al., 2017; Dai et al., 2019a). These functions use
basic soil properties that are widely available, such as texture,organic matter, and bulk density, to estimate hydraulic, solutetransport, thermal, and biogeochemical parameters (Van Looy
et al., 2017). The soil hydraulic properties (water retention
characteristic and the hydraulic conductivity curve) (Brooks etal., 1964; van Genuchten, 1980; Montzka et al., 2017; Gupta
et al., 2020; Gupta et al., 2021) are pivotal in describing
the flow and storage of water in soil and its availability forvegetation via root water uptake (Verhoef and Egea, 2014).
Besides applications in soil science, ecology, engineering, andhydrology, soil hydraulic functions and their PTFs are used
extensively to derive hydraulic parameters for land surface
models (LSMs). LSMs are embedded in global climate andQuarter 2 2021

Hydraulic Con𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 𝐾𝐾 [𝑐𝑐𝑐𝑐 𝑑𝑑 %8 ]Soil Water Content 𝜃𝜃 [𝑐𝑐𝑐𝑐$𝑐𝑐𝑐𝑐%$ ]Soil Matric Potential h [-cmH2O]

Soil Water Content 𝜃𝜃 [𝑐𝑐𝑐𝑐$ 𝑐𝑐𝑐𝑐%$ ]Figure 1. Summary of the SP-MIP effort. Comparison between experiments 1 and 2 will show the model diversity caused by the PTFs. Comparison
betweenexperiments
2 and
3 will showmodel
comes fromdifferent
soilformaps
aggregation
Experiment
4 serves:(i) toFigure
1. Soil water
retention
curvethe(left)and diversity
hydraulicthat
conductivity
curves(right)the and
USDA
sand classschemes.
for the 13
PTF (coloured
curves).
quantify the effect of spatial variability of soil parameters on model diversity, (ii) to assess the sensitivity of each model to soil hydraulic parameters,and (iii) to investigate to which degree the spatial variability of key water and energy balance outputs is controlled by soil properties.

numerical weather prediction (NWP) models to enable regional
to global coverage and calculation of soil- and land surfacerelated processes, pertaining to the water, energy, and carbon
balances. Consequently, uncertainty in basic soil properties
propagates into the derived soil parameters and can affectfluxes and state variables simulated by LSMs (Weihermüller
et al., 2021; Dai et al., 2019b). For example, the evaluationof European Centre for Medium-Range Weather Forecasts(ECMWF) soil moisture and temperature analyses against
in situ data collected on the Tibetan Plateau (TP) revealed
that the persistent systematic model bias is mainly caused bynonrepresentative values of saturated hydraulic conductivity
used in the model and simplifications of certain soil physicalprocesses (e.g., freeze-thaw) (Su et al., 2013; Yu et al., 2018).
A further investigation into the five existing soil texture
databases over the TP demonstrated a wide uncertainty range
across different climate regions of the TP (i.e., arid, semi-arid,
and subhumid) (Zhao et al., 2018). The results of these kindsof studies show that the choice of soil information and soil
databases matters for LSMs (Dai et al., 2019a; Montzka et al.,2017; Fatichi et al., 2020).
Information on basic soil properties such as soil texture andporosity is also used to derive the parameters required in theequations used to calculate thermal soil properties (thermal
conductivity and heat capacity) in LSMs (Dai et al., 2019a;
Johansen, 1977; Lu et al., 2007; He et al., 2020; Ghanbarianand Daigle, 2016; Dai et al., 2019c). Hence, the term PTF
can also be used in this thermal context. Furthermore, otherthan considering thermal property equations and their PTFs,
SoilWat ultimately aims to develop a unified underlying
theory to yield both thermal and hydraulic parameters in aphysically-consistent way (Lu and Dong, 2015). This will help
facilitate the harmonization of soil hydro-thermal properties
and their PTFs as used in LSMs, to avoid artifacts potentiallyoriginating from the choice of soil property maps and PTFs
(Weihermüller et al., 2021), and confusion with uncertainties
stemming from different model structures and physics.
5

For this purpose, SoilWat is conducting a systematic assessmentof the quality of resolved soil maps used in climate modeling, andthe effect of different soil maps on the prediction of land surface
fluxes and state variables. With this endeavor, we seek to improve
the quality and resolution of soil maps used in LSMs. Current
ongoing efforts include a “Soil Parameter Model Intercomparison
Project” (SP-MIP, see Fig. 2); this involves eight LSMs [i.e., theCommunity Land Model (CLM), the Interaction Sol-BiosphèreAtmosphère (ISBA) model, the Joint UK Land Environment
Simulator (JULES), the Jena Scheme for Biosphere Atmosphere
Coupling in Hamburg (JSBACH) model, the Organising
Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE)
model, the Minimal Advanced Treatments of Surface Interaction
and Run Off (MATSIRO) model, MATSIRO with Groundwater(MATSIRO-GW), and the Noah- Multiparameterization(Noah-MP) model] and the ISMC working groups of “SoilThermal Properties” and “Pedotransfer functions and land
surface parameterization” (https://soil-modeling.org/science-panels/
working-groups/pedotransfer-functions). Analysis of the SP-MIP
data is currently ongoing.
The recent paper by Weihermüller et al. (2021) explored theeffect of using different PTFs (Clapp and Hornberger, 1978;
Cosby et al., 1984; Pachepsky and Rawls, 2004) (see Fig. 1) on
key components of the water balance for an area in Germany(using a 30 year set of meteorological driving data). It employed
the widely used and tested HYDRUS model (Šimůnek etal., 2016) to avoid confounding effects of choice of PTF
and model structure. The effects on the evapotranspiration
were considerable, and they varied depending on the choiceof soil type, land cover (bare, grass, or crop), and whether
groundwater was present. Follow-on studies are currently
being conducted to test the effects with other independent soil
physical models, as well as the effects on the thermal regime.
Soil Infiltration Processes

Infiltration is a key soil process that partitions precipitation
Quarter 2 2021

of some of the common approaches presently used to deduce
this important land surface flux). The SEC implements a
simple and easy-to-use formulation of evaporation surface
resistance (Lehmann et al., 2018) that has been tested using
FLUXNET data and decadal lysimeter records from arid
regions (Lehmann et al., 2019). More recently, the SEC model
has been extended to include the competing effects of drainage
in an analytical framework (Lehmann and Or, under review).
This line of research not only fits into the SoilWat theme oflinking soil and climate processes, but it also offers readily
useable improvements for global models as has been shown
recently (Or and Lehmann, 2019; Decker et al., 2017; Mu
et al., 2021).
Figure 2. Soil water retention curve (left) and hydraulic conductivity
curves (right) for the United States Department of Agriculture (USDA)
sand class for 13 PTFs (colored curves).

into surface runoff and water that enters the soil profile. As partof the SoilWat initiative, Vereecken et al. (2019) reviewed thebasic principles and current approaches to describe infiltrationin LSMs, its numerical implementation, and its sensitivity tomodel parameters. They found a large variation in infiltration
approaches adopted by LSMs, also in relation to upscalingfrom the local to the grid-scale, in addition to the large variety
of approaches used to estimate soil hydraulic properties asmentioned above. They identified several processes not yet
considered in LSMs that influence infiltration into soils such as
soil structure (Fatichi et al., 2020), water repellency, crusting,freeze-thaw processes, hysteresis, flow instability, and swellingand shrinking. At present, we lack information on how,e.g., vegetation and land use may impact soil structure andthus soil hydraulic properties and the process of infiltration,although efforts by Gupta et al. (2020, 2021) have tried totake this into account.
Soil Surface Evaporation

Soil evaporation is an important component of the hydrologicalcycle that affects plant available water and the surface energybalance. Knowledge of surface evaporation (defined as
evaporation from soil pores and canopy interception, but nottranspiration) is important for separation of evapotranspiration
to its components (transpiration and evaporation) to betterlink the water and carbon cycles and for various aspects ofwater resource management (e.g., irrigation scheduling). The
dynamics of soil evaporation and internal drainage following
rainfall are sensitive to soil properties; in other words, soil type(as defined by its textural composition) affects the rates bywhich soil water percolates to deeper layers, and the surface
resistance to evaporation. It also affects a characteristic depth
below which water cannot be easily extracted by capillarity
and related physical processes (of course, plant roots can accessand extract soil water down to several meters). These common
but often overlooked soil effects on soil evaporation havebeen formulated in a conceptual model by Or and Lehmann(2019) termed the Surface Evaporation Capacitor (SEC), andthe model has been applied globally to improve climatic
surface evaporation predictions (highlighting shortcomings
6

Soil Water and Heat Flow

Most of the current operational LSMs describe water flow via
Richards’ equation and heat flow via Fourier’s law, and the soil
water and heat flow are “softly” coupled via soil heat capacity
and thermal conductivity, both of which are dependent on soil
water content. Although such model treatments of soil physics
can broadly capture the soil water-heat dynamics over humid
and subhumid regions, their performance over arid and semiarid lands (ASALs) (that make up >40% of the globe) arealways compromised. Such model deficiency over ASALs ismainly caused by the missing mechanism of vapor transfer,which is key for actively and dynamically coupling soil waterand heat flow, since it carries simultaneously mass and energy(Garcia Gonzalez et al., 2012; Zeng et al., 2011a; Zeng et al.,2011b; Shahraeeni and Or, 2010; Shahraeeni and Or, 2012;
Shokri et al., 2009).
A study with water vapor transfer incorporated into the
JULES model found that the water vapor flux contributes significantly to the water and heat transfer in the upper soil layers
over three semi-arid and temperate arid sites (Garcia Gonzalez
et al., 2012). The inclusion of vapor transfer has a direct andpositive impact on the diurnal evolution of evaporation, soil
moisture content, and surface temperature, which will contribute to the improved understanding and prediction of landair interactions. Furthermore, the consideration of soil vapor
flow can further our wider process understanding, including
land-atmosphere feedbacks at larger scales. For instance, wemight be better able to grasp why afternoon rain is more likely
over drier soils (Taylor et al., 2012), or why the semi-arid ecosystem plays a dominant role in the trend and variability of theland CO2 sink (Ahlström et al., 2015).
Afternoon rain falls preferentially over dry soils, particularly
over semi-arid regions, where surface fluxes are sensitive tosoil moisture and convective events are frequent (Taylor etal., 2012). Semi-arid ecosystems were found to account for
39% of interannual variability of global Net Biome Production (NBP), which is mainly driven by the local compensatory effects of soil water availability on NBP (Ahlström et al.,2015; Jung et al., 2017). To facilitate understanding the roleof soil physical processes within the above context, SoilWat is
conducting an in-depth survey on how key soil physical processes (water and heat flow) are represented in Earth System
Quarter 2 2021

and Land Surface Models.
Soil-Root Hydraulics

Since the root system is the plant organ that is responsible
for the acquisition of water and nutrients, its hydraulicproperties play a key role in the functioning of the vegetationand in the water, carbon, and nutrient cycles of terrestrial
ecosystems. Root hydraulics have been represented in LSMs
in different ways: assuming a vertical “big root” (Amenu andKumar, 2008; Tang et al., 2015) or assuming a network ofparallel roots that take up water near the root tips and thatare connected at the root collar (Sulis et al., 2019; Kennedy etal., 2019). These hydraulic root models are parameterized in
a “top-down” approach by first postulating a simplified root
hydraulic structure and then defining its parameters fromroot distributions and root segment hydraulic properties.
Another approach is to start with a 3-D root hydraulicarchitecture model and solve the corresponding flow
equations. This system of equations was scaled up to a 1-D
model and general properties of the root system that describe
total root water uptake and water uptake distributions as
a function of the soil water potential distribution can bederived (Vanderborght et al., 2021).
It was found (Vanderborght et al., 2021) that the upscaled
equations can be approximated by a form that is equivalent
to the parallel root model. This implies that parameters of theparallel root model can be derived in a bottom-up approachfrom 3-D root hydraulic architectures, whereas this was notpossible for the big-root model, which contains twice thenumber of parameters as the parallel root model (see Fig.
3, on cover). A parallel root model that was parameterized
in a bottom-up approach predicted the root water uptake
profiles better than parallel root or big root models that wereparameterized in a top-down approach. These results showthat simple 1-D root hydraulic models used in LSMs canrepresent uptake by complex 3-D root hydraulic architectures.
Root density profiles could be used to parameterize rootarchitecture models that are subsequently used to parameterize
the upscaled root water uptake model. The 3-D root hydraulicarchitecture is currently being coupled with a soil-root transfer
model that describes the water flow from the bulk soil to theroot surface and represents non-linearities of soil water flow
equations. First results show that upscaling the linear equationsof the 3-D root hydraulic architecture, and then coupling the
upscaled equations to the non-linear soil equations, results ingood predictions of the root water uptake profiles compared
to predictions by the full 3-D model coupled with the nonlinear soil equations. The upscaling reduces the number of
non-linear equations that need to be solved so drastically that
this approach may be feasible for use in operational LSMs.
Soil-Groundwater Interactions

The groundwater system is an essential component of the
global hydrological cycle, and is directly connected with thesoil profile via the deep drainage at the bottom of the soil
profile (groundwater recharge), and root water uptake and7capillary rise from the groundwater for relatively shallow
groundwater levels. Hence, groundwater can moderate soil
water content, and therefore the root water uptake and relatedtranspiration and evaporation processes. It also controls root
distribution patterns, together with local climate conditions(i.e., precipitation-infiltration depth) (Fan et al., 2007).
The first generation LSMs did not include simulation ofgroundwater and simply applied a free drainage boundary
condition to the bottom of a fixed soil column. More recently,
however, groundwater simulation has been included in a
number of LSMs (Yeh and Eltahir, 2005a, 2005b; Maxwell
and Miller, 2005), and tested at a range of scales from single
basins to the globe, with approaches ranging from lumped to
distributed models. Note that lateral groundwater flow is not
generally included in the majority of global hydrological models
and LSMs, with some exceptions, e.g., Felfelani et al. (2021).
Including water table dynamics has been shown to improveriver discharge simulations (Yeh and Eltahir, 2005b; Koirala
et al., 2014) and including capillary flux from groundwater
increases evapotranspiration, with the global mean simulated torise by up to 16% (Koirala et al., 2014; Niu et al., 2007; Yeh andFamiglietti, 2009; Anyah et al., 2008). Key SoilWat members,
together with members of the wider GEWEX hydrologicalcommunity, are currently in the process of resubmitting apaper on “Global groundwater modeling and monitoring:
Opportunities and challenges” (Condon et al., under review)that looks at the complexities of implementing realistic
groundwater modeling on the continental to global scale.
Summary and Outlook

There are a number of publications addressing relevant
issues linked to SoilWat aims. Ongoing studies include theimportance and influence of soil hydraulic and thermal
properties, pedotransfer functions accounting for soil structure,soil-plant interactions (via root hydraulics), soil evaporation,
coupled soil water and heat flow, as well as soil-groundwaterinteractions on land surface processes.
Outstanding issues are:•

Critically, how to systematically evaluate added value of
improved soil process representation in LSMs (see Bonetti etal., 2021 for some of the challenges of showing improvement)•

How to best consider relevant soil-process-based evaluation
metrics in Model Intercomparison Project (MIP) activities,e.g., via incorporation of suitable soil-related indices in theInternational Land Model Benchmarking Project (ILAMB,
https://www.ilamb.org/) or https://modelevaluation.org•

Soil root hydraulics, and plant hydraulics, depend on
the vegetation structure (leaf area index, canopy height,
rooting depth, root density). Since roots are hidden,
information about root distributions is scarce. Rootsystems adapt to the soil and environmental conditions.
This plasticity has been addressed in LSMs by considering“optimal” distributions. However, in managed systems
and when environmental changes occur faster than the
Quarter 2 2021

process that optimizes these distributions, optimality
cannot be assumed. Describing how vegetation will
respond to changing environmental conditions (either bymanagement or due to climate change) will therefore be
critical to predict land-surface processes.
••

How to better represent preferential (aka bypass) flows in
LSMs (Gharari et al., 2019), for example, as caused bymacropore flows (Rahman and Rosolem, 2017), fingered
flows, and funnel flows (Pales et al., 2018; Demand etal., 2019). This would also require concerted efforts incollaboration with the soil biological and related modelingcommunities.
In ASALs, soil water and heat transfer are strongly coupled,
and the soil vapor flow dominates (i.e., carrying both massand energy). Nevertheless, the soil vapor flow is largely
ignored in LSMs, and the water flow is only weakly coupledwith heat transfer through the heat capacity and thermal
conductivity functions (i.e., via soil water content). On
the other hand, land surface fluxes and convective eventsare very sensitive to soil moisture over dry lands, and so is
the interannual variability of NBP. Therefore, it is essential
to revisit the importance of representing soil vapor flow in
LSMs for understanding land-air interactions over ASALs.
Related to this issue is the topic of vapor adsorption andhow to represent this in LSMs for ASALs (Saaltink et al.,2020; Verhoef et al., 2006). Saaltink et al. show that, on a
yearly basis, inward vapor flux (adsorption) into soil canbe around a third of the outward vapor flux (evaporation).
Detailed soil physical models can reproduce these diurnal
evolutions of soil vapor flow, but only if the driest part ofthe soil water retention curve is represented properly, whichbrings us back to the importance of choice of hydraulic andthermal properties, and their PTFs.

SoilWat will organize workshops and seminars to facilitate
discussions on the above issues, and report back to the community on the outcomes.
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Quarter 2 2021

WCRP’s New Lighthouse Activities
Jan Polcher1 and the International GEWEX Project Office
Laboratoire de Météorologie Dynamique (LMD), Centre
National de la Recherche Scientifique (CNRS)/Institut
Pierre Simon Laplace (IPSL)

1Explaining andPredicting Earth
System Change

My Climate Risk

Safe Landing
Climates

WCRP Academy
Digital Earths

A diagram of the new WCRP Lighthouse Activities and their provisional
names, adapted from the Report of the 41st Session of the Joint ScientificCommittee (WCRP, 2020)

The World Climate Research Programme (WCRP) is chartinga fresh course, and its new Lighthouse Activities (LHAs) arebeacons to guide programming in renewed areas of focus.
There are five LHAs, each with a distinct emphasis and setof deliverables. The three main LHAs are Explaining andPredicting Earth System Change, My Climate Risk, and Safe
Landing Climates. The two supporting LHAs are Digital Earths
and WCRP Academy. GEWEX Panels and groups, with theirexpertise in process-level understanding from regional to global
scales, are well-positioned to contribute to these LHAs.
Explaining and Predicting Earth System ChangeThis LHA tackles the design and potential delivery of a system
for the quantitative observation, explanation, early warning,and prediction of Earth system change, on regional and global
levels and multi-annual to decadal timescales. It will consistof a research program that will devise, improve, and evaluate
individual components of the system and the of the systemitself, and it will advance fundamental understanding ofEarth system change on global and local scales. The expected
result of this LHA is an integrated capability planned with
input from WCRP, with Earth Year and Earth Observation
decade components. All four of GEWEX’s Panels, the Global
Atmospheric System Studies (GASS) Panel, the GEWEX Data
and Analysis Panel (GDAP), the GEWEX Hydroclimatology
Panel (GHP), and the Global Land/Atmosphere System Study(GLASS) Panel, contribute to process-level understanding andprediction of the climate and can offer support to this LHA.
My Climate Risk

The My Climate Risk LHA will develop a new framework
for assessing and explaining regional climate risk, delivering
meaningful climate information at the local scale. It will involve
several case studies in the form of labs, which will be dynamic,
exploratory, transdisciplinary environments rather than physicalinfrastructure, intended to provide a forum for bringing together
relevant stakeholders and partners. Examples include evaluation
10of different national or regional climate risk assessments andregion-specific risk assessments. This LHA aims to develop a
new practice to synthesize climate information and will lead tothe production of consolidated regional climate information,
global capacity exchange, and guides on best practice. Within
GEWEX, the Regional Hydroclimate Projects (RHPs) stand as
examples of regional climate research as they strive to improveunderstanding and prediction of local water and energy cycles.
Safe Landing Climates

This LHA will explore the routes to climate-safe “landing spaces”
for human and natural systems on multi-decadal to centennial
timescales, connecting climate, Earth system, and socioeconomic
sciences. It will study pathways for achieving key sustainable
development goals. This requires a worldwide research activity
that will plan, encourage, and coordinate relevant activities
globally; communicate and disseminate key findings; andfacilitate user-oriented climate-safe landing tools. We expect
several outcomes from this initiative, including new climate andEarth system models that will investigate how climate change
and its ramifications might impact society and natural systems.
GEWEX’s focus on extreme and compound events can help interms of Earth system feedbacks in safe landing spaces.
Digital Earths

The undertaking of this LHA involves the development ofa digital and dynamic representation of the Earth system,
blending models and observations to enable study of the past,present, and potential futures of the Earth system. This will bea joint activity, with external institutions providing the digital
infrastructure and WCRP implementing selected versions for
topics where significant progress is required. The result will
be generic software-hardware solutions that allow simulation
models and data assimilation to perform several orders ofmagnitude more efficiently. It will facilitate the extraction ofEarth-system sector specific information from vast amounts ofenvironmental data and allow exciting new ways of accessing
and using climate data and information. GEWEX is poised tohelp with the regional focus of the Digital Earths LHA.
WCRP Academy

This concept is more fluid than the other LHAs, with an overall
goal but no fixed approach yet established. The aim is to create
one or more targeted capacity exchange climate programs,
working with one or more of the other lighthouses and climate
education providers. The program has the potential for online
or in-person opportunities and either global or regional foci.
Capacity development is key, and this LHA intends to provide
knowledge and information to all regions worldwide to tackle
the issues and opportunities that climate change brings.
The names and details of the LHAs may change as concepts arerefined, and there has already been a good deal of evolution.
Online meetings and consultations are taking place to shape theLighthouse Activities, and the LHA leads are defining strategies
to move forward. Knowing where and when GEWEX groups
can contribute to each LHA will help their progress, and many
GEWEX community members are active participants in thedevelopment process.
Quarter 2 2021

Meeting/Workshop Reports
The Second Determining
Evapotranspiration Workshop
Virtual Meeting
10–12 February 2021
Oscar Hartogensis1 and Joan Cuxart2
Wageningen University, Wageningen, the Netherlands;
University of the Balearic Islands, Palma de Mallorca, Spain

12

1. Introduction: Aims and Antecedents

Within the framework of GEWEX activities, the need fora deeper understanding of evapotranspiration (ET) has
been identified. This objective is to be pursued through theanalysis of the basic mechanisms to a synthesis resulting inparameterizations that can be used in applications such as
numerical models or remote sensing estimations.
After an initial side meeting at the 2018 GEWEX OpenScience Conference, organizers began assembling the interested
community to determine the state-of-the-art in the field and to
define courses of action. The first activity was a reflection paper
published in GEWEX News in early 2019, structured on fourmain items related to ET: i) definition and understanding, ii)
in situ measuring, iii) parameterization, and iv) estimations byremote sensing and at the catchment scale. A first workshop washeld in Sydney in October 2019, where participants reflected on
ET along five views of the ET process: open water, landscapescale, interception, transpiration, and soil evaporation.
The second workshop was initially planned for August 2020 in the
Netherlands, but the COVID-19 pandemic finally caused it totake place online in February 2021, organized by the Meteorology
and Air Quality Group of the Wageningen University (WUR).
The workshop was dedicated to Jim Shuttleworth, a reference
researcher in the domain, who passed away shortly before theworkshop. A talk remembering his life and work was given by
the workshop attendee Rafael Rosolem, Jim's last Ph.D. student.
2. The Participants and the Organization

Arranging the online-only workshop presented a number ofchallenges for the organizing committee, such as ensuring
the attendance of scientists from a wide range of time zones,
coping with shorter attention spans in online meetings, andfostering lively interactions. With this in mind, the following
format was put together as an alternative to a “regular” format,
with 10 minute talks followed by 5 minute question sessions
for each participant:
1. three-hour sessions (14–17 CET) during three consecutive days,
2. presentations on the state-of-the-art in ET on the first day
and future developments on the second day, and3. an organized discussion on the third day.
11

Figure 1. Characterizations of the GEWEX-ET workshop audience

Participants were asked to send, in advance, three to fiveslides with their recent research highlights and an outlook
on the major challenges ahead. Three early career scientists
from Wageningen University organized this material into
presentations on the state-of-the-art (first day) and future
developments (second day), leaving plenty of time for open
conversation afterwards in the plenary session or break-outgroups. The third day’s discussion was guided by an online
questionnaire (www.mentimeter.com) with questions distilledfrom the first two days with the objective of seeing the different
opinions and the research trends for the coming years.
Overall, the participants were enthusiastic about the format,
and the Mentimeter poll provided a quantitative opinion of
the audience on a number of outstanding issues. We started
the questionnaire with some basic queries that characterized
the audience (Figure 1). It is clear that there was a good balance
between youth and experience, and enough interdisciplinarityand diversity of investigation methods.
3. The State-of-the-Art

WUR early career scientists Mary Rose Mangan, Femke Jansen,
and Xabi Pedruzo Bagazgoitia summarized the contributions of theparticipants submitted with some anticipation on the first day. They
did this broadly, according the various pathways of ET (vegetation,soil, open water, interception, atmosphere, and landscapeintegrated ET), combining the materials of the participants using
this main concept. Other dimensions of looking at the process,i.e., methodology (ground measurements, remote sensing, andcomputational models), study type (process understanding or
Quarter 2 2021

Method

ThemesPathway

Icon

Ground Measurements
Icon

Vegetation

Remote Sensing

Soil

Computational Models

Open Water

Study Typewater budgets. The determination of ET
at large scales relies mostly on reanalysis
products, remote sensing estimations, andgood validation data (e.g., by scintillometers).
4. Discussion and the Way Forward

The surface energy budget is considered
to be an outstanding issue since manyInterception
approaches rely on assuming its closure,
Practical Application
Atmosphere
while it is well known that a 20% imbalanceis quite usual. Even if recent studies indicate
Integrated Evapotranspiration
ScalesIcon
that local advections may partly explain
Spatialthe missing contributions, the issue is far
from conclusive, as methodologies are still
Temporalunder development. Furthermore, goodexperimental estimates of ET are needed, andthere is a call for better control of the factors
Figure 2. Dimensions along which the overview presentation by early career scientists was organized leading to errors. It is also worth mentioningthat some experimental estimates rely on
practical application) and scales (spatial and temporal) were added
similarity expressions that often do not work too well outside
as icons to each slide (Figure 2). In this way, a highly homogenized,
standard ranges. These issues impact the validation of remoteorganized, and comprehensive overview was assembled. Thesesensing developments, pointing to a need for an integrative
slides are available upon request from the authors.
approach considering all methodologies simultaneously(model values, in situ data, remote sensing estimates) and
It is beyond the scope of this report to provide a written
exploring ways to make them converge.
overview of this material, which is an overview in itself. We
will point out some of the main storylines here.
Expanding the applicability of the most widely-used approaches(Penman-Monteith and similarity theory) is considered vital.
Vegetation was the opening subject, which was split into
Participants discussed opposing views of the use of machine
discussions on processes at the root and leaf scales and thelearning techniques, considering possible good results and theland-atmosphere interactions at the canopy scale. Measurelack of understanding of those results. The maintenance andment strategies, radiation, and nighttime processes were mengrowth–if possible–of current networks such as FLUXNET
tioned. The field-scale implies dealing with terrain heterogeor the Integrated Carbon Observation System (ICOS) is
neity, both in observing and modeling approaches. Here theconsidered a cornerstone of future activities, since validation
Penman-Monteith approach and Monin-Obukhov similarity
at multiple sites is a necessary step for any new development.
relationships were discussed and the exploration of alternatives
recommended. Larger spatial scales require the use of mulHenk De Bruin indicated in his keynote speech that the standard
tiple sources of information, including numerical models andUnited Nations Food and Agriculture Organization (FAO)
remote sensing information. Participants also identified theequation based on Penman-Monteith needs to be revised. He
need for better monitoring and parameterization (representaalso stressed that for well-watered surfaces, ET depends in thetion) of irrigation and the link between ET and CO2 uptake tofirst degree on incident solar radiation, and air temperature andimprove ET estimates. Partitioning between evaporation andatmospheric water vapor deficit are a consequence, and not atranspiration is deemed necessary for parameterizations, anddriver of, the overall atmospheric boundary layer evolution.
using isotopes to investigate this is considered very promising.
The discussion on the third day using the Mentimeter tool was
Open water evaporation on land was discussed in terms of lakebased on statements from the general discussions of the firstmodels and the need for specific observations to test and improve
two days. A sample of the questions is given in Figure 3. An
them. In addition, the representation of flooded areas has also
important consideration is the surface energy balance (SEB)
proved difficult. The main challenges with soil evaporation areclosure, which affects the validation of flux parameterizations,the large heterogeneity in soil moisture, the thermal contrasts
implying that land surface coupling may be a way to partiallyresulting from it, and the circulations those thermal contrasts
overcome this limitation. Other ideas concern the improvement
can generate. Again, the impact of irrigation was considered.
of the closure through better observational methods or
A discussion on the interaction between soil moisture andmeasurement of missing contributions, and the open question
atmospheric water vapor deficit followed, and the coupling ofon what ET contributes to the total SEB imbalance.
both factors was deemed important. Other points addressed are
the reliability of the direct measurements of evapotranspiration
Despite the fact that we are aware that heterogeneity at the(eddy-covariance and lysimeters) and the estimation of thesurface is a major issue, the community indicated that 1D-models
horizontal transport of water vapor in the surface energy andare still a good tool to test new developments. Concerning
Icon

Process Understanding

12

Quarter 2 2021

separation of evaporation and transpiration, isotopes andlysimeters (on bare and vegetated surfaces) are considered tobe the most promising methods so far, but other options areon the table, such as sap flow and chamber measurements.
Distributed measurements and high-resolution modelingare seen as the most suitable approaches to estimate the roleof advection.
Concerning the determination of ET at the global scale,four issues are considered almost equally important: i) good
representation of land use and land cover, ii) taking irrigation
into account, iii) a better understanding of the impact ofincreased CO2 in the stomatal function, and iv) the responseof the system to extreme events.
As for the immediate practical challenges ahead, a difficult
but rewarding task involves improving the ET ground-truth
network to be able to monitor global ET and quantify the
impact of global change on ET. Some ideas for approaching
this task are to refine the partitioning of ET, to study
heterogenous landscapes, and to modify our conceptual
vision of the ET process. Machine learning poses anotherchallenge, as it is regarded as something that will happen withan unknown impact.
The community, when asked if any subjects were
underrepresented or ignored during the discussion, citedinterception, sublimation, dew, phenology, plant hydraulics,
stomatal conductance, and human water use. Participantsconsidered the Land surface Interactions with the Atmosphere
over the Iberian Semi-arid Environment (LIAISE) campaign,
13

Figure 3. Some example questions from the Mentimeter questionnairethat guided the discussion on day 3 of the workshop

which takes place during summer 2021, a good opportunity
to test some of the elements discussed.
The meeting ended with congratulations to the organizers fora well-conducted online workshop making use of innovative
ways of discussion, with one participant saying that it “was themost interesting online event of the past year for me; and wehave all been to too many.” The community will determine
how to organize itself as a GEWEX Hydrolimatology Panel(GHP) Cross Cutting Project in terms of working groups
and will follow up on workshop discussions and decide when
it will be useful to meet again. The first order of businessfollowing the workshop is the LIAISE campaign, as many ofthe participants will contribute to it.
Quarter 2 2021

2020 GLASS Panel Meeting
Virtual Meeting
23–25 November 2020
Michael Ek1, Kirsten Findell2, and Anne Verhoef3
Outgoing GLASS Panel Co-Chair, 2GLASS Panel Co-Chair,
Incoming GLASS Panel Co-Chair

1
3

The GEWEX Global Land-Atmosphere System Study(GLASS) Panel met virtually for three partial days at the endof November 2020 to share progress in the 15 months since
our last meeting. Despite the difficult circumstances of 2020,
there was much to share. Discussion during the meeting wasprimed by a set of pre-recorded screen-casts shared by the
project leads 1–2 weeks in advance of the meeting. Our core
projects stem from process-based inquiries, to benchmarking
activities, to global model intercomparisons, and now includes
the GEWEX/GLASS Land-Atmosphere Feedback Observatories (GLAFO) initiative. The goal of the GLAFO initiative isto establish a network of long-term observing stations focusedon the land-atmosphere system from groundwater, throughthe surface and vegetation, to the top of the boundary layer. In addition to the project updates summarized below, wewelcomed new members Volker Wolfmeyer and Yijian Zeng,heard from guests describing projects relevant to GLASS, andmarked the end of Mike Ek’s tenure as GLASS Co-Chair.
Anne Verhoef now joins Kirsten Findell as Co-Chair. We lookforward to continued progress in the year ahead, and hopefor an in-person meeting in later 2021 where we can properly
thank Mike for a job well done and thank Anne for taking on
the next four years!
Core Project Updates: Process-Oriented Projects
GLAFO (and New GLASS Panel Member Volker
Wulfmeyer)

New GLASS Panel member Volker Wulfmeyer shared somebackground on his journey from physics student to designerof new observational instrumentation to multi-faceted
meteorologist tackling a new frontier of boundary layer
observational capabilities. These strands of expertise contributeto the GEWEX Land Atmosphere Feedback Observatories(GLAFOs, Wulfmeyer et al., 2020). GLAFOs will observe therelevant processes and variables with respect to mass, energy,
water vapor, and momentum transport with unprecedented
spatial and temporal resolutions, from bedrock to the lower
troposphere. Volker presented how the GLAFOs will be
designed in order to characterize all relevant interactions
and feedback loops between the subsurface, vegetation, andthe atmosphere. The measurements will be realized throughthe synergistic use of in situ instruments and ground-based
scanning active remote sensing systems. This approach waspioneered during the Land-Atmosphere Feedback Experiment
performed at the Atmospheric Radiation Measurement
program’s Southern Great Plains (ARM SGP) site in August
2017 (Wulfmeyer et al., 2018). By observing profiles of the14mean, gradient, and turbulent fluctuations (if applicable) of
all relevant variables, unprecedented data sets will be provided
for the study of land-atmosphere (L-A) feedbacks. Such data
include surface and entrainment fluxes, advection, and theevolution of key variables in the other compartments of thecritical zone (such as ground water levels, soil moisture, andvegetation temperatures), from the diurnal cycle, via seasonal/
annual to ideally climatological time scales. Due to the highresolution and accuracy of the observations, the measurements
can also be used for operational data assimilation in numerical
weather prediction (NWP) models as well as for tests anddevelopments of parameterizations. It is envisioned that the
GLAFOs will be set up and operated in all climate zones withdifferent levels of complexity.
LoCo

The local land-atmosphere coupling (LoCo) project update waspresented by Joe Santanello. LoCo consists of 20 international
scientists across academic and government institutions.
LoCo has continued to focus on three near-term objectives
to enhance our ability to understand, quantify, and improveland-atmosphere coupling and prediction: (1) promote theimportance and development of improved observations of the
L-A system, namely in the planetary boundary layer (PBL), as
well as improved utilization of soil moisture and surface fluxes
measurements in models; (2) pursue adoption of LoCo metrics
by operational NWP and climate centers; and (3) expand
the scope and reach of LoCo in terms of processes and scales
beyond that of warm season thermodynamics and beyond
that of 1-D column assumptions. The GLAFO observational
measurement system and the Land surface Interactions with the
Atmosphere over the Iberian Semi-arid Environment (LIAISE)
field campaign both plan extensive PBL measurements, and anumber of initiatives are underway in response to the National
Aeronautics and Space Administration (NASA)’s Decadal
Survey prioritization of PBL measurements and spaceborne
mission concepts. In addition, the National Oceanic andAtmospheric Administration (NOAA)’s Climate Process
Team project (Coupling of Land and Atmospheric Subgrid
Parameterizations, CLASP), led by five GLASS members, isworking towards applying current and developing new LoCo
L-A coupling metrics for Global Climate Model (GCM)
development, as well as incorporating LoCo metrics into theInternational Land Model Benchmarking (ILAMB) package.
To advance L-A science, the LoCo Working Group (WG)
continues to convene annual American Geophysical Union(AGU) and American Meteorological Society (AMS) meeting
sessions (which have become some of the largest in thehydrology sessions and conferences). The WG also influences
and responds to agency solicitations that now reflect thepriorities of LoCo and supports outreach to the scientific,
modeling, and observational communities.
SoilWatAnne Verhoef gave an update on SoilWat, describing a
number of ongoing activities focused on providing reliablesoil hydraulic and thermal properties for land models, andQuarter 2 2021

exercises describing the impact of these properties on the land
fluxes. This also included recommendations to harmonize
pedotransfer functions (PTFs) used in land models and related
model intercomparison projects (MIPs) to avoid artifacts
originating from the choice of PTFs rather than from different
model structures. A number of corresponding publications
are near submission or have just been submitted. An ongoing
activity includes conducting a global soil hydraulic parameter
MIP that could leverage ILAMB (described below). A recentlystarted activity is the compilation of a data set of globallydistributed soil thermal properties, ideally combined withhydraulic properties, where the data set would serve to support
development and verification of models simulating thermal
properties. Near-future work includes testing both single
and combined influences of hydraulic and thermal properties
in models. Finally, establishing a global soil temperature
database for land model verification is planned, which would
also fit nicely with ILAMB work by providing independent
verification data. Yijian Zeng has now taken over from Anneas the SoilWat lead and rapporteur; his expertise spans detailed
mechanistic land surface modeling, remote sensing, and insitu observations (see below for more information). Finally,note that the SoilWat activity cuts across the GLASS andInternational Soil Modelling Consortium (ISMC, https://soilmodeling.org) communities.
Core Project Updates: Benchmarking Projects
PLUMBER2, modelevaluation.org, and Urban-PLUMBER

Gab Abramowitz described the Protocol for the Analysis
of Land Surface models (PALS) Land Surface Model
Benchmarking Evaluation Project, Phase 2 (PLUMBER2)
that is well underway, with contributions across land
surface, carbon cycle, hydrological, and empirical model
approaches contributing to a broad focus on land surface
flux predictability. Data from 170 flux tower sites have been
through a lengthy quality control process and some groups
are already producing final simulation suites, although many
have still yet to engage, with submission timelines extended
into 2021. Ten of approximately thirty groups have begun
submitting simulations via modelevaluation.org (http://modelevaluation.org/) to date. The modelevaluation.org web
application has faced a range of logistical challenges this year,with the ramp up of PLUMBER2 and Urban-PLUMBER
providing the first heavy-use cases. While these challenges
are being resolved and have not greatly affected the progressof these experiments, broader engagement beyond a single
institution to develop a shared, community-based model
benchmarking resource would likely improve its longer-term
security. To this end, the Centre of Excellence for Climate
Extremes (CLEX) at the University of New South Wales(UNSW, Australia) and the MetOffice (UK) are intending
to develop a modelevaluation.org-based automated testing
system for model development that is model agnostic,
incorporating details from process-based site diagnostics
to global ILAMB-based analyses. In regards to UrbanPLUMBER, the initial feedback for Phase 1 (a single urban
15site) has been provided to the 26 participants, and results arebeing finalized and written up. The project webpage publishes
submitted metadata and plots all requested variables, whereobserved energy fluxes will be compared with benchmarks.
Data for Phase 2 (~25 urban sites) is being quality-controlled
and standardized, and will be provided to participants startingin February and finishing in mid-2021.
ILAMB

Dave Lawrence provided an update on ILAMB, which is amodel data intercomparison and integration project designed
to improve the performance of land models and, in parallel,
improve the design of new measurement campaigns to reduce
uncertainties associated with key land surface processes.
The ILAMB package has been run on the Coupled Model
Intercomparison Project Phase 5 (CMIP5) and Phase 6(CMIP6) historical simulations. Across the majority ofmodels that participated in both CMIP5 and CMIP6, we areseeing general improvements from CMIP5 to CMIP6 acrosscarbon, water, energy, and surface climate metrics, indicating
improved model performance. ILAMB is also being utilized in
the assessment of land-only versus coupled model simulations
to help understand the role of climate biases in coupledmodels on key land carbon and water flux and state variables.
Finally, ILAMB continues to expand, with over 30 variables
being assessed against more than 80 global, regional, and sitelevel data sets and incorporating new metrics and methods toaccount for observational uncertainty.
Core Project Updates: Model Intercomparison Projects
LUMIP

Dave Lawrence also provided an update on modeling humanland-use activities that have resulted in large changes to the
Earth’s surface, with resulting implications for climate. The
Land Use Model Intercomparison Project (LUMIP) aims
to further advance understanding of the impacts of landuse and land-cover change (LULCC) on climate throughcoordinated model experiments. A series of manuscripts havebeen published or are in preparation, including key papers on
the new LULCC data set and on the multi-model response
to idealized deforestation (see list of publications and projects
athttps://www.cesm.ucar.edu/projects/CMIP6/LUMIP/).
Significant lack of agreement in terms of biogeophysical andbiogeochemical impacts of LULCC on land surface fluxes andvariables, as well as L-A feedbacks, persists across models.
GSWP3 and LS3MIP

Hyungjun Kim shared the current status of the Land Surface,Snow, and Soil Moisture Model Intercomparison Project(LS3MIP) of CMIP6. In LS3MIP, 13 modeling groups havesubmitted the tier one experiment results from the landonly configuration [i.e., land-hist, which shares the protocol
with the Global Soil Wetness Project Phase 3 long-term
retrospective experiment (GSWP3 EXP1)], and six of themsubmitted the results of the tier one coupled experiments (i.e,Quarter 2 2021

amip-lfmip-pdLC and amip-lfmip-rmLC). Three synthesis
papers are being developed describing the first analysis results:(1) overview and benchmarks (led by Hyungjun Kim), (2)land water and carbon balances (led by Ryan S. Padrón, Swiss
Federal Institute of Technology), and (3) simulation of cold
processes (led by Lawrence Mudryk, University of Toronto)
based on the land-hist experiment. Up-to-date data holding
status of LS3MIP can be checked at https://pcmdi.llnl.gov/
CMIP6/ArchiveStatistics/esgf_data_holdings/LS3MIP/index.
html.
Updates from Liaisons and Other Initiatives
GASS

John Edwards summarized progress on projects organized by
the GEWEX Global Atmospheric System Studies (GASS)
Panel. The results of the intercomparison of large-eddy
models participating in the GEWEX Atmospheric Boundary
Layer Study Phase 4 (GABLS4) project on the boundary
layer over the Antarctic Plateau have now been published(Couvreux et al., 2020). The models were able to simulate
the very stable boundary layer, provided high resolution
was used. Several new projects were launched following thesecond Pan-GASS Meeting in 2018. Recently-publishedresults from the COnstraining ORographic Drag Effects(COORDE) intercomparison of orographic drag schemes(van Niekerk et al., 2020) demonstrate the large variationin parameterized drag between models and show that it isfrequently underestimated over land. The Elucidating the Roleof Clouds-Circulation Coupling in Climate (EUREC4A) field
campaign was concluded successfully in February 2020 and anintercomparison of simulations of shallow convection basedon these data will shortly be initiated. Good progress has been
made in analyzing results from the intercomparisons of fog
modeling and (sub-)diurnal precipitation.
GHP

Craig Ferguson gave a short presentation on the activities of the
GEWEX Hydroclimatology Panel (GHP) before handing overthe baton of GLASS-GHP liaison to Josh Roundy. There were
three activities identified with particular relevance to GLASS.
The first is the proposed Regional Hydroclimate Project(RHP) called the Third Pole Environment, led by Yaoming
Ma. It has a large focus on measurements of land-atmosphereinteractions across the Tibetan Plateau (with hopes to expand
across Eurasia) and could be linked to the GLAFO project. The
second is the Quantifying Evapotranspiration (ET) crosscut,
led by Joan Cuxart, which aims to improve process-based
ET estimates. The second workshop relating to this cross-cut
topic took place remotely between 10–12 February 2021.
Finally, the emerging U.S. North American proposed RHP,
led by Timothy Schneider, aims to bring together multiple
U.S. agencies on a focused hydroclimatological study over the
U.S. This US-RHP is formulating an umbrella strategy of U.S.
continental-scale modeling paired with intensive field studies
and observational transects to better understand, represent,
and predict coupled water and energy processes at the Earth's
16

surface. Additional information can be found in a pair ofrecent GEWEX Quarterly articles (Schneider and van Oevelen,
2020; Scott and Schneider, 2020). For all of these projects,
links between GLASS and GHP will continue to be developedand strengthened over the next year.
LIAISE

Aaron Boone provided an update on the international LIAISE
project, where the overall objective of this new activity is
to improve our understanding of the impact of surface
heterogeneity (notably that induced by anthropization)on the water cycle in terms of land-atmosphere-hydrology
interactions in a water resource-limited bread-basket region.
However, the understanding of the impact of anthropizationand its representation in models have been inhibited due toa lack of consistent and extensive observations. The project
depends on an international intensive field campaign which
consists of in situ and remotely-sensed measurements of (1)land surface physiographic parameters, biophysical and landstate variables, and turbulent and radiative fluxes; and (2)atmospheric state and turbulence measurements within theatmospheric boundary layer. The campaign will extend overa Long Observation Period (LOP: April–September) and willconsist of continuous monitoring of the surface and the lowest
50 m of the atmosphere, and two Special Observation Periods(SOPs). The SOPs will also include both in situ and remotelysensed atmospheric measurements: five days in May or Junewith the objective of monitoring a typical dry-down event,and a 15-day SOP in late July with a focus on measurements
over the entire study domain. This allows the inclusion ofdifferent representative land cover types over two strongly
contrasting zones, one with extensive irrigation, the other overa bare-soil dry natural grass and rain-fed agricultural surface.
LIAISE was delayed by the COVID-19 pandemic, but the
research teams are tentatively moving forward for a campaign
in summer 2021, where there is a planned expansion of somesurface, atmospheric, and remotely-sensed measurements.
S2S

Paul Dirmeyer highlighted the International Subseasonalto-Seasonal (S2S) Prediction Project, which involves mostly
operational forecast modeling centers, with one of the
S2S sub-projects focusing on the role of the land surface
as a source of predictability beyond weather time scales.
Operational model output is being evaluated for model skill
and the potential role of soil moisture as a skill source fordroughts and heat wave forecasts. Land surface temperature
anomalies over elevated terrain is the focus of the ongoing
Impact of Initialized Land Temperature and Snowpack on
Sub-seasonal to Seasonal Prediction (LS4P) project of GASS,
which is shifting focus from the Tibetan Plateau to the
U.S. Rocky Mountains and high plateaus of western North
America. In the near future, in addition to the proposed
Vegetation-Global Land-Atmosphere Coupling Experiment(Veg-GLACE) modeling experiment (described below),
another relevant effort regarding S2S forecasting and therole of the land surface as a source of predictability is the
Quarter 2 2021

NOAA-community effort to develop and implement thenext generation Unified Forecast System (UFS) of models,
where medium to extended range and S2S are an emphasis
for the UFS.
Veg-GLACE

During discussion time, we learned from Souhail Boussetta
and Gianpaolo Balsamo about Veg-GLACE, a new multimodel experiment that has been proposed to examine S2S
predictability coming from improved vegetation modeling andbased on the experience from a previous GLACE2 experiment(Koster et al., 2011) and the Snow-Global Land-Atmosphere
Coupling Experiment (SNOWGLACE, Orsolini et al., 2013).
The concept is developed within a new European Horizon
project called the CONsistent representation of temporalvariations of boundary Forcings in reanalysES and Seasonal
forecasts (CONFESS, https://confess-h2020.eu/), and will allow
us to investigate the impact of a satellite-based vegetation
data set, where a set of experiments based on a multi-model[initially Météo-France, the European community EarthSystem Model (EC-Earth), and the European Centre for
Medium-Range Weather Forecasts (ECMWF)] will be
run to evaluate the impact of realistic vegetation variability
on subseasonal to seasonal forecast skill. The experimental
protocol of Veg-GLACE will be tested within the CONFESS
project and consolidated in 2022 to be proposed to WCRP/World Weather Research Programme (WWRP) and Working
Group on Subseasonal to Interdecadal Prediction (WGSIP)
modeling groups. The GLASS Panel Meeting welcomed theinitiative and different modeling groups have already expressed
an interest in contributing.
New GLASS Panel Member Yijian Zeng

New GLASS panel member Yijian Zeng presented his researchon “Soil-Water-Plant-Energy Interaction in Cold Regions–Tibetan Plateau as the Research Testbed”, with the goal ofunderstanding soil-water-plant-energy interaction in cold
regions. This uses an integrated modeling framework couplinga physically-based soil freeze-thaw model with groundwater andbiogeochemical models. This allows study of the uncertainty
in soil physical properties that cause uncertainties in soilhydro-thermal properties that then propagate further into theestimate of land surface states and fluxes. A comprehensive
forward observation simulator is therefore suggested (i.e., aphysically-based coupled-process model combined with aradiative transfer model) to assimilate remotely-sensed Earthobservations [e.g., from the Soil Moisture and Ocean Salinity(SMOS) and Soil Moisture Active Passive (SMAP) missions]for a physically-consistent estimate of soil properties and surface
states and fluxes. This then addresses the role of the land-surface
in understanding water, energy, and carbon cycles at both local
and global scales. Such an approach will in particular contributeto the GEWEX-ISMC SoilWat Initiative described above.
GDAP

GEWEX Data and Analysis Panel (GDAP) Co-Chair Rémy
17

Roca (with input from fellow GDAP Co-Chair Tristan
L’Ecuyer) provided an update on his Panel’s activities. He notedthat the precipitation data set from the centennial reanalysis
elaborated under GSWP3 has now been added to theFrequent Rainfall Observations on GridS (FROGS) database(http://frogs.ipsl.fr), where users can easily download many
precipitation products, including those from various reanalysis
products. They expect that this new addition will be includedin the future assessment run by GDAP as a way to provide
some feedback to GLASS on these unique data sets. Further
discussion will be facilitated in the coming year by the newlyappointed GLASS-GDAP liaison, Yunyan Zhang. There arealso plans that began at the GEWEX SSG meeting in January
2020 regarding a possible land-atmosphere energy and water
cycle closure and consistency assessment that could link localscale surface processes to global-scale data sets, specifically the
GEWEX Integrated Product. This was to have been discussed
further at a workshop later in Spain, but was postponed dueto the pandemic; we hope to address this in the near future asa GEWEX cross-Panel activity.
CORDEX Urban Project

Aude Lemonsu gave an update on the Urban CoordinatedRegional Downscaling Experiment (CORDEX Urban)
project, which began at the last International Conference
on Regional Climate (ICRC)-CORDEX conference whena group proposed a Flagship Pilot Study project dedicatedto urban areas and related issues. The proposal for "URBan
environments and Regional Climate Change" (2021–2025) iscoordinated by Tomas Halenka (Charles University) and Gaby
Langendijk (The Climate Service Center Germany, GERICS)
and involves more than 20 partners. The main objective isto understand the effect of urban areas on regional climate
and the impact of regional climate change on cities with thehelp of coordinated experiments with urbanized regional
climate models (RCMs). One important scientific question
is to identify what urban processes need to be included in
RCM simulations to account for urban effects and feedbacks
on higher resolution and convection-permitting scales. In a
related effort, Aude is coordinating a 4-year French research
project with the urban region of Paris as a case study that willcontribute to the preparation of the 2024 Olympic Games,
and includes a large experimental component dedicated to thestudy of surface and atmosphere interaction processes and theimpacts of urban areas on the dynamics of boundary layers andother meteorological phenomena. At the same time, a World
Meteorological Organization (WMO)-approved researchdemonstration project has started, and includes several
meteorological centers and research groups at the international
level with the objective to advance high-resolution modelingof urban areas, and the predictability of certain meteorologicalphenomena (storms, heat waves) in cities.
iLEAPS

Eleanor Blyth and Xianhong Meng briefed the GLASS Panelon the Integrated Land Ecosystem-Atmosphere Processes
Quarter 2 2021

Study (iLEAPS). The focus of iLEAPS is on the biosphereas a mediator of Earth system processes, with study of theland and atmosphere together as a system involving physical,
chemical, and biological processes. Land-atmosphere systems
include important feedbacks between atmospheric chemistry
and plants that have an impact on society and on the Earthsystem, where iLEAPS scientists also address key societal
challenges related to health, biodiversity, climate, food and fuelsecurity, and promote scientific excellence through developing
international science initiatives that are multi-disciplinary.
A Land Modelling Summit, co-sponsored by GEWEX,
iLEAPS, and the Analysis, Integration and Modeling of the
Earth System (AIMES) project, will be held 12–16 September
2022 in Oxford, UK, and the 6th Open Science Conference
sponsored by iLEAPS is planned for February 2022 in
Auckland, New Zealand. A Special Collection in the Journalof Advances in Modeling Earth Systems on “Next Generation
Land Surface Modeling” will tie iLEAPS, GEWEX (GLASS),
and AIMES together, with issues linking water and energy,
carbon and the anthropogenic impact. Xianhong Meng is the
new iLEAPS-GLASS liaison.
WGNE and WMAC

Mike Ek provided a general briefing on the WMO Working
Group on Numerical Experimentation (WGNE) and the
WCRP Modelling Advisory Council (WMAC), where
WGNE and WMAC are interested in improving global NWP
and climate models, and on coordinating high-level aspects ofmodeling across WCRP, respectively. The GLASS presence inthese groups focuses on improving land modeling and landatmosphere interaction in models. The new WCRP structure
calls for designing a new “model-data home” that brings
together WMAC and the WCRP Data Advisory Council.
WCRP Lighthouse Activities

Peter van Oevelen reported on the WCRP Lighthouse
Activities (LHAs, https://www.wcrp-climate.org/wcrp-ipla) that have been initiated to highlight major experiments
and high-visibility projects as part of the new WCRP, wheresuch activities truly integrate the capabilities (scientific,
technical, infrastructure) across WCRP and with partners.
Hyungjun Kim is serving on the Safe Landing Climates LHA(https://www.wcrp-climate.org/images/documents/WCRP_
Implementation_Plan/Safe%20Landing%20Climates.pdf),
and Kirsten Findell (as co-chair) and Mike Ek are serving on
the Explaining and Predicting Earth System Change LHA(https://www.wcrp-climate.org/images/documents/WCRP_
Implementation_Plan/Explaining%20and%20Predicting%20
Earth%20System%20Change.pdf), where they see their roleas leveraging GLASS and GEWEX activities on process-level
understanding and modeling.
References
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J. Edwards, B.J.H. van Stratum, C.C. van Heerwaarden, J. Huang, A.F.
Moene, A. Cheng, V. Fuka, S. Basu, E. Bou-Zeid, G. Canut, and E.

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Wood, 2011. The Second Phase of the Global Land–Atmosphere CouplingExperiment: Soil Moisture Contributions to Subseasonal Forecast Skill. J.
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Orsolini, Y.J., R. Senan, G. Balsamo, F. Doblas-Reyes, F. Vitart, A.
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Schneider, T., and P. van Oevelen, 2020. A Proposed Regional Hydroclimate
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gewex.org/gewex-content/files_mf/1590612006May2020.pdf.
Scott, R., and T. Schneider, 2020. AmeriFlux Should be an Integral Partof a GEWEX U.S. Regional Hydroclimate Project. GEWEX Quarterly
Vol. 30, no. 3, Quarter 3, 2020, 3–4. https://www.gewex.org/gewex-content/
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GEWEX QUARTERLY
Published by the International GEWEX Project OfficePeter J. van Oevelen, Director
Shannon F. Macken, Editor
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Quarter 2 2021