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# What have we learnt from high-frequency monitoring when increasing the number of years and solutes?

## Feedback from the Kervidy-Naizin catchment (western France)

Rémi Dupas and Ophélie Fovet

UMR SAS, INRAE, Institut Agro, Rennes, France
> AgrHyS: CZ Observatory of Agrohydrosystems

Kervidy Naizin site

* Headwater catchments 5km²

* Intermittent stream Strahler order 2

* Temperate oceanic climate

* Impervious bedrock (schist)

* Loamy soils

* 95% of area used for agriculture with mixed, pig and dairy farming

25 years of observations and
research on the response (time,
dynamics) of water and
biogeochemical (CNP) cycles to
changes in atmospheric and
agricultural inputs

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Observatoire
de Rennes

OCAR

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eLTERPLUS

INRAO

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# Long-term high frequency monitoring of Kervidy Naizin stream water concentration

## Historical monitoring

Daily samples from 2000 to 2022 at the stream outlet

* Majors anions (NO3), dissolved carbon, Phosphorus

Autosampler for flow events since 2007

* Majors anions (NO3), dissolved carbon, Phosphorus, C isotopes, MO fluorescence

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Base flow supplied by shallow hillslope GW poor in OM, SO4 rich in CL and NO3
Storm flow feed by riparian GW and soil water rich in OM and SO4 with a limited storage

## Previous conceptual understanding

from A. Aubert thesis 2013

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Soil water contribution decreases, relative contribution of riparian zone increases. Significant influence of near/in stream processes (denitrification, vegetation uptake). Reductive conditions in riparian zone which is the main contribution in storm flow
> Long-term high frequency monitoring of Kervidy Naizin stream water concentration

Recent (from 2010) advances for sub hourly monitoring

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**NO3 (+DOC) since 2010**

S:scan spectrolyser V2
Uv visible spectrophotometer

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Observatoire
de Rennes

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**TP (+TRP) since 2016**

Phosphax
Wet lab with on-line
analyser

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1) What do we learn when increasing the number of solutes monitored at HF?

**Major ions since 2018**

Riverlab: Field wet lab with ionic chromatography system

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2) What do we learn when increasing the number of years with HF monitoring?
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# What can we learn when increasing the number of solutes monitored at High Frequency?

Investigation of (a)synchrony between 7 major ions (Ca, Mg, Na, K, Cl, NO3, SO4)

* Looking for non-redundant information

* Storm events selection

* Classification of pairwise pattern based on the Cx-Cy plots

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Brekenfeld et al.
(in review on
EGUsphere)
> What can we learn when increasing the number of solutes monitored at High Frequency?

Investigation of (a)synchrony between 7 major ions

Brekenfeld et al.
(in review on
EGUsphere)

Very synchronous pairs of solutes

- synchronous concentration decrease of NO3, Cl, Mg and Na

- complex variations not synchronous with other solutes for SO4 and Ca

Inter-event synchrony : When solutes have synchronous storm variations, their relative variations among event are proportionnal in amplitude

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> What can we learn when increasing the number of solutes monitored at High Frequency?

Interpretation and perspectives

Synchronous variations of NO3, Cl, Na and Mg : only 2 end-members
contributions are sufficient to explain their storm dynamics

Brekenfeld et al.
(in review on
EGUsphere)

* 1 reflecting low concentrations of storm water (rain, surface or shallow flows)

* 1 reflecting the overall catchment signature (base flow)

Elemental ratio analysis

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> What can we learn when increasing the number of solutes monitored at High Frequency?

Interpretation and perspectives

Brekenfeld et al.
(in review on
EGUsphere)

Ratio at initial cond.

Ratio at peak

Ratio in rain

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* Cl, Na, Mg

* pre event and event water ratios similar: spatial homogeneity of sources

* Stream ratios > rainfall ratios: agricultural Cl

* NO3: higher differences between pre-event and event ratios and stream ratios >> rainfall ratios: high agricultural sources, higher spatial heterogeneity (reactivity?)
> What can we learn when increasing the number of solutes monitored at High Frequency?

Interpretation and perspectives

Brekenfeld et al.
(in review on
EGUsphere)

For synchronous solutes, 2 end-members sufficient to explain storm dynamics but ratio analysis help to specify the processes that are determinant for geochemical background of the catchment

Additional end-members (or event scale processes) are required to
explain asychronous solutes with complex storm variations: Classic
EMMA could be applied using these pairs of solutes

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INRAO
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# What can we learn when increasing the number of years of high-frequency monitoring?

## Nitrate and total phosphorus (TP) time series

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- Study-period: 2016-2023

- 30-min frequency

- 69 % data completion nitrate (S::CAN spectrolyser V2)

- 71 % data completion TP (Hach Lang Phosphax)

200 storm events identified, of which 142 monitored for nitrate and 155 monitored for TP

## Hydroclimatic controls of these extreme events?

### Will they increase in the Future?

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INRAO
> What can we learn when increasing the number of years of high-frequency monitoring?

Controls of water volume and rainfall intensity on nutrient fluxes

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* A positive relationship for most of the events: transport-limitation -> presence of legacy sources

* Non-linearity: for high water volumes, some degree of source limitation for nitrate and increased source availability for TP

* Outliers for TP: extreme events with high rainfall intensity

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> What can we learn when increasing the number of years of high-frequency monitoring?

Identification of events with extreme concentrations (Extreme Value Theory)

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- 10 extreme events for TP (spring season)

- 14 extreme events for nitrate (autumn season)

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> What can we learn when increasing the number of years of high-frequency monitoring?

The role of antecedent discharge and rainfall intensity

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For both TP and Nitrate, extreme concentration during storm events with high rainfall intensity following dry periods
> What can we learn when increasing the number of years of high-frequency monitoring?

Potential mechanisms & implications for the future

<figure><img src="image_39.png" /><figcaption>Photo EPAB Douarnenez</figcaption></figure>

* Seedbed preparation for spring crop -> vulnerable to erosion

* Recent manure/slurry application -> high risk of incidental losses

Climatic projection indicate increasing frequency of such extreme events in the future

Long-term monitoring will allow us to assess whether extreme concentrations will become more frequent in the future, or conservation practices will counterbalance the effect of climate change

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INRAO
> What have we learnt from high-frequency monitoring when increasing the number of years and solutes?

More years

* Analyse rare events and their hydroclimatic controls

* Anticipation of climate change effects

More solutes

* A lot of redundant information (synchrony)

* Some additional information (asynchrony)

* Perceptual model of main catchment compartments based on EMMA

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