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(1) BRGM – French Geological Survey Water Resources Management 3 avenue C. Guillemin BP 36009

(1) BRGM – French Geological Survey Water Resources Management 3 avenue C. Guillemin BP 36009 45060 Orléans Cedex 2 France t.klinka@brgm.fr. Determination of groundwater level indicators and thresholds levels for groundwater drought management # 240.

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(1) BRGM – French Geological Survey Water Resources Management 3 avenue C. Guillemin BP 36009

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  1. (1)BRGM – French Geological Survey Water Resources Management 3 avenue C. Guillemin BP 36009 45060 Orléans Cedex 2 France t.klinka@brgm.fr Determination of groundwater level indicators and thresholds levels for groundwater drought management #240 (1)Delphine Allier, Etienne Buscarlet, Thomas Klinka, Jean-Charles Manceau, Xavier Marly Speaker : Thomas Klinka Tuesday 24thSeptember 12:30 – 12:45

  2. Determination of groundwater level indicators • « Last summer in France … » Vigilance Alert Reinforcedalert Crisis • News A cornfield with drought in Montbert near Nantes (44), France Map of water restrictions on 17/09/2019 Source : http://propluvia.developpement-durable.gouv.fr In places there is no drop of water in the bed river of the Ardusson (10), France

  3. Grand Est region Determination of groundwater level indicators • Droughtsindicators in Grand Est region • Project funded by the Regional Delegation for Environment (DREAL), Water basins agencies (Rhin-Meuse, Seine-Normandie) • Step 1Review of the current drought indicators for groundwater in the region • Step 2Propose an approach for implementing groundwater level indicators and thresholds levels at a regional scale • Hydrogeologicalcontext • In Champagne-Ardenne : • 3 chalky water body, • The Champigny water table, • The Oxfordian and Portlandian limestones, • The Brienne groundwater (alluvial). • Each aquifer has its own hydrodynamic, cycle, recharge, withdrawals …

  4. Determination of groundwater level indicators • Step 1 : Review of the current drought indicators for groundwater in the region Brie and Champigny limestoneMécringes National ID : 01868X0030 • Different criteria among administrative areas • Thresholds on the groundwater levels (e.g. monthly mean), return period as expected frequency (e.g. 5 years dry, 20 years), etc. • Thresholds on the river flows, % on mean annual minimum flow (MAM), minimum flow for 3 consecutive days (VCN3), etc. • Discontinuities from one month to another • Differences between thresholds not significant • Flaws in the method: • No homogeneity, stationarity, independence on piezometric time-series • Precludes a probabilistic approach (return period) • Difficult to standardize a drought indicator applicable to all groundwater resources

  5. Determination of groundwater level indicators • Step 2 : How to implement groundwater level indicators and thresholds levels at a regional scale, with heterogeneous hydrogeological context ? • Proposed framework: • Characterisation of groundwater dynamics for the main aquifers of the Grand-Est region • Establishment of clusters of piezometers based on similar dynamics • Evaluation of most appropriate indicators and methodology for fixing thresholds • Dataset: Extraction of 1,490,499 dailymeasurements [French groundwater national portal, ADES, public data, https://ades.eaufrance.fr], on 344 piezometers. National ID : 01114X0042 « Ferriferous Lorrain » basin Levels (mAMSL)

  6. Determination of groundwater level indicators • Characterisation of groundwater dynamics - 1st tool : Periodogram • Objective :identify periodicities in groundwater level time series (seasonal, multi-annual) • Interest for drought :identify groundwater main sensibility, either to annual meteorological droughts or to droughts over several years • Method : Lomb-Scargleperiodogram (adapted to unevenly-sampled data) • Provides a Fourier-like power spectrum and allows detecting the main periodic component in the groundwater dynamics National ID : 01086X0011 Seno-Turonian chalk normalized spectral power Levels (mAMSL) Main periodicitiesdetected in the dataset : 1 year, # 6.5 years, # 16 years National ID : 04137X0022 Quaternary alluvium – Plain of Alsace normalized spectral power Levels (mAMSL)

  7. Determination of groundwater level indicators • Characterisation of groundwater dynamics - • 2ndtool : Groundwater recession curves & characteristic time • Objective :draw the groundwater level evolution with no recharge rate • Interest for drought :indicator of the resistance to effective rainfall deficiency • Method : in-house automatic procedure based on a manual procedure (Forkasiewicz & Margat, 1976) • Provides estimation of the of the base level, and of a characteristic recession coefficient (or halfdecay time) National ID : 02276X0050Lower Tithonian limestones from the Paris basin Levels (mAMSL) Levels (mAMSL) Times (days)

  8. Determination of groundwater level indicators • Characterisation of groundwater dynamics - • 2ndtool : Groundwater recession curves & characteristic time • Objective :draw the groundwater level evolution with no recharge rate • Interest for drought :indicator of the resistance to effective rainfall deficiency • Method : in-house automatic procedure based on a manual procedure (Forkasiewicz & Margat, 1976) • Provides estimation of the of the base level, and of a characteristic recession coefficient (or halfdecay time) National ID : 01116X0138 Oxfordianlimestonefrom the Paris basin Levels (mAMSL) Levels (mAMSL) Times (days)

  9. Determination of groundwater level indicators • Characterisation of groundwater dynamics - • 3rd tool : SGI (Standardised Groundwater level Index) [Bloomfield & Marchant, 2013] • Objective :quantify, in a standardized way, the groundwater level status between different observation sites of a given aquifer or at a larger scale • Interest for drought :indicator to detect, and quantify homogeneously hydrogeological droughts (magnitude, intensity) • Method : calculation of the SGI according to the method used in the French National Hydrological Situation report (Seguin, 2005) National ID : 004137X0066 Recentalluvium - Rhénanes National ID : 004137X0066Oxfordianlimestone Levels (mAMSL) Levels (mAMSL) 1-month SGI 1-month SGI

  10. Determination of groundwater level indicators • Establishment of clusters of piezometers based on similar dynamics • Similarity index based on the cross-correlation of pairs of time series • Clustering algorithm : k-medoids • Number of groups depending on the chosen level of discrimination • Identification of differences in terms groundwater dynamics : main periodicities & heterogeneity in terms of groundwater functioning (ex. Rhin alluvial groundwater associated to the nival flow regime)

  11. Determination of groundwater level indicators • Evaluation of most appropriate indicators and methodology for fixing thresholds - Which indicator ? • Monthly vs. annual indicator ? • Which low level indicator ? • Minimum groundwater level : impacted by outliers • Mean Annual Minimum (MAM-n days) groundwaterlevel : how to choose n ? • Choice of MonthlyMeanAnnual Minimum groundwaterlevel (annual minimum of the pz levelmonthlymeans) National ID : 01995X0012Alluvium - Rhénanes Levels (mAMSL) Levels (mAMSL)

  12. Determination of groundwater level indicators • Evaluation of most appropriate indicators and methodology for fixing thresholds - Which threshold methodology for which dynamics ? • If dominant periodicity is annual: a frequency analysis may be carried out • The threshold can be fixed based on return period (~probability of occurrence) • If dominant periodicity is multi-annual: precludes a frequency analysis • Thresholds could still be established based on observed frequency, associated with information on the main periodicity. • The observedfrequencyshould not beassimilated to return period (probabilisticmeaning) National ID : 01995X0012 Alluvium - Rhénanes Levels (mAMSL) National ID : 01987X0039 Pliocene – Plain of Alsace Levels (mAMSL)

  13. MANY THANKS FOR YOUR ATTENTION! Pierre Percée lake (Meurthe & Moselle)

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