Ground referencing GRACE satellite estimates of groundwater storage changes in the California Central Valley, USA

International audience[1] There is increasing interest in using Gravity Recovery and Climate Experiment (GRACE) satellite data to remotely monitor groundwater storage variations; however, comparisons with ground-based well data are limited but necessary to validate satellite data processing, especially when the study area is close to or below the GRACE footprint. The Central Valley is a heavily irrigated region with large-scale groundwater depletion during droughts. Here we compare updated estimates of groundwater storage changes in the California Central Valley using GRACE satellites with storage changes from groundwater level data. A new processing approach was applied that optimally uses available GRACE and water balance component data to extract changes in groundwater storage. GRACE satellites show that groundwater depletion totaled $31.0 6 3.0 km 3 for Groupe de Recherche de Geodesie Spatiale (GRGS) satellite data during the drought from October 2006 through March 2010. Groundwater storage changes from GRACE agreed with those from well data for the overlap period (April 2006 through September 2009) (27 km 3 for both). General correspondence between GRACE and groundwater level data validates the methodology and increases confidence in use of GRACE satellites to monitor groundwater storage changes

Uncertainty in evapotranspiration from land surface modeling, remote sensing, and GRACE satellites

International audienceProliferation of evapotranspiration (ET) products warrants comparison of these products. The study objective was to assess uncertainty in ET output from four land surface models (LSMs), Noah, Mosaic, VIC, and SAC in NLDAS-2, two remote sensing-based products, MODIS and AVHRR, and GRACE-inferred ET from a water budget with precipitation from PRISM, monitored runoff, and total water storage change (TWSC) from GRACE satellites. The three cornered hat method, which does not require a priori knowledge of the true ET value, was used to estimate ET uncertainties. In addition, TWSC or total water storage anomaly (TWSA) from GRACE was compared with water budget estimates of TWSC from a flux-based approach or TWSA from a storage-based approach. The analyses were conducted using data from three regions (humid-arid) in the South Central United States as case studies. Uncertainties in ET are lowest in LSM ET ( 5 mm/mo), moderate in MODIS or AVHRR-based ET (10-15 mm/mo), and highest in GRACEinferred ET (20-30 mm/month). There is a trade-off between spatial resolution and uncertainty, with lower uncertainty in the coarser-resolution LSM ET ( 14 km) relative to higher uncertainty in the finer-resolution ( 1-8 km) RS ET. Root-mean-square (RMS) of uncertainties in water budget estimates of TWSC is about half of RMS of uncertainties in GRACE-derived TWSC for each of the regions. Future ET estimation should consider a hybrid approach that integrates strengths of LSMs and satellite-based products to constrain uncertainties

Calibration and evaluation of a semi-distributed watershed model of Sub-Saharan Africa using GRACE data

International audienceIrrigation development is rapidly expanding inmostly rainfed Sub-Saharan Africa. This expansion underscoresthe need for a more comprehensive understandingof water resources beyond surface water. Gravity Recoveryand Climate Experiment (GRACE) satellites provide valuableinformation on spatio-temporal variability in water storage.The objective of this study was to calibrate and evaluatea semi-distributed regional-scale hydrologic model basedon the Soil and Water Assessment Tool (SWAT) code forbasins in Sub-Saharan Africa using seven-year (July 2002–April 2009) 10-day GRACE data and multi-site river dischargedata. The analysis was conducted in a multi-criteriaframework. In spite of the uncertainty arising from the tradeoffin optimising model parameters with respect to two noncommensurablecriteria defined for two fluxes, SWAT wasfound to perform well in simulating total water storage variabilityin most areas of Sub-Saharan Africa, which havesemi-arid and sub-humid climates, and that among variouswater storages represented in SWAT, water storage variationsin soil, vadose zone and groundwater are dominant. Thestudy also showed that the simulated total water storage variationstend to have less agreement with GRACE data in aridand equatorial humid regions, and model-based partitioningof total water storage variations into different water storagecompartments may be highly uncertain. Thus, future workwill be needed for model enhancement in these areas with inferiormodel fit and for uncertainty reduction in componentwiseestimation of water storage variations

Inclinometry and geodesy: an hydrological perspective

International audienceTwo orthogonal, precise and low drift tiltmeters have been installed in the Vosges mountains in order to study environmental surface loading. The first results show the great sensitivity (10¡10 radians), stability (negligible drift) of the instrument, and its ability to be used as a tool to study hydrological loading. This work focuses on local and regional hydrological physical modelling, with a stepwise refinement of mass balance calculations on a geodetic purpose. We show that meteorological forcing mainly drives stock variations inside a hydrological unit, it is therefore necessary to take great care of precipitation and evapotranspiration. Uncertainty assessment on stock variations is also raised, and shows that hydrological models bring good estimation of short term water stock variations, but that long term geodetic variations provide complementary information for stored water modelling

Time-Lapse Seismic and Electrical Monitoring of the Vadose Zone during a Controlled Infiltration Experiment at the Ploemeur Hydrological Observatory, France

The vadose zone is the main host of surface and subsurface water exchange and has important implications for ecosystems functioning, climate sciences, geotechnical engineering, and water availability issues. Geophysics provides a means for investigating the subsurface in a non-invasive way and at larger spatial scales than conventional hydrological sensors. Time-lapse hydrogeophysical applications are especially useful for monitoring flow and water content dynamics. Largely dominated by electrical and electromagnetic methods, such applications increasingly rely on seismic methods as a complementary approach to describe the structure and behavior of the vadose zone. To further explore the applicability of active seismics to retrieve quantitative information about dynamic processes in near-surface time-lapse settings, we designed a controlled water infiltration experiment at the Ploemeur Hydrological Observatory (France) during which successive periods of infiltration were followed by surface-based seismic and electrical resistivity acquisitions. Water content was monitored throughout the experiment by means of sensors at different depths to relate the derived seismic and electrical properties to water saturation changes. We observe comparable trends in the electrical and seismic responses during the experiment, highlighting the utility of the seismic method to monitor hydrological processes and unsaturated flow. Moreover, petrophysical relationships seem promising in providing quantitative results

GRACE satellite monitoring of large depletion in water storage in response to the 2011 drought in Texas

International audienceTexas experienced the most extreme one-year drought on record in 2011 with precipitation at 40% of long-term mean and agricultural losses of ~$7.6 billion. We assess the value of Gravity Recovery and Climate Experiment (GRACE) satellite-derived total water storage (TWS) change as an alternative remote sensing-based drought indicator, independent of traditional drought indicators based on in situ monitoring. GRACE shows depletion in TWS of 62.3 ± 17.7 km3 during the 2011 drought. Large uncertainties in simulated soil moisture storage depletion (14-83 km3) from six land surface models indicate that GRACE TWS is a more reliable drought indicator than disaggregated soil moisture or groundwater storage. Groundwater use and groundwater level data indicate that depletion is dominated by changes in soil moisture storage, consistent with high correlation between GRACE TWS and the Palmer Drought Severity Index. GRACE provides a valuable tool for monitoring statewide water storage depletion, linking meteorological and hydrological droughts

Physical modelling to remove hydrological effects at local and regional scale: application to the 100-m hydrostatic inclinometer in Sainte-Croix-aux-Mines (France)

International audienceNew inclinometers devoted to hydrological studies were set up in the Vosges Mountains (France). Two orthogonal 100-meter base hydrostatic inclinometers were installed in December 2004 as well as a hydrometeorological monitoring system for the 100-km² hydrological unit around the inclinometer. As inclinometers are very sensitive to environmental influences, this observatory is a test site to confront hydrological modelling and geodetic observations. Physical modelling to remove hydrological effects without calibrating on geodetic data is tested on these instruments. Specifically, two deformation processes are most important: fluid pressure variations in nearby hydraulically active fractures and surface loading at regional scale

Investigating the respective impacts of groundwater exploitation and climate change on wetland extension over 150 years

International audiencePeatlands are complex ecosystems driven by many physical, chemical, and biological processes. Peat soils have a significant impact on water quality, ecosystem productivity and greenhouse gas emissions. However, the extent of peatlands is decreasing across the world, mainly because of anthropogenic activities such as drainage for agriculture or groundwater abstractions in underlying aquifers. Potential changes in precipitation and temperature in the future are likely to apply additional pressure to wetland. In this context, a methodology for assessing and comparing the respective impacts of groundwater abstraction and climate change on a groundwater-fed wetland (135 km2) located in Northwest France, is presented. A groundwater model was developed, using flexible boundary conditions to represent surface-subsurface interactions which allowed examination of the extent of the wetland areas. This variable parameter is highly important for land management and is usually not considered in impact studies. The model was coupled with recharge estimation, groundwater abstraction scenarios, and climate change scenarios downscaled from 14 GCMs corresponding to the A1B greenhouse gas (GHG) scenario over the periods 1961-2000 and 2081-2100. Results show that climate change is expected to have an important impact and reduce the surface of wetlands by 5.3-13.6%. In comparison, the impact of groundwater abstraction (100% increase in the expected scenarios) would lead to a maximum decrease of 3.7%. Results also show that the impacts of climate change and groundwater abstraction could be partially mitigated by decreasing or stopping land drainage in specific parts of the area. Water management will require an appropriate compromise which encompasses ecosystem preservation, economic and public domain activities

Local and global hydrological contributions to gravity variations observed in Strasbourg

International audienceWe investigate the contribution of local and global hydrology to the superconducting gravimeter (SG) installed in the Strasbourg observatory.Adeterministic approach is presented to account for the contribution of water storage variations in the soils in the vicinity of the gravimeter: both amount and distribution of water masses are determined before calculating Newtonian attraction. No adjustment is performed on gravity time series. Two multi-depth Frequency Domain Reflectometer (FDR) probes have been installed to monitor the amount of water stored in the soil layer above the gravimeter. Since August 2005, they have been monitoring the variation of the water content of the entire soil thickness. Several investigations have been undertaken in order to estimate the distribution of water masses: a precise local DEM (Digital Elevation Model) has been determined using differential GPS. The geometry and heterogeneity of the soil layer have been evaluated thanks to geophysical and geomechanical prospections. The comparison between observed and modelled gravity variations shows that daily up to seasonal variations are in good agreement. For long-term variations, deep water storage and other processes have to be modelled to explain recorded gravity variations