Guadalajara, Léon, Zapopan, Aguascalientes,. Queretaro, Morelia and Toluca). For the last decades, overexploitation of the aquifer has led to environmental ...
GROUNDWATER DEFICIT AND LAND SUBSIDENCE IN CENTRAL MEXICO MONITORED BY GRACE AND RADARSAT-2 Pascal Castellazzi (1), Richard Martel (1), Jaime Garfias-Soliz (2), Angus I. Calderhead (3), Javier Salas-García (2), Jianliang Huang (3) and Alfonso Rivera (3) 1 2
Institut National de la Recherche Scientifique, Canada. Universidad Autónoma del Estado de México, Mexico. 3 Natural Resources Canada, Canada.
ABTRACT In the context of a lack of reliable data in assessing groundwater overexploitation, space borne sensors bring useful information. While space-borne SAR and gravimetric data are now used to study groundwater, their interoperability is still poorly studied. In this paper, we apply two cutting-edge techniques for the deficit assessment of one of Mexico’s most important watersheds. Space-borne gravimetry is used to extract total and groundwater storage variations from 2003 to 2013; meanwhile InSAR techniques allow the detection of groundwater deficit areas in 2012 and 2013. GRACE reveals the large-scale combined effect of local unconfined aquifer overexploitation revealed by InSAR. Results show a non-anthropogenic water deficit within the northern half of the basin. Important local decreases in groundwater storage are observed by InSAR within several cities of the southern part of the basin, but groundwater storage loss is partially compensated by surface water storage increase. Index terms: Groundwater deficit, land subsidence, RADARSAT-2, GRACE, Mexico 1. INTRODUCTION Groundwater is a main source of water supply in central Mexico. The Lerma-Santiago-Pacifico watershed is one of the main watersheds of the area. It extends over 133,484 km2 and provides water for eight of Mexico’s 35 most populated cities (Fig 1; Mexico, Guadalajara, Léon, Zapopan, Aguascalientes, Queretaro, Morelia and Toluca). For the last decades, overexploitation of the aquifer has led to environmental and socioeconomic issues, directly impacting more than 30 million Mexicans. The study aims to: (1) analyse the large-scale total and groundwater overexploitation, and (2) localize groundwater depletion areas through land subsidence
detection. To accomplish these objectives, gravimetric data from The Gravity Recovery and Climate Experiment (GRACE) satellites, radar data from RADARSAT-2, and field data are used.
Fig. 1: Major cities and population density within the Lerma-Santiago-Pacifico watershed. 2. METHODOLOGY Since 2002, GRACE satellite mission has been measuring temporal variations of earth gravimetric field. GRACE data can be used to detect large-scale groundwater deficits [1-3]. In this study, GRACE data from January 2003 to December 2013 are used. From raw GRACE data, total water storage (TWS) signal can be extracted [4,5]. TWS is interpreted as the combination of four components: Soil moisture storage (SMS); Surface water storage (SWS); Groundwater storage (GWS); Snow and ice storage (SIS).
A proper estimation of the variations within 3 compartments allows to extract the remaining parameter from TWS: ΔGWS = ΔTWS – (ΔSWS + ΔSMS + ΔSIS) SMS is extracted using Land Surface Models (LSM). Several versions within the Global Land Data Assimilation System (GLDAS) [6] and one from the Climate Prediction Center (CPC) [7] are used. SWS is extracted using field data and SIS is nonexistent in the region.
groundwater volume loss within the areas localized through InSAR. In this project, over 126 RADARSAT-2 Fine and Ultrafine images are processed using D-InSAR. The extent of the SAR dataset is shown on Fig. 3. Images were ordered taking into account population density (Fig. 1 and 2) and presence of a locally documented groundwater deficit. Several types of field data are available, and used to complete and validate the remotely-sensed data products: (1) Two extensometers placed in Toluca Valley to validate InSAR measurements; (2) three soil moisture sensors placed at different depth to optimize and validate the removal of SMS signal from GRACE TWS, and (3) groundwater static-level variations from 1968 to 2012 in more than 250 wells. Field data reinforce the eventual link between land subsidence and groundwater pumping. Groundwater overexploitation is revealed, and its large-scale effects are compared with GRACE results.
Fig. 2: Temporal evolution of the average depth (in meters) of the main water extraction wells within 3 valleys. Averages were calculated from 164 extraction wells in Aguascalientes valley, 43 in Queretaro and 88 in Toluca. Mexico City is one of the World’s most known land subsidence case, while other cities in Mexico are still poorly studied. Differential InSAR (D-InSAR) has been used to reveal groundwater overexploitation in Toluca valley [8, 9]. Recently, Persistent Scatterer Interferometry (PSI) was applied to Mexico City [10]. The use of InSAR in central Mexico has shown that the phenomenon was generalised to almost every major city of Central Mexico [11, 12]. With the exception of Mexico City and Toluca, the relationship to groundwater deficit is likely, but still not thoroughly studied. Static level drawdowns in unconfined aquifers are implying: (1) land subsidence caused by clay interbeds pore pressure decrease and (2) groundwater volume losses contributing to gravimetric variations. Decomposition of gravimetric time series based on the water cycle compartments (i.e. surface water, groundwater and soil moisture) is used to infer
Fig. 3: Coverage of the Fine and Ultrafine RADARSAT-2 InSAR time-series (population density as background, and groundwater level data shown on Fig. 2 as dots). 3. RESULTS Fig. 4 to 7 show GRACE TWS measurements and the decomposition of influencing signals in Water Thickness Equivalents (WTE). Fig. 8 shows TWS trend rates from 2003 to 2013. Examples of D-InSAR results revealing land subsidence caused by groundwater level drawdown are shown. Aquifers are overexploited through municipal, industrial or agricultural wells (Fig. 9 and 10).
Results show a small TWS loss of 0.28 km3 per year over the watershed (Fig. 8). Among the most likely causes of this TWS loss are: (1) the decrease in rainfall over the last decade, especially in the northern part of the watershed; and (2) the groundwater overexploitation within the main cities.
Fig. 8: CSR RL05 GRACE TWS trend rates (cm/year) .
Fig. 4: CSR RL05 version of GRACE TWS variations (cm of water thickness equivalents - WTE) over the watershed (black line) with error estimates (blue dash lines).
Fig. 5: Time averaged SMS variations (cm of WTE) from six Land Surface Models from GLDAS and CPC.
Fig. 6: Time averaged SWS variations (cm of WTE) reported on the watershed area.
Fig. 7: Time averaged GWS variations (cm of WTE) over the watershed extracted with GRACE TWS signal decomposition. ΔSMS is extracted using CLM10 (Fig. 5).
Fig. 9: D-InSAR over Aguascalientes valley (11 orbital cycles or 264 days). Evolution of the depth to groundwater level is shown on Fig. 2. Despite groundwater overexploitation revealed by InSAR in the southern part of the basin (Figures 2, 3, 9 and 10), TWS is almost stable. Surface water restocking policies and several dam constructions in the state of Nayarit (Fig. 3 and 6) have played an important role in keeping TWS stable in the southern part of the basin. First estimation shows a large scale groundwater annual loss of over 0.5 cm of WTE or 0.67 km3 (Fig. 7) over the whole watershed.
Fig. 10: D-InSAR over the city of Celaya (6 orbital cycles or 144 days). Fractures cause severe infrastructure damages. 4. DISCUSSION AND PERSPECTIVES Combining InSAR and GRACE shows a great potential in assessing causes, consequences and extent of the groundwater overexploitation. Further studies will be focused on: (1) in-depth analysis of aquifer reactions to decreases of hydraulic head level through PSI and Small Baseline Subset Interferometry (SBASInSAR), (2) investigation of the link between local or regional and watershed-scale deficits and (3) the built of a 3D groundwater flow model calibrated with both InSAR and GRACE results. Novelties of the study include: (1) the first large-scale groundwater storage monitoring study in Mexico using signal decomposition of GRACE time-series, (2) the use of InSAR as a way to localize groundwater deficit area calculated at a larger scale with GRACE TWS signal decomposition. REFERENCES [1] M. Rodell, J. Chen, H. Kato, J. S. Famiglietti, J. Nigro, and C. R. Wilson, "Estimating groundwater storage changes in the Mississippi River basin (USA) using GRACE," Hydrogeology Journal, vol. 15, pp. 159-166, 2006. [2] M. Rodell, I. Velicogna, and J. S. Famiglietti, "Satellite-based estimates of groundwater depletion in India," Nature, vol. 460, pp. 999-U80, Aug 2009. [3] J. Huang, J. Halpenny, W. van der Wal, C. Klatt, T. S. James, and A. Rivera, "Detectability of groundwater
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