Eugene Yan

Integration of GRACE (Gravity Recovery and Climate Experiment) data with traditional data sets for a better understanding of the time-dependent water partitioning in African watersheds

Monthly (71 months) Gravity Recovery and Climate Experiment (GRACE) gravity field solutions acquired over North and Central Africa (August 2002–July 2008) were destriped, smoothed (250 km; Gaussian), and converted to equivalent water thickness. These data were analyzed in a geographic information system environment together with relevant data sets (e.g., topography, geology, remote sensing) to assess the utility of GRACE for monitoring elements of hydrologic systems on local scales. The following were observed over the Niger, Congo, and Nile Basins: (1) large persistent anomalies (standard deviation, SD > 10 cm) on SD images over periods of 2–7 yr; (2) anomalous areas originate at mountainous source areas that receive high precipitation, extend downslope toward mountain foothills, and often continue along main channels, wetlands, or lakes that drain these areas; (3) time-series analyses over anomalous areas showed that seasonal mass variation lags behind seasonal precipitation; and (4) seasonal mass variations showed progressive shift in phase and decrease in amplitude with distance from the mountainous source areas. Results indicate that (1) the observed temporal mass variations are largely controlled by elements of the hydrologic cycle (e.g., runoff, infiltration, groundwater flow) and have not been obscured by noise, as previously thought; and (2) it is possible to use GRACE to investigate the temporal local responses of a much larger suite of hydrologic systems (watersheds, lakes, rivers, and marshes) and domains (source areas and lowlands) within watersheds and subbasins worldwide.

Aquifer recharge, depletion, and connectivity: Inferences from GRACE, land surface models, and geochemical and geophysical data

Data from the Gravity Recovery and Climate Experiment (GRACE) and outputs of the CLM4.5 model were used to estimate recharge and depletion rates for large aquifers, investigate the connectivity of an aquifer’s subbasins, and identify barriers and preferred pathways for groundwater flow within an aquifer system. The Nubian Sandstone Aquifer System and its subbasins (Dakhla, Northern Sudan Platform, and Kufra) in northeast Africa were used for demonstration purposes, and findings were tested and verified against geological, geophysical, remote sensing, geochronologic, and geochemical data. There are four major findings. (1) The average annual precipitation data over recharge areas in the southern Kufra section and the Northern Sudan Platform subbasin were estimated at 54.8 km 3 , and 32.8 km 3 , respectively, and knowing the annual extraction rates over these two areas (∼0.40 ± 0.20 km 3 ), recharge rates were estimated at 0.78 ± 0.49 km 3 /yr and 1.44 ± 0.42 km 3 /yr, respectively. (2) GRACE-derived groundwater depletion rates over the Dakhla subbasin and the Northern Kufra section were estimated at 4.44 ± 0.42 km 3 /yr and 0.48 ± 0.32 km 3 /yr, respectively. (3) The observed depletion in the southern parts of the Dakhla subbasin is apparently caused by the presence of the east-west−trending Uweinat-Aswan basement uplift, which impedes the south-to-north groundwater flow and hence reduces replenishment from recharge areas in the south. (4) A major northeast-southwest−trending shear zone (Pelusium shear system) is apparently providing a preferred groundwater flow pathway from the Kufra to the Dakhla subbasin. Our integrated approach provides a replicable and cost-effective model for better understanding of the hydrogeologic setting of large aquifers worldwide and for optimum management of these groundwater resources.

Integrated solutions for hydrologic investigations in arid lands

Hydrological assessment studies across vast regions of the arid world are often hindered by the inaccessibility of these areas and the paucity of data sets, as well as the high expenses and diffi culties entailed in acquiring these data sets, their unpublished nature, and their varying scales, projections, and datum. Using the Eastern Desert (ED) of Egypt (225,000 km 2 ) and the Sinai Peninsula (61,000 km 2 ) as test sites, we demonstrate practical and cost-effective integrated (geochemistry, geophysics, and modeling) solutions that utilize web-based geographic information system (GIS) (http://www.esrs .wmich.edu/webmap) technologies and take advantage of readily available global remote sensing data sets. Adopted methodologies allowed: (1) development of conceptual models for hydrogeologic settings conducive to groundwater entrapment and augmentation, including groundwater in fractured basement aquifers, groundwater impounded by dike swarms crosscutting alluvial aquifers, and groundwater residing in alluvial aquifers associated with ascending deep-seated fossil waters; (2) selection of criteria to identify and validate the preferred distribution of each of these aquifer types and usage of the selected criteria and observations from the GIS data sets to identify, test, and refi ne potential well locations; and (3) construction and calibration of hydrologic models to estimate average annual recharge over the major watersheds in the Sinai (463 ◊ 10 6 m 3 /yr) and ED (171 ◊ 10 6 m 3 /yr) and the average modern contributions to Nubian fossil aquifers (Sinai: 13 ◊ 10 6 m 3 /yr), and to model the partitioning of precipitation as a function of precipitation amounts. The successful application of the integrated and cost-effective methodologies developed for the study areas should invite similar applications in arid regions elsewhere.