Satellite and Ground-based Approaches for Monitoring Impacts of Agriculture on Groundwater Resources
University of Texas at Austin
Increased intensity of the hydrologic cycle predicted by climate change models should reduce reliability of precipitation, soil moisture, and stream flow because of longer-term droughts interspersed with intense droughts. As a result, reliance on groundwater may increase in the future. Irrigated agriculture is the dominant consumer of global groundwater resources. Sustainable development of groundwater resources requires quantitative estimates of the rates that groundwater is being renewed or groundwater recharge rates. Impacts of land use change and climate variability/change on groundwater recharge need to be considered. In addition, water quantity and water quality issues need to be addressed to fully assess water availability. This presentation will focus on various techniques that can be used to assess groundwater resources with examples from different regions globally. The Gravity Recovery and Climate Experiment (GRACE) provides a valuable tool for monitoring changes in total water storage (TWS), including surface water, soil moisture, and groundwater at 10 d to monthly timescales. By comparing GRACE TWS with output from the Global Land Data Assimilation System, temporal variations in groundwater storage can be estimated. Use of GRACE to monitor seasonal changes in groundwater storage in the U.S. High Plains related to irrigation will be compared with groundwater depletion estimates from NW India published recently. In addition, uncertainties related to GRACE processing and GLDAS modeling will be examined. Groundwater hydrographs provide valuable information on changes in groundwater storage in response to land use change. Increasing groundwater levels in response to changes from natural grassland/shrubland ecosystems to rain-fed agroecosystems in the U.S. High Plains will be described. Large scale groundwater level declines (? 1.4 m/yr) have also been recorded in areas of irrigated and indicate that irrigation pumpage is not sustainable in some parts of the High Plains aquifer. Groundwater solute hydrographs also record changes in water quality in response to cultivation with increasing salinity and nitrate levels in the High Plains aquifer. Unsaturated zone profiles provide a time-integrated record of past impacts of land use change on recharge and solute fluxes at decadal to millennial timescales. Lack of recharge under natural ecosystems in the U.S. High Plains is shown by bulge-shaped accumulations of chloride in the subsurface in response to changing climate from Pleistocene to Holocene times about 10,000 to 15,000 years ago. Increased recharge after conversion to cropland is recorded by downward displacement of these salt bulges and development of nutrient bulges. Irrigation in this region results in accumulation of salts and nutrients that are similar in magnitude to those beneath natural ecosystems but accumulated over decades rather than the millennia represented by bulges under native vegetation. Unsaturated zone data provide an extremely valuable record of water, salt, and nutrient fluxes that link land surface practices and groundwater quantity and quality. The various approaches for monitoring groundwater quantity and quality cover a range of space and timescales and can be used to determine approaches toward sustainable management of groundwater resources. Recharge estimates under different land use settings can be used to determine what level of irrigation application is sustainable in semiarid regions.