Water balance investigations of groundwater depletion in Asia : information needs and uncertainty analysis

Abstract

Irrigated agriculture is an essential component of global food production. In many regions of the world, and Asia in particular, groundwater is a major source of irrigation water. Over-pumping of groundwater aquifers has depleted storage in some areas. Quantification of the degree to which groundwater is being used unsustainably is critical to our understanding of the stability of irrigated agriculture. In this study, we investigate the feasibility of estimating groundwater storage changes using the water balance equation and currently available sources of global data. We employ multiple measurements of each water balance component and a constrained least squares estimation method in order to reduce uncertainties. Global datasets of precipitation, evapotranspiration, and runoff were obtained and evaluated for suitability for use in the water balance. In order to fill gaps in the existing data, we developed a new estimate of evapotranspiration based on NDVI measurements and land use information. Results are presented for the continental U.S. and for our study region in Asia. We also show that multiple regression of runoff against basin characteristics can provide additional runoff information in ungaged basins. The water balance can be used as a screening tool for poor quality data, and we are able to identify problematic basins in Asia. In addition, the least squares water balance estimator can be used to reduce the uncertainty in estimates of each component of the water balance.

(cont.) However, the uncertainty which remains in estimates of groundwater depletion precludes definitive statements about the sustainability of resource use in Asia for the time being. Groundwater depletion may be easier to detect using the water balance technique in drier climates, and forthcoming data products from advances in remote sensing may help to alleviate problems with the current data. However, we are unable to effectively evaluate the sustainability of groundwater use in many parts of Asia using the currently available data. This has implications for our ability to evaluate water and food security throughout the region.