A new technique to estimate net groundwater use across large irrigated areas by combining remote sensing and water balance approaches, Rechna Doab, Pakistan

Over-exploitation of groundwater resources threatens the future of irrigated agriculture, especially in the arid and semi-arid regions of the world. In order to reverse this trend, and to ensure future food security, the achievement of sustainable groundwater use is ranking high on the agenda of water policy makers. Spatio-temporally distributed information on net groundwater use-i.e. the difference between tubewell withdrawals for irrigation and net recharge-is often unknown at the river basin scale. Conventionally, groundwater use is estimated from tubewell inventories or phreatic surface fluctuations. There are shortcomings related to the application of these approaches. An alternative methodology for computing the various water balance components of the unsaturated zone by using geo-information techniques is provided in this paper. With this approach, groundwater recharge will not be quantified explicitly, but is part of net groundwater use, and the spatial variation can be quantitatively described. Records of routine climatic data, canal discharges at major offtakes, phreatic surface depth fluctuations, and simplified information on soil textural properties are required as input data into this new Geographic Information System and Remote Sensing tool. The Rechna Doab region (approximately 2.97 million ha), located in the Indus basin irrigation system of Pakistan, has been used as a case study. On an annual basis, an areal average net groundwater use of 82 mm year(-1) was estimated. The current result deviates 65% from the specific yield method. The deviation from estimates using tubewell withdrawal related data is even higher.