Assessing variability of evapotranspiration over the Ganga river basin using water balance computations

Abstract

AbstractA thorough assessment of evapotranspiration (ET) pervades several important issues of the 21st century including climate change, food‐security, land‐management, flood and drought prediction, and water resources assessment and management. Such a proper assessment is of particular importance in the Ganga river basin (GRB) with its backdrop of a rapidly increasing population pressure and unregulated use of water resources. Spatially averaged ET over the GRB is computed as the residual of atmospheric and terrestrial water budget computations using a combination of model simulations and satellite and ground‐based observations. The best estimate of monthly ET is obtained as the monthly mean of atmospheric and terrestrial water balance computations for the period 1980–2007. The mean monthly average of ET from these various estimates is 72.3 ± 18.8 mm month−1. Monthly variations of ET peak between July and August and reach a minimum in February. For the entire study period, the rate of change of ET across the GRB is −11 mm yr−2 (i.e., mm/yr/yr). Alongside a notable influence of the 1997–1998 El Niño, results allude to the existence of interim periods during which ET trends varied significantly. More specifically, during the period of 1998–2002, the rate of decline increased to −55.8 mm yr−2, which is almost 5 times the overall trend. Based on the correlation between ET and independent estimates of near‐surface temperature and soil moisture, we can infer that the ET over the GRB is primarily limited by moisture availability. The analysis has important potential for use in large‐scale water budget assessments and intercomparison studies. The analysis also emphasizes the importance of synergistic use of mutliplatform hydrologic information.