Evapotranspiration variations over major Indian River basins in the past three decades

Abstract:

Evapotranspiration (ET) plays a vital role in the Earth’s climate system by linking water, carbon, and energy cycles. Here we compute ET estimates and assess the factors controlling its variability over major Indian River basins. Basin-averaged monthly ET estimates are computed employing the terrestrial water balance approach for the period of 1982-2014. However, instead of using terrestrial water storage changes (TWSC) from those Gravity Recovery and Climate Experiment (GRACE) satellites we use simulations from the WaterGAP Global Hydrology Model (WGHM)). ET estimates computed in this study (ET computed using TWSC estimates from WGHM) show strong similarity with GRACE-derived ET estimates (average R = 0.90) impart significant confidence on the approach. Further, these ET estimates are compared against ancillary ET estimates obtained from a variety of different approaches including atmospheric water balance, purely satellite-based observations (MODIS) and land surface model simulations. Trend analysis shows significant increase in annual ET over Ganga (2.72 mm/yr) and Krishna (3.90 mm/yr) River Basins, while in Godavari and Mahanadi River Basins the observed trends are insignificant. In order to assess the factors controlling ET, we perform Hierarchical Partitioning Analysis by integrating estimates of precipitation, soil moisture, temperature and NDVI. Results reveal that ET variance is largely controlled by the availability of water (represented by precipitation and soil moisture) in all the river basins. Precipitation (soil moisture) accounts for 65% (16%), 70% (20%), 77% (15%) and 67% (18%) of the variability in monthly ET over the Ganga, Godavari, Krishna and Mahanadi River Basins, respectively. Results from this study will contribute to a better understanding of the appropriateness and discrepancies amongst different ET products and provide important information on the factors controlling ET variability that is essential for long-term water resource assessments.