Assessment of Spatio-Temporal Variability of Green and Blue Water Evapotranspiration for Wheat Crop over Upper Ganga Command Area

Green and blue evapotranspiration (GWET & BWET) analysis over the command area is crucial for agricultural water resources planning and management and improving crop water productivity. The GWET and BWET refer to the fractions of ET contributions from the water obtained from precipitation and irrigation, respectively. The Upper Ganga command area (UGCA) is India's oldest canal irrigation system with a geographical area of 2 Mha, among which 0.9 Mha is the culturable command area (CCA). The UGCA has a 0.35 Mha irrigable command area in which 76.2 % is under wheat crop, and irrigation water use efficiency varies from 33 to 38%. Over the past few decades, water scarcity in the UGCA has been a massive concern for the government and policymakers. The present research aims to assess and investigate long-term spatial and temporal evapotranspiration for wheat crops from1951 to 2019 using high-resolution gridded data. The rainfall (at 0.25o × 0.25o resolution) and temperature (1 o × 1 o) were obtained from the Indian Meteorological Department (IMD) Pune. Firstly, the temperature data were resampled to 0.25° × 0.25° spatial resolution using the inverse distance weighted (IDW) method.

Furthermore, based on the data availability, reference evapotranspiration (ETo) and the crop coefficients (Kc) were estimated using the Hargreaves and Samani (1985) and the thermal unit (TU) method, respectively. In addition, the GWET and BWET were estimated and mapped for each grid in the ArcGIS environment using estimated crop evapotranspiration and effective rainfall. The crop coefficients for the wheat crop's initial, mid, and late crop stage are 0.42,1.24 and 0.41, respectively. The overall decreasing trend in the crop evapotranspiration with a rate of 0.305 mm per season was observed. The total GWET during the rabi season (November to mid-April) varies between 13.15-42.87 mm and 3.53-49.30 mm whereas BWET varies between 437.59-617.86 mm and 482.14-60.7.03 mm spatially and temporally, respectively across the study region. The present study will be beneficial to policymakers for effective planning and management of irrigation water.