This paper analyzes the sensitivity of reference evapotranspiration (ET o ) to climatic variables for different agro-ecological regions of India: semi-arid (Kovilpatti and Parbhani), humid (Mohanpur), and sub-humid (Ludhiana and Ranichauri). The FAO-56 Penman-Monteith (FAO-56 PM) method is used to estimate ET o , and sensitivity of ET o has been studied in terms of change in maximum air temperature (T max ), minimum air temperature (T min ), solar radiation (R s ), average relative humidity (RH avg ), and wind speed (W s ). Sensitivity analysis is performed by increasing and decreasing the climate variables such as T max , T min , R s , and RH avg by one unit of increment and decrement, respectively, up to five units (except for W s ) while keeping the other variables and parameters constant. However, wind speed W s (km h −1 ) is only increased with an increment of one km h −1 up to five km h −1. The results showed that the change in ET o is linearly related to change in all climate variables (r 2 =0.97 in most cases) at all sites. Further, ET o is most sensitive to R s at Kovilpatti, Mohanpur and Ranichauri, and to W s at Ludhiana and Parbhani. However, the sensitivity of ET o to the same variable shows considerable variation from site to site and at the same site within the year. ET o is less sensitive to RH avg followed by T min at all sites.
The evapotranspiration of the wheat crop grown in Tarafeni South Main Canal (TSMC) irrigation command area of West Bengal, India was estimated based on Normalized Difference Vegetation Index (NDVI) from LANDSAT images. The crop evapotranspiration (ETc) of wheat crop was estimated using the crop coefficient (Kc) maps and the reference evapotranspiration (ETo) in the TSMC irrigation command area. The ETo was estimated from the well known temperature based ETo estimation method, i.e. FAO-24 modified Blaney-Criddle method using measured maximum and minimum air temperatures data during January 2011 in the command area. The Kc maps were mapped in ARC GIS software using procured LANDSAT images for the study period. The area under wheat crop was clipped from land use/land cover map generated from LANDSAT image of January, 2011 for winter season. Further, the crop evapotranspiration map was obtained by multiplying Kc map with the estimated ETo value i.e., 5.76 mm/day for a particular day. The maximum crop evapotranspiration computed for Rabi crop was 5.57 mm/ day, whereas minimum was 1.59 mm/day for the TSMC command area.
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