Crop water requirement at Narpatganj in Kosi command area of Bihar

Crop water requirement (CRW) is the depth of water needed to meet the water loss through evapotranspiration (ETc) of a disease free crop, growing in large fields under non-restricting conditions of soil, water and fertility and achieving full production potential under the given growing environment. To estimate ETc, climatological methods have been suggested by Penman, 1948. Under the present study, evapotranspiration has been computed for agro-climatic conditions of Narpatganj in Kosi command area of Bihar by Penman method modified by Doorenbos and Pruitt (1977) and verified by Penman-Monteith method (Allen et al. 1998). It is observed to be maximum (94.71 mm) during 10^th fortnight and minimum (26.52 mm) during 1^st fortnight of the year with a variation of 12.88% and 9.34% (Penman-Monteith method) and average variation of 10.21%.

Based on FAO guidelines, crop coefficients (Kc) and crop water requirements have also been computed on fortnightly basis for existing and potential crops of the region. Crop coefficient value was maximum (1.18) for wheat crop followed by 1.15 for tomato crop during the first fortnight. Average fortnightly irrigation requirement has been observed to be 27.4 mm/fortnight (maximum) for chilli crop with crop duration extending between 22^nd to 5^th fortnights. This is based on assured expected fortnightly minimum rainfall at 70% probability level.