The objectives of this study is to estimate the CWR and IWR and to developed cropping pattern in the study areas for the selected crops to ensure the optimum use of available irrigation water and the optimum use of the available land. So assessment of evapotranspiration loss, effective rainfall and the percentage of total area covered by each crops is very essential. Again to ensure the minimum loss of irrigation water crop coefficient at different stages with their stage lengths and various climatic parameters are adjusted for the study areas.
a) Study Area Selection: Two districts Rangpur and Dinajpur and an upazila named Saidpur of Nilphamari districts in the northwestern zone of Bangladesh are selected as the study area.
b) Data Collection Various climatological data such as monthly maximum and minimum temperature, monthly average humidity, monthly average sunshine hours, daily average wind speed, monthly total rainfall, altitude, latitude, longitude are collected from Bangladesh Meteorological Department (BMD), Agargaon, Dhaka. Total area, total cultivable area, total area under irrigation, planting date, harvesting date, stages length, rooting depth and percentage of the total area covered by each crops are collected from Department of Agricultural and Extension (DAE), Rangpur for all the study areas. The other data such as crop coefficients at various stages, yield response, critical depletion factors are collected from FAO table and then adjusted.
c) Data Processing The estimation of all the climatic parameters are made based on Least Square Parabolic Method. In case of any missing data for a year, value of that parameters for that year are predicted from the previously available data.
d) Reference Evapotranspiration (ET0) Although over the last 50 years several empirical and semi empirical method has been developed for the determination of ET0 but the FAO Penman-Monteith is now sole recommended. The Penman-Monteith Equation for determining reference evapotranspiration (ET0) is given by the following equation (FAO 1998 a).
e) Cropping Pattern To get the irrigation water requirement on daily or monthly basis for all the crops, a cropping pattern with planting and harvesting date with the total percentage of cultivable land covered by the each crops are inserted in the cropping pattern menu of the Cropwatv8.0.
f) Total irrigation Water Requirement After inserting cropping pattern, CROPWAT v 8.0 provide net scheme irrigation water requirement for that particular study area and for all the crops at a time.