Agriculture is the largest user of water in Pakistan. During the last fifty years, water used for irrigation has substantially increased due to increased cropping intensities. Spatial and temporal estimation of potential water requirements for agriculture will permit to assess the expected level of water stress. It will help in improved planning, allocation of water resources and sustainable groundwater management. The variation in climate at the regional scale effects the selection of crop and the evaporation needs of crops. Based on these variations of climate and crops with respect to their culture, intensity and patterns, Indus basin has been divided into three agro-ecological and seven agro-climatic zones. Some of agro-climatic zones exhibit different cropping pattern and crop periods within the zone. To accommodate significant variations in cropping pattern and periods within agro-climatic zones, this study divides all main canal commands of Indus into 11 groups. The reference evapotranspiration is estimated at different stations by using Penman Montieth equation. The spatial variability of ETo across the canal commands is accumulated using GIS. The upper and northeastern part of the basin has lower reference evapotranspiration (1200-1300 mm) because of mild climate, whereas, the lower part of the basin, Southern Punjab and Sindh have much higher ETo values (1700-2100 mm). It has also been observed that ETo is maximum in the month of June and minimum in December. The computation of TAM consultants for CRBC and IWMI using Penman are quite close though different climatic data sets are used for the calculations. A bigger difference is shown by Penman (IWMI 2000) with Delta II model (SPMP, 1988) and Priestley method (Bastiaanssen, 98), respectively 11 and 19 percent. The variation of reference evapotranspiration along with different land forms results in diversified agriculture with respect to cropping pattern and crop period. For developing crop co-efficient curve for different crops, periods of planting and harvesting, crop duration and crop growth stages are determined on the basis of primary and secondary information about cropping practices across the basin. For the major crops, planting and harvesting period extends for 2-4 weeks. The potential water requirements of Rabi crops are in the same range of 240 mm to 462 mm. Whereas Kharif crops have quite variable requirements, 341 mm to 1004 mm. The spatial variation in potential evapotranspiration of different crops is from 14 to 50 percent in the basin The 10-daily water requirements of fully cultivated 1000 ha with existing cropping patterns show that water demand of lower Indus is about 50% higher than the upper Indus. Despite of a scatter in peak demand, the higher and lower water demand periods do not show a big shift across the Indus.
