Rainfall variability has a great impact on agricultural production and irrigation water requirements in the monsoon countries. For agricultural development as well as for planned river water management, an irrigation system was developed in the Eastern Terai of Nepal known as Kankai Irrigation System (KIS) in the lower part of the river Kankai. In the study, irrigation water requirement in different growing seasons for main crop paddy is calculated with the help of the Cropwat (8.0) model using climate data, crop coefficient and soil characteristics. The FAO (Food and Agricultural Organisation) CROPWAT 8.0 Simulation software and the CLIMWAT 2.0 tool are used to calculate the crop water requirement, irrigation water requirement and irrigation schedules for the crop paddy for the monsoon and spring season. Canal water demand and supply, irrigation schedules and planning depend on the inter-seasonal and intra-seasonal rainfall variability. The Penman-Monteith method is used to estimate the reference evapotranspiration and crop evapotranspiration (ET0) and the US Department of Agriculture (USDA) soil conservation (SC) method is used to find out the effective rainfall. The study results show that the ET0 varied from 3.12 to 7.99 mm (average 4.9 mm) and the effective rainfall varied from 3.6 to 167.8 mm. The net irrigation requirements are 276.2 and 980.5 mm for paddy in monsoon and spring rice, respectively. There is a higher demand for irrigation in winter (Rabi) and spring season (Zaid) and lower demand during the wet season (Kharif). Correlation between rainfall variability and irrigation requirement is highly positive. The value for correlation coefficient (r2) is 0.98, slope 7.93 and the intercept 15.