A study was carried out to estimate the weekly rainfall and evaporation at various probability levels and to assess surplus and deficit periods at different risk levels by using 27 years (1981-2007) rainfall and evaporation data. Extreme value Type I distribution was used to forecast the rainfall and evaporation at 10-90% probability levels. At 10% to 50% probability level, the expected rainfall is not scare at all the weeks. Surplus/deficit analysis for forecasted and 27 year mean weekly rainfall and evaporation were carried out. The surplus/deficit period which is found by 27 year mean weekly rainfall and evaporation shows similar trends of forecasted rainfall and evaporation at 60% risk levels. Water resources planning, crop and drainage planning should be done based on minimum risk levels. 40% risk level can be adopted for any planning because at 10% and 20% risk levels the surplus rainfall is very low and at 30% risk level, the deficit of rainfall is higher than the 40% risk. This analysis helps to assess supplementary irrigation period during deficit of rainfall and drainage period during the surplus of rainfall.