Water availability plays a key role in securing agricultural production and sustaining the income of farming households. Nepal is one of the countries most dependent on agriculture; more than 80% of the population works in agriculture, contributing to 35% of its total gross domestic product (GDP). As one of the longest rivers in Nepal, Koshi River is one of the main water supplies for agricultural activities. In recent years, due to the population growth and the climate change, there has been increasing stress on the water resources in Koshi River basin. Therefore, a comprehensive investigation of water availability in the basin area is required, prior to an effective strategy for water resources allocation and management. In this study, we provide a quantitative assessment of available water resources in Koshi River basin and highlight the trend of water availability for agricultural use. Moreover, we discuss the potential water-related risks for farming households in the basin area. The contribution of this study is to provide the basis for efficient water management strategies in Koshi River basin.