Crop water requirements assessment in the irrigation-scarce Terai plain: a case study of Rohini Rural Municipality, Nepal

ABSTRACT Agriculture in Nepal's Terai plain, particularly in Rohini Rural Municipality, faces growing challenges from seasonal water scarcity, erratic rainfall, and inadequate irrigation infrastructure. This study evaluates crop water requirements (CWRs), irrigation water requirements, and irrigation scheduling for key crops – rice, wheat, potato, mustard, pulses, and winter vegetables – using the CROPWAT 8.0 model and FAO Penman–Monteith method. Local climate data, crop characteristics, and loam soil conditions were used to estimate reference evapotranspiration (ETo), crop evapotranspiration (ETc), and effective rainfall. Results indicate high seasonal variation in irrigation demand, peaking in December and declining during the monsoon. Rice had the highest CWR (740.3 mm), followed by wheat (357.3 mm). The study highlights the significance of site-specific irrigation planning, as local factors significantly influence water needs. Spatial analysis via ArcGIS identified dominant rain-fed areas and zones with water deficits, underscoring the urgency for improved irrigation strategies. The study validates CROPWAT's utility in data-scarce regions and provides practical irrigation schedules suited to local agro-climatic conditions. These insights are valuable for enhancing irrigation efficiency, conserving water, and boosting crop productivity – contributing to informed agricultural planning and sustainable water resource management in Nepal's Terai region.