Potential Impacts of Cropping and Management Interventions on Resilient and Sustainable Irrigation Development in Western Nepal

Abstract

Insufficient irrigation with existing water resources constitutes a major reason for low agricultural productivity levels in Nepal. Since large irrigation projects are costly and often prove difficult to manage, small and farmer-led irrigation practices are critical for improving agricultural productivity and profitability. However, the potential impact and sustainability of irrigation expansion and intensification scenarios remains poorly understood. We address this knowledge gap, through a participatory modelling exercise that uses the Soil & Water Assessment Tool (SWAT) model to assess how stakeholder defined scenarios affect the agricultural and water systems with special focus on groundwater us and sustainability. The scenarios were developed based on stakeholder engagement meetings and include closing yield gaps in rice-wheat system, cultivating vegetables between kharif and spring rice, replacing lentil and fallow by irrigated maize, triple cropping instead of double cropping system, and replacing rainfed rabi crops by horticultural crops. Groundwater irrigation was sustainable if the surface water irrigation was applied to the monsoon crops and dry season crop was provided with groundwater irrigation. Excessive extraction of groundwater adversely affects streamflow leaving downstream users with insufficient water, mostly during periods of low flow. Groundwater sustainability can be achieved through implementation of sustainable land management practices such as composting/mulching, conservation tillage, plantation of cover crops to reduce evaporation and promote groundwater recharge. Such practices can enhance the groundwater recharge by 23-36% and reduce risks of groundwater overexploitation over time. Moreover, crop yield and production of crops grown during winter and spring season such as winter-wheat and winter-maize were low even after sufficient irrigation and fertilizer were provided due to short growing period and temperature stress. Hence, the cultivation of such crops in dry seasons by expanding costly irrigation facilities require farmers to time their planting and harvest activities much better through ensured input availability and adequate support. Rather crops like mungbean, lentils, and vegetable which have short growing period can be suitable as winter crops as they also have higher economic value and environmental benefits.

Keywords: SWAT, streamflow, Groundwater recharge, irrigation, Water resources sustainability, Crop productivity