Rice, wheat, maize, millet, and barley are the five major staple cereal crops in Nepal. However, their yields are low, and imports are needed to meet domestic demand. In this study, we quantify the gap between current and potentially attainable yields in Nepal, estimate how much additional fertilizer and irrigation are required to close the gap, and assess if self-sufficiency can thus be achieved. For this, we first test the ability of the crop model EPIC to reproduce reported yields in 1999–2014 accurately. On average, simulated and reported yields at the national level were in the same range, but at the district level, the error was large, as the resolutions of the available climate and soil input data were not high enough to depict the heterogenic conditions in Nepal adequately. In the main study, we show that average yield gaps in Nepal amount to 3.0 t/ha (wheat), 2.7 t/ha (rice), 2.9 t/ha (maize), 0.4 t/ha (barley), and 0.5 t/ha (millet). With additional irrigation and fertilization, yields can be increased by 0.1/2.3 t/ha (wheat), 0.4/1.3 t/ha (rice), 1.6/1.9 t/ha (maize), 0.1/0.3 t/ha (barley), and 0.1/0.4 t/ha (millet), respectively. The results show that providing reliable and affordable access to fertilizer should be a priority for closing yield gaps in Nepal.

Nepal is expanding its irrigation facilities as an adaptive measure to climate change; however, the current canal conveyance efficiency (CCE) is low with significant water losses. In this study, we assess the potential impact of increasing CCE on the productivity of rice, maize, and wheat under different climate change scenarios (SSP1-2.6, SSP3-7.0, and SSP5-8.5), utilizing three bias-adjusted general circulation models. The study simulates potential yields at ecoregion levels for two periods: near future (2023 to 2050) and end-century (2075 to 2100). Management scenarios include the following: (1) business as usual, (2) CCE at 30%, (3) CCE at 50%, and (4) CCE at 70%. The results indicate that increasing CCE to 30%, coupled with expanded irrigated areas and adjusted fertilization rates, could boost yields by three tons per hectare across all three crops at the national level. Further increasing CCE to 50% could yield additional increases of up to 0.6 t/ha of maize and 1.2 t/ha of rice in the terai region. A CCE of 70% results in further increases of up to 2.1 t/ha of rice and 1.2 t/ha of maize. The benefits of improved CCE vary by location, with the subtropical terai region experiencing the most and the mountain regions showing the least. We conclude that there is potential to increase yields by increasing CCE to 70% in the terai region, 50% in the hill region, and 30% in the mountains. Wheat appears to benefit the least from improved CCE. This work highlights efficient irrigation as a reliable adaptive measure for future climate change in Nepal.

Rice, wheat, and maize are the most important staple food crops in Nepal. Due to the complex topography and climate of the country, and a lack of agricultural inputs, the productivity of the crops has remained low over the last decades, with only moderate increases in recent years. National production cannot meet the national demand for the three crops, and food imports are necessary to close the gap. Climate and demographic change will most likely exacerbate the problem. It is therefore an objective of the Nepalese government to develop strategies to increase the productivity of the crops permanently and sustainably. A first step in this endeavour is to analyse the existing yield gap and how it may be closed, for which we use the biogeophysical crop model EPIC. We divided Nepal into 3430 homogeneous simulation units (based on climate, altitude, soil, and slope class, overlaid by district boundaries) and simulated current management practices on all units for the years 2000-2014. We then compared the resulting yields to crop production data from the Nepalese Ministry of Agricultural Development and calibrated the model until a good fit was achieved. Subsequently, we estimated maximum potential yields by simulating crop growth without nutrient or water stress, and lastly determined the yield gaps by subtracting the yields under current management practices from the maximum potential yields. We found considerable yield gaps for all three crops 2 t/ha for rice, 4 t/ha for wheat, and 4 t/ha for maize. If we compared the yield gaps between current yields and yields simulated without nutrient stress, but under rainfed conditions, the gaps were smaller, indicating that increasing fertilizer application rates should be the first step in closing the yield gap. However, due to the complicated topography of Nepal, yields and yield gaps of the crops vary considerably between regions, and measures to close the gaps will have to be customized to local conditions. This includes expanding the irrigated area in the lowland Terai regions and valleys in hilly areas where precipitation patterns change and temperature increase under climate change. The findings of this study may support policy-makers in their goal to increase grain production and ensure food security in Nepal. Keywords: yield gap, water management, climate change adaptation