Towards sustainable agricultural land use in Nepal under climate change: The role of efficient irrigation and fertilizer application

Abstract

The increase in global food demand is expected to rise by 35% to 56% from 2010 to 2050, which poses a significant challenge for humanity, particularly in the face of a growing population and climate change. Achieving a balance between meeting, this increased demand for food and feed while ensuring sustainable agricultural systems without encroaching on natural ecosystems and safeguarding scarce natural resources such as water is imperative, particularly in light of projected climate change impacts. A considerable part of the world’s agricultural land is currently not realizing its maximum productivity potential. Nepal is among these regions, where inadequate mineral nutrient use, insufficient management options, and inadequate irrigation water supply significantly contribute to the existing yield gaps. Enhancing food production to achieve food self-sufficiency is the foremost objective in Nepal. Nepal’s Ministry of Agriculture Development had proposed an Agriculture Development Strategy to transform the agricultural sector, which focuses on raising agricultural productivity through efficient fertilizer use, expanding irrigated areas, improving irrigation efficiency, and promoting efficient and sustainable farming practices. In this dissertation, I endeavored to model some of the objectives outlined in the Agriculture development strategy and investigate current and future agriculture potential on a national scale in Nepal. Specifically, the primary goal of this thesis is to analyze the current yield gap of major cereal crops in Nepal, proposing ways to narrow it down, and then evaluate different scenarios of the sustainable utilization of water and mineral nutrients in the future, considering the impacts of climate change on agricultural croplands. To achieve this objective, the study is structured into three main chapters: (1) Model-based yield gap assessment in Nepal’s diverse agricultural landscape, (2) Effect of irrigation canal conveyance efficiency enhancement on crop productivity under climate change in Nepal and, (3) Assessment of the effect of Alternate Wetting and Drying (AWD) and Continuous flooding (CF) in rice production in Terai Nepal. First, I quantified the gap between current and potentially attainable yields in Nepal and determined the additional amount of fertilizer and irrigation required to bridge this gap. I then assessed the country’s potential to achieve self-sufficiency in cereal food production. The findings suggest that Nepal could attain food self-sufficiency if there is consistent and effective management of mineral nutrients and water supply. Results showed considerable average yield gaps in Nepal which are 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). The pathways to narrow down this yield gap are by efficiently using additional irrigation water and fertilizer application. Cereal yields can be improved by 0.4 to 1.3 t/ha (rice), 0.1 to 2.3 t/ha (wheat), 1.6 to 1.9 t/ha (maize), 0.1 to 0.4 t/ha (millet), and 0.1 to 0.3 t/ha (barley) with irrigation water and fertilizer application, respectively. I then assessed the potential impact of expanding irrigated areas in the near future (2023-2050) and far future (2075-2100) periods with three canal conveyance efficiency (30%, 50% and 70%) on the productivity of rice, maize, and wheat under three climate change scenarios (SSP1-2.6, SSP3-7.0, SSP5-8.5), utilizing three bias-adjusted Coupled Model Intercomparison Project based general circulation models (GFDL-ESM4, MPI-ESM1-2-HR, and IPSL-CM6A-LR). Results indicated that increasing canal conveyance efficiency to 30%, coupled with expanded irrigated areas and adjusted fertilization rates, could boost yields up to 3t/ha per hectare across all three crops at the national level in the future. The results also indicated that in future, improving canal conveyance efficiency from 30% to 50% increases yields up to 0.6 t/ha for maize and up to 1.2 t/ha for rice, while further improvement to 70% results in increases of up to 1.2 t/ha for maize and 2.1 t/ha for rice in the Terai region. The benefits of improved canal conveyance efficiency vary by location, with the subtropical Terai region experiencing the most and the mountain regions showing the least. I found that achieving canal conveyance efficiencies of 70% in the Terai region, 50% in the hill region, and 30% in the mountains is sufficient to enhance crop yields. Finally, I found that adopting the climate-friendly water-conserving measure of Alternate Wetting and Drying over the conventional Continuous Flooding method for rice in the Terai region in the near future (2023 – 2050) has resulted in significant improvements across multiple aspects. First, rice yields increased by 0.2 to 1.4 t/ha after implementing alternate wetting and drying practices. Additionally, irrigation amounts were reduced by 350 to 450mm under the Alternate Wetting and Drying method, which suggests improved water use. In addition, there was a considerable decrease in methane emissions by 23 to 27 Gg CH4 yr-1 in the Terai region with the Alternate Wetting and Drying practice, demonstrating this approach's environmental benefits. In summary, this doctoral thesis aims to contribute to the ongoing discourse of feeding a growing national population through domestic production, taking into account food self-sufficiency and food security in Nepal, a low-input use developing country with rugged terrain and multiple agro-climatic zones. The pathways to achieve this food self-sufficiency and food security in a sustainable manner were analyzed. These pathways encompass, first, the closing yield gaps of major cereal crops of Nepal through the efficient utilization of fertilizers and water; second, expansion of irrigated areas and enhancement of canal conveyance efficiency; and third, adoption of climate-smart adaptation measure alternate wetting and drying methods in rice cultivation.