Kaini, Santosh (2021) Climate change impacts on river water availability for irrigation, crop irrigation water requirements and canal system capacity needs in an irrigation scheme in Nepal. PhD thesis, Victoria University.

Abstract

The impacts of climate change on water resources and agriculture, accompanied by a growing population, have contributed to increasing food and water scarcity. Due to the continuing growth in population and changes in food requirement habits, the demand for agricultural products is increasing continuously. It has been projected that the rise in food demand will increase by 50-100% between 2009 to 2050. The irrigation sector plays a crucial role in the agricultural food production system, utilizing about 70% of the world‘s total annual water consumption. About 16% of the world‘s cropland is irrigated, accounting for about 44% of the world‘s food production. Climate variability influences water availability for agriculture, crop water demand, and crop grain yield, rendering global food security vulnerable to climate change. Research has shown that South Asia will face negative impacts on agriculture due to climate change, and food scarcity will increase if adaptation measures are not considered. In this regard, there is a need to investigate existing irrigation schemes by assessing the impacts of climate change on both the supply and demand sides of irrigation water simultaneously to cope with changes in future water availability and food scarcity. This research aims to holistically investigate the climate change impacts on both the supply and demand sides of irrigation water. The methodology developed in this research investigated climate change impacts on the supply and demand sides of irrigation water in the Sunsari Morang Irrigation Scheme in the Koshi River basin of Nepal. The irrigation command area is 68,000 hectares. With this background, the objective of this research is to assess the climate change impacts on the supply and demand sides of irrigation water. This research is divided into four major components. 1. Selection of global climate models and downscaling of global climate model outputs to assess climate change impacts on daily rainfall and temperature (minimum and maximum) in the river basin and irrigation command area. 2. Future impacts of climate change on river water availability at the main irrigation canal intake. 3. Crop water requirements due to climate change. 4. The irrigation canal system‘s hydraulic capacity requirements for irrigation water supply in the climate change context. Climate change is the main driver in assessing river water availability for irrigation, crop irrigation requirements, and canal system capacity needs for the future. In this study, climate change scenarios Representative Concentration Pathways (RCPs) 4.5 and 8.5 for the short-term (2016–2045), mid-century (2036–2065), and end-of-century (2071–2100) periods were considered. Representative General Circulation Models (GCMs) were selected for the study area under each climate change scenario and study period. Daily precipitation and temperature data based on selected GCMs were downscaled to a higher resolution (10 × 10 km2). The downscaled daily precipitation and temperature data were applied to assess the climate change impacts on water availability in the river, and irrigation water demand in the irrigation command area. The irrigation canal system capacity assessment was based on water availability in the river, and irrigation water demand. The selection of global climate models for a specific geographical location, with high capacities to represent the past and to project the likely future climate, is a crucial step when assessing climate change impacts. An advanced envelope-based selection approach for the selection of a representative global climate model has been used in this research to select a representative climate model for the Koshi River basin. A total of 105 GCM simulations and 78 GCM simulations were taken for RCP4.5 and RCP8.5 scenarios respectively for the initial selection of GCMs. The GCMs selection process involved three steps: (a) initial model selection considering changes in climatic means (mean air temperature and annual precipitation), (b) refined model selection based on projected changes in climatic extremes, and (c) final model selection based on past performance. One GCM/ensemble was selected at each corner of four climate extremes (cold/dry, warm/dry, cold/wet, and warm/wet) for RCP4.5 and RCP8.5 in the short-term (2016-2045), mid-century (2036-2065), and end-of-century (2071-2100) periods. After the selection of representative GCMs/ensembles, quantile mapping was applied for bias correction at a finer resolution of 10 km × 10 km. The Soil and Water Assessment Tool (SWAT) hydrological model was used for hydrological modelling, and was calibrated and validated using observed river flow data measured near the headworks (intake) of the Sunsari Morang Irrigation Scheme in the Koshi River. Impacts of climate change on the flow of the Koshi River were projected for the short-term, mid-century, and end-of-century periods considering climate change scenarios RCP4.5 and RCP8.5 using downscaled daily precipitation and temperature data. The Agricultural Production Systems Simulator (APSIM) crop model was selected for crop modelling, and was calibrated and validated using measured field data which included phenological development, biomass yield, and grain yield for the winter wheat crop in the Sunsari Morang Irrigation Scheme command area over two years. Impacts of climate change on the irrigation water demand, biomass yield, and grain yield were predicted for the short-term, mid-century, and end-of-century periods considering climate change scenarios RCP4.5 and RCP8.5, using downscaled daily precipitation and temperature data. In addition, the irrigation demand (mm/cropping period) required to reach potential wheat grain yields under current climate conditions was compared with observed irrigation practices and crop grain yield. The hydraulic capacity of the main canal networks in the Sunsari Morang Irrigation Scheme, in terms of water losses and flow carrying capacity, were assessed using the Personal Computer Stormwater Management Model (PCSWMM) hydraulic model, which was calibrated and validated using measured canal characteristics, discharge, flow velocity, and water depth data. Information on daily water availability at the headwork of Sunsari Morang Irrigation Scheme in the Koshi River, drawn from hydrological assessments, was used to estimate water intakes into the canal network system. Based on irrigation water availability at the headwork, and the amount of irrigation water required for winter wheat crops, (both present and future), the winter wheat crop area coverage and the water carrying capacity of the main canal were assessed. The key innovation of this research is the development of a comprehensive methodology to assess the climate change impacts on the supply and demand sides of irrigation water. The research has demonstrated its effectiveness through its successful application in the Sunsari Morang Irrigation Scheme in the Koshi River of Nepal. The methodology and outcome of the research could be adapted to similar physical-climatic conditions around the world to holistically assess the climate change impacts on both the supply and demand sides of irrigation water. The findings of this research are beneficial to water practitioners, the agricultural community, policymakers, planners, and researchers in Nepal and internationally. The findings on representative General Circulation Models (GCMs) selection for the Koshi River basin could also be used by research and scientific communities. Findings on climate change impacts on precipitation and temperature, and projected Koshi River flows could be used by the National Planning Commission, Nepal and Water and Energy Commission Secretariat, Nepal, for sectoral and water resources project planning, and in formulating water resources policies and basin plans for the Koshi River basin respectively. Findings on climatic changes and their potential implications could be used by the relevant sectors for the development of adaptation strategies, including the National Planning Commission, Nepal. It could also be used by the Department of Water Resources and Irrigation, Nepal, for planning and management of irrigation projects and the expansion of the irrigation command areas. The findings on projected climate change impacts on water resources, irrigation water demand and hydraulic assessment of the irrigation canal network could be used by the Department of Water Resources and Irrigation, Nepal, to manage irrigation projects in the region and by local farmers to increase crop yield in study area. It is also hoped that the challenge of agricultural production for the growing population in the developing world could be addressed with some insights provided by this research, despite the negative impacts of climate change on the irrigation and water resources sector.

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