Climate-Induced Changes in Crop Water Use and Food Production in South Asia: Evaluating the Impacts during Sensitive Crop Growth Phases of Rice and Wheat

Avoiding water scarcity and ensuring sustainable food production have become pressing challenges under changing climate and rapid economic development. In the Indus, Ganges, and Brahmaputra (IGB) river basins of the Hindu-Kush Himalaya (HKH), understanding the links between climate change and water availability, demand, and supply—particularly during sensitive crop growth phases—is vital for designing effective adaptation strategies. This PhD improves the knowledge base of crop-water resources under climate variability and extremes, focusing on rice and wheat production. The central research question is: How can high-resolution climate impact assessments enhance water resource management and food production in the HKH river basins? The dissertation assesses vulnerabilities in irrigation demand and supply, crop productivity, and the impacts of climate extremes. It also explores large-scale adaptation strategies, especially phase-specific approaches to improve crop-water use efficiency and promote climate-smart practices. Chapter 1 introduces the research background, objectives, and challenges. Chapter 2 presents the study area, methods, and models. A novel crop-phenology-based data extraction approach is applied, using the LPJmL hydrological and vegetation model with downscaled, bias-corrected CMIP5 climate projections under RCP4.5–SSP1 and RCP8.5–SSP3 scenarios. Chapter 3 highlights how climate variability explains a large share of yield variation: 27–72% in wheat and 17–55% in rice due to temperature, and up to 39% in wheat and 75% in rice due to precipitation. This underlines the importance of high-resolution, phase-specific assessments for adaptation planning. Chapter 4 shows that irrigation demand rises during wheat’s vegetative phase (+6 mm) and rice’s reproductive phase (+26 mm), but decreases in other phases, emphasizing the need for dynamic irrigation management. Chapter 5 examines climate extremes. Warm and dry conditions significantly increase irrigation water demand—over 50% of future wheat irrigation changes are due to such extremes during ripening. Yields also decline, up to 36% in wheat’s reproductive phase and more than 20% in rice’s vegetative phase. These findings stress the vulnerability of crop growth phases to extremes and the urgency of adaptation. Chapter 6 explores supplemental irrigation as a strategy to enhance water productivity. Results show that applying only 4% of total irrigation requirements can maintain 56% of harvest, while 23% sustains 84%. Such optimization demonstrates how strategic water allocation can buffer against climate stress and improve resilience. Chapter 7 synthesizes the research under the HI-AWARE project, quantifying how climate variability and socio-economic development affect irrigation water demand, supply, and yields in the IGB basins. It highlights the urgent need for phase-specific adaptation strategies, such as optimized irrigation scheduling, to secure water and food systems. Overall, this PhD provides the first comprehensive analysis of crop-phase-specific water use under climate extremes in the IGB region. The findings underscore that climate-smart, phase-specific strategies are critical to sustaining water and food security. The evidence also supports regional climate policy alignment with SDGs and highlights the need for continued improvement in crop-water modeling for resilience building.