Climate Impacts on the Water-Food Nexus

Food demands are rising due to an increasing population with changing food preferences, placing pressure on agricultural production. Additionally, climate extremes can negatively impact crop production, and climate change is expected to affect the frequency and severity of extremes, thus highlighting the vulnerability of the agricultural system to climate variability. To adapt to a changing climate, people have been trying to build a more resilient and reliable water resource management system for food production, and the application of irrigation is vital with the potential to avoid crop yield decline or even bring markedly increased crop yields. However, irrigation also causes the depletion of available water resources, especially for streamflow in those major rivers in growing regions with irrigating equipment. This dissertation investigates first the effect of global climate change, especially change of climate extremes, on agricultural and then focuses on the water-food nexus in the United States, which is a representative large food producer and also a big agricultural water consumer.A derived 1° dataset of growing season climate indices and extremes is compiled over the major growing regions for maize, wheat, soybean, and rice for 1951-2006 in the first study. From analyzing this dataset, it is found that before 1980, temperature-related indices had few trends; after 1980, statistically significant warming trends exist for each crop in the majority of growing regions worldwide. Anomalous temperature and precipitation conditions are shown to often occur concurrently. This leads to the confluence of a variety of climate conditions that negatively impact crop yields. The United States is very representative, as there is an increasing trend for hot days and maximum growing season temperature since 1980, indicating a rapidly warming climate. Additionally, precipitation has intensified temporally and spatially, causing less rainy days and longer dry spells that could be problematic to the water availability during growing seasons.Based on the findings about how climate has changed in the US, the next study seeks to find how irrigation impacts the large-scale response of crops to varying climate conditions and how we can explicitly account for uncertainty in yield response to a changing climate. This work quantifies the impacts of climate change on crop production with or without irrigation. Five CMIP5 climate projections are also applied to simulate future crop yield response to climate. The comparison between projected crop yield time series for rainfed and irrigated cases indicates that irrigation can buffer against climate variability that could lead to negative yield anomalies. Through trend analysis of the predictors, the trend in crop yield is mainly driven by projected trends in temperature-related indices, and county-level trend analysis shows regional differences are negligible. This framework provides estimates of the impact of climate and irrigation on US crop yields for the 21st century that account for the full uncertainty of climate variables and the range of crop response.Crop production in the US has benefitted greatly from irrigation, which also leads to streamflow depletion that could greatly increase water stress. The third study combines irrigation water demand and a streamflow routing model to evaluate how irrigation has been impacting streamflow through the Mississippi River Basin, which covers the majority of the conterminous US and many major irrigated growing regions. In this study, optimal irrigation, defined as the climatological water demand, is simulated and the streamflow depletion is estimated afterwards. It is found that irrigation could lead to up to 20% of depletion for annual mean streamflow over certain regions. For example, the Missouri River Basin could see more than 5% of streamflow depletion all year round. The interannual variability of depletion due to irrigation also highlights the problem of increasing water stress risk, especially during drought years.This dissertation begins at the global scale, looking at climate change, especially climate extremes over major growing regions during the last few decades. Then based on the finding that United States has been one of the hot spot crop producers that are significantly challenged by a changing climate, I estimate the impacts of the climate change on rainfed and irrigated US crop yields, both historically and for the future 90 years. Through this study irrigation is shown to be effective in buffering against climate. However, the tradeoff between water and food, namely irrigation and crop production, still requires rigorous planning to help us benefit most generally. The third study attempts to obtain a big picture of how irrigation has impacted streamflow over the Mississippi River Basin. The results could inform initial policy making through the prioritization of agricultural water use where the hydrologic impacts are minimized and the yield increases due to irrigation are maximized. More importantly, this study enables future work that involves irrigated yield gains, defined as increase in crop yields due to irrigation, and thus provides an improved understanding of how people could better handle the tradeoffs in the water-food nexus.