On the Climate-Agriculture-Water Nexus at the Regional Scale

Abstract

Development of large-scale agriculture is one of the most significant anthropogenic global change processes during the 20𝑡h century. In this thesis, connections between climate, agriculture, and water availability are investigated, using examples from Egypt and the Central United States.

Agriculture reflects and impacts water availability at local, regional, and global scales. A bottom-up, reconstruction of agricultural water consumption in Egypt illustrates how this process is driven by socioeconomic trends and constrained by ecological limits. This analysis shows that Egypt is currently withdrawing most of the Nile’s annual flow, 61.5 km3, and in the coming few years will be importing that same volume as virtual water, to satisfy the growing population and economy.

Agricultural land-use change in the Central U.S. is found to be the dominant factor in shaping regional climate during the 20𝑡h century. Agricultural development (expansion, intensification, irrigation) accounts for observed July-August temperature decreases (0.2-0.3 ∘C) from 1920-1949 to 1970-1999 and about 30% of precipitation increases (0.2 to 0.3 mm/day). These agriculturally driven cooling and wetting trends in the historical period have led to a modification of summer (May-August) water availability (represented through Precipitation-Evapotranspiration (P-E)) with increases of about 17 mm from 1915-1944 to 1975-2004, and significant increases in summer relative humidity and rainfall over a highly productive region spanning Iowa, Illinois, Indiana, and Ohio.

In the future, projected climate change will impact the same agricultural systems significantly, requiring adaptation of existing drainage infrastructure to manage projected excess springtime soil moisture, and development of supplemental irrigation systems to manage the drier summers. Temperature increases, consistent with global warming, and associated reductions in summer relative humidity are shown in multi- model ensembles of CMIP5 and CMIP6 to lead to summer drying: a P-E reduction of 12 mm and 36 mm respectively.

Finally, this thesis investigates impacts of agriculture on the climatology of heat waves in the Central U.S. Non-irrigated (irrigated) agriculture has increased June-September average daily maximum wet-bulb temperatures by 0.3 (0.7) ∘C in the historical period. In the future, irrigated agricultural areas will be 0.9 ∘C hotter than without irrigation. These enhancements of heat stress will exacerbate projected impacts of climate change.