Declines in historical yield variability and the contributions of irrigation

Abstract

Model-based studies indicate that climate change will increase agricultural yield variability, with major implications for price volatility and food security. Contrary to this expectation, we examine historical trends in the detrended coefficient of variation and find declines in yield variability in the late 20th and early 21st centuries in the US Midwest and India. Taking advantage of detailed annual yield and irrigation data, we use a fixed-effects model to show that irrigation emerges as a major observable factor driving declining historical county-level trends. We also quantify and visualize the impact of irrigation across both time and space. Findings are consistent when controlling for variability in growing and killing degree days, as well as when permitting for nonlinearities. We also find that increases in irrigation are associated with reduced variability in unirrigated yields within the same county, suggesting that irrigated fields confer benefits to unirrigated fields. We then explore these patterns in India, documenting a widespread increase in irrigation across several crops, and spatial consistency in areas that are irrigated and areas with low coefficient of variation. Like in the US Midwest, we find statistically significant associations between irrigation and yield variability; however, the strength of the relationship varies by crop. Our results underscore the importance of irrigation for stabilizing agricultural productivity. However, they also highlight that yield volatility should be expected to increase if irrigation declines due to groundwater depletion. The combination of increasing future yield variability and declining ability to irrigate poses a major threat to global food security.