A Model-Based Evaluation of the Effects of Irrigation Expansion on Regional and Global Land Surface Climate

Abstract

Irrigation is essential for enhancing agricultural productivity and satisfying global food demand. The challenge of feeding a growing population without increasing cultivated land can be addressed partly by expanding irrigation to currently rain-fed croplands. At the same time, water-scarce regions with currently unsustainable irrigation practices may experience a reduction in irrigated area. While the evaporative cooling effect of irrigation on regional climate has been extensively studied, it is still unclear how different irrigation expansion (or contraction) scenarios would affect global and regional near-surface climatic conditions. Here we evaluate the hydro-climatic impacts of several irrigation expansion scenarios. To assess changes in evapotranspiration $\left(ET\right)$ and land surface temperature $\left(LST\right)$ under various irrigation scenarios, we incorporated and validated an irrigation model representing four main irrigation methods: sprinkler, drip, furrow, and flood-paddy, into the NASA Catchment land surface model. Globally, the increase in $ET$ from irrigation and the associated impact on average yearly $LST$ are negligible, with an average global cooling of $-0.03°C$ in the case of current irrigation (with respect to a baseline without irrigation). Compared to the current irrigation scenario, the maximum irrigation expansion scenario can lead to an additional $LST$ cooling of $-0.01°C$, while the net irrigation contraction in a sustainable irrigation scenario is associated with a $0.02°C$ warming. While global and regional impacts of irrigation on $LST$ are relatively modest, locally, the cooling effect can be substantial, with a maximum yearly average $LST$ cooling as strong as $-5°C$ and monthly cooling peaks of up to $-10.2°C$ in heavily irrigated periods.