Impacts of Irrigation on Land-Atmosphere Coupling Strength Under Different Evapotranspiration Characteristics

The Budyko curve displays that the magnitude of evapotranspiration (ET) is limited mainly by the availabilities of energy and water, i.e., under wet conditions, ET is primarily controlled by the available energy, while under dry conditions, ET is primarily controlled by the available water. Land-atmosphere coupling (LAC) strength, which relates to the Budyko curve, is widely discussed because of its contribution towards the improvement in seasonal climate forecasts. For example, the "hot spots" of LAC, where the soil moisture anomalies strongly affect the local precipitation, are found in the transition zones between wet and dry climates. ET of these transition zones is limited by the available water, but at the same time, the surface latent heat flux is large enough to trigger moist convection. Recently, the impacts of irrigation have gained lots of attention, including the change in LAC. Badger and Dirmeyer (2015) analyzed the climate response of Amazon forest replacement by crop with consideration of irrigation in model simulations, discovering negative relationship between added irrigation water and coupling between the soil moisture and the latent heat flux. In addition, Lu et al. (2017) found remarkable decreases of LAC strength with the increase of irrigated cropland percentage in the Great Plains of America. The two studies show that irrigation is possible to affect land-atmosphere coupling strength. However, whether the irrigation process leads to the reduction of coupling strength in other regions of the world remains unclear. This study aims to compare the differences of irrigation impact on land-atmosphere coupling strength between five selected locations undergoing intense irrigation: India, North China Plain, Southwest Europe, Great Plains and Middle East. The spatial divergence of the factor that limits the ET (e.g., either by the available energy or water) will be the focus in this study. Both offline simulation (Community Land Model) and couple simulation (coupled with Community Atmosphere Model) are used to explore the direct change and the subsequent shifts in land-atmosphere interactions. Also, three LAC indices are adopted to quantify the coupling strength between land and atmosphere.