Modeling study of irrigation effects on land surface fluxes and water recycling in the Southern Great Plains

Description

Remarkable changes in cultivation of farmland together with insufficient precipitation in the warm season have required the extensive irrigation to sustain crop growth in the Southern Great Plains (SGP). In this study, we incorporated an irrigation scheme into the NOAH land surface model utilized as part of WRF model, which is informed by satellite-measured potential irrigation area data. Daily irrigation is triggered in early morning when root-zone soil moisture availability is below a specific threshold over croplands or pastures during the growing season. We conducted four sets of regional simulations using the WRF model, with or without irrigation included, over the SGP for a typical dry and wet summer, respectively. The results show that irrigation reduces model bias in simulating the soil moisture, surface air temperature, latent heat (LH), and sensible heat (SH) fluxes, suggesting it is critical to include the irrigation process in modeling the land surface fluxes and land-air interaction.

Irrigation brings additional water to the surface, leading to the increase of soil moisture. As a result, the evapotranspiration and corresponding LH increased at the land surface. The surface air cools because of the decreased SH, compensating with the increased LH. The experiment results in this study show the irrigation caused LH (SH) increase (decrease) by 5-15 W/m2, cooling the surface by 0.3–0.5°C and increasing the surface air specific humidity by 0.3–0.6 g/kg, averaged over the SGP domain during a drier warming season (2006). Those changes mainly occurred during daytime.

We found that the irrigation-induced increase in evapotranspiration and decrease in surface temperature led to two competing processes affecting the evolution of convective clouds and precipitation. The increase in evapotranspiration resulted in the formation of unstable conditions associated with increased water vapor that could enhance convective cloud formation, and therefore increase the chance of larger amounts of precipitation. Meanwhile, the surface cooling reduced the convective available potential energy and created a more stable atmosphere acting to suppress convective cloud formation and decrease the chance of precipitation. As a result of these two opposite mechanisms, the spatial pattern of simulated precipitation change induced by irrigation is very inhomogeneous, and the averaged total precipitation over the SGP domain only slightly increased during irrigation season. The irrigation intensity and corresponding impact on the surface fluxes and precipitation is much smaller in a wetter year (2007). The model results also indicate the irrigation-caused soil memory from the previous period results in wetter and cooler surface air lasting for more than one month, suggesting the impact of irrigation on regional climate and water recycling could be at scales from intraseasonal to seasonal.