Irrigation, while being an important anthropogenic factor affecting the local to regional water cycle, is not typically represented in regional climate models. An irrigation scheme is incorporated into the Noah land surface scheme of the Weather Research and Forecasting (WRF) Model that has a calibrated convective parameterization and a tracer package is used to tag and track water vapor. To assess the impact of irrigation over the California Central Valley (CCV) on the regional climate of the U.S. Southwest, simulations are run (for three dry and three wet years) both with and without the irrigation scheme. Incorporation of the irrigation scheme resulted in simulated surface air temperature and humidity that were closer to observations, decreased depth of the planetary boundary layer over the CCV, and increased convective available potential energy. The result was an overall increase in precipitation over the Sierra Nevada range and the Colorado River basin during the summer. Water vapor rising from the irrigated region mainly moved northeastward and contributed to precipitation in Nevada and Idaho. Specifically, the results indicate increased precipitation on the windward side of the Sierra Nevada and over the Colorado River basin. The former is possibly linked to a sea-breeze-type circulation near the CCV, while the latter is likely associated with a wave pattern related to latent heat release over the moisture transport belt.

Land–atmosphere interactions are a critical component of precipitation processes within the Amazon basin and La Plata River basin (LPRB) in South America. Two of the possible pathways through which the land surface can affect precipitation are 1) by changing the amount of moisture available for precipitation (moisture recycling) and 2) by changing the atmospheric thermal structure and consequently affecting circulation patterns. In this study, the Weather Research and Forecasting (WRF) Model with embedded water vapor tracers (WVT) is used to disentangle these relative contributions, with a particular focus on the precipitation of LPRB. Using WRF-WVT we track the moisture that originates from the Amazon basin over a 10-yr period. It is estimated that Amazon evapotranspiration (ET) contributes to around 30% of the total precipitation over the Amazon and around 16% over the LPRB. Focusing on large-scale circulation patterns that transport moisture into the LPRB, we show that land surface conditions in northwestern Argentina are critical for the meridional transport of moisture to higher latitudes via Chaco jet events (CJEs). Warm surface air temperature associated with dry soil moisture over northwestern Argentina is linked to enhanced CJE northerly low-level winds that intensify moisture transport by changing continental-scale circulation patterns. WRF sensitivity tests confirm that soil moisture variations over this region affect meridional moisture transport.

A vast number of weather forecast and climate models have a common warm-and-dry bias, accompanied by the underestimation of evapotranspiration and overestimation of surface net radiation, over the central United States during boreal summer. Various theories have been proposed to explain these biases, but no studies have linked the biases with the missing representation of human perturbations, such as irrigation. Here we argue that neglecting the impact of irrigation contributes to the longstanding warm surface temperature and lack of precipitation biases over this region. By using convection-permitting multi-season simulations over the contiguous United States coupled with an operational-like irrigation scheme, we show that irrigation increases surface evapotranspiration and decreases surface temperature by increasing evaporative fraction. By increasing the frequency of mesoscale convective systems, irrigation reduces the summertime model precipitation deficit and improves the simulated precipitation diurnal cycle over the Great Plains. The increased precipitation also alleviates the warm bias in our simulation setup, likely by damping the positive feedback between soil moisture and temperature.

An irrigation scheme is implemented in the Weather Research and Forecasting (WRF) model to investigate irrigation impacts over the Continental U.S. (CONUS). Four major irrigated regions and two downwind regions were chosen to understand irrigation impacts over different climate regimes with a focus on irrigation‐induced changes on the water and energy cycles. The Dynamic Recycling Model (DRM) is employed to quantify precipitation induced by irrigation and the precipitation recycling ratios over each irrigated region. With the irrigation scheme, WRF improves the simulated precipitation, surface skin temperature, and energy fluxes compared to reference datasets. For the energy cycle, irrigation increases latent heat flux over the irrigated regions along with reduced sensible heat flux. The evaporative cooling effect induced by irrigation leads to a cooler surface and less outgoing longwave radiation at the surface. Irrigation also intensifies the hydrological cycle over the irrigated regions, reflected by the increased precipitation, evapotranspiration, recycling ratio, and moisture export. Downwind regions exhibit increased precipitation and evaporation, decreased moisture flux divergence, and less consistent variations in recycling ratio. The precipitation increases over the irrigated regions can be partly explained by the more unstable low‐level conditions, while reduced net moisture export is coincident with the precipitation increases over the downwind regions.