The Effects of Great Plains Irrigation on the Surface Energy Balance, Regional Circulation, and Precipitation

Abstract

Irrigation provides a needed source of water in regions of low precipitation. Adding water to a region that would otherwise see little natural precipitation alters the partitioning of surface energy fluxes, the evolution of the planetary boundary layer, and the atmospheric transport of water vapor. The effects of irrigation are investigated in this paper through the employment of the Advanced Research (ARW) Weather Research and Forecasting Model (WRF) using a pair of simulations representing the extremes of an irrigated and non-irrigated U.S. Great Plains region. In common with previous studies, irrigation in the Great Plains alters the radiation budget by increasing latent heat flux and cooling the surface temperatures. These effects increase the net radiation at the surface, channeling that energy into additional latent heat flux, which increases convective available potential energy and provides downstream convective systems with additional energy and moisture. Most noteworthy in this study is the substantial influence of irrigation on the structure of the Great Plains Low-level Jet (GPLLJ). The simulation employing irrigation is characterized by a positive 850-mb geopotential height anomaly, a result interpreted by quasi-geostrophic theory to be a response to low-level irrigation-induced cooling. The modulation of the regional-scale height pattern associated with the GPLLJ results in weaker flow southeast of the 850-mb anomaly and stronger flow to the northwest. Increased latent heat flux in the irrigated simulation is greater than the decrease in regional transport, resulting in a net increase in atmospheric moisture and a nearly 50% increase in July precipitation downstream of irrigated regions without any change to the number of precipitation events.

Observational Evidence that Great Plains Irrigation Has Enhanced Summer Precipitation Intensity and Totals in the Midwestern United States

“…This is especially true during the summer, when the characteristics of the PBL can be important in determining the probability of rainfall occurrence (Findell and Eltahir 2003a,b). The downwind enhancement of irrigation on precipitation has already been demonstrated on local scales (see above) and remotely (e.g., DeAngelis et al 2010;Harding and Snyder 2012b;Lo and Famiglietti 2013;Wei et al 2013;Huber et al 2014), but uncertainty remains in the degree and location of irrigation impacts on more regional scales. Average winds during July and August at the 850-hPa pressure level, which is often used to diagnose moisture transport (Harding and Snyder 2012a;Huber et al 2014), are generally southwesterly over eastern Nebraska, where extensive cropland irrigation occurs.…”

“…In particular, the expected decreases in the LCL over the irrigated areas may be greater than those of the LFC because the LCL is closer to the surface and therefore would likely respond more readily to increased irrigation. The greater decrease of the LCL is evident in the irrigation-induced changes to the atmospheric vertical profile depicted in Huber et al (2014). If the LCL of a parcel is lowered more than its LFC, then the vertical distance over which a saturated air parcel must rise to reach its LFC, attain positive buoyancy, and utilize CAPE would increase.…”

“…This moistening of the PBL over irrigated areas, which has been documented in studies of changes in dewpoint temperature (Mahmood et al 2006(Mahmood et al , 2008 and specific and relative humidity (Brown and DeGaetano 2013;Qian et al 2013), lowers the lifting condensation level (LCL; cloud base) (Betts 2004;Sun et al 2007), allowing rising air parcels to more readily form clouds and potentially reach their level of free convection (LFC). Increased moisture in the PBL also enhances convective available potential energy (CAPE; Pielke and Zeng 1989;Pielke 2001;Qian et al 2013;Huber et al 2014), which represents the conduciveness of the upper-air environment for convective development. However, it is also possible that irrigation-induced radiative changes may reduce the potential for convection over the irrigated areas.…”

“…likelihood of deep, moist convective development and precipitation (Huber et al 2014;Im et al 2014). Additionally, if the height of the LCL is lowered more than the height of the PBL, it is possible that shallow convective clouds would preferentially develop (Qian et al 2013).…”

“…Since enhanced CIN hinders the development of convective precipitation, these shallow, nonprecipitating clouds may block incoming solar radiation, thereby reducing net surface radiation and creating a negative feedback on the initial flux perturbations caused by irrigation (Qian et al 2013). Such mechanisms may account for the results of some studies showing reduced frequency of summer precipitation events over irrigated areas (Harding and Snyder 2012a;Huber et al 2014;Im et al 2014).…”