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.
Highlights
The impacts of land cover change make it a first-order climate forcing at local and regional scales [1,2]
In order to assess the primary impacts of irrigation in the central United States, we focus on monthly mean differences between irrigation simulation (IRR) and control simulation (CTL) simulations
The results presented here confirm those of previous studies that extreme values of irrigation alter the surface energy balance, boundary-layer budgets of heat and moisture, and precipitation
Summary
The impacts of land cover change make it a first-order climate forcing at local and regional scales [1,2]. The primary impact of irrigation is due to changes in the local radiation budget via providing more surface moisture for latent heat (LE). The local cooling due to the increased LE results in a decreased sensible heat flux (H). Adegoke et al [11] investigated the impact of irrigation using the Regional Atmospheric Modeling System (RAMS) and showed a 36% increase in latent heat and a 15% decrease in sensible heat flux between irrigated and non-irrigated simulation in Nebraska. The increase in LE accompanying irrigation leads directly to a decrease in surface temperatures, a result that has been established observationally [12]. The decrease in sensible heat flux leads to a reduction in the rate of boundary layer growth and entrainment of dry free-tropospheric air, which prevents the high-MSE boundary layer air from becoming diluted [14,15,16]
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