Over the last five decades, the total acreage under irrigation in the U.S. High Plains increased from less than 3 million acres to over 20 million acres. The rapid development of irrigation in this area, which stretches from Nebraska through western Kansas to the Texas Panhandle, enabled the transformation of the area into one of the major agricultural areas of the United States. Large-scale land use changes of this nature could alter transpiration and evaporation thus generating complex changes in the lower atmosphere (PBL) radiation budget. In particular, changes in water vapor flux into the atmosphere, can modify cloud and precipitation, by increasing the available convective potential energy (CAPE). This study evaluates the changes in the summertime surface energy budget due to irrigation in Nebraska using the Colorado State University Regional Atmospheric Modeling System (RAMS).

Three 15-day simulations were conducted: one using a 1997 satellite-derived estimate of farmland acreage under irrigation in Nebraska (control-run), the second using the OGE vegetation dataset (dry-run), and the third the Kuchler vegetation dataset (natural vegetation-run) as lower boundary conditions in RAMS. In the first simulation, the topsoil in the irrigated locations, up to a depth of 0.2 meters, was saturated at 00Z (GMT) each day for the duration of the experiment (July 1-15, 1997). In both the dry and natural vegetation runs, the soil was allowed to dry out, except when replenished naturally by rainfall. Identical observed meteorology was used for lateral boundary conditions in the three cases. The results presented here are the area-averaged model-derived quantities over a 10-km grid centered over Nebraska July 6-15,1997. This grid was nested inside a larger 40-km grid, which extends over most of the central US.

Over the two-week period of the RAMS integrations, the most significant inner domain area-averaged difference between the control and dry runs was a 36% increase in the surface latent heat flux. At 500 meters above the ground, a 28% increase in water vapor flux and a 2.6 oC elevation in dew point temperature were also observed. Surface sensible heat flux of the control-run was 15% less and near-ground temperature was 1.2 oC less compared to the dry-run. The cumulative effect of the increase in water vapor and dew point temperature, and the decrease in temperature is reflected in an observed increase in the control-run theta-e within the boundary layer. These differences are amplified in the control versus natural vegetation runs. For example, near-ground temperature was 3.3 oC warmer and the surface sensible heat flux was 25% more in the natural vegetation-run compared to the control-run. These results indicate that the structure of the CBL and the partitioning of the available surface energy in this region have been altered by human modifications to the land surface. These land cover conversions have implications for short- to medium range weather prediction, especially the prediction of cumulus convective rainfall in the U.S. High Plains.