| Popis: |
Ground-water recharge and consumptive irrigation requirements (CIR) in the Great Plains and adjacent areas largely depend on an environment extrinsic to the ground-water system. This extrinsic environment, which includes climate, soils, and vegetation, determines the water demands of evapotranspiration, the availability of soil water to meet these demands, and the quantity of soil water remaining after these demands are met for potential ground-water recharge. The geographic extent of the Great Plains contributes to large regional differences among all elements composing the extrinsic environment, particularly the climatic factors. Mean annual potential evapotranspiration ranges from about 26 inches in northeastern North Dakota to about 68 inches in eastern New Mexico. Mean annual precipitation ranges from about 10 inches in north-central Montana to about 48 inches in central Arkansas. Cool-season precipitation, the seasonal precipitation most critical to recharge, is even more regionally variable and ranges from about 2 inches in northeastern Montana to about 23 inches in Arkansas. Variability of soils and vegetation tends to modify local soil-water balances. A soil-water simulation program, SWASP, which synthesizes selected climatic, soil, and vegetation factors, was used to simulate regional soil-water conditions during 1951-80. The output from SWASP consists of several soil-water characteristics, including surface runoff, infiltration (effective precipitation), consumptive water requirements, actual evapotranspiration, potential recharge or deep percolation under various conditions, consumptive irrigation requirements, and net fluxes from the ground-water system under irrigated conditions (NFI). Simulation results indicate that regional patterns of potential recharge, CIR, and NFI are largely determined by evapotranspiration and precipitation. The local effects of soils and vegetation on potential recharge, however, cause potential recharge to vary by more than 50 percent in some areas having similar climatic conditions. The following are some of the more significant simulation results: (1) infiltration in the study area is typically 90 to 95 percent of the mean annual precipitation, (2) potential recharge under nonirrigated conditions (DPD) ranges from about 0.25 inch in the western parts of the Great Plains to about 10 inches in parts of northeastern Texas and Arkansas, and (3) DPD as a percentage of mean annual precipitation ranges from about 1 percent in the drier western parts to about 25 percent in parts of eastern North Dakota, northeastern Texas, and Arkansas. CIR for high-water-demand row crops (including corn) in intensively irrigated areas ranges from about 10 inches in northeastern Nebraska to 19 inches in parts of southwestern Kansas. Net losses from the ground-water system (NFI) in irrigated areas range from about 2 inches in northeastern Nebraska to more than 21 inches in northeastern New Mexico and southeastern Colorado. |
