Submitted to: ASAE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 7/9/2000
Publication Date: 7/9/2000
Citation: Interpretive Summary: In 1997, a large number of measurements were made to determine the rate at which water enters the soil, the degree to which the soil is packed, and the amount of sand, silt and clay in the soil. The measurements of these soil characteristics were taken at the experimental watersheds at the USDA- Grazinglands Research Laboratory, El Reno, Oklahoma. Four watersheds are cultivated in winter wheat, and another four watersheds are in native grass prairie. The measurements were used to determine the average value each soil characteristic within a watershed and the range of values that each soil characteristic displayed within a watershed and between watersheds. This information is needed to study changes in the amount of water in the soil throughout the year. The study found that, due to the large range of values of the soil characteristics within a watershed, a relationship between the three soil characteristics could not demonstrated for the watersheds under consideration. Nevertheless, average values of each soil characteristic did show similarities between selected winter wheat watersheds, as well as between selected native grass watersheds. However, the similarities were not strong enough to conclude that the four winter wheat and the four native grass watersheds could be viewed as identical representations of one another. It is speculated that differences in livestock grazing and other management practices between individual watersheds are the reasons for the differences between watersheds.
Technical Abstract: Variability of saturated infiltration, surface bulk density, and surface soil particle size composition between experimental watersheds; spatial variability based on hillslope position; and, temporal variability during the growing season have been quantified from measurements completed in 1997 on the Water Resources and Erosion (WRE) experimental unit source watersheds at the USDA-ARS Grazinglands Research Laboratory at Fort Reno, Oklahoma. The study assessed the field-scale variability of these properties for modeling applications of the water balance in the surface and unsaturated zone. A direct relationship was not found between bulk density, particle size distribution, or saturated infiltration. Native warm season grass fields exhibited average saturated infiltration rates greater than wheat fields. The average dry bulk density of the surface layer soil was lower on the native warm season grass fields than on wheat fields. Hillslope position did not influence infiltration rates or surfac soil bulk density. Soil particle size composition was found to be dependent on hillslope position. Temporal differences of saturated infiltration under wheat were not detected. Overall, caution should be exercised in treating these watersheds as a single unit or replicates. However, observed differences are small and the assumptions of similarity in soil properties between fields may be acceptable for selected hydrologic modeling applications and objectives.