Determination of Hydrologic Parameters for Selected Soils in Arizona and New Mexico Utilizing Rainfall Simulation

Estimation of runoff and sediment yield from small, ungaged watersheds is a difficult hydrologic task. Watershed models are useful in this regard, but require some information which is directly related to the hydrologic processes. Rainfall simulation is an important experimental technique for gathering such information, particularly in the pinyon-juniper forest lands of the Southwest.

This technical report contains the results from a study to determine hydrologic parameters for selected soils in Arizona and New Mexico. A small area (approximately 1 square meter or 10 square feet) spray-type simulator was used to collect water and sediment runoff from four pinyon-juniper forest sites. A total of 108 plot experiments were performed at the four sites. The sites were located near existing permanent runoff plots or at locations where the United States Department of Agriculture (USDA) Forest Service plans to conduct simulation experiments using 32.5 square meter (350 square foot) plots to enhance the data base being collected for the USDA – ARS Water Erosion Prediction Project.

Data analyses indicate that the infiltration characteristics among the sites are quite similar. Differences can be attributed to the amount of organic ground cover on a plot, the antecedent water content of the soil prior to the experiment, and the surface roughness as measured with a point frame. Clay content of the soil had an observable but minor effect on infiltration characteristics. Cover density had a noticeable effect on infiltration rates. Based on visual estimates in the field, plots were designated as having “high” or “low” cover, or were scraped bare of all cover. Some of the scraped plots were covered by window screen during the experiments. Relative to the unprotected scraped plots, the high, low and protected scraped plots exhibited infiltration rates which were 3.3, 2.2, and 1.9 times higher, respectively. Sediment yields per unit area were 0.19, 0.36, and 0.56 as much for the high, low and screened plots, respectively, compared to the unprotected bare plot.

Numerous water chemistry samples were analyzed to determine potential nutrient loadings from forest lands to water bodies. Total phosphorus and organic solids are related to inorganic sediment yield. Total nitrogen also appears to be related to inorganic and organic sediment yields.