Hydrologic Parameters for Selected Soils in Arizona and New Mexico as Determined by 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 five pinyon-juniper forest and range sites. A total of 104 plot experiments were performed at the five sites. Two sites had been experimental areas during a previous study and revisited during this study to determine year-to-year variability in hydrologic parameters. The other three sites were new to this study. At two of the three new sites, a recently constructed rainfall simulator was used.

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 soil’s percent silt fraction. Based on visual estimates in the field, plots were designated as having “high” or “low cover. At the two sites revisited during this study, there were no statistical differences in hydrologic parameters between the previous and current experiments. Differences which appeared to exist in the data could be attributed to soil moisture and other transient site conditions.

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