Impact of incremental surface soil depths on infiltration rates, potential sediment losses, and chemical water quality.

A study was conducted between October 1974 and August 1976 to measure the effects of incremented surface soil depths on infiltration rates, potential sediment production, and chemical quality of runoff water. The treatments were incremental removals of 7.6-cm soil layers to a depth of 30.5 cm on two pinyon-juniper sites in Utah. Hydrologic parameters were measured at each 7.6~cm incremental soil depth using a Rocky Mountain infiltrometer. With one exception, no significant differences occurred in infiltration rates among treatment depths during either 1975 or 1976 at either the Blanding (southeastern Utah) or Milford (southwestern Utah) site. A significant change in infiltration capacities was noted between the 1975 and 1976 field seasons when data from both treatment depths and study sites were pooled. There were no significant differences in potential sediment production between sites or among treatment depths at a site. In terms of chemical water quality, a significant change in phosphorus content of runoff waters was observed at the Blanding site between the 1975 and 1976 field seasons. Significant differences in potassium concentrations were found between sites and among soil depths. Nitrate concentrations were very low in runoff waters from all soil depths at both sites. With continued intrusions by man on wildlands through increasing use of offroad vehicles, grazing of domestic livetack, other agriculture, strip mining, especially in the semiarid rangelands, a more comprehensive understanding of the cause and effects of soil erosion is essential. Many published reports have dealt with soil erosion by water (including the effects of vegetal cover, soil textures, slope, aspect, etc. on erosion) and on infiltration-erosion relationships. Little or no information, however, has been published on how incremental soil losses (or perhaps soil profile reconstruction in mining reclamation work) affect plant production and related processes and hydrologic parameters such as infiltration rates, sediment production, and the chemical water qualities of runoff water. The objective of this study was to determine if infiltration rates, sediment losses, and chemical qualities of surface runoff waters would be affected by various incremental surface soil depths at two pinyon-juniper sites (Pinus spp.Juniperus spp.) in Utah. Authors are graduate research assistant and professor, Rangeland Hydrology, Watershed Science Unit, College of Natural Resources, Utah State University, Logan, 84322. Mr Lyons is currently hydrologist with the Bureau of Land Management, Montrose,‘Colorado. This study was supported in part by the Utah Agricultural Experiment Station (Proj. 749) and in part by the Office of Water Research and Technology, Dept. of Interior (Grant A-022-Utah), in cooperation with the Bureau of Land Management, Journal Paper 2378, Utah Agricultural Experiment Station, Logan. 84322. Manuscript received November 16, 1978. 186 Methods and Procedures Site Description Two Utah sites were selected for this study, one in the southeast near Blanding, and one in the southwest near Milford. Blanding Site The Blanding site is about 70 km west of Blanding, Utah, in the N.E. ‘/4 of T38S, R18E, near Coyote Flat. Elevation is about 1,981 meters, with a 30-year (193 l1950) average annual precipitation of about 28 cm per year, of which about 13 cm falls between May and September. The soils are derived from sandstone-siltstone and are of the Aridic Argiustolls-Typic Argiustrolls association. The texture is sandy loam and the color is red (Gifford 1973). The pH ranges from 7.7 to 8. I with a total nitrogen percent of 0.13 to 0.2 1. The area was double-chained in the fall of 1967 and broadcast seeded to crested wheatgrass (Agropyron cristatum)at a rate of 9.1 kg per hectare. Grazing has been excluded. Mlford Site The Milford site is about 72.4 km west of Minersville, Utah, in the S. ‘/z of Section 7, T30S, R 15W. Elevation is 2,100 meters. The annual precipitation averages about 36 cm, with 15 cm falling from May to September. The soils are from the Xerollic CalciorthidsXerollic Paleorthids assocation and have a silt loam texture (Gifford and Tew 1969). The pH ranges from 7.2 to 7.4, and organic carbon ranges from 0.7% to I . 7%. This site was double-chained in the fall of 1967 and broadcast seeded to crested wheatgrass. Grazing has been excluded. Sampling Procedure The dependent variables of interest in this study were: (1) infiltration rates, (2) potential sediment losses, and (3) phosphorus, potassium, and nitrate concentrations of runoff waters. These parameters are all highly related, and through the use of a rainfall simulator (or infiltrometer), data for all three may be obtained simultaneously. We used a Rocky Mountain infiltrometer (Dortignac 1951). This infiltrometer is equipped with a sprinkler head that has three Type F nozzles, a 0.25-m* (76.2 cm X 30.5 cm) plot frame that is driven approximately 2.5 cm to 7.0 cm into the ground, a wind screen that shelters the water drops, collection buckets and hoses for recovery of runoff water with sediment, and a water supply tank mounted upon a trailer. All plots were pre-wet approximately 16 to 20 hours prior to the application of simulated rainfall to eliminate confounding effects of any antecedent moisture. Rainfall was simulated at an average rate of 8.2 cm per hour for 28 minutes. Runoff water was collected and measured 3 minutes after rainfall began and thereafter at 5minute intervals to determine infiltration rates. Integrated subsamples of runoff waters were analyzed for phosphorus, potassium, and nitrate contents. An additional 0.95 liters of each integrated subsample was used to determine total sediment production (kilograms of oven-dry sediment per hectare). Thirty large soil pits were dug at each of the two sites during the JOURNAL OF RANGE MANAGEMENT 33(3), May 1980 early summer of 1975. Each pit contained three random 7.6-cm 7 incremental soil sampling depths (randomized from the 0, 7.6, 15.2, 22.9, and 30.5-cm soil depths). Fifteen soil pits allowed, therefore, nine replications of each treatment. Fifteen soil pits were utilized the first summer (1975) for infiltration, sediment 6 production, and chemical water quality measurements. The remaining nine replications ( 15 soil pits) of each treatment remained exposed to the environment, but untreated for one year, and were then utilized during the summer of 1976. Z f5 Results and Discussion E 3 Infiltration : Examination of the analysis of variance for infiltration 6 4 rates revealed a site by treatment-depth interaction and a 2 rainfall duration by year by site interaction. Figure 1 shows the relationship between the Blanding and Milford sites and $ between depths at a site. A significant difference existed 6 3 between sites for all depths except at 30.5 cm. The high 5 infiltration capacities at the Blanding site are attributed E primarily to soil texture, the Blanding site being a sandy I loam while the Milford site is silt loam in texture. _