Changes in water infiltration capacities following burning of a ponderosa pine forest floor

Abstract

The practice of burning large acreages of forest lands in the West for the reduction of fuel hazards is currently accepted by certain land management agencies. Although these prescribed burns frequently accomplish their objectives, little attention has been given to the possible effects these have on watershed conditions. In Arizona, where the demand for water is increasing each year, and where large burning programs are in effect, the influences of burning on water infiltration into the soil is of particular importance. Literature on burning and infiltration relationships is inconclusive. Detrimental and beneficial effects to soils and infiltration capacities have been reported. During the summer of 1963, four sites were selected in the ponderosa pine region of east central Arizona. This study area was located five miles east of McNary on the Fort Apache Indian Reservation, where a large prescribed burning program has been underway since 1948. In July, a light burn and a heavy burn treatment, which approximated prescribed burning and wildfire conditions, respectively, were conducted on each of the four sites. Temperatures during treatments were measured with fusion pyrometers. Surface soil temperatures for the light burns did not exceed 200°F. Maximum temperatures at the soil surface for heavy burns ranged from 350°F. to 550°F. An infiltrometer plot, one by four feet in size, was installed in the center of each treatment area (unburned control, light burn, and heavy burn) on each site. A modified North Fork infiltrometer with constant head tank was utilized to conduct infiltration measurements. The twelve infiltrometer plots remained in place for 25 months, through two overwintering periods. Two infiltration runs were conducted on each plot in late summer,. 1963, and three series of runs were made in both the summers of 1964 and 1965. Infiltration curves were plotted for each run from runoff data programmed into a computer and incremental digital plotter. Infiltration capacity values were obtained directly from these curves. Light and heavy burns produced highly significant decreases in infiltration capacities immediately following burning. No statistically significant differences due to burning were detected between the burning treatments and controls during the second and third summers. It was concluded that the burning programs conducted in late fall on the Fort Apache Indian Reservation, when followed by an overwintering period with freezing and thawing conditions, cause no appreciable effect on watershed conditions. Additional results showed that nearly all the 96 infiltration curves plotted exhibited a pronounced dip after five to fifteen minutes of water application. Soil wettability or a resistance to wetting is felt to be an important factor in causing this depression; however, the extent and practical significance of this new phenomenon is uncertain. Increases in soil pH, carbon, and total nitrogen percentages for the surface two inches of soil were detected immediately following light and heavy burning treatments. These increases were still evident two years after treatment but to a lesser extent. A statistically significant increase in the bulk density of the surface one inch of soil, which was found immediately after burning, was not found after one overwintering period. Changes in the physical and chemical properties of the silt loam soils in the study area following burning were considered to be negligible.