Monday, 17 November 2003: 5:00 PM
Runoff and erosion on a semi-arid grassland after a wildfirePoster PDF (327.2 kB)
Land maígers and BAER teams need to be able to assess the effects of wildfires on semi-arid grasslands to be able to calculate the on and offsite risks due to potential increases in runoff and erosion. Currently in southeastern Arizoí, peak runoff and erosion rates following a grassland wildfire are estimated using TR55 (USDA-NRCS, 1972) and Universal Soil Loss Equation (ULSE) (Wischmeier, 1959). However, both these methods have uncertainties in parameter estimation and questions regarding their applicability to semi-arid rangelands. The Ryan Fire burned over 17,000 ha of southwestern semi-arid grassland and oak woodland areas in Southeastern Arizoí in April and May 2002. Rainfall simulator experiments were conducted on two semi-arid grassland Ecological Sites (ES) in June 2002 immediately following the Ryan Fire and one year later to measure and quantify post wildfire peak runoff and erosion rates. Rainfall simulator experiments were conducted at the two ESs, Loamy Upland and Limey Slopes, at two different soil moisture conditions (initial and wet) using a range of rainfall intensities between 50 and 180 mm/h. Runoff and erosion rates were measured for each rainfall intensity. The plot characteristics including surface cover, basal gap intercept and microtopography were also measured at both sites. The measured hydrologic and erosion responses from the two years of simulations on the burned sites are compared with each other and with results from similar unburned ESs. The results from the rainfall simulator experiments immediately following the Ryan fire showed an increase in the runoff ratio (runoff/rainfall) from 5 to 74% and in the sediment yield ratio (sediment yield/runoff/slope) from 399 to 2230% for the Limey Slopes and Loamy Upland ESs, respectively. The first look at the recovery of the burned sites, comparing results from 2002 and 2003, showed a decrease in sediment yield, however, there was an increase in the runoff. These results indicate that there may be a decrease in the productivity of the site or a longer recovery rate than anticipated. This paper presents the first stage in quantifying wildfire effects on hydrology and erosion in semi-arid grasslands in southeastern Arizoí and in developing a dataset that can be used to extend the use of USFS Disturbed WEPP as a post fire evaluation tool for southwestern semi-arid grassland watersheds.