Water quality as affected by season and prescribed burning, Post Oak Savannah, Texas

Abstract

Variation in nutrient and sediment loss via runoff is responsive to precipitation patterns, site characteristics, and disturbance. Fire is necessary for natural maintenance of most grasslands and savannahs. Prescribed burning is an effective management tool which suppresses invading brush species and controls undesirable forb and grass species. The impact of rangeland prescribed bums on infiltration, erosion, and water yield is well documented on an individual storm event basis, but little information is available on how this affects seasonal patterns of runoff quantity and quality. Three adjacent 0.16 ha catchments with the same slope (4%) and soil were established 3 km west of College Station, Texas, in 1989 on rangeland dominated by little bluestem (Schizachrium scoparium (Michx.) Nash) and brownseed paspalum (Paspalum plicatulum Michx.). These catchments were monitored between December 1990 and November 1996 to determine the seasonal patterns of hydrological response and water quality associated with winter burning in the Post Oak Savannah region of Texas. Seasonal nutrient and sediment loss was generally greatest in winter and decreased through the following fall. Seasonal patterns of runoff and water quality during the four-year period with burning were not significantly different from the patterns established during the no-bum period. The water quality effects of burning were limited to an increased concentration of many insoluble constituents in the first runoff event following the bum. For example, total phosphorus (TP) concentration in first post-bum runoff averaged about 500 jig L-1 compared with 100 [tg L-1 TP concentration typical of winter season runoff. Relatively low standard deviations associated with mean seasonal concentrations of TP and total Kjeldahl nitrogen (TKN) imply that a strong correlation exists between seasonal runoff volume (M3 ) and nutrient loss (kg ha-'). These data indicate that prescribed winter burning for maintenance of pasture in the Post Oak Savannah is an acceptable, sustainable management technique with no threat to maintaining water quality.