The effects of clearcutting, intensive site preparation, herbicide release and fertilization on surface water quantity and quality at the Alto Experimental Watersheds in east Texas

Nine small (2.5 ha) and four large (70-135 ha) watersheds were instrumented in 1999 in order to evaluate the effects of intensive silvicultural practices on water quality and quantity. Two treatments were evaluated: a conventional treatment involving clearcutting with herbicide site preparation, and a more intensive treatment that added subsoiling, fertilization, and a release herbicide application. After about three years of watershed calibration, harvest occurred in spring, 2002, with planting in December, 2002. First-year post-treatment response was measured on the treated watersheds. Small watershed effects were contrasted with the results from a similar water quality study on the same watersheds in 1981, in which two combinations of harvesting and mechanical site preparation without BMPs were evaluated.; Total storm runoff increase ranged from 0.94 to 13.73 cm in 2003 on all six small watersheds following harvest due to the reduction in evapotranspirational demand. Runoff increases were not statistically significant on the large watersheds. Total first-year sediment loss was significantly greater on two of the conventionals (SW2 and SW9) and one of the intensives (SW6). The greatest first-year increase of 540.1 kg/ha observed in 2003 was measured on SW6. This value was still only one-fifth of that observed from shearing and windrowing in 1981 without BMPs. First-year sediment loss was significantly greater on the intensive large watershed following harvest, but not on the conventional large watershed. Fertilization resulted in increased losses of nitrogen and phosphorus, but total annual loss rates were very low and only accounted for a small percentage of the applied nutrient. Herbicide concentrations peaked immediately after application and fell below the detection limit of 1 ppb within 6-9 months of application. Peak concentrations were several orders of magnitude below LD50 values. These data suggest that intensive silvicultural practices with BMPs do not significantly impair water quality.