APEX MODEL VALIDATION FOR FORESTED WATERSHEDS

摘要

Forested watersheds are generally associated with higher water quality than watersheds with other major land uses (USEPA, 1995). However, the amount of sediment and nutrients leaving forested watersheds may be subject to short-term increases due to certain silvicultural practices such as timber harvesting, mechanical treatments, and fertilization (Yoho, 1980). Silvicultural practices have changed over the last 20-30 years. However, field studies conducted specifically to exam effects of these combinations of mechanical and chemical treatments on water quality are limited. The considerable expense and collection difficulties in forestry studies caused by the time duration, natural rainfall variation, substantial land area requirements, field personnel, and automated sampling equipment requirements often make field studies unfeasible .Therefore, hydrologic/water quality computer models tested with measured data can provide a much more efficient and effective way to evaluate the effects of silvicultural practices on water quality than what is feasible through monitoring by itself. The Agricultural Policy/Environmental extender (APEX) model (Williams et al., 2005) has been modified to enhance factors associated with forestry conditions such as rainfall interception by canopy, litter, subsurface flow, nutrient movement, and routing enrichment ratios in Saleh et al. (2004). The objective of this study is to validate, the modified APEX using flow, sediment, nutrient, and herbicide losses data collected from 1999 to 2004 at nine small watersheds (2.58-2.74 ha) with three replicates of each of the following treatments: a) undisturbed control; b) clear-cut followed by herbicide site preparation, replanting, and herbicide herbaceous release (conventional); and c) clear-cut followed by herbicide site preparation, sub-soil, fertilizer application, replanting, and herbicide herbaceous release (intensive) located in southwest Cherokee County in east Texas.