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This study was conducted to determine if the Agricultural Policy/Environmental Extender (APEX) model could reasonably replicate the effects of silvicultural practices on streamflow and loading of sediments and nutrients. APEX was modified to enhance factors associated with forestry conditions such as rainfall interception by canopy, litter, subsurface flow, nutrient movement, and routing enrichment ratios. Historical data from the Alto watershed forestry project in east Texas were used to calibrate and test APEX. The historical data included measured flow, sediment losses, and nutrient (NO3-N, organic N, total N, PO4-P, organic P, and total P) losses from nine small (2.6 to 2.7 ha) watersheds, with three replicates of each of the following treatments: (1) clearing, shearing, windrowing, and burning (SHR); (2) clearcutting, roller chopping, and burning (CHP); and (3) undisturbed control watersheds (CON). In addition, the modified APEX model was applied to two of the watersheds to demonstrate its capabilities in simulating an important sediment source (roads) and an effective best management practice (streamside management zones, or SMZs).

The simulated and measured storm runoff, peak flow rates, and average annual sediment and nutrient losses were in reasonable agreement. Simulated storm runoff per mm of rainfall increased six times for SHR and five times for CHP watersheds during the first post-treatment year as compared to CON watersheds. Consequently, the sediment concentration increased about 13 times for SHR and doubled for CHP watersheds. The nutrient loading also increased during the first post-treatment year in SHR and CHP watersheds. However, storm runoff and sediment and nutrient losses were reduced during the second post-treatment year due to rapid vegetation growth. Storm runoff, along with sediment and nutrient losses from both SHR and CHP watersheds, approached those of CON watersheds during the fourth and fifth post-season years. In general, the modified APEX performance was reasonable considering that forestry losses are generally one or two orders of magnitude lower than agricultural losses. Further APEX simulations demonstrated that SMZs decreased the average annual runoff and sediment loss, while forest roads along with greater slope increased runoff and sediment loss from forested land.