Application of APEX for Forestry

Abstract

This study was conducted to determine if the Agricultural Policy/Environmental eXtender (APEX) model could reasonably replicate the effects of silvicultural practices on stream flow and loading of sediments and nutrients. APEX was modified to include factors associated with forestry conditions such as rainfall interception by canopy, litter, subsurface flow, and nutrient movement and 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 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 watersheds (CON). The simulated and measured storm runoff, peak flow, 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, APEX performance was encouraging considering that forestry losses are generally one or two order of magnitudes lower than agricultural losses.