APPLICATION OF APEX FOR FORESTRY

Abstract

This study was conducted to determine if the Agricultural Policy/Environmental Extender (APEX) model couldreasonably replicate the effects of silvicultural practices on streamflow and loading of sediments and nutrients. APEX wasmodified 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 wereused to calibrate and test APEX. The historical data included measured flow, sediment losses, and nutrient (NO3-N, organicN, 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 ofthe 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 thewatersheds to demonstrate its capabilities in simulating an important sediment source (roads) and an effective best managementpractice (streamside management zones, or SMZs).<br><br>The simulated and measured storm runoff, peak flow rates, and average annual sediment and nutrient losses were inreasonable agreement. Simulated storm runoff per mm of rainfall increased six times for SHR and five times for CHPwatersheds during the first post-treatment year as compared to CON watersheds. Consequently, the sediment concentrationincreased about 13 times for SHR and doubled for CHP watersheds. The nutrient loading also increased during the firstpost-treatment year in SHR and CHP watersheds. However, storm runoff and sediment and nutrient losses were reducedduring the second post-treatment year due to rapid vegetation growth. Storm runoff, along with sediment and nutrient lossesfrom 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 ordersof magnitude lower than agricultural losses. Further APEX simulations demonstrated that SMZs decreased the averageannual runoff and sediment loss, while forest roads along with greater slope increased runoff and sediment loss from forestedland.