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Application of APEX for Forestry

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Pp. 595-605 in Total Maximum Daily Load (TMDL) Environmental Regulations: Proceedings of the March 11-13, 2002 Conference, (Fort Worth, Texas, USA)  701P0102.(doi:10.13031/2013.7614)
Authors:   Saleh, A., J.R. Williams, J.C. Wood, L. M. Hauck, W.H. Blackburn
Keywords:   Model, Sediment, Flow, Nutrients, Nitrogen, Phosphorous, Forestry, Management

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). In addition, the 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 (BMP) (filter strips). 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. The APEX simulation demonstrated that filter strips decreased the average annual runoff and sediment loss while forest roads along with greater slope increased runoff and sediment loss from forested land.

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