Test of APEX for Nine Forested Watersheds in East Texas X. Wang,* A. Saleh, M. W. McBroom, J. R. Williams, and L. Yin ABSTRACT Hydrologic/water quality models are increasingly used to explore management and policy alternatives for managing water quality and quantity from intensive silvicultural practices with best management practices (BMPs) in forested watersheds due to the limited number of and cost of conducting watershed monitoring. The Agricultural Policy/ Environmental eXtender (APEX) model was field-tested using 6 yr of data for flow, sediment, nutrient, and herbicide losses collected from nine small (2.58 to 2.74 ha) forested watersheds located in southwest Cherokee County in East Texas. Simulated annual average stream flow for each of the nine watersheds was within 6 7% of the corre- sponding observed values; simulated annual average sediment losses were within 6 8% of measured values for eight out of nine watersheds. Nash-Sutcliffe efficiency (EF) values ranged from 0.68 to 0.94 based on annual stream flow comparison and from 0.60 to 0.99 based on annual sediment comparison. Similar to what was observed, simulated flow, sediment, organic N, and P were significantly increased on clear- cut watersheds compared with the control watersheds. APEX rea- sonably simulated herbicide losses, with an EF of 0.73 and R 2 of 0.74 for imazapyr, and EF of 0.65 and R 2 of 0.68 for hexazinone based on annual values. Overall, the results show that APEX was able to predict the effects of silvicultural practices with BMPs on water quantity and quality and that the model is a useful tool for simulating a variety of responses to forest conditions. T HE availability of clean, contaminant-free water is increasingly crucial with ever increasing demands on finite resources. 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 harvest- ing, mechanical treatments, and fertilization (Moore and Norris, 1974; Yoho, 1980; Binkley et al., 1999; McBroom et al., 2001; Ice et al., 2003). Silvicultural practices have been changed over the last 20 to 30 yr. Contemporary silvicultural practices increasingly involve the use of herbicides for site preparation and weed control, fer- tilization, and soil amelioration methods such as bed- ding and tillage. Moreover, best management practices (BMPs) now include streamside management zones (SMZs) on intermittent streams (Texas Forestry Asso- ciation, 2000). However, field studies conducted specif- ically to examine effects of these combinations of mechanical and chemical treatments with the imple- mentation of contemporary BMPs on water quality are limited. The considerable expense and collection diffi- culties in forestry studies caused by the time duration, natural rainfall variation, substantial land area require- ments, field personnel, and automated sampling equip- ment 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 sil- vicultural practices on water quality than what is feasi- ble through monitoring by itself in forestry studies. The Agricultural Policy/Environmental eXtender (APEX) model (Williams et al., 2000) was developed to evaluate various land management strategies includ- ing sustainability, erosion (water and wind), water sup- ply and quality, soil quality, plant competition, weather, and pests. APEX has been modified to enhance factors associated with forestry conditions such as rainfall inter- ception by canopy, litter, subsurface flow, nutrient move- ment, and routing enrichment ratios as reported in Saleh et al. (2004). Historical data (1980–1985) of measured flow, sediment losses, and nutrient (NO 3 –N, organic N, PO 4 –P, organic P) losses from nine small watersheds in East Texas, with three watersheds for each of the three treatments (without BMPs): (a) control (CON); (b) clear-cut followed by shearing, windrowing, and burn- ing (SHR); and (c) clear-cut followed by roller chop- ping and burning (CHP) were used to test APEX. Saleh et al. (2004) concluded that the modified APEX was able to reasonably simulate water quality and quantity from a variety of forest conditions including mature forest, harvested, site prepared, replanted, and forest regrowth scenarios. The flexibility of APEX has led to its adop- tion within the Conservation Effects Assessment Project (CEAP) for national assessment. The purpose of the national assessment is to estimate the benefits obtained from USDA conservation programs at the national level. At the CEAP survey sample points, there were no mea- sured responses of runoff, sediment, and/or nutrient loss to calibrate the model. Model parameterization has to be based on previous experience or studies near the sample points with closely matched field characteristics, manage- ment, and observed weather. As part of the CEAP mod- eling effort, the objective of this study was to test the APEX model using flow, sediment, nutrient, and her- bicide losses collected from 1999 to 2004 for the same X. Wang, Texas Agricultural Experiment Stn., Blackland Research and Extension Center, 720 E. Blackland Rd., Temple, TX 76502 USA. A. Saleh, Texas Inst. for Applied Environmental Research, Tarleton State Univ., Stephenville, TX 76401 USA. M.W. McBroom, Arthur Temple College of Forestry and Agriculture, Stephen F. Austin State Univ., Nacogdoches, TX 75925 USA. J.R. Williams, Texas Agricultural Experiment Stn., Blackland Research and Extension Center, Temple, TX 76502 USA. L. Yin, Dep. of Soil Science, Nanjing Agricultural Univ., Nanjing, 210095, P.R. China. Received 2 Mar. 2006. *Corre- sponding author (swang@brc.tamus.edu). Published in J. Environ. Qual. 36:983–995 (2007). Technical Reports: Surface Water Quality doi:10.2134/jeq2006.0087 ª ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: APEX, Agricultural Policy/Environmental eXtender; EPIC, Environmental Policy Impact Calculator; BMPs, best manage- ment practices; SMZ, streamside management zones; PE, percent error; EF, model efficiency or Nash-Sutcliffe efficiency. Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved. 983 Published online May 25, 2007