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TECHNICAL REPORTS

Surface Water Quality

Test of APEX for Nine Forested Watersheds in East Texas

^a Texas Agricultural Experiment Stn., Blackland Research and Extension Center, 720 E. Blackland Rd., Temple, TX 76502 USA
^b Texas Inst. for Applied Environmental Research, Tarleton State Univ., Stephenville, TX 76401 USA
^c Arthur Temple College of Forestry and Agriculture, Stephen F. Austin State Univ., Nacogdoches, TX 75925 USA
^d Texas Agricultural Experiment Stn., Blackland Research and Extension Center, Temple, TX 76502 USA
^e Dep. of Soil Science, Nanjing Agricultural Univ., Nanjing, 210095, P.R. China

^* Corresponding author (swang{at}brc.tamus.edu)

Received for publication March 2, 2006. 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 ± 7% of the corresponding observed values; simulated annual average sediment losses were within ± 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 reasonably simulated herbicide losses, with an EF of 0.73 and R² of 0.74 for imazapyr, and EF of 0.65 and R² 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.

Abbreviations: APEX, Agricultural Policy/Environmental eXtender • EPIC, Environmental Policy Impact Calculator • BMPs, best management practices • SMZ, streamside management zones • PE, percent error • EF, model efficiency or Nash-Sutcliffe efficiency