An integrated approach to modeling the impact of timber harvest on streamflow: A GIS-based hydrologic model

An integrated modeling approach to assess the impact of timber harvest on peak, mean daily flow and timing of streamflow was developed using GIS (ARC/INFO), C programming and Utools visualization. The model embraces the newly evolving focus on landscape level management. The integrated approach was used to test hypotheses that timber harvesting patterns impact peak, volume, and timing of streamflow. The model dealt with three watershed levels, two modeling approaches and four timber harvest scenarios. The model was calibrated and validated using two separate sets of precipitation events using ASCE recommendation.; This study in Hoh watershed revealed that timber harvesting significantly impacts peak and mean daily flow of streamflow at watershed, sub-basin and basin level in descending order. The difference in results of the impact of timber harvesting on streamflow is also attributable to difference in methodology. If about 27% of the basin was harvested (extracting 46.9 million m{dollar}sp3{dollar} of timber) the impact on peak would increase by 44 to 51% at basin, 34 to 41% at sub-basin and 55 to 141% at watershed level in medium and high hydrologic conditions. By reducing the timber harvest by 26%, the corresponding reductions in peak at the basin level, sub-basin level and watershed level were 9 to 10%, 7 to 9% and 16 to 18% respectively. Regarding mean daily flow, harvesting 27% of the basin would result in increase in mean daily flow by 30 to 47% at the basin level, 22 to 36% at the at the sub-basin level and 77 to 137% at the watershed level in medium and high hydrologic conditions. When the timber harvest was 34.6 million m{dollar}sp3{dollar} the corresponding reduction in impact on mean daily flow at basin, sub-basin and watershed level would be 7 to 10%, 5 to 8% and 14 to 20% respectively. Reforestation reduced impact on peak and mean daily flow in medium and high hydrologic conditions. The impact on timing to peak was more governed by modeling approach and hydrologic condition than timber harvest options. UTOOLS visualization was useful in identifying additional harvest criteria and communicating model results.