Click the link to read the article on the USFS website (Sara A. Goeking and David Tarboton):
Forest disturbance is expected to lead to increased streamflow – but in very dry watersheds, the opposite is often true.
Forest disturbance is typically expected to lead to increased runoff – and therefore more water available for aquatic ecosystems and people – because loss of forest vegetation results in less water being taken up and transpired by plants. However, recent studies in the western U.S. have found no change or even decreased streamflow following forest disturbance due to drought and insect epidemics.
We investigated streamflow response to forest cover change using hydrologic, climatic, and forest data for 159 watersheds in the western U.S. during 2000–2019. Forest change and disturbance were quantified in terms of net tree growth (total growth volume minus mortality volume) and mean annual mortality rates, respectively, from the U.S. Forest Service’s Forest Inventory and Analysis database. Annual streamflow was analyzed using multiple methods to understand the contributions of precipitation, temperature, aridity, and tree mortality.
Many watersheds exhibited decreased annual streamflow even as forest cover decreased. This decreased streamflow was not attributable to precipitation and temperature changes in many disturbed watersheds, yet streamflow change was not consistently related to disturbance, suggesting drivers other than disturbance, precipitation, and temperature.
Multiple regression analysis indicated that although change in streamflow is significantly related to tree mortality, the direction of this effect depends on aridity. Specifically, forest disturbances in wet, energy-limited watersheds (i.e., where annual precipitation exceeds potential evapotranspiration [PET]) tended to increase streamflow, while post-disturbance streamflow more frequently decreased in dry, water-limited watersheds (where the PET to precipitation ratio exceeds 2.35).
- While streamflow often increased following forest disturbance, it decreased in some watersheds.
- The direction of streamflow response to forest disturbance (increase vs. decrease) is dependent on aridity.
- Tree mortality during 2000-2019 was highest in arid watersheds – the same watersheds where disturbance tends to result in decreased streamflow.
- Forest disturbances in wet, energy-limited watersheds tended to increase streamflow, while post-disturbance streamflow more frequently decreased in dry, water-limited watersheds.
Disturbance effects on water yield in western coniferous forests
Using Forest Inventory & Analysis data for broad-scale assessments of vegetation effects on water resources
Variable streamflow response to forest disturbance in the western US: A large-sample hydrology approach
Goeking, Sara A. ; Tarboton, David G. , 2022