Variable Streamflow Response to Forest Disturbance in the Western US: A Large‐Sample Hydrology Approach

• Published in: Water resources research 2022-06, Vol.58 (6), p.n/a
• Main Authors: Goeking, Sara A., Tarboton, David G.
• Format: Article
• Language: English
• Subjects: Annual; Annual precipitation; Aquatic ecosystems; Aridity; CAMELS; Climate change; Disturbance; Drought; Evapotranspiration; forest disturbance; Forest ecosystems; forest inventory; Forest management; Forest vegetation; Forests; Hydrology; Insects; Interception; large sample hydrology; Mortality; Multiple regression analysis; Potential evapotranspiration; Precipitation; Precipitation-temperature relationships; Regression analysis; Runoff; Stream discharge; Stream flow; Streamflow; Temperature; Temperature changes; Transpiration; Tree growth; Trend analysis; water yield; Watersheds
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2021WR031575

Forest cover and streamflow are generally expected to vary inversely because reduced forest cover typically leads to less transpiration and interception. 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. from the CAMELS data set for the period 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: Mann‐Kendall trend analysis, time trend analysis to quantify change not attributable to annual precipitation and temperature, and multiple regression to quantify contributions of climate, mortality, and aridity. Many watersheds exhibited decreased annual streamflow even as forest cover decreased. Time trend analysis identified decreased streamflow 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 potential evapotranspiration [PET] is less than annual precipitation) 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). Plain Language Summary 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. We examined streamflow and forest change in 159 watersheds in the western U.S. to test this expectation. We found that not all disturbed watersheds experienced increased streamflow. Very dry watersheds were more likely to produce less runoff following forest disturbance and were also more likely to experience forest disturbance. Key Points Large‐sample analy