Investigating streamflow response to forest changes in the Western United States using a modelling approach

Mountain forest catchments supply most of the water in the Western United States (US). These catchments are experiencing forest changes that can result in alteration of vegetation cover and soil characteristics, causing changes in streamflow. This paper describes research to understand streamflow response to forest changes in the Western US using the HBV (Hydrologiska Byråns Vattenbalansavdelning) hydrological model. The study was conducted using data from 100+ CAMELS (Catchment Attributes and Meteorology for Large Sample Studies) watersheds from 1990 to 2019. The HBV model was applied to analyze streamflow changes during two distinct periods: the control period (October 1990 to September 2009) and the assessment period (October 2009 to September 2019). This analysis thus focuses on large scale decadal changes. Changes in streamflow were analyzed using (1) Reconstruction of assessment period streamflow based on control period calibration, (2) Comparison of behavioral model parameter sets between control and assessment periods and (3) Comparison of simulations using control period and assessment period parameter sets. Differences in model simulations were related to forest change data, as reported by the US National Forest Inventory dataset, for 2010-2019. Results indicated that several watersheds experienced an increase in streamflow during the assessment period compared to the control period. Conversely, some watersheds showed a decrease in streamflow during the same period. Associations between changes in streamflow with determinative factors such as aridity, disturbance severity, and process indicators inferred from model parameters, were investigated.