WET Hydrology & Hydrogeology

Cumulative forest change and climate variability are two dominant drivers of hydrological alterations in forested watersheds. Assessing their impacts on streamflow is critical for watershed management and ecosystem protection. Previous studies, however, focused on the cumulative effects (CEs) of forest disturbance on total streamflow, with limited attention given to baseflow and surface runoff. Five-nested watersheds (34.6~5580 km²) with significant forest disturbance, located in the southern interior of British Columbia, Canada were selected. The modified double mass curves were applied to each watershed to separate the relative effects of forest disturbance and climate variability on streamflow components. The analyses showed that cumulative forest disturbance increased streamflow components in all nested watersheds, while climate variability decreased them, despite inconsistent responses among study watersheds. The CEs of forest disturbance on streamflow at various spatial scales of watersheds have been extensively studied. Researchers and land managers are seeking transfer functions for extrapolation of knowledge and data from one scale to another. In spite of substantial progress on scaling properties of hydrological variables, no studies have investigated scaling properties of the CEs of forest disturbance on streamflow. In this study, two methods including product moments and probability weighted moments were used to determine the scaling properties. The results showed that the CEs of forest disturbance on streamflow components obeyed the simple-scaling, which indicates the log-log linear relationship between the CEs and watershed areas. In addition, the coefficient of variation of the CEs of forest disturbance on streamflow was independent of spatial scales. More importantly, the scaling exponents of the CEs of forest disturbance on streamflow components were greater than 1, suggesting that the CEs increased with watershed size. This conclusion is contrasted to the commonly-held perception of scaling exponents being less than 1 as larger watersheds normally are hydrologically resilient to forest change. The counterintuitive finding might be due to increased synchronization effects in snow-dominated systems, large variations in topography, and interactions between forests and climate in large watersheds. This study provided critical information for managing the CEs of forest disturbance on hydrology in the context of future forest disturbance and climate change.