Streamflow Response to Wildfire Differs With Season and Elevation in Adjacent Headwaters of the Lower Colorado River Basin

Abstract

Fires increasingly impact forested watersheds, with uncertain water resources impacts. While research has revealed higher peak flows, longer‐term yields may increase or decrease following fire, and the mechanisms regulating post‐fire streamflow are little explored. Hydrologic response to disturbance is poorly understood in the Lower Colorado River Basin (LCRB), where snowmelt often occurs before the growing season. Here, we quantify annual streamflow changes following what have been, before 2020, two of the largest wildfires in the modern history of the contiguous United States. We evaluate nine nested watersheds with >50 years records within the Salt River Basin to evaluate fire impact over ranges of elevation, climate, vegetation, burned area, and spatial scale. We employ double‐mass comparison of paired watersheds, pre‐ and post‐fire runoff ratio comparison, multiple linear regression of climate and fire, and time‐trend analysis. Precipitation and streamflow are decoupled during dry periods; therefore we conduct separate change detection for wet and dry periods. Post‐fire summer streamflow increased by 24%–38% at all elevations. While winter/spring streamflow remained constant in the highest, coldest headwaters, winter flows declined in lower‐elevation headwaters. As a result, basin annual streamflow declined. These results support emerging understanding that warm semiarid watersheds respond differently to disturbance than well‐studied, colder watersheds. Asynchrony between winter snowmelt and summer evaporative demand is likely important when considering long‐term impacts of forest management and disturbance on water supply in the LCRB.

Subseasonal to seasonal streamflow forecasting in a semiarid watershed

“…During the cool season in particular, there is considerable variability of streamflow, which increases nonlinearly as precipitation increases (Figure 2). In agreement with previous studies (Biederman et al, 2022; Robles et al, 2021), we find that streamflow remains low until cool season precipitation reaches close to ~200 mm in the Salt River and Tonto Creek. The Verde River is somewhat different, as streamflow increases more gradually after 200–300 mm of precipitation has fallen.…”

A review of wildfire impacts on stream temperature and turbidity across scales

“…Because wildfires perturb land surface characteristics (Vega et al, 2020), reducing rainfall interception (Shakesby and Doerr., 2006), creating hydrophobic soils (Certini, 2005), and reducing recharge rate (Ball et al, 2021), fireaffected watersheds exhibit flash flow, increasing peak flows and reducing low flows (Hallema et al, 2017). Thus, without substantial groundwater input, burned watersheds are likely to show an accelerated warming during summer low flow seasons in the western United States (Biederman et al, 2022).…”

“…Because wildfires perturb land surface characteristics (Vega et al, 2020), reducing rainfall interception (Shakesby and Doerr., 2006), creating hydrophobic soils (Certini, 2005), and reducing recharge rate (Ball et al, 2021), fire-affected watersheds exhibit flash flow, increasing peak flows and reducing low flows (Hallema et al, 2017). Thus, without substantial groundwater input, burned watersheds are likely to show an accelerated warming during summer low flow seasons in the western United States (Biederman et al, 2022). Additionally, black carbon deposition on snowpack due to wildfire can decrease snow albedo and accelerate snowmelt, which has potential cooling ability to regulate stream thermal sensitivity if it can sustain until summer (Gleason et al, 2019; Yan et al, 2021).…”

Modeling Post‐Wildfire Hydrologic Response: Review and Future Directions for Applications of Physically Based Distributed Simulation

“…Evapotranspiration shifts after wildfire can also contribute to differences in watershed annual yield (i.e., total flow), especially in the western United States (e.g., Blount et al., 2020; Hallema, Sun, et al., 2018; Saxe et al., 2018). There is, however, substantial variability in wildfire effects on annual yield (A. P. Williams et al., 2022; Biederman et al., 2022), and compensatory uptake by unburned downgradient vegetation can reduce measured wildfire effects on streamflow (Collar et al., 2022).…”