Presenter/s: Natalie Collar
Symposium Session: 2023 - 08 - Restoration as Risk Reduction: Wildfire
Topics covered: california, colorado, groundwater, idaho, modeling, mountain west, risk and resilience, stream, and wildfire
The impact of wildfire on water availability and stream function are critical issues in the western United States. Because actual evapotranspiration (ETa) constitutes the largest loss in the terrestrial water budget, it has been suggested that fire-induced ETa reductions are primary drivers of the modified runoff response often observed in and downstream of burn scars. To evaluate this, ten gaged watersheds with burns exceeding 5% of their total contributing drainage areas were selected from California, Oregon, Montana, Utah, New Mexico, and Colorado. Discharge data were compiled and gridded 30-meter ETa estimates were generated with the Operational Simplified Surface Energy Balance (SSEBop) model. Fire-induced ETa shifts were quantified with statistical tests that compared pre and post-fire monthly ETa in burned and unburned pixels. Because streamflow data are point measurements that aggregate a large spatial area, additional statistical methods were required to isolate the effect of fire from climate on baseflow and runoff.
This talk will present the key takeaways most relevant to stream restoration practitioners. First, fire-induced streamflow shifts were seasonably variable. This means that annual time series likely mask or dilute the most acute effects of landscape disturbances on streamflow magnitude and timing and as such, restoration engineers should carefully consider the time-step of any post-disturbance hydrologic time series used to inform design. Second, where fire-induced streamflow increased significantly, surplus water from ETa reductions was sufficient to account for increased discharge volumes. However some basins experienced fire-induced ETa reductions and no detectable shift in streamflow, suggesting compensatory uptake pathways may have consumed the excess water before it reached the gage. We show that by understanding where and how compensatory uptake pathways develop (e.g., the position of intact vegetation relative to the disturbed area) or what constrains it (e.g., whether basin ETa flux is energy or water-limited), practitioners can better predict how their watersheds may respond to future hydroclimatic or landscape disturbances. Finally, when recharge/runoff generation zones were observed along elevation gradients, drought and wildfire caused zones to contract and expand in similar manners. This suggests that burn scar hydrology may provide insight into potential future states and better position stream restoration efforts for success.