LINKING FIRE-INDUCED EVAPOTRANSPIRATION SHIFTS TO RECHARGE/RUNOFF AND VEGETATION CONVERSION IN THE WESTERN UNITED STATES

The impact of forest disturbance on water availability is a critical issue in Colorado. Because actual evapotranspiration (ETa) constitutes the largest loss in terrestrial water budgets, 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 the western US. Discharge data were compiled and gridded 30-meter ETa estimates were generated from Landsat imagery in Google Earth Engine. The effect of fire-induced ETa shifts on streamflow magnitude, timing, and groundwater recharge/runoff generation zones were evaluated with various statistical methods and mass balances. First, 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 consumed 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) or what constrains it (e.g., whether basin ETa flux is energy or water-limited), practitioners can better predict how watersheds may respond to future hydrologic disturbances. Second, 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.