Wildfire and the water balance

Abstract:

Burning can dramatically alter surface vegetation, soil properties, and sediment loading to streams. These changes should modify how water is partitioned between the subsurface, surface, and atmosphere, which in turn affects streamflow magnitude. Yet while some studies document changes in streamflow after wildfire, others have found no discernable differences in annual total streamflow. Using a water balance framework, this presentation will explore the factors that determine whether or not wildfire impacts annual streamflow. With examples from fire-affected watersheds across the state of Colorado, we examine fire effects on snow accumulation and persistence and how these effects vary between snow zones; how loss of trees can impact transpiration, soil moisture, and groundwater; and how sediment loading to streams may alter the timing and magnitude of streamflow due to changes in surface-groundwater exchange. We explore whether the impacts on individual water balance components are likely to have additive or compensating effects on streamflow. Additive effects are those that combine to force streamflow either higher or lower, whereas compensating effects are those that have inverse impacts on streamflow. Finally the presentation will conclude with a strategy for linking the spatial scale of wildfire impacts with magnitudes of additive and compensating effects to predict whether or not a given wildfire is likely to modify streamflow.