July 27, 2026
By Abdisa Kebebew, Washington State University
After a wildfire, a watershed may no longer store and release water in the same way. Vegetation and litter that once intercepted rainfall and protected the soil may be severely reduced. Burned vegetation exposes the soil, leaving ash and loose sediment on the surface. During the next storm or snowmelt event, water that once moved slowly through vegetation and soil may instead run off downslope more quickly, carrying sediment toward streams and reservoirs. This is a problem in watersheds that supply municipal water to cities like Seattle and Portland. But how should watershed protection in pre-fire conditions and rehabilitation efforts after a wildfire best be deployed to reduce these impacts?
Runoff does not begin everywhere at the same time. Some parts of a watershed may stay relatively dry, while others become wet enough to repeatedly produce runoff. Hydrologists call this “variable source area hydrology.” The idea is simple: only certain parts of a watershed become active runoff sources during a storm or snowmelt event.
For example, water moving through the soil often collects in lower-slope areas of a watershed. When those soils become saturated, they have little room to store more water. Additional rainfall or snowmelt may then run across the surface and enter the stream. In contrast, an upper slope usually remains dry or disconnected from the channel, so it may contribute less runoff during that event.
This distinction matters because sensitive runoff source areas are often where sediment, ash, nutrients, and other particles are mobilized and delivered downstream. These source areas are not always in the same place, however. Burned areas on the hillslope may contain loose sediment, but that sediment becomes a larger water-quality concern when runoff dislocates and delivers it to a stream. The greatest risk occurs where three conditions overlap: sediment is available, runoff is generated, and a connected pathway carries that material into the drainage network.
My research uses the Water Erosion Prediction Project model, or WEPP, to make these hidden runoff patterns more visible. WEPP is a process-based hydrology model used to simulate runoff and erosion from hillslopes and watersheds. A central part of my work is improving how hillslopes are represented inside the model. Instead of treating an entire hillslope as one uniform area, I divide it into smaller sections from the upper slope to the channel. These sections can represent differences in topography, soil, vegetation cover, and burn severity.
This may sound like a modeling detail, but it changes the kind of information managers can use. A simple model might estimate how much runoff or erosion could leave a burned watershed. That is useful, but it still leaves managers asking where to act. A more spatially detailed model can produce maps that highlight the parts of the watershed most likely to generate runoff and deliver sediment. In practice, this can help managers decide where to place monitoring equipment and where to prioritize erosion-control treatments.
The more detailed approach also helps clarify between two important runoff mechanisms. Infiltration-excess runoff happens when the rainfall rate is greater than the soil’s ability to absorb water. This can occur on bare, water-repellent, or severely burned soils. Saturation-excess runoff happens when soils are already wet and push out additional water (Figure 1). The first mechanism points to actions that protect the surface of the soil and help water infiltrate. The second one points towards actions to restore wet lower slopes, riparian areas, and places where subsurface water reconnects with the surface.
When most information is based only on what comes off an entire watershed or hillslope, managers can become locked into treating the watershed as one large unit. Models that simplify hillslopes can reinforce that same view. The result is that limited post-fire resources may be spread broadly, even though only some areas are in need, locking managers into a less efficient use of resources.
A spatially detailed WEPP approach can help shift that decision-making. Instead of asking only, “How much erosion might occur from this burned watershed?” managers can also ask, “Which parts of the burn scar are most likely to send sediment to the stream during the next storm?” That shift supports faster, more targeted, and more defensible post-fire responses, potentially transforming how effective they can be at limiting the input of sediment into a municipality’s drinking water system.
Figure 1: Example WEPP output shows how runoff frequency and soil saturation can vary across a watershed and between seasons. A simplified figure with winter and summer panels can show the main contrast: wetter near-channel areas are more active in the wet season, while summer conditions may highlight drier slopes and more limited runoff connectivity.