| 13 Jan 2026
Wildfire-induced disruptions to evapotranspiration, runoff, and water balance closure across California's water supply watersheds
Abstract. Wildfire activity has intensified across forested mountain watersheds globally, yet the basin-scale hydrologic consequences of large, high-severity fires remain poorly quantified. Here we integrate four decades of satellite-derived evapotranspiration (ET), precipitation (P), full natural flow (FNF) records, and spatially explicit fire-perimeter data to evaluate how wildfire alters ET, basin outflow, and water-balance closure across major water-supply basins in California. High-severity fires consistently suppressed ET by 100–250 mm in the first postfire year, with recovery strongly modulated by vegetation traits, moisture availability, and disturbance recurrence. Structurally diverse and moisture-rich basins recovered 75 % of prefire ET within 4–5 years, whereas drier, conifer-dominated systems required up to a decade. Although interannual P remained the dominant control on basin outflow, reduced ET partially offset drought-year declines in FNF within heavily burned sub-basins, indicating a localized compensatory effect. Water-balance analysis revealed systematic negative residuals (P − ET − FNF) during years with substantial fire disturbance, demonstrating measurable departures from steady-state closure. Basin-specific diagnostics showed that these deviations arise from both disturbance-driven hydrologic shifts and observational uncertainties, including precipitation underestimation and stream-gauge bias. Proportional and two-parameter adjustments improved closure across most basins, underscoring the need for disturbance-aware calibration in regional water-balance assessments. Collectively, our findings reveal that wildfires act as short-term hydrologic shocks that suppress ET, alter basin outflow patterns, and distort modeled water budgets across fire-prone headwater systems. Incorporating fire history, disturbance intensity, and ET-recovery patterns into hydrologic models and reservoir operations will be essential for improving postfire flow prediction and sustaining long-term water-supply reliability in an increasingly disturbance-affected climate.
This manuscript presents a comprehensive and well-executed analysis of wildfire impacts on evapotranspiration, basin outflow, and water-balance behavior across major California watersheds. The long-term perspective, integration of multiple datasets, and focus on water-supply relevance make this a valuable contribution to the postfire hydrology literature. The results are generally convincing and clearly presented. I have a few comments below that I believe would help strengthen the methodological clarity and interpretation:
The watershed descriptions highlight volcanic terrain, groundwater-fed baseflow, and snowmelt-driven recharge, suggesting substantial subsurface storage capacity in several basins. While FNF integrates both surface and subsurface discharge, the manuscript does not explicitly discuss potential changes in basin water storage (ΔS). Although I am not deeply familiar with California's montane aquifer and soil storage dynamics, postfire changes in infiltration and recharge could plausibly lead to transient storage effects that influence P–ET–FNF residuals. Clarifying whether storage changes are assumed negligible, and over what timescales, would strengthen the interpretation of the water-balance analysis.
Several analytical thresholds appear somewhat arbitrary and would benefit from additional justification or brief sensitivity testing. These include the 75% recovery benchmark, the 3% burned-area threshold used to define high-fire years, and the 500 m buffer for selecting unburned reference areas. Providing a short explanation or supporting references for these choices would improve methodological transparency.
The CECS ET product is derived from internal water-balance calculations and is then used in the basin-scale closure analysis. Because ET, P, and runoff are therefore not fully independent within this framework, it would be helpful for the authors to clarify how assumptions within the CECS product may influence the observed P–ET–FNF residuals. A brief discussion of potential error propagation or circularity would strengthen confidence in the closure diagnostics.
The manuscript primarily attributes closure deviations to precipitation underestimation and stream-gauge bias, which is plausible given the use of gridded 30 m precipitation data. However, this interpretation remains largely inferential. Comparison with one or more independent precipitation products (where available) could help further support this conclusion and strengthen attribution of closure imbalances.
Most fires affected relatively small portions of the basins, which likely limits detectability of hydrologic responses at downstream gauging stations. While this issue is acknowledged, additional discussion of potential scale mismatch between burned areas, ET aggregation, and FNF stations would help clarify the limits of inference, particularly for weaker runoff responses.