Water quality and forest restoration in the Lake Tahoe basin: impacts of future management options

Abstract

Land managers in the Lake Tahoe basin are considering increasing the use of prescribed fire and forest thinning to restore conditions that will be more resilient to wildfires. However, such restorative treatments also constitute disturbances that could increase sediment and nutrient loads. We examined whether the water-quality impacts from future treatments are likely to be lower compared to the potential impacts from future wildfires under various climate change scenarios. We applied an online interface for the Water Erosion Prediction Project (WEPP) model in combination with a landscape change model (LANDIS-II) to evaluate the effects of different combinations of thinning and prescribed burning on fine sediment (< 2 mm), very fine sediment (< 16 µm), and phosphorus over time. First, we generated results based on historic weather data for soil disturbance conditions, including: an undisturbed baseline, a uniform thinning treatment; a uniform prescribed fire treatment; and uniform low, moderate, and high wildfire burn severity. Residual ground cover declined in that order, and expected loads of sediment and phosphorus increased. We then combined the estimated loads from hillslopes with projected management-disturbance regimes across each decade of the next century. We found that expected sediment and phosphorus loads were lower under the scenario that emphasized thinning, whereas scenarios that increased prescribed burning resulted in loads that were comparable to scenarios that involved less treatment. These results reflect the finding from the WEPP analysis that prescribed burning is expected to reduce ground cover more than is thinning. Our analysis supports efforts to increase fuel reduction treatments to mitigate future wildfires, but it also suggests that preventative treatments may not avoid a long-term decline in water quality as wildfires increase with climate change.

WEPPcloud hydrologic and erosion simulation datasets from 28 watersheds in US Pacific Northwest and calibrating model parameters for undisturbed and disturbed forest management conditions

“…In the Lake Tahoe Basin, we were interested in applying the WEPPcloud interface to all 63 watersheds that flow into the lake and further run the models for disturbed conditions (thinning, prescribed fire, wildfire, simulated fire) [ 1 , 2 ], however, the model calibration was performed only for 17 watersheds with long-term USGS data. Therefore, we manually distributed the calibrating parameters to the remaining watersheds based on the watersheds' similarities, parent material, and proximity ( Fig.…”

“…Part II. Model performance assessment and applications to forest management and wildfires [1] and are also part of an additional study on the impacts of future forest management options on water quality in the Lake Tahoe basin, California/Nevada [2] . Fig.…”

Keeping up with the landscapes: promoting resilience in dynamic social-ecological systems

“…We used the model results to speak directly to the effects on forest dynamics, fire dynamics, and associated forest outcomes (structure, composition, carbon; see Maxwell et al 2022a,b). We then layered on additional statistical and simulation modeling frameworks to evaluate changes and outcomes for wildlife habitat, water, air quality, and economics (see White et al 2022, Slauson et al 2022, Dobre et al 2022. Our modeling did not delve into questions related to the effects of different combinations or sequences of treatments.…”

Evaluating pathways to social and ecological landscape resilience

“…Decadal summaries of LANDIS-II ground disturbance by type were used to derive estimates of sediment and nutrient loading using Watershed Erosion Prediction Project (WEPP) models (Conroy et al 2006) calibrated for the study area (see Dobre et al 2022 in this issue for more details). Water quantity and timing A qualitative measure of increased water yield and delayed runoff to downstream water bodies and meadows.…”

“…This likely reflects the exceptional importance of restoring and maintaining the ecological and hydrological quality of Lake Tahoe, although most efforts to date have focused on urban sources of impact rather than forest lands (Riverson et al 2008). Although increased management activity modestly increased delivery of fine sediment particles and phosphorus to waterways, those increases generally offset loads from wildfires; as a result, the differences across scenarios were not substantial (Dobre et al 2022). This finding is consistent with previous work suggesting that forest treatments mitigate wildfire impacts to water quality (Buckley et al 2014).…”