Hemlock Project

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Due to decades of active fire management in the Sierra Nevada range, forest densities are much higher than previous “unmanaged” values which has resulted in many watersheds having very high fuel loads. High fuel loads, when combined with a warmer and drier climate, often results in an increased frequency of catastrophic wildfires. Strategies are being developed to restore forests to sustainable levels and adopt science-based strategies to reduce forest densities through selective thinning. Forest thinning also effects watershed function by 1) reducing water loss from evapotranspiration, 2) influences the timing of snowmelt and runoff, and 3) increases snow accumulation on the forest floor. Forest thinning also reduces evapotranspiration that results in a higher fraction of precipitation leaving the forest as runoff rather than as evaporation to the atmosphere. Forest thinning can also influence the timing of snowmelt and runoff. An open tree canopy could allow more snow to reach the ground rather than be held in the canopy; and strategic spacing of forest openings would limit early season sunlight reaching the forest floor and retard snowmelt. Thus increasing snow accumulation and extend snow storage (i.e. delay snowmelt). The purpose of the Hemlock Project is to quantitatively evaluate the effects of different forest-stand structures on wildfire resiliency and water yield which is quantified via implementation of catchment measurements in the snow-rain transition zone. In addition, the study would provide quantitative assessments of the water-cycle impacts of forest vegetation density, structure, disturbance and management actions that could be scaled across the Sierra Nevada (and other forests). The study would also collect data and develop quantitative tools that could assess vegetation densities which optimize hydrologic benefits and wildfire resiliency. These tools would be used to guide forest management, particularly in important watersheds serving the Central Valley Project and the State Water Project. Preliminary estimates of runoff increases if forest density is optimized in Sierra watersheds is approximately1 million acre feet of additional runoff in average water years. The objective of this project is to quantitatively evaluate the effects of differences in stand structure on wildfire resilience, water yield and the water cycle in the snow-rain transition zone by a program of field measurements, integration of data using hydrologic modeling, and assessment. The project is currently in the permitting and hydrologic data acquisition-planning phase. University of California, Merced is scheduled to install two complete weather stations, three soil moisture, metric potential, and snow monitoring clusters, and four water-stage recorders this summer/fall.

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