This report describes and evaluates WATBAL, a model used to estimate changes in sediment delivery and water yield in response to logging, road activities, and fire on the Clearwater National Forest (CNF) in Idaho. Sources of inaccuracy in WATBAL’s estimates of sediment delivery and water yield are of interest for several reasons.

Roads and activities associated with logging contribute to increases in sediment delivery. Studies of the WATBAL’s accuracy corroborate that, prior to its calibration by the CNF circa 2006, the model generally underestimated measured sediment yield from watersheds on the CNF. WATBAL’s inability to accurately estimate sediment delivery from activities that are near streams, which is important because a significant portion of the road network on the CNF is hydrologically connected or proximate to streams; WATBAL’s failure to factor in surface and mass erosion from logging landings; WATBAL’s inability to estimate erosion and sediment delivery from mining and grazing in watersheds subjected to those land uses.

The sediment delivery component of WATBAL was calibrated by the CNF circa 2006. The CNF’s own assessment of WATBAL’s sediment delivery component indicates that the model likely has an accuracy of plus or minus 30-40% with respect to watershed scale sediment yields. This level of uncertainty in the model indicates that the impacts of activities on sediment delivery can be substantially greater than forecast by WATBAL. WATBAL’s water yield component also has several limitations. Research has shown that the methods used in WATBAL water yield component underestimate changes in water yield in response to logging and roads. The water yield methods used by WATBAL are unable to estimate the effects of roads and logging on peakflows.

This report provides several recommendations to improve WATBAL.