Required accuracy of input data for WRENSS to determine silvicultural treatment effects on annual water yield

The WRENSS handbook contains a hydrologic procedure for estimating increased water yield from silvicultural treatments. The difference in annual evapotranspiration for a forested watershed under harvested and unharvested conditions approximates the annual water yield increase. Annual water yield increases predicted by WRENSS require area precipitation and general forest stand characteristics. However, forest stand characteristics had little effect on predicted water yields. Annual water yield estimates were only as accurate as the basin wide precipaitation data. The WRENSS predicted water yield increases were accurate for the snow dominated region. Potential model upgrades include improved estimates of snow vaporization, evapotranspiration rates, indexed precipitation or orographic functions, and assessment of streamflow generation processes from alpine areas for water yield increases in alpine-subalpine basins.