Streamflow models are an important tool in assessing climate impacts on water resources by simulating the streamflow associated with climate change scenarios. Essential climate forcings are temperature and precipitation, which are required at high spatial resolution (0.125 degrees latitude and longitude). Climate models, however, are run at much coarser resolution (2 degrees) and do not resolve important mesoscale processes controlling precipitation distribution.

In the Pacific Northwest, the surface orography creates dramatically different precipitation zones over the horizontal distance of one or two climate model grid cells. In order to create precipitation fields appropriate to force a streamflow model, additional information must be added to the climate simulation to account for the mesoscale variations, using methods refered to as "downscaling". Various methods have been developed for precipitation, which will be summarized.

The streamflow in mountainous regions, where there is considerable storage in snowpack, is determined both by temperature, which controls melting, and precipitation, which affects the total available water and also directly controls streamflow where there is no snow storage. In order to isolate the effects of precipitation, streamflow simulations are presented for the Yakima River, a tributary of the Columbia on the east side of the Cascade Mountains, using observed temperature and various precipitation data for the period 1958-1993.

The Yakima basin is useful test region since it is affected by surrounding topography and is small enough that the large-scale forcing is reasonably uniform across the region. To evaluate the down-scaling methods, comparisons are made to base calculations using observed precipitation and cyclic climatological mean precipitation.