7997 - Laserfiche WebLink

Home Browse Search

occur on the scale of tens or hundreds of square kilometers -- a scale several orders of magnitude finer than current GCM resolution.6 Second, hydrologic parameterizations in GCMs are very simple and often do not provide the detailed information necessary for water-resource planning (WMO, 1987). For example, the GCM soil- moisture budget is typically computed by the so-called "bucket method", in which the field capacity of the soil is assumed to be uniform everywhere (Manabe. 1969a,b). Runoff occurs when the soil moisture exceeds this capacity, and the rate of evaporation is determined as a simple function of the soil moisture and the potential evaporation rate (Manabe and Wetherald, 1985). Efforts are being made to improve GCM hydrology (Dickinson, 1984; IPCC, 1990), including improvements in vegetation parameterizations and the behavior of soils. Until such improvements occur, however, other methods must be used to evaluate hydrologic impacts. Temperature predictions are considered to be the most reliable GCM output relative to precipitation, and other climatic variables (IPCC, 1990). More generally, GCM predictions of changes In temperature, precipitation, and other climatological variables are considered much more reliable than predictions of runoff or soil moisture (IPCC, 1990; WMO, 1987). Consequently, several investigators have emphasized using temperature and precipitation estimates for a doubled-C~ environment as inputs to more detailed regional models (e.g., USEPA, 1990; Lettenmaier and Gan, 1990; Bultot, et al., 1988; Gleick, 1987a,b). Under the guidance of the U.S. Environmental Protection Agency, a set of climate-change scenarios was developed for use in evaluating the impacts of the greenhouse effect on water availability in 6.y-his is not meant to imply that increasing GCM resolution alone will resolve the bulk of the problems with GCMs, which suffer from several other limitations. Nonetheless, the resolution problem is critical for hydrologic analysis, particularly in regions where hydrologic processes are dominated by orography. 8