Extrapolation of synoptic meteorological data in mountainous terrain and its use for simulating forest evapotranspiration and photosynthesis

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Abstract
A model for calculating daily microclimate conditions in mountainous terrain is presented. Maximum-minimum daily air temperatures, precipitation, and dew ponit are extrapolated from valley stations to adjacent mountain slopes, after making elevation- and aspect-related corrections. The model (MT-CLIM) produces estimates of daily incoming shortwave radiation, air temperature, humidity and vapor pressure deficit, and precipitation for any mountain study site. MT-CLIM was tested against measured meteorological data on six study sites consisting of three north-south slope pairs in western Montana [USA] Correlations between predicted and observed daily conditions on the six study sites for the period April-November 1983 were air temperature, R2 = 0.88-0.92 with standard errors < 0.48.degree.C; relative humidity R2 = 0.63-0.72 with standard errors < 15%; radiation R2 = 0.60-0.78 with standard errors < 100 W m-2. To test the utility of the model, data from MT-CLIM were used to run the DAYSTRANS/PSN model of evapotranspiration (ET) and photosynthesis (PSN) for western coniferous forests. Seasonal ET calculated from MT-CLIM data deviated by less than 5% from ET calculated from measured meteorological conditions on the six sites. Estimates of seasonal PSN agreed to within 10% of estimates derived from measured meteorological data on all sites except one. For each of the north-south slope study pairs, the combined MT-CLIM DAYTRANS/PSN simulation predicted that north slopes would have higher (5-31%) seasonal PSN than the comparable south slopes, which agrees with observed patterns of forest productivity in this semiarid region.