Mathematical Models for Estimating Evapotranspiration of a Hill Evergreen Forest at Kog Ma Watershed, Doi Pui, Chiang Mai Province; 2. Determining Probable Effects of Land Use Change

Abstract

An application was studied of a mathematical model coupling the Penman, Penman-Monteith and Rutter’s models to estimate the probable effect on the evapotranspiration process of converting a hill evergreen forest in Huai Kog Ma, Doi Pui, Chiang Mai province, northern Thailand to agricultural area.  Related model parameters including the maximum stomatal conductance (C^*_leaf), leaf area index ( LAI), initial canopy drainage rate (Ds), canopy drainage coefficient (b), maximum canopy storage (S), vegetation height (Z) and crown cover (p) were adjusted based on an assigned scenario.  It was found that C^*_leaf, LAI, and p were more sensitive to a change in evapotranspiration than the other parameters.  When either C^*_leaf or LAI were increased, the transpiration increased, resulting in higher evapotranspiration.  The average rate of change of both parameters was similar (2.44%).  The opposite result was found if C^*_leaf and LAI were decreased, which resulted in an average rate of change equal to 2.71 and 2.54 percent, respectively.  Increasing Ds or b resulted in higher evapotranspiration with an average rate of change of 0.42 and 0.41 percent, respectively and a decrease resulted in average rates of change of 0.94 and 0.62 percent, respectively.  When S was increased, the average rate of evapotranspiration change was equal to 0.36 while when S was decreased the value was 0.46 percent.  Evapotranspiration decreased if the vegetation height and crown cover were reduced, with the average rate of change being 0.38 and 1.97 percent, respectively.  Based on the previous results, if a hill evergreen forest is changed to an agricultural area where C^*_leaf is higher while the LAI, vegetation height and crown cover are lower, then evapotranspiration will decrease.

Keywords:  evapotranspiration, mathematic model, Huai Kog Ma watershad, hill evergreen forest, land use change