Characteristics of evapotranspiration in the southwestern slope of Mt. Merapi

Abstract

Mt. Merapi is an active volcano in Indonesia and has several watersheds in its southwestern slope, e. g., the Blongkeng, the Putih, the Batang and the Bebeng watershed. The evapotranspiration in this region was estimated whose spatial and temporal resolution was 50× 50m and one month, respectively. The method was based on the Penman Monteith Equation, which requires the information on five elements, i. e., effective radiation, air temperature, vapor pressure, aerodynamic resistance and canopy resistance. Using air temperature data observed by the Indonesian Volcano Observatory, the relationships between altitude and air temperature were approximated by linear lines. Effective radiation was calculated with the result of numerical analysis about the shadow of the mountain and remote sensing analysis using GMS ISCCP-B2 and SPOT. Aerodynamic resistance was calculated from the representative vegetation heights based on the ground truth. Canopy resistance was calculated with NDVI from the data of satellite SPOT. The area at low altitude had a higher value of evapotranspiration than that of mountainous areas. The annual amount of evapotranspiration was 1, 210mm in the Putih watershed, and was approximately 1, 400mm in the other three watersheds. Every watershed had a high peak on September, a small peak in April and the lowest value in January. The lowest evapotranspiration rate each month was recorded at the Putih watershed. The Bebeng watershed had a high evapotranspiration at the altitude of 800 through 1, 000m, while the Putih watershed had no such high peak. The Bebeng watershed had a high peak of NDVI at the altitude of 800 through 1, 000m, which correspond to high evapotranspiration. The difference of vegetation between these watersheds was concluded to have caused the difference of evapotranspiration between these watersheds.