Effects of Land Cover Types and Forest Age on Evapotranspiration Detected by Remote Sensing in Xiamen City, China

Evapotranspiration (ET) is one of important components of surface energy and water cycles. The accurate information of ET is valuable for water management. This study was conducted to investigate the dependence of ET on land cover types, forest species and forest age in Xiamen City, China using remote sensing data. The information of forest species and age was retrieved from the forest inventory database produced in 2003. Remote sensing data of Landsat-5 TM acquired on November 5, 2006 was used to produce a land cover map and to retrieve ground surface albedo, normalized difference vegetation index (NDVI), and land surface temperature which were employed in conjunction with meteorological data (air temperature, relatively humidity, and sunshine hours) to estimate daily ET at a 30 m resolution using an empirical model on the basis of the energy balance principle for the study area. Derived ET shows distinct spatial variations, mainly caused by land cover types, species and development stages of forests. The daily average ET of water, forest, and built-up/spare soil, cropland is 5.59, 3.91, 2.92, and 2.73mm, respectively. The averages of daily ET are 4.37, 4.36, 4.30, 4.11, 4.00, and 2.85 mm for Chinese Fir, Schimacrenata, Slash pine, Tea trees, Masson pine, and Longan, respectively. The 5-year binned averages of daily ET increases with forest age at the rate of 0.20 mm d(-1) (10a)(-1) for all forests with ages in the range from 1 to 60 in this study area (R-2=0.73). However, the changes of ET with forest ages differ among different species. The changes in daily average ET values of Chinese Fir, and tea trees with ages are not detectable. The averages of daily ET values of Slash pine, Schimacrenata, and Masson pine increase with age significantly. Daily ET of Longan increases fast during the early development stage and then decreases gradually with tree age above 6. The daily ET of Longan aged above 15 does not show obvious trend.