Distributed hydrological model for mapping evapotranspiration using remote sensing inputs

A distributed hydrological model [Wigmosta, M.S., Vail, L.W., Lettenmaier, D.P., 1994. A distributed hydrology-vegetation model for complex terrain. Water Resource Research 30 (6), 1665-1679] is further developed to simulate the detailed spatial and temporal variation patterns of evapotranspiration (ET) around a flux tower site. In addition to meteorological, topographical and soil data, the model utilizes optical remote sensing data (Landsat TM at 30 m resolution) to characterize the distributions of vegetation types and the leaf area index (LAI). The use of LAI allows process-based modeling of major hydrological processes including transpiration, precipitation interception, and evaporation from vegetation and soil. Water flows within and between five strata (overstore, understore, moss/litter, soil unsaturated zone, and soil saturated zone) are modeled on a daily basis. A moving window of nine pixels is used to consider the lateral subsurface flow. The model is applied to a small watershed of dimension of about 16 km×12 km in Saskatchewan, Canada. The temporal variations of simulated ET are compared with eddy-covariance ET measurements over a black spruce stand located within the watershed. The stand was the Old Black Spruce in the Southern Study Area during the Boreal Ecosystem-Atmosphere Study (BOREAS) in 1994. Although the black spruce site is located in a flat area with less than 1.5 m topographical variation within 150 m of the flux tower, there was about 10.5 mm water loss through saturated subsurface flow during the growing season of 1994, accounting for 5.7% of the rainfall in same period. Even though the watershed studied had gentle terrain variations, the topography had considerable influence not only on the water table but also on the soil moisture and saturated water redistribution. This suggests the importance of modeling hydrological processes as influenced by topography in mapping ET.