A distributed hydrological model: TerrainLab

TerrainLab is a distributed hydrological model, which is used to simulate the detailed spatial and temporal variation patterns of evapotranspiration (ET). This model is based on the physically based, distributed hydrology-vegetation model of Wigmosta et al. (1994). The original Wigmosta's model provides an effective system to consider the flow of water in the soil-plant-atmosphere continuum, and is similar to many other models (Beven and Kirkby, 1979; Band et al., 1993; Paniconi and Wood, 1993). In addition, the model also includes a modeling framework in which the water flow is simulated among neighboring spatial units in a regular grid on varying topography. This framework is particularly suitable for applications to remote sensing data as pixels in remote sensing images can be treated as the spatial units.

Model Structure

Basic model simulations of the physical and biological processes are made at the pixel scale. According to the need of simulating hydrological processes, a pixel is vertically divided into five strata, i.e. overstorey, understorey, litter or moss layer, soil unsaturated zone, and soil saturated zone (Fig. 1).

The framework of the model, separated into input, main model and output domains, is illustrated in Fig. 2. Precipitation, solar radiation, topographic parameters, land cover, leaf area index (LAI), and soil properties are the major inputs to the model. All input parameters are spatially resampled to every pixel. The use of remotely derived parameters, such as LAI and land cover, allows for process-based modeling of evapotranspiration (ET) using the Penman-Monteith equation (Monteith, 1965) as well as other hydrological and energy components such as precipitation interception and its evaporation. The major outputs are ET, soil moisture, and water table. Horizontally, a moving window of 3 X 3 pixels is used to estimate the lateral, saturated, base flow according to topography and water table (Fig. 3).

At each daily time step, this window is moved across the modeling domain to update the water table of each pixel as a result of the net lateral base flow in all eight cardinal directions. As the hydraulic conductivity in saturated soils is generally less than 1 m per day (Ward and Robinson, 2000), the daily time step is adequate for estimating the lateral water flow for 30 m X 30 m pixels.

Several major assumptions are made in the model: (i) all physical and biological properties are homogeneous within the basic modeling unit, i.e. the pixel; (ii) soil texture is vertically invariant (although structure changes with depth); (iii) as the model is generally run at daily time steps, a sinusoidal diurnal variation pattern of the incident solar radiation is assumed in deriving daily radiation components; and (iv) at the daily time step, surface runoff does not occur until the whole soil profile is saturated.


© Revised: Mar., 2005