Background for the distributed physically based models:
The use of distributed physically based models in environmental analysis is becoming more commen as greater demands are placed on hydrological models, particularly for problems involving prediction of future hydrological conditions resuting from changes in land use or cliamte. Although most hydrological analysis is still performed using lumped conceptual models, physically based process models are better suited for many problems, particularly thoses involving the prediction of future hydrological conditions induced by land use or climate change. Futhremore, the distributed models are also better suited to utilize currrent spatial data products such as NEXRAD rainfall and AVHRR snow cover data, or real-time point measurements for input and testing.
The biref introduction of the DHSVM model
The DHSVM is shorted for the distirbuted hydrology soil vegetation model. It presents the hydrological process integratedly at the spatial scale (typically 10 – 90 m horizontal resolution) described by digital elevetion model (DEM) data. It can explicitly describe the effects of topography and vegetation on water fluxes through the land scape. Originally developed in the early 1990s (Wigmosta et al. 1994), the model code has been further developed by a wide cast of characters at the university of Washington under the direction of Dennis P.Lettenmanier, and at pacific northwest notional laboratory. The recent updated version of this model is DHSVM v.3.1.2 (2014/9/29). The code source of this model whic was procuded by the computer language of Care opended for the public (See http://www.hydro.washington.edu/Lettenmaier/Models/DHSVM/)
The applications of this model
(1) DHSVM model has been utilized in a number of research activities invovling hyrological analysis and modelling. (2) The model has also been used to study the interactions between climate and hydrology, and the potential impacts of climate change on water resources. (3) In addition, a handfull of researchers apply this model to explore the forest management activities on water process.
The principles of this model
The modeled landscape is divided into computational grid cells centred on DEM nodes (See figure below). The characterization of topography is used to model topographic controls on absorbed shortwave radiation, precipitation, air temperature, and downslope water movement. The vegetation characteristics and soil properties which can vary spatilly throughout the basin are assigned to each model grid cell. At each time step, the model provides simutaneous solutions to energy and water balance equations for evry grid cell in the watershed. Individual grid cells are hydrologically linked through surface and subsurface flow routing.
Conopy snow interception and release is modeled using a one-layer mass and energy model. Snow accumulation and melt below the canopy (or in open) are simulated using a two-layer mass and energy model that explicityly incorperates the effects of topography and vegetation cover on the energy and mass exchange at the snow surface. Evopertranspiration is presented using a two-layer canopy model with each layer partioned intio wet and dry areas. Unsaturated soil moisture movement through multiple rooting zone soil layers is calculted using Darcy’s Law. Discharge from lower routing zonge recharged the local (grid cell) water table. Each grid cell exchanges water with its adjacent neighbors as a function of local hydraulic conditions resulting in a transient, three-dimensional representaiton of surface and saturated subsurface flow.
The drianage network is presented as a series of connected reaches with each reach passing through one or more DEM grid cells. As surface or subsurface water is routed downslope toward a stream channel, it may intercepted by a road network. A road reach begins to intercept subsurface flow when grid water table rise abovel the elevation of the associated road drainage ditches. Surface water in roadside ditches is routed through the road drainage network until it reaches a culvert or stream channel. The discharge from a culvert without a defined channel is allowed to reinfiltrate as it moves downslope below the culvert. The active road drainage/channel network may expand and contract as grid cell water table rise and fall below their channel beds. Flow in road drainage ditches and stream channels is routed using a cascade of linear channel reserviors.