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Evapotranspiration

Introduction

The primary ET model is based on empirically derived equations that follow the work of Kristensen and Jensen (1975), which was carried out at the Royal Veterinary and Agricultural University (KVL) in Denmark.

MIKE SHE also supports the Soil-Vegetation-Atmosphere Transfer (SVAT) model, which calculates evapotranspiration using the Shuttleworth-Wallace method. This advanced approach captures detailed energy balance and vegetation interactions. This is ideal for sub-daily simulations, in-depth analysis of evepotranspiration in vegetation landscapes and dynamic surface hydrology.

MIKE SHE includes a simplified ET model that is used in the Two-Layer UZ/ET model. The Two-Layer UZ/ET model divides the unsaturated zone into a root zone, from which ET can occur and a zone below the root zone, where ET does not occur. The Two-Layer UZ/ET module is based on a formulation presented in Yan and Smith (1994). Its main purpose is to provide an estimate of the actual evapotranspiration and the amount of water that recharges the saturated zone. It is primarily suited for areas where the water table is shallow, such as in wetland areas.

Kristensen&Jensen method

With the Kristensen & Jensen method, the ET processes are split up and modelled in the following order:

  1. A proportion of the rainfall is intercepted by the vegetation canopy, from which part of the water evaporates.
  2. The remaining water reaches the soil surface, producing either surface water runoff or percolating to the unsaturated zone.
  3. Part of the infiltrating water is evaporated from the upper part of the root zone or transpired by the plant roots.
  4. The remainder of the infiltrating water recharges the groundwater in the saturated zone where it will be extracted directly if the roots reach the water table, or indirectly if capillarity draws groundwater upwards to replace water removed from the unsaturated zone by the roots.

This method requires as an input data only precipitation and potential evapotranspiration.

SVAT

SVAT method is based on the work of Jesper Overgaard (2005) using Two-layer Land-Surface modelling approach. This approach is based on solution of the energy balance at the land surface. It is one-dimensional column model that provides information on the fluxes of heat and water between the land surface and atmosphere. It requires different set of input data:

  • Precipitation Rate
  • Air Temperature
  • Wind Speed
  • Global Radiation
  • Relative Humidity
  • Air Pressure
  • Reference Height Above Ground

The primary method provides as a result Actual Evapotranspiration. The method is more conceptual; it allows spatially and time variable inputs and requires relatively low computational power. The SVAT method provides several fluxes as a result, as such being more sophisticated and detailed in terms of climate data and processes. Such level of detail requires more focus on input data, and it is computationally more demanding. As such, it is more useful for large scale more focused models where detailed energy transfers are among required results.

One of the main feature in the SVAT module is that it allows feeding of land surface changes and hydrological properties changes back to atmosphere. This coupling of advanced hydrological model to a meso-scale atmospheric model through a shared land-surface model provides fully dynamic interactions allowing for more accurate scenario simulations dealing with changes in hydrological and land-surface properties.