1.   Purpose and objectives

     To simulate evapotranspiration and vertical soil moisture transport in the unsaturated zone in the presence of a shallow water table.

2.   Description

     The unsaturated soil profile is schematized into a root zone and a subsoil. The upper boundary of the model is the soil surface and for the lower boundary a level just below the lowest water table is selected. The root zone acts as a reservoir from which water is taken up by the roots. The depth of the root zone is constant in time.

     Vertical transport of soil moisture takes place through the subsoil. The transient flow process is simulated by a succession of steady flow situations. The flow situation during each time step is represented by a combination of two steady-state profiles corresponding to the flux across the upper or the lower boundary of the subsoil. This approach differs from many other models for transport in the unsaturated zone in that it does not simulate the flow in great detail. The time steps are in the order of 
magnitude of days and the exact soil moisture distribution is not calculated. This less detailed approach is in general acceptable in view of the accuracy of the available soil physics data and the horizontal heterogeneity in the field.  As a result the model MUST uses very little computer time and allows simple data management for its execution.

     The upper boundary condition is specified by the rainfall and potential evapotranspiration data. Meteorological data for the computation of potential evapotranspiration using Penman's equation adapted for a cropped surface may be used instead.

     For the lower boundary a drainage module is available to simulate flow to a complex system consisting of open (collector) drains, pipe drains and trenches. Instead, the lower boundary may be specified by the flux, the water table depth, or a relation between the two.

     The ability of the model to simulate the depth of the fluctuating water table and to estimate evapotranspiration rates has been verified in field experiments in The Netherlands. The model does not need to be calibrated.

3.   Input

     Soil moisture characteristics end hydraulic conductivity relations for each soil layer. Meteorological data to describe the upper boundary condition as well as the time-varying data for the lower boundary.  The model includes a number of default values related to evapotranspiration of specific crops that can be adapted by the user.

4.   Output

     The output includes computed actual evapotranspiration rates, water table depth, matric pressure in the root zone, fluxes across the lower boundary of the root zone and the subsoil and other variables depending on how the model is used. There is no graphical output.

5.   Operational requirements and restrictions

     The program runs in a DOS environment and requires a PC with 640 Kb RAM. A mathematical coprocessor is recommended.

6.   Form of presentation

     A diskette with the executable version of the program which includes a preprocessor. The diskette also contains an example data set and soil physical data for an international series of soil groups. A user's manual written in English is available.

7.   Operational experience

     The model has been applied successfully in several countries in Western Europe. It is known that the model has been used outside Europe but references to these applications are not available.

8.   Originator and technical support

     Dr P.J.M. de Laat. International Institute for Hydraulic and Environmental Engineering (IHE), Delft, The Netherlands.

9.   Availability

     From the HOMS National Reference Centre for The Netherlands.

10.  Conditions on use

     The executable version on diskette and the manual are available for the cost of reproduction and mailing. The source code by arrangement with originator.