SWACROP (Soil WAter and CROP production model) is a transient one-dimensional finite difference model for simulation of the unsaturated zone. It incorporates the process of water uptake by roots. The soil profile is divided into several layers (containing one or more compartments of variable thickness) having different physical properties. The partial differential equation for flow in the unsaturated system is solved using an implicit finite difference scheme.

An explicit linearization of the hydraulic conductivity (K) and soil water capacity (C) is used. Knowing the initial conditions (i.e. water content or pressure head distribution profile) and top and bottom boundary conditions, the system of equations for all the compartments is solved for each (variable) time step by applying the so-called Thomas tridiagonal algorithm. The integration procedure within each time step allows calculation of all water balance terms for each time period selected.

For the top boundary, data on rainfall, potential soil evaporation, and potential transpiration are required. When the soil system remains unsaturated, one of three bottom boundary conditions can be used: pressure head, zero flux, or free drainage. When the lower part of the system remains saturated, one can either give the ground-water level or the flux through the bottom of the system as input. In the latter case, the ground-water level is computed. The rate of vegetation growth, both potential and actual can be simulated in the crop growth submodel which is linked to the main water model in a dynamic way. This submodel supplies information about the vegetation characteristics to the main water model throughout the simulation period. However, both models can be run separately.

The program runs in batch mode. Example data sets are provided, which can be copied and edited with a text editor for individual problems. Results are saved in text files. SWACROP is distributed on DD DOS-formatted diskettes containing source code, executable image, a graphical analysis package for the water balance, and example data sets. The user's manual contains installation instructions, a list of symbols used, and various papers covering the theory and applications of SWACROP.

SYSTEM REQUIREMENTS

• Intel 80i86 based computer
• 640 Kb RAM
• hard drive with about 2 Mb free disk space
• DOS 3.0 or higher
• math coprocessor

Authors: J.G. Wesseling, P. Kabat, B.J. van den Brock, and R.A. Feddes (The Netherlands)