SCIENCES of SOILS Vol. 2, 1997 -

WaterMod 1.6®

A simulation model for soil water dynamics in agricultural and environmental systems

Review by Jon Arah,

Soil and Water Sciences Division, International Rice Research Institute, P.O. Box 933, Manila 1099, PHILIPPINES

Since this review was written, WaterMod 2 has been released. This major upgrade now includes the Richard's equation for describing infiltration, and a crop growth model that responds to soil water content.

The excellent online help asserts that WaterMod is intended for those interested in studying the dynamics of soil water in agricultural systems. The price suggests that educational institutes, rather than interested individuals, are the main target group. Suggested uses include:

The model incorporates rainfall and irrigation inputs, runoff, transpiration, evaporation from the soil and from the canopy, infiltration and drainage. It presents its output in readily-assimilable form (graphs which may be printed and copied as metafiles or bitmaps). Typically, a user specifies soil, crop, climatic and irrigation parameters using a purpose-built front-end utility called the WaterMod Builder; climatic inputs can also be supplied via a met data file. When the model is run, daily moisture content depth profiles are displayed beside time-courses of two out of the following system properties: precipitation, irrigation, soil evaporation, canopy evaporation, evapotranspiration, drainage, runoff. The model runs fast enough (between 5 and 10 simulated days per second on a 66MHz 486 with 8 MB RAM) to encourage enquiry along the lines of the questions it sets itself to address. As such, it serves its purpose well.

Its treatment of many processes is necessarily simple. Uptake of water by roots occurs according to the "distributed model" in which available water in each soil layer is withdrawn in proportion to the root density in that layer multiplied by the total evapotranspirative demand, the "preferential model" in which surface layers are exploited first, or the " compensating model " in which the previous two models are combined (distributed then preferential). Runoff is simply the excess of precipitation and irrigation over infiltration plus a user-defined surface storage. More significantly, water movement throughout the soil profile is treated according to a (conceptually-incoherent) version of the tipping-bucket model: specifically, the amount of water that moves down from one layer to the next is equal to the mass of water in excess of field capacity in the upper layer multiplied by the ratio of the saturated hydraulic conductivity of the upper layer to the thickness of that layer.

This last is a gross simplification. It may or it may not lead to significant errors in simulation, depending on the relative importance of other factors (where evapotranspiration dominates, soil hydrology may not matter much). Nevertheless, I would like to see a version of the program in which soil-water-movement paradigms other than the tipping-bucket (however modified) are on offer. I have in mind Richards' equation or Darcy's law, with any of the various current approaches to theta and k-psi. These more rigorous treatments may be more computationally-demanding but they are also more accurate: they should be on the menu. A great deal of work has been directed, successfully, toward making WaterMod 1.6 user-friendly. I would like to see the next version offer a few more realistic treatments of water movement in soil.

WaterMod is supplied for Microsoft Windows 95 (full 32 bit application), as well as Microsoft Windows 3.1x. Single user license $320, group licences available. The Windows 3.1x and 95 versions of WaterMod have the same model features.
Postal address:
Greenhat Inc., PO Box 1590, 101 Cooks Road, Armidale NSW 2350, Australia

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