SahysMod is a computer program for the prediction of the salinity of soil moisture, groundwater and drainage water, the depth of the watertable, and the drain discharge in irrigated agricultural lands, using different hydrogeologic and aquifer conditions, varying water management options, including the use of ground water for irrigation, and several crop rotation schedules, whereby the spatial variations are accounted for through a network of polygons.

[1] There is a need for a computer program that is easier to operate and that requires a simpler data structure then most currently available models.

Therefore, the SahysMod program was designed keeping in mind a relative simplicity of operation to facilitate the use by field technicians, engineers and project planners instead of specialized geo-hydrologists.

It aims at using input data that are generally available, or that can be estimated with reasonable accuracy, or that can be measured with relative ease.

Although the calculations are done numerically and have to be repeated many times, the final results can be checked by hand using the formulas in this manual.

The depth of the water table, the rainfall and salt concentrations of the deeper layers are assumed to be the same over the whole polygon.

For this purpose, the seasonal water-balance factors given with the inpu] are reduced automatically to daily values.

Groundwater levels and soil salinity (the state variables) at the end of the season are found by accumulating the daily changes of water and salt storage.

The program then calculates the surface area of each polygon, the distance between the nodes and the length of the sides between them using the Thiessen principle.

The depth of the water table, the rainfall and salt concentrations of the deeper layers are assumed to be the same over the whole polygon.

The other water balance components (like actual evaporation, downward percolation, upward capillary rise, subsurface drainage, groundwater flow) are given as output.

[7] The input data on irrigation, evaporation, and surface runoff are to be specified per season for three kinds of agricultural practices, which can be chosen at the discretion of the user: The groups, expressed in fractions of the total area, may consist of combinations of crops or just of a single kind of crop.

To obtain rotations over the years it is advisable to introduce annual input changes as explained When a fraction A1, B1 and/or U1 differs from the fraction A2, B2 and/or U2 in another season, because the irrigation regime changes in the different seasons, the program will detect that a certain rotation occurs.

If one wishes to avoid this, one may specify the same fractions in all seasons (A2=A1, B2=B1, U2=U1) but the crops and irrigation quantities may be different and may need to be proportionally adjusted.

Creative combinations of area fractions, rotation indexes, irrigation quantities and annual input changes can accommodate many types of agricultural practices.

[8] Variation of the area fractions and/or the rotational schedule gives the opportunity to simulate the effect of different agricultural practices on the water and salt balance.

(In a future version of Sahysmod, the upper soil reservoir may be divided into two equal parts to detect the trend in the vertical salinity distribution.)

The three soil reservoirs can be assigned different thickness and storage coefficients, to be given as input data.

This, in turn, leads to a change of the water-balance, which again influences the height of the water table, etc.

By installing a drainage system with zero capacity one obtains the opportunity to have separate water and salt balances in the transition above and below drain level.

In Sahysmod, the salt concentration is expressed as the EC of the soil moisture when saturated under field conditions.

The dissolution of solid soil minerals or the chemical precipitation of poorly soluble salts is not included in the computation method.

If required, farmers' responses to waterlogging and soil salinity can be automatically accounted for.

The user can also introduce farmers' responses by manually changing the relevant input data.

The program runs either with fixed input data for the number of years determined by the user.

Some input parameters should not be changed, like the nodal network relations, the system geometry, the thickness of the soil layers, and the total porosity, otherwise illogical jumps occur in the water and salt balances.

In some cases of incorrect changes, the program will stop and request the user to adjust the input.

The program offers only a limited number of standard graphics, as it is not possible to foresee all different uses that may be made.