SaltMod is computer program for the prediction of the salinity of soil moisture, groundwater and drainage water, the depth of the watertable, and the drain discharge (hydrology) in irrigated agricultural lands, using different (geo)hydrologic conditions, varying water management options, including the use of ground water for irrigation, and several cropping rotation schedules. The water management options include irrigation, drainage, and the use of subsurface drainage water from pipe drains, ditches or wells for irrigation.
Soil salinity models The majority of the computer models available for water and solute transport in the soil (e.g. Swatre, DrainModDrainmod ) are based on Richard's differential equation for the movement of water in unsaturated soil in combination with a differential salinity dispersion equation. The models require input of soil characteristics like the relation between unsaturated soil moisture content, water tension, hydraulic conductivity and dispersivity.
These relations vary to a great extent from place to place and are not easy to measure. The models use short time steps and need at least a daily data base of hydrological phenomena. Altogether this makes model application to a fairly large project the job of a team of specialists with ample facilities.
Simplified salinity model: SaltMod
Literature references (chronological) to case studies after 2000:
Older examples of application can be found in: Salinity in the Nile DeltaR.J. Oosterbaan, and M. Abu-Senna, 1990. Using SaltMod to predict drainage and salinity in the Nile Delta, Egypt. In: Annual Report 1989, p. 63-74. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. See "Case study Egypt" in the SaltMod manual, on line: , or directly: Integration of irrigation and drainage managementSaltMod: A tool for interweaving of irrigation and drainage for salinity control. In: W.B.Snellen (ed.), 1997, Towards integration of irrigation, and drainage management. ILRI Special report, p. 41-43. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. Free download from: or directly at
Rationale There is a need for a computer program that is easier to operate and that requires a simpler data structure than most currently available models. Therefore, the SaltModod 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 the manual.
SaltMod's objective is to predict the long-term hydro-salinity in terms of general trends, not to arrive at exact predictions of how, for example, the situation would be on the first of April in ten years from now.
Further, SaltMod gives the option of the re-use of drainage and well water (e.g. for irrigation) and it can account for farmers' response to waterlogging, soil salinity, water scarcity and over-pumping from the aquifer. Also it offers the possibility to introduce subsurface drainage systems at varying depths and with varying capacity so that they can be optimized. Other features of Saltmod are found in the next section.
Principles Seasonal approach The computation method Saltmod is based on seasonal water balances of agricultural lands. Four seasons in one year can be distinguished, e.g. dry, wet, cold, hot, irrigation or fallow seasons. The number of seasons (Ns) can be chosen between a minimum of one and a maximum of four. The larger the number of seasons becomes, the larger the number of input data required. The duration of each season (Ts) is given in number of months (0 < Ts < 12). Day to day water balances are not considered for several reasons: daily inputs would require much information, which may not be readily available; the method is especially developed to predict long term, not day-to-day, trends and predictions for the future are more reliably made on a seasonal (long term) than on a daily (short term) basis, due to the high variability of short-term data; even though the precision of the predictions for the future may still not be very high, a lot is gained when the trend is sufficiently clear; for example, it need not be a major constraint to design appropriate soil salinity control measures when a certain salinity level, predicted by Saltmod to occur after 20 years, will in reality occur after 15 or 25 years.
Hydrological data The method uses seasonal water balance components as input data. These are related to the surface hydrology (like rainfall, evaporation, irrigation, use of drain and well water for irrigation, runoff), and the aquifer hydrology (like upward seepage, natural drainage, pumping from wells). The other water balance components (like downward percolation, upward capillary rise, subsurface drainage) are given as output. The quantity of drainage water, as an output, is determined by two drainage intensity factors for drainage above and below drain level respectively (to be given with the input data), a drainage reduction factor (to simulate a limited operation of the drainage system), and the height of the water table, resulting from the computed water balance. Variation of the drainage intensity factors and the drainage reduction factor gives the opportunity to simulate the effect of different drainage options.
Agricultural data
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: A: irrigated land with crops of group A B: irrigated land with crops of group B U: non-irrigated land with rainfed crops or fallow land The groups, expressed in fractions of the total area, may consist of combinations of crops or just of a single kind of crop. For example, as the A type crops one may specify the lightly irrigated cultures, and as the B type the more heavily irrigated ones, such as sugarcane and rice. But one can also take A as rice and B as sugarcane, or perhaps trees and orchards. The A, B and/or U crops can be taken differently in different seasons, e.g. A=wheat+barley in winter and A=maize in summer while B=vegetables in winter and B=cotton in summer. Un-irrigated land can be specified in two ways: (1) as U=1−A−B and (2) as A and/or B with zero irrigation. A combination can also be made. Further, a specification must be given of the seasonal rotation of the different land uses over the total area, e.g. full rotation, no rotation at all, or incomplete rotation. This occurs with a rotation index. The rotations are taken over the seasons within the year. To obtain rotations over the years it is advisable to introduce annual input changes. When a fraction A1, B1 and/or U1 in the first season differs from fractions A2, B2 and/or U2 in the second season, because the irrigation regimes in the seasons differ, 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 have to be adjusted in proportion. Cropping rotation schedules vary widely in different parts of the world. Creative combinations of area fractions, rotation indexes, irrigation quantities and annual input changes can accommodate many types of agricultural practices. 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.
Soil strata
Saltmod accepts four different reservoirs, three of which are in the soil profile: a surface reservoir an upper (shallow) soil reservoir or root zone an intermediate soil reservoir or transition zone a deep reservoir or aquifer. The upper soil reservoir is defined by the soil depth from which water can evaporate or be taken up by plant roots. It can be equal to the rootzone. The root zone can be saturated, unsaturated, or partly saturated, depending on the water balance. All water movements in this zone are vertical, either upward or downward, depending on the water balance. (In a future version of Saltmod, the upper soil reservoir may be divided into two equal parts to detect the trend in the vertical salinity distribution.) The transition zone can also be saturated, unsaturated or partly saturated. All flows in this zone are vertical, except the flow to subsurface drains. If a horizontal subsurface drainage system is present, this must be placed in the transition zone, which is then divided into two parts: an upper transition zone (above drain level) and a lower transition zone (below drain level). If one wishes to distinguish an upper and lower part of the transition zone in the absence of a subsurface drainage system, one may specify in the input data a drainage system with zero intensity. The aquifer has mainly horizontal flow. Pumped wells, if present, receive their water from the aquifer only.
Water balances
The water balances are calculated for each reservoir separately as shown in the article Hydrology (agriculture). The excess water leaving one reservoir is converted into incoming water for the next reservoir. The three soil reservoirs can be assigned a different thickness and storage coefficients, to be given as input data. In a particular situation, the transition zone or the aquifer need not be present. Then, it must be given a minimum thickness of 0.1 m. The depth of the water table, calculated from the water balances, is assumed to be the same for the whole area. If this assumption is not acceptable, the area must be divided into separate units. Under certain conditions, the height of the water table influences the water balance components. For example, a rise of the water table towards the soil surface may lead to an increase of evaporation, surface runoff, and subsurface drainage, or a decrease of percolation losses from canals. This, in turn, leads to a change of the water balance, which again influences the height of the water table, etc. This chain of reactions is one of the reasons why Saltmod has been developed into a computer program. It takes a number of repeated calculations (iterations) to find the correct equilibrium of the water balance, which would be a tedious job if done by hand. Other reasons are that a computer program facilitates the computations for different water management options over long periods of time (with the aim to simulate their long-term effects) and for trial runs with varying parameters.
Drains, wells, and re-use The sub-surface drainage can be accomplished through drains or pumped wells. The subsurface drains are characterized by drain depth and drainage capacity factor . The drains are located in the transition zone. The subsurface drainage facility can be applied to natural or artificial drainage systems. The functioning of an artificial drainage system can be regulated through a drainage control factor. When no drainage system is present, installing drains with zero capacity offers the opportunity to obtain separate water and salt balances for an upper and lower part of the transition zone. The pumped wells are located in the aquifer. Their functioning is characterized by the well discharge. The drain and well water can be used for irrigation through a re-use factor. This may affect the salt balance and the irrigation efficiency or sufficiency.
Salt balances