Salinity control of soils in (irrigated) agricultural land

Title

Controlling the problem of soil salinity, reclaiming salinized agricultural land.

Description/Definition

Soil salinity control entails a combination of measures aiming at:

1. the prevention of soil degradation by salinization
2. the reclamation of already salty (saline) soils (see also land reclamation)

to ascertain sustained crop yields unaffected by salinity problems and an excessive salt content of the soil.
Retrogression and degradation are two regressive evolution processes associated with the loss of equilibrium of a stable soil. ... Soil salination results from the accumulation of free salts to such an extent that it leads to degradation of soils and vegetation. ... Land reclamation is either of two distinct practices. ... This article or section does not cite any references or sources. ...

Soil reclamation is also called soil improvement, rehabilitation, remediation, recuperation, or amelioration.

The soil salinity problem

Reference ^[1] .
Salty (saline) soils (see soil salinity) are soils having a high salt content in which sodium chloride (NaCl, "table salt") predominates. A saline soil is also a sodic soil
Salty soils are a common feature in irrigated lands of the aridand semi-arid regions and have poor to no crop production.
The problems are often associated with the occurrence of high water tables, indicating a lack of natural subsurface drainage to the underground owing to: It has been suggested that this article or section be merged with soil salination. ... Jordanian and Israeli salt evaporation ponds at the south end of the Dead Sea Sodium chloride, also known as common salt, table salt, or halite, is a chemical compound with the formula NaCl. ... An arid environment has an extremely low yearly precipitation, receiving much less rain or snowfall annually than would satisfy the climatological demand for evaporation and transpiration. ... Semi-arid generally describes regions that receive low annual rainfall (25 to 50 cm /10 to 20 in) and generally have scrub or grass vegetation. ... Cross section showing the water table varying with surface topography as well as a perched water table The water table or phreatic surface is the surface where the water pressure is equal to atmospheric pressure. ... Look up Underground in Wiktionary, the free dictionary. ...

1. insufficient transport capacity of the aquifer, and/or
2. lack of free outflow conditions of the aquifer because, for example, the waterlogged area is situated in a topographical depression.

The prime cause of salinization is the fact that irrigation water brought in from the rivers contains salts. All irrigation water, however "sweet", bring salts that remain behind in the soil after evaporation.
For example, assuming irrigation water with a low salt concentration of 0.3 g/l (equal to 0.3 kg/m3 corresponding to an electric conductivity of about 0.5 dS/m) and a modest annual supply of irrigation water of 10000 m3/ha (almost 3 mm/day) already brings 3000 kg salt/ha each year. In the absence of sufficient natural drainage (as in waterlogged soils) and without a proper leaching and drainage program to remove salts, this would lead in the long run to a high soil salinity and reduced crop yields.
Many irrigation waters have higher salt contents and in many irrigation projects the annual supply of irrigation water is more than mentioned in the example (e.g. sugar cane needs about 20000 m3/ha per year), so that the import of salts is often more than 3000 kg/ha per year and it can go up to 10000 kg/ha/year. An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well. ... Waterlogging is a verbal noun meaning the saturation of such as ground or the filling of such as a boat with water. ... Topography, a term in geography, has come to refer to the lay of the land, or the physiogeographic characteristics of land in terms of elevation, slope, and orientation. ... Soil salination results from the accumulation of free salts to such an extent that it leads to degradation of soils and vegetation. ... Irrigation is the artificial application of water to the soil. ... This bridge across the Danube River links Hungary with Slovakia. ... This article or section is in need of attention from an expert on the subject. ... Leaching is the process of extracting a substance from a solid by dissolving it in a liquid. ... Drainage is the natural or artificial removal of surface and sub-surface water from a given area. ... In agriculture, crop yield (also known as agricultural output) is a measure of the yield per unit area of land under cultivation. ... Species Ref: ITIS 42058 as of 2004-05-05 Sugarcane is one of six species of a tall tropical southeast Asian grass (Family Poaceae) having stout fibrous jointed stalks whose sap at one time was the primary source of sugar. ...

The secondary cause of salinzation is the enormous change of the natural water balance of the irrigated lands. In irrigation projects it is impossible to achieve 100% irrigation efficiency (meaning that all the irrigation water is consumed by the plants). The maximum attainable efficiency is about 70% but usually it is less than 60%. This means that minimum 30%, but usually more than 40% of the irrigation water is not evaporated and it must go somewhere.
The main part of the water losses are stored in the underground and they change the original aquifer hydrology (geohydrology) considerably. Many aquifers are unable to cope with these quantities of water and, as a result, the water table rises and creates a problem of waterlogging.
The adverse effect of water logging is two-fold: (1) it reduces the yield of most crops and (2) it is a symptom of insufficient natural drainage to the underground so that the salts brought in with the irrigation water cannot be removed and accumulate in the soil. Look up Underground in Wiktionary, the free dictionary. ... An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well. ... Water covers 70% of the Earths surface. ... Hydrogeology (hydro- meaning water, and -geology meaning the study of rocks) is the part of hydrology that deals with the distribution and movement of groundwater in the soil and rocks of the Earths crust (commonly in Aquifers). ... Cross section showing the water table varying with surface topography as well as a perched water table The water table or phreatic surface is the surface where the water pressure is equal to atmospheric pressure. ... Waterlogging is a verbal noun meaning the saturation of such as ground or the filling of such as a boat with water. ...

Normally, the salinization of agricultural land affects a considerable part of the irrigation project, to the tune of 20 to 30%. When the agriculture in such a fraction of the land is abandoned, a new salt and water balance is attained, a new equilibrium is reached, and the situation becomes stable.
In India alone, millions of hectares have been severely salinized. China and Pakistan do not lag much behind (perhaps China has even more salt affected land than India), and world wide the score is tens of millions of hectares.
A regional distribution of the 323 million ha of saline land world wide is shown in the page Soil salination. This article or section does not adequately cite its references or sources. ...

Although the principles of the processes of salinization are fairly easy to understand, it is more difficult to explain why certain parts of the land suffer from the problems and other parts do not, or to predict accurately which part of the land will fall victim. The main reason for this is the variation of natural conditions in time and space, the usually uneven distribution of the irrigation water, and the seasonal or yearly changes of agricultural practices.
Only in lands with undulating topography the explanation and prediction is pretty simple: the depression areas will degrade the most. The preparation of salt and water balances for distinguishable sub-areas in the irrigation project, or the use of agro-hydro-salinity models can be helpful in explaining or predicting the extent and severity of the problems.

Principles of soil salinity control

The governing principle of salinity control is to establish a drainage system in the affected or to be affected parts of the land (see also Land drainage). The system should permit a small fraction of the irrigation water (about 10 to 20 percent, the drainage or leaching fraction) to be drained and discharged out of the irrigation project.
In a stable salinity situation, the salt concentration of the drainage water is normally 5 to 10 times higher than that of the irrigation water, so that with the given drainage fraction the salt import will equal the salt export and no salt accumulation will occur. A drainage system is the pattern formed by the streams, rivers, and lakes in a particular watershed. ... Look up Discharge in Wiktionary, the free dictionary. ...

When reclaiming already salinized soils, the salt concentration of the drainage water will initially be much higher than that of the irrigation water (say 50 times higher) and the salt export will be much more than the import, so that with the same drainage fraction a rapid desalinization occurs. After one or two years, the soil salinity is decreased so much, that the salinity of the drainage water has come down to a normal value and a new, favorable, equilibrium is reached.

In regions with pronounced dry and wet (rainy) seasons it is worthwhile to consider limiting the drainage function of the system to the wet season, and close the system during the dry season. This practice of checked drainage saves irrigation water.

The discharge of salty drainage water problem may pose environmental problems to downstream areas. The environmental hazards must be considered very carefully and, if necessary mitigating measures must be taken. If possible, the drainage must be limited to wet seasons only, when the salty effluent does inflict the least harm. The environmental issues will not be further discussed here.

The drainage system designed to evacuate salty water also lowers the water table. To reduce the cost of the system, the lowering must be reduced to a minimum. The highest permissible level of the water table (or the shallowest permissible depth) depends on the irrigation and agricultural practices and kind of crops.
In many cases a seasonal average water table depth of 0.6 to 0.8 m is deep enough. This means that the water table may occasionally be less than 0.6 m (say 0.2 m just after an irrigation or a rain storm). This automatically implies that, in other occasions, the water table will be deeper than 0.8 m (say 1.2 m). The fluctuation of the water table helps in the breathing function of the soil while the expulsion of CO2 produced by the plant roots and the inhalation of fresh oxygen is promoted. Cross section showing the water table varying with surface topography as well as a perched water table The water table or phreatic surface is the surface where the water pressure is equal to atmospheric pressure. ...

The establishing of a not too deep water table offers the additional advantage that excessive field irrigation is discouraged, as the crop yield would be negatively affected by the resulting elevated water table, and irrigation water may be saved.

The reader is requested to be aware of the generality of the statements made above on the optimum depth of the water table, because in some instances the water table can be still shallower than indicated (for example in rice paddies), while in other instances it must be considerably deeper (for example in some orchards). The establishment of the optimum depth of the water table is in the realm of the agricultural drainage criteria. (See also Watertable control).

Soil salinity models

The majority of the computer models available for water and solute transport in the soil (e.g. Swatre ^[2], DrainMod ^[3] ) are based on Richard's differential equation for the movement of water in unsaturated soil in combination with a differential salinitity 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 hydrologic 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

1.INTRODUCTION

1.1. General

Saltmod is computer program for the prediction of the salinity of soil moisture, ground water and drainage water, the depth of the water table, and the drain discharge 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.
The model can be freely downloaded from www.waterlog.info. The manual is also available.^[4]
Examples of application can be found in

1. Integration of irrigation and drainage management ^[5]
2. Salinity in the Nile Delta ^[6]

1.2. Rationale

There is a need for a computer program that is easier to operate and that requires a simpler data structure then the currently available models. Therefore, the Saltmod program was designed keeping in mind a relative simplicity of operation to promote its use by field technicians, engineers and project planners.
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.

Saltmod's objective is to predict the long-term hydro-salinity in terms of general trends, not in 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 and it can account for farmers' responses to water logging, 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 capacities so that they can be optimized. Other features of Saltmod are found in the next section.

2. PRINCIPLES

2.1. 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 is 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:

1. daily inputs would require much information, which may not be readily available;
   
2. 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;
   
3. 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 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.

2.2. 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 impact of different drainage options.

2.3. 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 are 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 are a fractions and/or the rotational schedule gives the opportunity to simulate the impact of different agricultural practices on the water and salt balance.

2.4. Soil strata

Saltmod accepts four different reservoirs, three of which are in the soil profile:

1. a surface reservoir
2. an upper (shallow) soil reser­voir or root zone
3. an intermediate soil reservoir or transition zone
4. 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 root zone.
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.

2.5. Water balances

The water balances are calculated for each reservoir separately. 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 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 impacts) and for trial runs with varying parameters.

2.6. 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. 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 have an impact on the salt balance and the irrigation efficiency or sufficiency.

2.7. Salt balances

The salt balances are calculated for each reservoir separately. They are based on their water balances, using the salt concentrations of the incoming and outgoing water. Some concentrations must be given as input data, like the initial salt concentrations of the water in the different soil reservoirs, of the irrigation water and of the incoming ground water in the aquifer.
The concentrations are expressed in terms of electric conductivity (EC in dS/m). When the concentrations are known in terms of g salt/l water, the rule of thumb: 1 g/l -> 1.7 dS/m can be used. Usually, salt concentrations of the soil are expressed in ECe, the electric conductivity of an extract of a saturated soil paste. In Saltmod, the salt concentration is expressed as the EC of the soil moisture when saturated under field conditions. As a rule, one can use the conversion rate EC : ECe = 2 : 1.
Salt concentrations of outgoing water (either from one reservoir into the other or by subsurface drainage) are computed on the basis of salt balances, using different leaching or salt mixing efficiencies to be given with the input data. The effects of different leaching efficiencies can be simulated by varying their input value.
If drain or well water is used for irrigation, the method computes the salt concentration of the mixed irrigation water in the course of the time and the subsequent impact on the soil and ground water salinities, which again influences the salt concentration of the drain and well water. By varying the fraction of used drain or well water (to be given in the input data), the long term impact of different fractions can be simulated.
The dissolution of solid soil minerals or the chemical precipitation of poorly soluble salts is not included in the computation method, but to some extent it can be accounted for through the input data, e.g. by increasing or decreasing the salt concentration of the irrigation water or of the incoming water in the aquifer.

2.8. Farmers' responses

If required, farmers' responses to water logging and salinity can be automatically accounted for. The method can gradually decrease:

1. the amount of irrigation water applied when the water table becomes shallower;
2. the fraction of irrigated land when the available irrgation water is scarce;
3. the fraction of irrigated land when the soil salinity increases; for this purpose, the salinity is given a stochastic interpretation.
   

Response 1. is different for ponded rice and "dry foot" crops.

The responses influence the water and salt balances, which, in their turn, slow down the process of water logging and salinization. Ultimately an equi­librium situation will be brought about.
The user can also introduce farmers' responses by manually changing the relevant input data. Perhaps it will be useful first to study the automatic farmers' responses and their effect and thereafter decide what the farmers' responses will be in the view of the user.

The responses influence the water and salt balances, which, in their turn, slow down the process of water logging and salinization. Ultimately an equilibrium situation will be brought about.

The user can also introduce farmers' responses by manually changing the relevant input data. Perhaps it will be useful first to study the automatic farmers' responses and their effect and thereafter decide what the farmers' responses will be in the view of the user.

2.9. Annual input changes

The program may run with fixed input data for the number of years determined by the user. This option can be used to predict future developments based on long-term average input values, e.g. rainfall, as it will be difficult to assess the future values of the input data year by year. The program also offers the possibility to follow historic records with annually changing input values (e.g. rainfall, irrigation, agricultural practices), the calculations must be made year by year. If this possibility is chosen, the program creates transfer files by which the final conditions of the previous year (e.g. water table and salinity) are automatically used as the initial conditions for the subsequent period. This facility renders it possible to use various generated rainfall sequences drawn randomly from a known rainfall probability distribution and obtain a stochastic prediction of the resulting output parameters. If the computations are made with annual changes, not all input parameters can be changed, notably the thickness of the soil reservoirs and their total porosities as these would cause illogical shifts in the water and salt balances.

2.10 Suggestions

The selection of the area to be analysed by Saltmod should be governed by the uniformity of the distribution of the cropping, irrigation and drainage characteristics over the area. If these characteristics are randomly varied in space, it is advisable to use a larger area and the area average values of the input parameters. If, on the other hand, more uniform sub-areas can be identified, it is advisable to use the sub-areas separately for the analysis. It is also possible to use first the larger area approach and to use some of the outputs as inputs in the restricted area approach.
For example, an area may have non-irrigated, fallow, land next to irrigated land. The resulting capillary rise in the fallow land can be obtained as output from the larger area approach, and used as ground water input in a separate analysis for the fallow or irrigated land.
If the user wishes to determine the effect of variations of a certain parameter on the value of other parameters, the program must be run repeatedly according to a user-designed schedule. This procedure can be used for the calibration of the model or for the simulation runs.

2.10 Warnings

Soil salinity

Some of the input data are interdependent. These data can, therefore, not be indiscriminately varied. In very obvious illogical combinations of data, the program will give a warning. The correctness of the input remains the responsibility of the user.

2.11 Output data

Capillary rise

The output of Saltmod is given for each season of any year during any number of years, as specified with the input data. The output data comprise hydrological and salinity aspects.
The data are filed in the form of tables that can be inspected directly or further analyzed with spreadsheet programs.
The interpretation of the output is left entirely to the judgement of the user.
The program offers the possibility to develop a multitude of relations between varied input data, resulting outputs and time.
Different users may wish to establish different cause-effect relationships. The program offers only a limited number of standard graphics, as it is not possible to foresee all different uses that may be made.
Although the computations need many iterations, all the end results can be checked by hand using the equations presented in the manual.
The program is designed to make use of spreadsheet programs for the detailed output analysis, in which the relations between various input and output variables can be established according to the scenario developed by the user.

References

1. ^ Abrol I.P., Yadav J.S.P, Massoud F. 1988. Salt affected soils an their management, Food and Agricultural Organization of the United Nations (FAO), Soils Bulletin 39.
2. ^ Swatre
3. ^ Drainmod
4. ^ R.J.Oosterbaan, 2000, SaltMod, Agro-Hydro-Salinity Model. Description of Principles, User Manual, and Case Studies, 80 pp. ILRI Special Report (International Institute for Land Reclamation and Improvement), Wageningen, The Netherlands. Free download from ILRI-Alterra under "Other publications, nr. 14" or from www.waterlog.info.
5. ^ R.J.Oosterbaan, 1997. SaltMod: A tool for interweaving of irrigation and drainage for salinity control. In: W.B.Snellen (ed.), Towards integration of irrigation, and drainage management. ILRI Special report, p. 41-43. Free download from ILRI Alterra under "Other publications, nr. 3"
6. ^ R.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. ILRI, Wageningen, The Netherlands.

See also

• Salt
• Watertable control
• Drain spacing equation
• Well drainage

For other uses, see Salt (disambiguation). ... Energy balance has the following meanings in several fields: In physics, energy balance is a systematic presentation of energy flows and transformations in a system. ...

External links

• The FAO presents an article on salty soils in this document.

• A leading research institute is the US Salinity Laboratory at Riverside

• The following private website gives free downloads of articles and softwares on soil salinity

Miscellaneous