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Irrigation has been used as tool by humans for over 8000 years. Irrigation started in the Nile valley where humans attempted to modify the way that the river seasonally flooded their fields in order to make cropland more productive (van Schilfgaarde, 1994). From 1940 to 1989 the amount of land being irrigated around the world has increased at a rate of 2.7% per year. In 1940 there were 95 M ha in irrigation while by 1989 there were over 280 M ha (van Schilfgaarde, 1994). This large increase in land under irrigation occurred at the same time as an exponential jump in human population, and increases in both are greatest in the arid and sub-arid regions of the Earth. Our population has therefore become reliant on irrigation to fend off large scale hunger (Abrol et al., 1988).
As rain falls it carries no dissolved salts. Once this water strikes the earth and travels as surface runoff or in ground water it will come into contact with and carry dissolved salts. Any water used for irrigation carries ions in solution and by depositing this water on our fields in the form of irrigation we can effect the concentration of salts in our croplands. If these salts become too concentrated it can lead to salinization. Salinization can reduce yields in it's earliest stages and eventually lead to the destruction of fertility in the soil. Currently the Earth is losing 3 ha of arable land a minute to the effects of salinization (Abrol et al., 1988). Can we stop this loss? Will we be able continue using arid lands to meet our food needs?
The Secondary Salinization Process
Salinization has a direct effect on both plant growth and the structure of the soil. If the soil is saline a plant will have to expend energy bringing water into it's cells because it is forced to work against osmotic potential. The cation exchange complex (CEC) effects the stability of colloid size particles in the soil. The cation's positive charge will be attracted to the negative charge found on clay particles which make up most of the colloid fraction. Di-valiant cations(Ca, Mg) will allow the colloidal particle to get close enough together that Van Dehr Wahls forces will cause the clays to flocculate, or form stable aggregates. Sodic soils, whose CEC is dominated by mono-valiant sodium cations, will tend to be dispersed and not form stable aggregates.
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The quality of the water used for irrigation has an effect on soil salinization. Surface water can vary greatly in the amount of salts that it carries in solution. Most large rivers in the world carry about .11 to .18 g of dissolved material per liter of water. Small water courses tend to have more dissolved material and rivers tend to increase in dissolved load as they travel from their source (Szabolcs, 1986). In some regions of the world irrigation water is predominantly pumped from aquifers and the dissolved load of ground water will vary greatly due to local lithology. Shale lithologies tend to create highly saline ground water (Schwartz et al., 1987). In many areas of the world brackish water is used for irrigation. If brackish water is used, it must be recognized that the effects of salinization and sodification will be accelerated (Bajwa et al., 1989).
Proper irrigation management can slow down or stop salinization. If fields are excessively watered or if large amounts of water are allowed to seep away from canals, the ground water surface under the field and under the region can be raised (Sommerfeldt et al., 1980). If the ground water surface rises to within 3 m (depending on soil type) of the surface the upper portion of the saturated zone will experience evaporation which will cause dissolved salts in the ground water to become concentrated in the soil profile (Szabolcs, 1986). In arid areas irrigation can cause the aquifer recharge rate to experience a forty fold increase even with wise water conservation. The ability for the soil and bedrock to absorb and transport this increase without raising the water table will allow salts which leach through the soil profile to be carried away. If the soil has layers which possess low hydraulic conductivity a perched water table could be created over the regional aquifer. If drainage is poor due to bedrock, soil or topography the water table will rise and water and salts will eventually reach and be concentrated in the root zone leading to destruction of fertility (Schwartz et al., 1987).
The type of crop being raised in the field seems to have little effect on soil salinization and sodification. Studies done in the San Joquin Valley, an area which is currently cultivated with grapes, orchards, vegetables and various truck crops showed little variation in soil salinity levels under fields of all types. The minor variation seen is due to the amount of water which each field uses. Crops which required more water had soils which were more saline (Nightingale, 1974). Plants in general have little ability to remove sodium from the soil. Less then .5% of sodium which reaches the field in dissolved form is removed during the harvest of crops while nearly 50% of dissolved calcium can be consumed by the yearly crops. Plants will therefore tend to make the soil more sodic as irrigation continues (Prunty et al., 1991).
Salt Balance and Leaching Requirement
Salt balance and leaching are tied to one another because as we will see in the next section leaching is the only effective way of removing salinity from the soil. Salt balance is used to measure the amount of salt which is being introduced into the soil, the amount which is being removed and the amount of salt which is currently within the soil and ground water (Szabolcs, 1986). Although it would be nice to be able to remove all salts from the soil which are the result of irrigation it would prove very costly to do so. The amount of salinization which crops can endure and still produce fairly high yields can be measured (Richards, 1995). Therefore if we are to use irrigation on a sustainable basis we will have to manage the soil to maintain the salt balance under critical salinity and sodicity levels.
The leaching requirement is the amount of water which will have to travel through the soil and into the ground water in order to leach to desired amount of salts. In general one volume unit of fairly high quality irrigation water can leach 80% of salts out of that same volume of soil. This is only a general rule and there is a lot of variation caused by different soil types (Abrol et al., 1988). Irrigation water must serve two proposes . Some of the water will be used by plants(transpiration) and some will be lost to unavoidable evaporation. Both of those processes will raise the salt concentration in the soil. The second purpose of irrigation water is leach the excess salt, created by evapotranspiration, away and therefore maintain the salt balance.
Soil Amendments are additives which are spread on the surface or injected into the soil of a field. In the case of sodic soils, gypsum is often used. In sodic soils leaching may not prove to be completely effective on it's own because leaching is not very capable of removing Na from the exchange complex. The addition of gypsum will force Na out of the exchange complex and replace it with Ca. The change from a mono to di-valiant cation will allow the colloids to flocculate and give the soil more structure. Municipal solid waste (MSW) has been added to sodic soils and has proven to increase flocculation and structure. The increase in structure leads to increased permiability and therefore more effective leaching. Soil amendments must be used along with a salt balance management program and will not be effective on their own. Soil amendments are generally not required on soils which are only effected by salinization and not sodicity (Lax et al., 1994).
The most effective and most costly remediation technique is the use of artificial drainage. This drainage can come in the form of tube wells which require pumping and therefore a power source or drainage trenches which don't require pumping put do require a natural depression to drain into (Abrol et al., 1988). Although this does solve the immediate problem in the field, it has proven difficult to dispose of the drained water. Many areas, such as the Imperial Valley in California and the Punjab in Pakistan are closed basins and therefore it is impossible to remove the salts from the region. If drainage is allowed to return to rivers it will increase the salinity for those who are trying use the river below. A current example of this is the Colorado River (van Schilfgaarde, 1994).
Perhaps the most effective remediation technique is an increase in irrigation efficiency. Any water which has been lost to evaporation has increased salinity, but not contributed to plant growth. Therefore, techniques which minimize evaporation also minimize salinization. Often times placing mulch or plastic over the field can decrease evaporation significantly (Lax et al., 1994). Leaching is more effective if it occurs under unsaturated conditions. In order for leaching to occur without saturation water would have to be applied slower then the infiltration rate. Therefore any method of applying water to the field which does not require flooding or ponding will lead to more efficient leaching (Abrol et al., 1988).
There are a few methods of remediation, although practiced, which have little real effect. Scraping involves physically removing saline crust from the surface of the field and flushing involves running water over the surface of a field which has developed impermeable salt crust in order to dissolve and remove them. Both of these are desperation acts and show that fertility of the soil is in extreme jeopardy (Abrol et al., 1988).
Methods of Living with Soil Salinity and Brackish Water Irrigation
If remediation efforts prove ineffective or too costly, a farmer has the choice of growing crops which are resistant to saline environments. Crops such as barley, cotton and sugar beets can be grown in salt effected soils and will do well when other, less resistant crops have failed. The planting of deep rooted perennials, which will use large amounts of water year round and will minimize evaporation, will allow the soil to slowly recover. However, this process is slow and perennial crops are not large income earners. Therefore it seems likely that operators will be slow to place their fields in perennials without subsidy from a government agency (Richards, 1995).
It had long been hoped that genetic engineering and breeding would grow varieties of common crop species which would be resistant to salt effects. There has been little real progress and only six salt resistant varieties have been released. Salinization and sodic effects in soil are rather patchy, laterally variable conditions. If a field is beginning to become salt-effected and yields are marginal a farmer would have better overall yields if he or she used a standard, non-salt resistant, high yield variety. This would maximize yield in the less effected micro sections (Richards, 1995).
In some irrigated areas of the world, the only water available is of the poor quality brackish sort, or good quality water is available in very limited quantities. If a small amount of good water is available test plots have shown that if one-third of a field's irrigation is done with good quality water good yields can be achieved. This is especially the case if the good water can be used to leach the soil. If a farmer can obtain good quality water only once or twice a year he or she can use that water to leach out most of the salts implaced by the brackish irrigation (Bajwa et al., 1989).
Even if brackish water is the only irrigation water available new techniques and good management can make irrigation a somewhat long term reality. If the field can be saturated or flooded at the time of seed germination, much of the salinity can be leached out, if only for a brief period. Most species are more susceptible to salt effects during germination and more resistant during other portions of their life cycle. With this system, the soil salinity will remain below the crop's damage threshold throughout the year, although during some times in the season salt levels may become quite high (Pasternak et al., 1985).
The Future of Irrigated Agriculture
Most irrigation projects are by necessity large scale affairs tied to a government agency. Central management with planned water deliveries to fields may be convenient, but crops are very sensitive to timing due to weather variation. Delivery schedules need to have some built in flexibility and final word about timing should work from the bottom to the top, instead of the other way around, because the people working the field will know what is best (van Schilfgaarde, 1994).
Most irrigation projects receive large subsidies from government. The water which irrigates fields cost much less then it is worth (Babu et al., 1996). Because the water is not expensive it does not cost very much to waste it and there is little incentive to conserve or to be as efficient as possible. In order for agriculture to be sustainable the farmer must pay the true price for the water that he or she uses on agriculture. He or she must also pay the price to deal with negative environmental impacts, even those impacts which do not occur on agricultural property. When he or she gets done paying those costs, the farmer would then charge the people the true price of the harvest. As the consumer, we could then decide if the true cost is worth it. The farmer or the consumer may decide that because the true cost of water is rather high perhaps water which is currently used for agriculture should be diverted to other uses. Of course some developing nations should continue to subsidize agriculture in order to form self-sustaining economies, however subsidy should be the exception and not the rule.
The human race is creating less additional irrigated farmland then we did just ten years ago (van Schilfgaarde, 1994). At the same time land is being removed from production due to salinization and water logging. With current management techniques I believe that it would be possible to maintain the current production of irrigated agriculture around the world for some time to come. However, we would have to continually exploit new lands as we slowly but inevitably destroyed the fertility of some of our current production areas. Eventually we would run out, because on the human time scale ground water and soil are non-renewable resources, at least at the rate we are currently consuming them. I doubt greatly that the production of irrigated agriculture can keep pace with the rise in human population for very much longer. If irrigated agriculture has a hope of long term survival it will be in determining the real price of resource destruction caused by it and finding people who are willing to pay that price for remediation in the products they consume.
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