The problem of salinity is important in arid and semiarid areas because the rain contributes little to the lixiviation of salts, in addition to which the quality of the irrigation water available is usually poor.
To avoid reductions in yield when the concentration of salts exceeds crop tolerance, the excess salts must be lixiviated below the root zone. Therefore, an additional fraction of water must be added to the net irrigation requirement. This quantity, called a leaching requirement, is calculated as a function of the soil and water salinity levels, to allow salt leaching through drainage (see Sections 4.1, 5.7, and 5.8).
When leaching requirements are applied, it must be remembered that excessive leaching can lead to the lixiviation of nutrients. When irrigation is managed under saline conditions, the concentration of salts in the irrigation water, crop tolerance to salts, dependable rainfall, depth of the groundwater, and ease of drainage all must be taken into consideration.
The leaching frequency has been studied by several researchers. The results indicate that the most tolerant crops allow for a delayed leaching and for a relatively high concentration of salts in the root zone, with a minimum effect on yield as long as a low salt content is kept in the zone where most of the root water uptake occurs. It also has been found that plants are capable of making up for low water absorption in a more saline zone by increasing absorption in a less saline zone within their reach without decrease in yield. How much salt can be stored in the root zone before leaching is required and how often leaching requirements are to be applied are questions still unanswered , despite progress of research.
Crop tolerance to salts grows throughout the season. If the salinity level is sufficiently low during the seedling stage and the proper quantities of water with low salt content are applied, leaching may not be necessary and the salt level may be allowed to continue to increase until the end of the cycle. Winter rainfall or preplanting irrigation may saturate the soil and leach the salts accumulated so that there may be no need for leaching in the following irrigation.
On the contrary, when the water is saline, winter rainfall is scarce, and preplanting irrigation is light, leaching throughout the season will be necessary to prevent yield from being affected. Remember, however, that leaching is only required when the concentration of salts exceeds a threshold value .
For a long time, it has been assumed that very frequent irrigation would reduce the impact of salinity. A high water potential may partly reduce osmotic tension, but no increase in yield has been observed that would corroborate this assumption . If the surface of the soil is moistened frequently, evapotranspiration will be high most of the time and salts will be concentrated in shallow upper layers of the soil. In addition, water absorption by roots will tend to take place in shallow layers when they are moistened frequently, whereas if the surface of the soil is allowed to dry with less frequent irrigation, absorption also will take place in deeper layers. Both water absorption and evapotranspiration processes tend to concentrate salts near the surface under frequent irrigation conditions. Trickle irrigation systems are an exception because localized water displaces salts beyond the limits of the wet bulb. In this case, lixiviation prevails over evapotranspiration and water absorption, but a leaching fraction also has to be considered. The only acceptable measure to control salinity seems to be a controlled increase in the quantity of water applied, associated with drainage of the leachates.
When water resources are limited and nonsaline water costs become prohibitive, irrigation with saline water can be carried out on crops with moderate to high tolerance to salts, particularly during the latest growing stages. A mixture of saline and nonsaline water can be used for irrigation purposes, but this is a questionable practice. It may be more advisable to use nonsaline water during the most sensitive phases of the crop cycle and saline or mixed water during the rest of the time.
The use of saline water for irrigation generally calls for the selection of crops tolerant to salts, the development of appropriate irrigation scheduling, and the maintenance of the relevant soil physical properties to ensure better hydraulic conductivity.
Under saline conditions, irrigation scheduling requires better accuracy for the estimation of the components in the soil water balance, to better estimate the leaching requirements. With regard to irrigation management, it is advisable to consider how uniformly the irrigation water will be distributed so as to decide which part of the field is receiving the required leaching fraction.
The indiscriminate use of saline water for irrigation should be avoided because it leads to soil salinization. With regard to the various irrigation systems, trickle irrigation is the one that offers the best advantages under saline conditions . Sprinkler irrigation affects the leaves of sensitive crops but it is appropriate for leaching with water of good quality. Basin irrigation has the advantage, when compared to other surface irrigation systems, of ensuring evenly distributed leaching as long as the basins are sized and leveled correctly. In furrow irrigation, salts tend to accumulate near the seed, because leaching takes place down from the furrow. To control salinity in these irrigation systems, special attention has to be paid to the depth applied and to the uniformity of the application. Subsurface irrigation produces a continuous upward flux from the water table, which leads to the accumulation of salts on the surface and is not appropriate to control salts. Salts cannot be lixiviated with these systems and regular leaching will therefore be needed in the form of rainfall or surface irrigation .
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