5. SALINITY PROBLEMS OF THE DRYLAND REGIONS

5.1 Mode of formation
5.2 Factors influencing formation of saline seeps
5.3 Prevention and management of saline seeps

Apart from irrigated areas, salinity poses a major management problem in many unirrigated areas where cropping is done under rainfed conditions. Dryland salinity has been a threat to the land and water resources in several parts of the world although only in recent years has the seriousness of the problem become widely known. Dryland salinity is an acute management problem in western Australia and in the Great Plains region of North America. In Canada it occurs extensively in the prairie provinces of Manitoba, Saskatchewan and Alberta and in the United States in the states of Montana, North and South Dakota. Dryland salinity is also said to occur in South Africa, Iran, Afghanistan, Thailand and India and it probably exists in other countries. Saline spots or areas occurring in the dryland fields have been known by several local names, but most commonly as saline seeps. These problem soils range from a slightly saline soil condition which reduces crop growth to extensive areas where cultivation is almost impossible. Problems relating to the origin and management of dryland saline soils have been discussed at several meetings in the recent past (Anonymous 1976, 1978; Holmes and Talsma 1981).

5.1 Mode of formation

Dryland saline seepage is generally considered to be a manifestation of salt accumulation in seepage spots at low points or side slopes in the landscape developed when water infiltrates into soil to somewhat impermeable layers, and moves laterally downslope. Eventually at lower elevations, the water seeps laterally out at the soil surface and evaporates leaving the salts behind. Thus the development of saline seeps involves two areas in the field - the recharge and the discharge areas. In the recharge areas, water in excess of the retention capacity of the root zone soil percolates beyond the root zone, reaches the groundwater and increases the flow to the discharge area (Figure 39). The groundwater flow is mainly lateral and downslope and occurs most often over a shallow, less permeable layer. The groundwater travelling to the discharge area dissolves salts from the soil. In the discharge area the groundwater rises to the soil surface creating a seep. As the water evaporates from the seepage area salts accumulate.

Figure 39 Schematic diagram of a recharge and a seepage area (zone of salt accumulation)

5.2 Factors influencing formation of saline seeps

5.2.1 Excess water
5.2.2 Soil characteristics

A combination of climatic, geologic, soil and cultural conditions determines the nature, extent and intensity of saline seeps.

5.2.1 Excess water

Many dryland areas which are now cropped, existed under forest or grass cover. The ecosystem under these conditions was balanced - the grasses and trees utilizing all the precipitation in their respective areas and keeping the groundwater tables low. Clearing deep rooted perennial vegetation and its replacement by pastoral species and crops decreased plant water requirements, causing a surplus exceeding the retention capacity of the root zone. Any land use practice which allows excess moisture to migrate downward through the soil profile beneath the root zone can contribute to the formation and extension of dryland salinity. Some of the more important factors and practices that encourage excess moisture are:

i. Fallowing By far the most important contributing factor in many areas, e.g. Northern Plains of North America is the widespread alternate crop-fallow farming system. When fallowing is practised, the capacity of the soil to retain rain water is reduced resulting in greater losses through seepage.

ii. Rainfall It has generally been observed that salinity problems are more severe in years of high rainfall due to greater recharge and therefore accumulation of salt-laden water in the recharge zone.

iii. Water and snow accumulation Practices which result in the accumulation of water/snow in the recharge areas may aggravate the saline seep problem. These include shelter belts, single row wind breaks, railways, roads and highways, etc. Natural and constructed ponds, dugouts and small farm reservoirs can be important locally if they are located in the recharge areas and leak significant quantities of water.

iv. Overgrazing Range that is overgrazed utilizes and holds in place less moisture than before resulting in increased runoff and deep percolation.

5.2.2 Soil characteristics

Light textured soils having low moisture retention and high permeability in the recharge zone are more conducive to the formation of saline seeps at low points in the landscape. In some places the bed rock is of marine origin and its overburden is rich in soluble salts. Dissolution of this salt and its transportation with percolating water causes serious salinity problems. In many areas having saline seep problems the bed rock or the overburden are not of marine origin but the salts are derived from the normal weathering of the mineral constituents of the soil. Excess salts are rarely a problem in the place of their origin.

5.3 Prevention and management of saline seeps

5.3.1 Practices for the recharge area
5.3.2 Practices for the discharge areas

Since the cause of saline seeps is nearly always increased recharge in an upper area, any long-term solutions to the problem must include regional land use changes with the objective of at least partly restoring the original hydrological state. Apart from measures to restore the hydrologic equilibrium, site specific treatments of salt-affected land are required to restore their productivity. Since the problem is invariably complex, it is essential to devise an effective strategy for each region involving both regional land use changes and site treatment. For such strategies detailed investigations of the processes, sources and movement of salts must be worked out. Some of the more frequently recommended practices are discussed below.

5.3.1 Practices for the recharge area

Any practices that will increase water use in the recharge areas or in other ways decrease the excess water which ultimately contributes to the seepage will help in the control of saline seeps.

i. Intensive cropping Intensive cropping and elimination of long fallows result in increased water use in the recharge area, decreased seepage flow and therefore a reduced salinity problem.

ii. Growing deep rooted perennials Crops differ significantly in their rooting depth and seasonal water use (Figure 40). Research in Montana (Brown and Miller, 1978) showed that among the crops tested, alfalfa had the deepest rooting system (6.10 m) and depleted the maximum soil water (787 mm). These scientists studied the effect of six years of cropping with alfalfa on a saline seep recharge discharge area. In 1971, the watertable depth was 0.30 m below the soil surface in the discharge area and at 6.2 m in the recharge area. Six years later the watertable dropped by 2.9 m in the discharge area and by 2.0 m in the recharge area. Research is also underway to test several other crops and cropping sequences in relation to control of saline seeps.

iii. Drainage Surface drainage of natural and man-made ponds in the recharge area is an effective, important and often inexpensive method of controlling excess water in the seepage areas. Drainage of ponded water in the recharge area should be preferred over drainage measures in the seep area. In the latter, additional problems are encountered which include disposal of saline effluents, lack of adequate outlets, etc. While surface drainage may be adequate for removing excess water from the recharge areas, it is often necessary to resort to subsurface drainage in the discharge areas which is a costly proposition.

Figure 40 Plant available soil water content as influenced by various cropping systems (Brown et al., 1976)

5.3.2 Practices for the discharge areas

i. Cropping As in the case of recharge areas, intensive cropping involving perennial and deep rooted crops having a high water use helps in maintaining the watertable at greater depths and checks salinization.

ii. Drainage The objective of drainage in the discharge areas is to lower the watertable and desalinize the root zone through leaching. Artificial subsurface drainage and leaching by applied water are generally not economic in the dryland areas. Even so, drainage works have been installed with variable performance in limited areas. Sommerfeldt et al. (1978) experimented with mole drains which are made by pulling a bullet shaped object through the soil with a mole plough. It consists primarily of a beam with a perpendicular blade extending downwards. On the end of the blade is the bullet shaped object which leaves a channel. The drains are quickly installed and the installation cost is low. Mole drains perform best in soils with sufficient clay content to be cohesive and where the soil is wet enough to be plastic. The disadvantages of the mole drains are the limited depth (60 to 70 cm) to which these can be made and their relatively short life span (2 to 3 years).

According to Van der Pluym (1978) and Miller et al. (1981) subsurface drainage of most saline seep discharge areas is unfeasible because of extremely low permeability of soils resulting from high content of montmorillonite clay and often high SAR of the soil solution, problems of waste disposal, poor accessibility for heavy drainage equipment and difficulty in locating saline seep focal points. Also the cost of materials and installation is relatively high and is often more than the land price. From these considerations, it would be better to provide drainage and adopt excess water control measures in the recharge area rather than to control the watertable through drainage in the discharge areas.

iii. Salt-tolerant crops Planting salt-tolerant species is an effective way of obtaining some economic returns while efforts are being made to improve the saline seeps. Investigations on the relative tolerance of plant species in several Australian states have indicated the value of growing the following in the saline seep areas (Mitchel et al. 1978):