Salinization and related drainage problems are commonly associated with irrigated agriculture, especially in arid and semi-arid regions. In the dryland agriculture practiced in humid regions, most of the water in the crop root zone originates from rainfall, which has low salinity. The small amount of salt that enters the system is flushed through the root zone by the relatively large amount of excess water (water exceeding crop evapotranspiration and natural evaporation) available. By contrast, the water applied to irrigated crops is obtained from surface runoff, which picks up salts or gets more concentrated by evaporation as it flows over the land and into surface waters or subsurface aquifers. If all irrigation water is taken up by the crop (i.e., there is no excess), the salt contained in this water will accumulate in the root zone until the salinity reaches levels that will inhibit crop growth. If excess water is applied, as is usually the case, it will carry the salt through the root zone to deeper soil layers or, when the soil is highly impermeable, laterally to drainage ditches. If the water table is deeper than a few meters, the salt in the deeper soil layers will be transported downwards until it reaches ground-water. In many cases, the salt carried by groundwater later enters surface waters which may serve as irrigation sources for downstream users. If the water table is within a few meters of the ground, the salt held in soil moisture below the root zone during the growing season may be drawn upwards into the root zone by evaporation (capillary rise) during the fallow season. This salt must be leached out of the root zone during the next growing season or the salinity will eventually accumulate to harmful levels.
Since irrigation with slightly saline water requires, by its very nature, that water must be applied at levels exceeding crop evapotranspiration, it is important that the excess water (drainage) be removed from the crop system. Otherwise, it may raise groundwater levels and increase salt accumulation in the root zone. The problem of rising groundwater levels coupled with salinization is widespread in irrigated regions of the western United States, Asia, and our case study region in Australia. Although it is possible to install drainage systems which collect and divert much of the excess water used to leach salt, these systems can be expensive to construct and maintain. Furthermore, they just displace the problem of salt accumulation to another location (or user). Since the drainage water is more saline than the applied water, it is less desirable for irrigation. If the drainage water salinity is too high, this water must be treated as a waste product.
The scope and magnitude of irrigation-related salinization problems is well documented in Ghassemi et al.5 An informative example is the Murray-Darling river basin located in southeastern Australia. This semiarid basin covers about one-seventh of Australia and contains some of the continent's most productive agricultural land. It is distinguished by very low runoff (only one percent of precipitation) and high evapotranspiration. Salinization problems are important throughout the basin but have received particular attention in the lower Murrumbidgee River watershed, a 40,000 ha region planted predominantly in irrigated paddy rice, wine grapes, citrus, and dryland pasture. It is estimated that approximately 25% of cropland in the lower Murrumbidgee is slightly salinized while up to 15% is severely salinized. Moreover, the amount of affected land is increasing each year.
Annual rainfall in the lower watershed averages around 400 mm/yr, too little to support dryland cultivation of most food grains. Much of the irrigation water used to grow crops in this area is diverted from the Murrum- bidgee River, which rises in the more humid upland areas to the east. Irrigation related salinization problems in the Murrum- bidgee watershed are aggravated by the naturally high salinity of the shallow groundwater, which is within two meters of the surface over extensive sections of cropland, and by gradually increasing salt inflows to the river. These inflows are the result of agricultural drainage in the lowlands and dryland salinization (induced by deforestation) in the uplands.
In the Murrumbidgee watershed, artificial drainage systems are generally installed in fields planted to high value crops such as grapes and citrus. The effluent from these systems goes to downstream users in the watershed and then either to the Murrum-bidgee River or to evaporation ponds. The paddy rice is generally grown on low permeability soils which permit the paddies to be flooded throughout the growing season. Much of the salt entering the paddies eventually leaves when they are drained at the end of the growing season. However, the recharge to groundwater is sufficient to have created extensive groundwater mounds beneath rice growing areas in the Murrumbidgee. The local water table rise has aggravated salinization due to capillary rise, especially in neighboring non-rice growing areas. Crops grown in these areas (especially grapes) generally cost more to grow than rice but consume less water and can be more profitable. As more salt accumulates in the root zone during the fallow season, more water must be used to flush the soil before planting. This puts further stress on the limited water supplies available for irrigation.
Rising water tables and associated problems of salinization have induced growers, government agencies, and other groups interested in the lower Murrumbidgee watershed to develop plans to insure that agriculture in the region can be sustained over the long term. The current plan specifies that rice can be grown only in rotation on approved land (with low infiltration rates), up to a limit of 30% of total farm area. However it is not clear either:
1. That these restrictions will insure that the region's agriculture is sustainable; or
2. That this is the most economically efficient way to achieve sustain-ability, particularly when external costs to downstream users and ecosystems are considered.
A study currently being carried out by the authors is examining both issues in more detail.
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