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By Root 2170 0

irrigation water fixture at the base of each fruit tree or crop row and allows just enough water to drip out as the tree’s or crop’s roots can absorb, then little water is wasted, and there is no problem. But if the farmer instead follows the commoner practice of “broadcast irrigation,” i.e., flooding the land or else using a sprinkler to distribute the water over a large area, then the ground gets saturated with more water than the roots can absorb. The unabsorbed excess water percolates down to that deeper layer of salty soil, thereby establishing a continuous column of wetted soil through which the deep-lying salt can percolate either up to the shallow root zone and the surface, where it will inhibit or prevent growth of plants other than salt-tolerant species, or else down to the groundwater table and from there into a river. In that sense, the water problems of Australia, which we think of as (and which is) a dry continent, are not problems of too little water but of too much water: water is still sufficiently cheap and available to permit its use in some areas for broadcast irrigation. More exactly, parts of Australia have enough water to permit broadcast irrigation, but not enough water to flush out all the resulting mobilized salt. In principle, problems of irrigation salinization can be partly mitigated by going to the expense of installing drip irrigation instead of broadcast irrigation.

The other process responsible for salinization, besides irrigation salinization, is dryland salinization, potentially operating in areas where rainfall suffices for agriculture. That’s true especially in the areas of Western Australia and parts of South Australia with reliable (or formerly reliable) winter rains. As long as ground in such areas is still covered with its natural vegetation, which is present all year, the plants’ roots take up most of the rain falling, and little rainwater remains to percolate down through the soil to establish contact with the deeper salt layers. But suppose a farmer clears the natural vegetation and replaces it with crops, which are planted seasonally and then harvested, leaving the ground bare for part of the year. Rain soaking the ground when it is bare does percolate down to the deep-lying salt, permitting it to diffuse up to the surface. Unlike irrigation salinization, dryland salinization is difficult, expensive, or essentially impossible to reverse once the natural vegetation has been cleared.

One can think of salt mobilized by either irrigation or dryland salinization into soil water as like a salty underground river, which in some parts of Australia has salt concentrations three times those of the ocean. That underground river flows downhill just as does a normal above-ground river, but much more slowly. Eventually, it may seep out into a downhill depression, creating hypersaline ponds that I saw in South Australia. If a farmer on a hilltop adopts bad land management practices that cause his land to become salinized, the salt may slowly flow through the ground to the land of farms lying downhill, even if those farms are well managed. In Australia there is no mechanism whereby the owner of a downhill farm that has been thus ruined can collect compensation from the owner of an uphill farm responsible for his ruin. Some of the underground river doesn’t emerge in downhill depressions but instead flows down into above-ground rivers, including Australia’s largest river system, the Murray/Darling.

Salinization inflicts heavy financial losses on the Australian economy, in three ways. First, it is rendering much farmland, including some of the most valuable land in Australia, less productive or useless to grow crops and raise livestock. Second, some of the salt is carried into city drinking water supplies. For instance, the Murray/Darling River provides between 40% and 90% of the drinking water of Adelaide, South Australia’s capital, but the river’s rising salt levels could eventually make it unsuitable for human consumption or crop irrigation without the added expense of desalination. Even more