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Here is a response from Ron Stork, long-time FOR Conservation staffer... a bit more technical.
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Drip irrigation creates a wetting front (like any type of irrigation). A wetting front is the portion of the soil where water from the drip irrigation stops and the soil beyond is dryer. This is where soluble minerals accumulate and may precipitate. You can see this with furrow irrigation too. The water in the furrows rises and white minerals accumulate at the top of and in the center of a furrow. Such local salination is only a problem if sensitive plant tissue must grow into the salinated zone (mostly a problem with seedlings). When this local or micro salination is a problem, more uniform irrigation like rain or sprinkler irrigation is used to ensure that a problem wetting front does not appear on the surface of the ground.
The drip-irrigation wetting front is, of course, disrupted by rainfall, which drives soluble minerals downward. So the notion that drip causes long-term local surface micro-salination along the drip-irrigation wetting front is not true where winter rainfall is reasonably significant.
Soluble minerals can come from minerals in solution, either in the irrigation water or the soil profile itself. It can also come from dissolving minerals of the soil – although this process is slow.
Success of irrigated agriculture requires that soluble minerals are not allowed to build up to toxic levels in the root zone of the soil. This is not usually a micro-salination problem, but a larger scale issue. Ideally enough water is applied (by whatever method) to move excess minerals out of the root zone. There are two places for successfully removed minerals in excess of plant requirements to go: saline drainage water or the ground away from the root zone. If the salts cannot move away from the root zone, then the root zone will become increasingly less suitable to grow commercial crops.
Since drip irrigation reduces the amount of water that evaporates from the ground, drip irrigation means less water can be used to grow a crop—sometimes significantly so. Since water contains dissolved minerals, irrigated areas that receive less imported water will receive less dissolved minerals, reducing the need for saline (and sometime toxic) drainage (either into rivers or into deeper soils that can receive water). Drip also allows more precise application of water to the ground, which provides another tool to manage the location of problem groundwater.
In a nutshell, drip irrigation makes salt-management easier to handle, and is particularly helpful when water supplies are short, expensive, saline, highly valued, or the groundwater table is high and saline (and therefore cannot or should not receive more water). Still, drip irrigation cannot solve every problem inherent in irrigated agriculture. Some lands just don’t have a long-term future in irrigated agriculture.
On the groundwater allocation issue, there are institutional mechanisms (courts, agreements) to divide up a groundwater supply fairly. Of course, ideally, important or troubled groundwater resources would be subject to regulatory authority of the state and local governments, but in most cases the authority to do so is either unclear or unused. Nevertheless, where there the motivation to allocate the groundwater resource fairly is strong, court adjudication or settlements have been implemented.
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