(Perma)Culture and Sanity
Soil conservation swales are designed to slow or stop soil erosion in pastures by halting downhill flow of rainwater runoff. They are placed across sloped pastures, almost on contour, at intervals of around 50 to 100 feet going down the slope. Soil conservation swales are common in the Southwestern US, where the erosive effects of grazing cattle are high relative to the land's ability to recover.
Since soil conservation swales are designed for conventional pastures, however—that is, pastures lacking trees—if they held onto the water they collected instead of allowing it to escape, harmful waterlogging of the soil could occur. Grasses by themselves simply do not remove enough water from the soil to prevent waterlogging where swales stop and hold all runoff.
By allowing collected water to flow freely out of one end of the ditch instead of infiltrating, waterlogging is prevented. This is why soil conservation swales are sloped 1% (one inch per 8 feet or so) in one direction, and left open on one or both ends. A 1% slope is sufficient to allow collected water to leave the swale, yet at the same time is gentle enough to give any eroded soil time to settle out of the water before leaving the swale (hence the words 'soil conservation' in the name).
In arid and semi-arid areas, however, rainwater is too precious to release unnecessarily. Where rainwater is scarce, therefore, planting trees along water-holding, on-contour swales prevents waterlogging while still making productive use of the water. Trees absorb and transpire huge amounts of water through their leaves, not only preventing soil waterlogging but also humidifying the air and helping maintain a healthy rain cycle.
Most arid and semi-arid land has plenty of water to allow people to thrive comfortably—just not enough to waste through treating it in what have become the accepted ways in wetter areas of the US. Methods of wasting water considered 'normal' include treating rainwater as a disposal problem, drinking chemically-treated polluted water, defecating and urinating in clean water, using more land to grow lawn grass than any other agricultural crop, etc.
Reprinted from (Perma) Culture and Sanity Website