Duty of amount of land which can be irrigated with a given-quantity of water, or the relation which these bear to each other, is commonly expressed by the term duty of water. The investigation of the duty of water is one of the most complicated problems of irrigation. There is such a difference in meth ods of measurement, soils, crops, climatic con dition, ways of application of water, and fre quency of watering that the statements made by different persons are almost irreconcilable. In general, more water is used, or the duty is less, on the newer land than on that which has been cultivated by irrigation for some years. The rainfall also affects the quantity used, and as this is exceedingly irregular, the amount of water applied each year fluctuates. Seepage likewise complicates matters, for a field may often receive considerable water indirectly and require less by direct application. The duty of water is quoted at from 50 to 500 acres or more to the second-foot. For convenience the unit of 100 acres to the second-foot has been consid ered as indicating careful irrigating, although in the more southwestern portion of the and region this would be considered low, and in the northern part high.
Since the value of water per second-foot varies largely with its duty, it will be recognized that this value is exceedingly difficult to esti mate. However, it is necessary to arrive at certain averages in order to approximate the possible values of a river, or of a reservoir, in the future development of the country. It has been estimated that a perpetual water-right is worth from $25 to $50 per acre in a grain or grazing country, and as high as from $100 to $500 per acre for fruit land, rising in southern California for the best citrus lands even to $1,000 or more per acre. Assuming an annual supply of water as being worth $50 per acre irrigated and a duty of 1 second-foot to 100 acres, this quantity would be worth $5,000 and a stream furnishing a steady supply of 500 second feet would have a value to the com munity of $2,500,000. Considering stored water as having a value of $100 per acre of reclaimed land, producing fruit or other valuable crops and with a duty of two and one-half acre feet of stored water to each acre, then a storage reservoir holding 250,000 acre feet would jus tify an expenditure of $10,000,000.
Alkali and The excessive use of water in irrigation above referred to is a source of injury not only through the loss of the valuable life giving fluid but because this when in excess produces swampy conditions, reducing or preventing crop growth and, in many localities, causing a development of alkali upon the surface. Taking the arid region as a whole from 15 to 20 per cent or more of the agricultural lands formerly irrigated have been abandoned because of waste of water, much of which was largely preventable. The alkali con sists of mixtures of various kinds of earthy salts in different proportions, the most common of these consisting mainly of sulphate of lime or gypsum, known as white alkali or of sodium carbonate generally known as black alkali be cause of the burned appearance of the soil where it has been brought to the surface. In addition there are usually found in the alkali the ordinary table salt, chloride of sodium, salts of magnesium and similar minerals.
These have been set free by disintegration of rocks, but owing to .the scanty rainfall have not been washed away or leached out of the soil as has happened in more humid regions.
The accumulation of alkali in irrigated lands presents one of the most serious problems en countered in this method of agriculture. The injuries from the presence in excess of earthy salts are usually evident in the corrosive action on the tender bark, especially at the rootcrown. Experiments made in California show that apple trees are severely injured by the presence of 3,000 pounds of common salt per acre, this amount being disseminated through four feet in depth. On the other hand, the olive thrives where the soil contains as high as three tons of salt per acre, and the date palm flourishes on a soil so alkaline that hardly any other vege table survives. Alfalfa, when young, is easily killed by alkali, but it has been found to thrive in soil containing as much as 6,000 pounds of common salt, 3,000 pounds of carbonate and over 1,000 pounds of sulphate, per acre, distrib uted through six feet of depth. Sugar-beets also have been known to grow well where a large amount of alkali is present. Grapes ap parently are little affected by small amounts of alkali, while peaches and lemons are more sus ceptible to injury because of its presence. The recently introduced saltbush is notable for its ability to grow in alkaline lands, and sorghum and alfalfa, especially when the latter has reached maturity, are almost equally vigorous.
The most effective way of removing alkali is by under-drainage through tile laid at a depth of from three to five feet, the drainage water being allowed to escape into a stream, or into a well from which it can be removed by pumping. The troubles caused by alkali yield to careful treatment. The tiling of land for relief from surplus water and for washing out the alkali costs little more than drain-tiling as practised on eastern farms. In the govern ment demonstrations at Salt Lake City, Utah, where the percentage of alkali was very high, the lines of tile are 150 feet apart. The water descending into the soil dissolves large quanti ties of alkali near the surface and carries it off in solution. Land so tiled, even if badly alka line, can be returned to profitable cultivation in time, if heavily irrigated, and within one year can be used for the production of some crops suited to the climate. Large areas of alkali land in the West may be reclaimed at a cost below the actual increase in the value of the land. Drainage-works are a necessary adjunct of irrigation. On bench-lands or gently sloping hillsides the water which escapes from one man's farm should be caught and used by his neighbor below, under good management none being left to stagnate, the surplus from the cul tivated lands being often of great value in watering the lower meadows. There are many conditions where the question of disposing of the water is as important as that of obtaining it, particularly on the nearly level lands, where the subsoil has been filled to saturation by the water which has no opportunity to escape.