Dune sands

Due to the process of wind-loom movement of dunes ("saltation"), dune sands fall within a narrow range of particle size. Some 90% or more of a typical dune is sand, in the range of 0.1 to 0.5 mm. The remainder is fine silt, with a very small proportion of clay. Fertility and water retention capacity are low. Infiltration rate is very high, with consequent problems in irrigation application, also in leaching of fertilizer. This is obviously not a soil which would be given priority in selection of areas for irrigation development, if there were other options. In some situations, however, other options are not available, an example being the lower end of the Rajasthan Canal system in India. Fortunately the fine silt and colloidal material brought in with water from the supply canal, in the Rajasthan case, results in substantial improvement in the character of the virgin dune sand, and viable levels of productivity can be obtained. However, several factors remain, which make irrigation management in such an area difficult.

Sand is obviously highly susceptible to wind erosion. The same forces which formed the dunes will immediately begin to re-form dunes on levelled areas, if left unprotected. The most effective protection is a crop, or crop residue from a harvested crop. This implies that dune areas should be levelled only at a rate with which the cultivator can keep pace, i.e. keep under active cultivation. It also implies that, for an extended period, an area newly coming under irrigation will still have undeveloped dunes as islands within the already levelled and cropped portion of the area. From these dunes and dunes around the permanent perimeter of the irrigation area, hot wind-blown sand can destructively erode adjacent crops, particularly at the seedling stage. There is no simple solution to this problem in an area newly under development, as the only remedy involves planting of wind-breaks or secondary cover-crops on the dunes, and this may require a limited amount of irrigation by pumping and delivery by hose or movable sprinkler system. Such facilities are unlikely to be available to a small cultivator newly arrived in the area.

The very high infiltration rate of sand poses a problem in ensuring uniform distribution of water on the field. The solution adopted by some experienced cultivators is to divide a field (nominally a 50 m x 50 m basin) into narrow strips 2 m wide, by temporary ridges, and to direct the whole flow of some 2 ft³/sec (56 liters/sec) into each such strip in turn, completing the delivery in a matter of three or four minutes. The imponded water then infiltrates uniformly. Field application efficiencies as high as 75% can be obtained on dune sands which have been under canal irrigation for several years.

Sprinkler irrigation is a classical method of water application in such high infiltration rate soils. It is employed in large scale irrigation of desert areas. It is not, however, a solution readily available to the small cultivator, nor is it particularly efficient in areas of frequent high winds.

In the tertiary distribution system in dune-sand areas the problem is again wind-blown sand. An unlined tertiary, or water course, has two disabilities in such circumstances. First seepage losses are very high, compounding problems of rising watertable. Second, the channel can be filled and obliterated overnight in a sand storm. A lined channel can also be filled, but the in-filling sand can be removed and the channel restored to use. Clearing an unlined channel constructed in sand poses a problem as there is no evidence of when clearing has reached the original floor or sides of the channel; the original geometry of the channel is lost.

The use of covered lined channel, or pipe, can nominally avoid the problem of windblown sand. However, water from the supply canal carries sand and silt, which can deposit in the covered lined channel or pipe, as the flow velocities are necessarily small in this situation, due to low head and relatively flat gradient. A sectionalized removable cover on a lined channel would permit clearing of such a channel, but this procedure is not applicable with a pipe. A covered desilting cistern at the intake could be a solution, but its maintenance would need to be assured.

With regard to lining of small channels in dune sand areas, the lowest-cost solution, a trapezoidal section with sides supported by the fill, has been found troublesome, as the supporting sand may be eroded by wind, resulting in collapse of the linings. The alternative of a rectangular channel with structurally self-supporting sides is more satisfactory in this respect. Responsibility for cleaning of sand from small channels (water courses and minor canals) is a critical question. In view of the very considerable length of channel involved, particularly wafer courses, it is highly desirable that such maintenance be carried out by the beneficiary cultivators. This does not present a problem in a well developed area with well organized cultivator groups.- However, it can be a considerable problem in the early stages of settlement when some cultivators have not yet taken up their allotments, and a cultivator at the downstream end of a channel kilometers in length may find that the channel is blocked upstream where it traverses as yet unoccupied holdings. A dune sand area can be a very hostile environment for a cultivator, particularly in the early stages of its development. The farmer deserves particularly close institutional support.