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close this book Design and operation of smallholder irrigation in South Asia
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Soil surveys and land classification

Soil surveys classify the physical and chemical characteristics of the soils of an area. Irrigability surveys (land classification) add a further dimension, i.e. the potential economic productivity of the lands of the area in question. Land classification came into vogue during the major campaign of irrigation development in the Western United States, carried out by the Bureau of Reclamation in the early 1900's. At one point Congress decided that in some cases public funds were being spent on bringing irrigation to lands which, for reasons of soil deficiency or other factors, could not provide a reasonable living to the irrigator nor an economic return on the capital expenditures involved. It was then decreed that future project proposals should include information on the economic irrigability of the lands concerned. The system of irrigability classification evolved by the Bureau at that time has remained a key feature of irrigation planning, and is still widely used. The term "land classification" was -employed, rather than "soil classification. as other factors aside from the type of soil (pedology) were involved, including the cost of bringing water to the particular lands concerned. Criteria for a number of factors including soil depth, infiltration rate etc. were established, also specifications for the field surveys. The soil surveyor who previously had confined himself to soils now became a member of a multi-disciplinary team mapping economic land capability. When the Bureau extended its activities to the investigation of overseas projects the same approach was employed, although irrigability criteria were modified to suit the local situations.

Irrigation of smallholder areas involves the same soil factors (pedology) as does irrigation of larger holdings. However, with respect to irrigability classification, there are substantial differences, the principal one being the prospective input of the cultivator to the development of his holding. This may far exceed conventional economic limits. The wellbeing of the smallholder and his family is irrevocably determined by the productivity of his holding, and he has little other opportunity for bettering his situation than improvement of his land. The low opportunity cost of labor of the farmer and his family, given time, can accomplish wonders. An apparently barren boulder-strewn area with unproductive subsoil of a few inches depth can be changed, with patience and labor, into a fertile field. Steep slopes can be converted to small terraces, each with carefully constructed stone pitching.

It is not intended to convey that conventional economic irrigability classification is irrelevant in smallholder areas. It is very relevant in determining whether or not to bring irrigation into a particular project area. However, once a decision has been made to proceed with a project, irrigability classification is less important in determining whether to provide service to local areas with particular deficiencies. A small cultivator within the project perimeter but unfortunate enough to have relatively poor soil would be doubly unfortunate if he were to be excluded from supply of water. Provided that there is the technical possibility of substantially improving his holding, particularly under irrigation, it can be argued that equity demands that the cultivator in question be supplied with water and given the opportunity to make that improvement.

Irrigability classification involves making certain assumptions regarding the irrigation practices which will be followed. A case in point is the classification of lands as suitable, or unsuitable, for cultivation of wet-land rice (paddy). This involves consideration of infiltration rates. Rates of more than 2-3 cm per day are usually considered excessive for wet-land (flooded) paddy. Such lands would normally be classified as unsuited to that crop. However, smallholders in traditionally rice-eating areas may irrigate paddy in soils with infiltration rates ten times that amount, using semi-wetland techniques, i.e. without continuous flooding, rather than growing a more appropriate crop requiring less water.

Incorporation of soils data in an irrigability classification, without also reporting on the soils data separately, may be quite appropriate in a feasibility report prepared by a major organization which is also responsible for detailed design and execution of a project, as well as its investigation. However, where project design is subject to review and possible modification by agencies other than the one which carried out the original field investigation, such as prospective financing institutions, a separate soils survey report should also be provided. It is not readily possible to extract basic pedological data from an irrigability report (to "unscramble the omelette"). Pedology is basic, while irrigability classification involves judgement on many factors other than those related to soils, judgements on which other agencies subsequently involved may not always concur.

Irrigation of many soils, including the commonly-occurring silty or sandy loams, is relatively straight forward and does not call for extensive knowledge of pedology on the part of the irrigation engineer. Problem soils may be encountered, however, and these present the engineer with the difficulty that soils science is a complex subject, obscured by an esoteric nomenclature ("taxonomy") which is intimidating to other than a soils scientist. There is no middle ground in the literature, which either stops at simple soils water relationships, or requires a depth of background in soil chemistry and physics which only a soils scientist would have time or inclination to acquire.