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Show VIII. Basic Data Pertaining to Land Classification for Agriculture A. General Nature of Land Classification Land may be classified for many purposes. In a broad sense, land classification may include on the one hand rough subdivisions of the earth's surface into plains, hills, and mountains, and on the other a careful and highly detailed classification, acre by acre, to show suitability for a specialized use. This report is concerned with certain basic data used in land classification required for land use, irrigation, drainage, clearing, and soil erosion control. Classification groups the characteristics of land, surface and subsurface as they affect amounts and availability of waters that fall as rain and snow; that affect the disposition of this prime supply to soil water, to ground water and to stream flow; that affect its availability for use, control, storage, and conservation; and that set bounds to practical and economic projects and programs in development of interrelated water and land resources of the Nation. B. Basic Data for Land Classification Basic data required for land classification in a water resource program will vary with the immediate purpose of that classification and also with stages of development. Data already available in the form of topographic, geologic, soil classification, and vegetal cover maps, and from climatic, hydrologic, and other records are interpreted and evaluated in terms of their application to the needs of a particular type of land classification. Such an analysis should integrate various items of basic data necessary for the land classifications required in planning, design and operation of water resource programs. As statements cov- ering these items of basic data are contained elsewhere in this report, the following discussion will deal principally with the status of available information of the kinds ob- tained through field surveys of land classification.8 After a thorough study has been made of the informa- tion supplied by the pertinent items of basic data referred to above, a field survey is required to integrate soils and other data and to collect supplemental data necessary for the type of land classification desired. As a part of the field survey, information must also be obtained as to use made of the land, whether forest, grassland or cropland. Supplemental facts will be collected, such as type, density and condition of vegetation, degrees of erosion, and effec- tiveness of measures, if any, of runoff retardation and soil conservation. In initiating field surveys, it is important that require- ments of the various action programs be given considera- tion in developing the specifications and map legends in order that basic data obtained will prove adequate. This will involve such things as determination of scale of map- 1 For a more complete study of the entire subject, see "Land Classification in the United States." National Re- sources Planning Board, March 1941. ping and expression of all factors in appropriate class ranges, either qualitative or quantitative and their integration in the field survey. C. Status of Available Information The status of the most widely used types of land classi- fication is briefly discussed. They include classifications for land capability of agricultural, range and forest lands, for irrigation, for drainage, for land clearing and for occurrence and hazards of soil erosion. The coverage of field surveys adequate for classifying land according to its capability and for planning runoff retardation and conservation measures on farms and ranches is shown in figure 35. Coverage amounts to about 322,000,000 acres, mostly of farm and ranch land in soil- conservation districts. Land classification surveys of existing and proposed irrigation projects have been made to determine the extent and degree of suitability of land for sustained irrigation farming. Figure 38 shows the coverage of these surveys. About 38,800,000 acres have been classified as to irri- gability. The classification involves primary consideration of (1) productive capacity, (2) costs of land development, and (3) costs of production associated with each land tract. The land class assigned to a specific parcel of land is a composite evaluation of all the interrelated physical and economic factors which have an influence on the land's value for irrigation. Attention is called to the fact that limited information is available on irrigability of land in the western part of the country. It is true generally in the United States that the amount of basic data available on the public lands is generally inadequate. D. Land Use Capability Classification Land classification for programs of safe land use should recognize inherent land characteristics that affect physical suitability of land for use and stability or for permanence of land while it is in use. A combined interpretation of this kind is made in the land-capability classification, which is used in the program of technical assistance to farmers in soil conservation districts, in water flow re- tardation by watershed management, and in related activities. In the land-capability classification, bodies of land are grouped for easy reference in determining safe land uses and in recommending land use and conservation practices. The grouping is made at four levels: (1) Land suited or not suited for cultivation; (2) eight land-capability classes distinguished according to the degree that safe, sustained land use with a given technology is limited by essentially permanent land features; (3) subclasses distinguished according to kind of limitation; and (4) significant 371 |