OCR Text |
Show variations called land-capability units within the sub- classes. See figures 36 and 37. E. Land Classification for Irrigation Land classification for irrigation projects is accomplished in large part through the application of physical and chemical criteria and the consideration of economic factors. The latter are important because, in establish- ing the land classification specifications for a given project, it is necessary to evaluate each class of land not only in terms of sustained production under irrigation, but also in terms of its probable use and the income which will be available to meet land development and project costs. Additional data must be obtained, beyond those which are already available. See figure 38. Whenever land classification for irrigation is contem- plated, specifications are developed in terms of "arable" land. This is defined, in light of all information avail- able at the time, as land which in adequate units and when properly provided with essential improvements of leveling, drainage, building, irrigation facilities, and the like, will have sufficient productive capacity for crops, under sus- tained irrigation agriculture, to meet all production ex- penses. Included in the latter are a reasonable return on the investment, repayment of reasonable irrigation and improvements costs, and a satisfactory level of living for the farm family. As the project investigations are ex- tended and additional information obtained, the "irri- gable" lands become the specific lands which would be served with water under the final project plan. Three types of land classification for irrigation purposes are recognized, each representing a standard scale of operation, i. e., reconnaissance (1 : 24,000), semi detailed (1 : 12,000), and detailed (1 : 4,800). The detailed classification is applicable to reappraisal of existing projects, development of the final plan for projects approaching construction, and feasibility investigations for project authorization. Depending upon local conditions, the reconnaissance or the semidetailed classification may be used in the preliminary analysis and in delimiting areas for the detailed classification. The semidetailed classification may be used also as the basis for project authorization but it is considered inadequate for develop- ment of the final project plan. Six classes of land, four arable and two nonarable, are recognized as follows: Class 1.-Lands which are highly suitable for irrigation farming, being capable of producing sustained yields of a wide range of climatically adapted crops at reasonable costs. They are smooth lying with gentle slopes. The soils are deep and of medium to fairly fine texture with mellow open structure allowing easy penetration of roots, air, and water and having free drainage, yet good water holding capacity. Land development such as leveling and providing the farm distribution and drainage system is relatively simple to accomplish and can be attained at reasonable cost. Class 2.-Lands measurably below class 1 lands in pro- ductive capacity, adapted to somewhat narrower range of crops and more costly to prepare for irrigation or to farm. They have one or more deficiencies in soils, topography or drainage, either susceptible of correction, at justifiable costs, or provide lowered yields or increase production costs. Class 3.-Lands which are inferior to class 2 lands because of greater deficiencies in soils and have unsuit- able topography or drainage. Although approaching marginal utility for general crop production they are still considered suitable for irrigation especially if intermingled with better class lands. Class 4.-Lands having an excessive deficiency or re- stricted utility but which special engineering and economic studies show to be suited to irrigation under certain conditions. Class 5.-Lands temporarily held as nonarable pending completion of economic or engineering studies to deter- mine their suitability or feasibility for irrigation. Class 6.-Lands not meeting the minimum requirements of the next higher class and small areas of suitable land lying within larger bodies of nonarable land. F. Land Classification for Drainage Land classification for drainage partakes of many of the same elements found in both land capability and irrigation. Its purpose is to determine those wet lands which are or may be made suitable for agricultural use through drain- age. It takes into account the potential productive capacity, the costs of development, and the costs of production. Land classification for drainage is developed through consideration of pertinent basic hydrologic and soils data with reference to the potential crop productive capacity of the area where drained. Thus specifications are de- veloped which provide a measure of determining not only the suitability of land for crop production, but also whether the potentialities of the land are sufficient so that the benefits resulting from drainage will equal or exceed the development costs. The classification of the land for drainage purposes follows with certain modifications those outlined above for irrigation. At present about 5,000,000 acres have been classified, which is 10 percent of the lands potentially available for drainage. G. Land Classification for Land Clearing The clearing of land suitable for agriculture but now covered with trees and other growth requires a type of land classification comparable to those outlined above. There are about 10,000,000 acres of such land in the United States, most of which has not been classified, and approximately 7,000,000 in Alaska which should be classified in the near future. Its primary purpose is to determine those wooded or brush-covered lands which may be suited for agricultural production when cleared of impeding vegetation. Such a classification takes into account the same elements of productive capacity, costs of clearing and associated development, and the cost of production. Land classification for land clearing purposes is de- veloped by consideration of climatic, hydrologic, topo- graphic, and soils information and in reference to the capability of the land to produce crops when cleared. 91160&-50------27 375 |