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Show Specifications are developed to meet the particular needs for information as to whether the land can meet the costs involved in making the land ready for use. These are comparable in many ways to those indicated as needed in considering land for irrigation. H. Land Classification of Soil Erosion A land classification of soil erosion is needed to delineate those lands which have been subjected to erosion as well as those which are now or may be eroded. Such a classifi- cation provides the basis for determining critical sediment source areas that are the cause of damaging sedimenta- tion. Such land classification is particularly essential for the public lands for which information is generally lacking. Classification of soil erosion is generally developed in field mapping of soils and of land capabilities, together with interpretation of aerial photos. The classification takes into account the extent, degree, and character of erosion as they are related to such factors as topography, geology, soil, precipitation, hydrology, climate, and char- acter and use of vegetal cover. Specifications are estab- lished for both geologic and accelerated erosion based on the kind and extent of sheet, gully, and wind erosion. They are applied to the surface of the land and to stream chan- nels, and are related to the kind, character, condition, and use of the vegetal cover or land. Maps showing erosion are prepared according to purpose to be served, usually on a scale of 1:24,000 or 1:12,000. For detailed operation programs, a scale of 1:4,800 may be used. See figure 39. I. Conclusions Basic data requirements for land classification will vary with the immediate purpose of the classification and the stage of development of the program under consideration. With respect to water resource and related land develop- ment, however, there are many basic data needs common to the various applicable land classifications. Many elements of the primary data needed are included in other sections of this document. Essential to most of these classifications are such common basic factors as topography, geology, soils, climate, and vegetation, and various elements con- sidered under hydrology and economics. There are in most classifications various special elements which are re- quired to meet the particular purpose for which classifica- tions are made. J. Recommendations It is recommended that: The collection and analysis of basic soils and other data used in land classification surveys for agricultural purposes be accelerated; Needs for basic or specialized data be anticipated and all available and applicable information be assembled and studied in preparation of legends for field surveys; Basic data be collected to classify public and private lands for agricultural purposes according to their capabil- ities, irrigability, the yield and use of water, the degree and extent of erosion, and the kind and character of improvements and management to be accorded them. IX. Basic Data on the Relation of Land Management to Runoff and Erosion Land management, as used herein, refers primarily to the manipulation of vegetal cover by forest culture, grazing management, and agricultural practices. The evaluation of the effect of these practices on peak flows, water yield, ground water recharge, erosion, and sedimentation is a major hydrologic problem. The relation of land management to floods, water yield, ground water recharge, erosion, and sedimentation of reservoirs has been the subject of much discussion and study for a number of years. While the net result has been to demonstrate that land management does affect the disposition of precipitation and the incidence of erosion and production of sediment, there is still much to be learned on the subject. The need for further data has become particularly apparent in view of the ever increasing demands for water, the depletion of the land resource by erosion, and the threat of rapid and costly sedimentation to many of our reservoirs. Evaluation of the effects of land management requires an understanding of movements of water as it relates to the earth, and requires collection of data on a nearly limitless series of combinations of such factors as plant cover (both type and condition), cultural practices, soil, slope, drainage area, and drainage pattern. These factors and their interrelations may vary with different conditions of climate, including character of precipitation, tempera- ture, humidity, radiation and wind, and also from one topographic feature to another. A. Basic Data Available The basic data available for evaluating the effects of land management on rates and amounts of runoff and 377 |