| OCR Text |
Show area, and drainage pattern. These factors and their interrelationships may vary with different conditions of climate, including character of precipitation, temperature, humidity, radiation and wind, soils, types of land management, and also from one topographic feature to another. The effect of land management upon runoff and erosion from plots and small watersheds is well established for a number of combinations of soil, cover, condition, and climate. But little has been done to bridge the gap between results on these small areas and those on large water- sheds. Special attention should now be given to the development of methods for accurate estima- tion of effects of land management for larger watersheds. Erosion has received considerable attention from the st andpoint of the effect of different types and densities of cover on rates of soil loss, but little attention has been paid to the erosion process itself. Too little information is available on the natural Ia^vs involved in the detachment and transportation of soil particles. Likewise, facts are lacking on the effects of plant cover, litter, and biological organisms in the soil, and on the stabilization of soil particles. Infiltration rates have been determined, with various types of equipment, for a relatively large number of plant soil complexes, but application of results has often been difficult because of the unexplained variation in rates encountered. Bet- ter utilization of available data and improved methods of determining infiltration rates will re- quire greater knowledge of the mechanism of the infiltration process and its application to plant soil complexes not yet investigated. There is a lack of sufficient information to de- termine surface storage characteristics of different kinds of land for various rates of rainfall and runoff. More data are needed on the influence of land management on subsurface storage and on the rate at which storage opportunity is affected by evapo-transpiration. More data are required on geological formations and structure as they affect storage, movements, and recharge of under- ground basins. Although considerable data have been accumu- lated on the rate of transpiration of different individual types of vegetation, much of it is not applicable in water balance computations for combinations of types of cover. Data are needed on rates of evapo-transpiration for various field conditions, the mechanisms of the evapo-trans- piration processes, particularly in relation to avail- able supplies of water, and the interrelation of evaporation and transpiration under different site conditions. Further studies are needed to determine the disposition of precipitation after it enters the soil in order to arrive at the amount which goes to ground water reservoirs and the influence of watershed management practices on water yields. Enough is known from sporadic experiments thus far to justify the establishment of a network of 20 to 30 pilot drainage basin hydrologic sta- tions to determine practical effects of land uses and conservation measures on runoff retardation, on recharge of soil and ground waters, on control of soil erosion and sediment production, and on yield of water from river basins in important regions of the country. Land Classification Land may be classified for several important purposes. Certain basic data used in land classi- fication are required for land use, irrigation, drainage, clearing, and for appraisal and control of soil erosion. Land classification is one of the important methods of interpretation of basic data in the development of projects and river basin programs. Characteristics of land, surface and subsurface, are grouped as they condition amounts and avail- ability of waters that fall as rain and snow; as they affect the disposition of soil water to ground water and stream flow; as they affect its availability for use, control, storage, and conservation; and as they set practical bounds to projects and programs in the development of interrelated water and land resources of the Nation. Basic data required for land classification in a water resources program will vary with the im- 104 |
