PROBLEMS FROM LAND OCCUPANCY 163 



and impermeable clays and bare rocks permit negligible infil- 

 tration of water into the ground. Some slopes are so steep that 

 appreciable runoff may occur even though the surficial ma- 

 terials are moderately permeable; other areas so flat that water 

 from precipitation may remain long enough to permit infiltra- 

 tion into relatively impermeable soils. Some regions are so 

 arid that the soil is powder-dry at nearly all times, others so 

 wet that the ground is saturated practically to the surface. 



Enlightened programs of land management, watershed 

 management, and soil-and-water conservation are devoted to 

 the correction of the untoward effects of man's past unwise use 

 of the land. These programs include a wide variety of opera- 

 tions to achieve a corresponding variety of results in specific 

 localities. Grassed waterways and some channel modifications 

 are designed to permit overland flow but to prevent soil re- 

 moval. Strip cropping, contour plowing, and terracing are de- 

 signed to slow up and deflect overland flow on sloping, culti- 

 vated terrain. Dense vegetative cover of forest, brush, or grass 

 may prevent overland flow and soil loss; but in some places 

 the soil is porous enough to permit subsurface storm flow, 

 which reaches streams rapidly enough to be included in floods. 

 The various practices that impede overland flow provide in- 

 creased time for infiltration, and where these are combined 

 with practices that increase the rate of infiltration, a greater 

 amount of moisture is bound to find its way into the soil. 

 Ground-water reservoirs will be replenished if infiltration 

 continues after the soil has reached field capacity, but not 

 until then. 



A change in the vegetative cover or of tillage practices is 

 likely also to cause a change in the rate at which water is re- 

 turned to the atmosphere by transpiration and evaporation. 

 All plants use water and thus deplete the supply of soil mois- 

 ture. In some areas of semiarid climate a common water- 

 conservation practice is to allow fields to lie fallow — bare of 

 vegetation — in alternate years; the basis is that the reduction 

 in transpiration results in an increase of soil moisture avail- 

 able for crops in the intervening years. 



