PROBLEMS FROM LAND OCCUPANCY 185 



bursts in the hot summer months. The stream flow from the 

 area, as from other areas in the Salt River drainage basin, 

 comes chiefly from the winter precipitation. The contribu- 

 tion to stream flow from summer precipitation is limited to 

 that from channel interception and overland runoff. 

 As summarized by Rich: 



Vegetation or evaporation from bare soil could use more than 

 all the summer precipitation in the Southwest, and consump- 

 tive use is dependent on total precipitation less the small 

 amount of surface runoff that results from summer thunder- 

 storms, and part of this is dissipated from dry stream channels 

 by riparian vegetation and evaporation. Use during the winter 

 period depends on growing conditions. The water that is sur- 

 plus first satisfies the soil-moisture deficit and the balance is 

 yielded as stream flow. Water use during the spring and early 

 summer depends on the precipitation and the amount of mois- 

 ture held in the soil from winter precipitation. Thus consump- 

 tive use by forest and range vegetation depends on the amount 

 and distribution of the rainfall, the climate, topography, and 

 the storage capacity of the soils of the watershed as well as the 

 type of vegetation and the degree to which its use overlaps the 

 period of water surplus. The principal way in which man can 

 influence water yield is by sound management and use of vege- 

 tation cover, through proper livestock grazing, timber harvest- 

 ing, etc., so that consumptive use overlaps and interferes least 

 with the water-yielding period. 



From a water-yield standpoint, the major question is not 

 whether watersheds bare of vegetation would yield most water, 

 but which type of vegetation interferes least with water yield 

 and still controls sediment. In the Southwest grasses are dor- 

 mant and do not use water during the winter water-yielding 

 period. Use is largely confined to the summer period when there 

 is no surplus water, and surface evaporation alone could con- 

 sume all the precipitation. During the winter, high transpira- 

 tion by half-shrubs, winter annuals, and evergreen shrubs, all 

 characteristic of a deteriorated vegetation, tends to reduce water 

 yield. Total annual use by grasses is slightly higher than evapora- 

 tion from bare ground but lower than losses from a deteriorated 

 vegetation. The benefit of well developed and maintained plant 



