OCR Text |
Show PART II NATURE, USE AND MANAGEMENT OF WATERSHED LANDS Soils Distribution of broad groups of soils within the region are shown on the General Soil Map in Appendix VI, Land Resources and Use. Within these broad groupings certain soil characteristics have major signifi- cance in determining the use and management of watershed lands. These selected characteristics are: (1) available water-holding capacity; (2) slope; (3) erodibility; (4) salinity and alkalinity; (5) water table or wetness; and (6) rock, stones, cobble, or gravel on the surface. Available water-holding capacity of a soil is mainly determined by the texture of the soil profile and the effective soil depth or rooting zone. Total available water-holding capacity of deep clayey soils is high as contrasted to deep sandy or gravelly soils. Shallow soils less than 20 inches to underlying bedrock have limited available water-holding capacity regardless of the texture of the profile. Watershed treatment practices to increase water intake and depth of penetration are dependent upon soil texture and depth. Consequently, the proportions and distri- bution of sandy, loamy, and clayey soils by depth classes within the region have significance in watershed management. Shallow soils dominate steep slopes and are extensive along canyons and in mountain foothills. Application of watershed management practices must take into account the low available water-holding capacities of the soils as well as the generally low annual precipitation. The semi-desert valleys have a high proportion of moderately deep and deep soils but low annual precipitation is a limiting factor. High proportions of stones, cobble, and gravel throughout the soil profile on a volume basis limit available water-holding capacities even though the soils are deep. Mountain soils have significant acreages of soils with coarse fragments exceeding 50 percent of the soil volume. Slope gradients within all units on the General Soil Map range from gently sloping to very steep. However, canyon lands, foothills, and mountains constitute the major acreage of steeply sloping soils. Mechan- ical treatment of soils on slopes exceeding 35 percent is difficult or impossible due to operating limitations of equipment. Valleys, mesas, and plateaus contain a higher proportion of gently sloping soils than the other land forms. Water erosion is closely associated with steep slopes, lack of vegetal cover, and rapid runoff rates. This is particularly true of sheet erosion which is most evident on cropland. Severe gully erosion within the region is most devastating to deep, alluvial soils in narrow valleys. This is due to a concentration of runoff from adjacent steeper slopes where vegetal cover has been depleted. The high agricultural production of these alluvial soils under irrigation justifies costly protection measures on the watershed. Both management and structural practices to control water erosion are needed. |