| OCR Text |
Show The rapid spread of undesirable vegetation on overgrazed range is creating another water problem. Although very little is known about the effect of these changes on water supply, it is generally be- lieved that many of the invaders use more water than the grasses they replace. Certainly they cause a greater loss by evaporation, as the plants are usually taller than the grasses. Some, like juniper and mesquite, also prevent rain from reaching the ground. The total effect of these invaders upon the water supply is relatively small in comparison with losses by riparian vegetation, but is probably a sig- nificant factor in the total water supply of the basin. One of these invading weeds is halogeton, highly poisonous to sheep. There is no question that these invading plants are generally less effective in stabilizing soil than the former grasses. Resultant erosion has a direct bearing on water supply, reducing its quality and in- creasing sedimentation. And increased sedimenta- tion enlarges the area susceptible to invasion by salt cedar and similar vegetation. Conclusions Several types of weed-tree growth, especially salt cedar, are consuming large amounts of water in the basin. This draft on the available water supply is now probably over 1 million acre-feet annually and will undoubtedly increase as the various species con- tinue to invade the watershed. This loss is serious in a basin where competition for water already is intense. Among the plants of primary concern are salt cedar, cottonwood, and willow, which occupy riparian lands, and mesquite, juniper, cheat-grass, and nonpalatable weeds which occupy upland range areas. Although a number of agencies are study- ing the problem, its magnitude and its bearing upon the water resources are such as to justify prosecution of an immediate and adequate research program de- signed to determine the amount of water lost through transpiration by various types of plants and to suppress all nonbeneficial weed-tree or shrub growth. 10. Toxic Salts in Irrigation Water The Problem Effects of the use of irrigation water containing toxic salts. The Situation Water draining from all of the arid areas absorbs alkalis from the soil and carries them to the ocean. When water is used for irrigation and is dissipated to the air by evaporation or transpiration, the salts are left in the soil. Continuous deposits of such salts, if not washed from the soil3 will reach a con- centration sufficient to be toxic and even lethal to vegetation. The amount of flow required in the Central Arizona Project to carry away toxic salts from the basin at present is 154,000 acre-feet a year, and when the proposed project is completed 376,- 000 acre-feet will be required.74 This is based on the necessity for holding the concentration of salts in the underground storage at less than 5.5 tons of salts per acre-foot. In the Bureau of Reclamation's report, it is estimated that the salts brought into the Central Arizona Project with the irrigation water will total 2,070,000 tons each year. On the same basis, any other irrigated area must use part of the water diverted for irrigation to carry off the salts continually being left on the land by evaporation and transpiration. This also raises a question as to the advisability of large diversions of water free of alkali from the basin. (See Problem C-13.) One salt that may have serious ultimate effect on irrigated areas is boron. Although a valuable "trace" element in fertilization, in concentrations of even a few parts per million it is lethal to some crops, and is cumulative. These salts usually reach stream channels from a few isolated springs. As such, they can often be controlled and the salts pre- vented from reaching the streams. Conclusions A full program for the basin must include careful provision for flushing of salts from irrigated areas. While this need has been given due attention by the Bureau of Reclamation in individual project plans, a full study is needed of the cumulative effects of all possible project development on water quality, particularly in the lower basin, and upon asso- ciated water requirements for prevention of alkali accumulation. Authorization of a basin-wide water resources program, or of project units thereof, should be made in the light of a complete analysis of irrigation water quality requirements. Insofar "Bureau of Reclamation. Table 5, H. Doc. 136, 81st Gong., 1st sess. 441 |
