PART VII — WATER RESOURCES, FORESTRY, AND AGRICULTURE 



heat may be feasible? Is it conceiv- 

 able that per capita power require- 

 ments may be reduced by reducing 

 power requirements in the home and 

 industry? Can climate control be 

 achieved? Will future urban centers 

 require less energy and water for 

 airconditioning? 



Navigation — Navigation is not an 

 extremely heavy user. Evaporation 

 losses from reservoirs from which 

 water is released to maintain navi- 

 gable depths downstream constitute 

 the primary consumptive use. The 

 quantity is probably so small that 

 it deserves little consideration as com- 

 pared with other demands on our 

 water resource. However, it is appro- 

 priate to ask what future transporta- 

 tion technology may be expected. 

 Will relatively slow, bulk transport 

 by water continue to be a favored 

 procedure? Will high-speed surface 

 or air transport encroach on the 

 market for bulk transport to the 

 point where future expansion of navi- 

 gation facilities may stop? 



Recreation — Like navigation, rec- 

 reation is not presently a heavy con- 



sumer of water. Primary water use 

 by recreation is evaporation from 

 reservoirs constructed solely for wa- 

 ter recreation or from an increased 

 water surface area in reservoirs be- 

 cause of projected recreation. It is 

 unlikely that reservoirs will be built 

 solely for recreational purposes in 

 water-short areas. Recreation does 

 not appear to be a factor of great 

 uncertainty with respect to future 

 water use. However, it may be ap- 

 propriate to mention here the pos- 

 sibility of evaporation suppression 

 from water surfaces by the use of 

 film-forming chemicals or covers. If 

 successful techniques for evapora- 

 tion suppression could be achieved, 

 requirements for many of the uses 

 discussed above could be reduced. 



Fish and Wildlife — It is currently 

 accepted that the maintenance of 

 fish and wildlife requires that a con- 

 tinued flow be maintained. A sub- 

 stantial part of this flow is eventually 

 discharged into the oceans where it 

 can no longer be used. Water re- 

 quirements for this purpose are 

 surely not well known. The mech- 

 anisms by which a reduction in dis- 



charge into estuaries may affect 

 marine life need to be established. 

 This need derives from two com- 

 peting aspects. We need to know 

 how much water must be permitted 

 to flow to the oceans in order to 

 maintain fisheries for both economic 

 and sports purposes, and the extent 

 to which this fresh-water flow in- 

 fluences other estuarine and oceanic 

 resources. We also need to know 

 the consequences of excessive flood 

 flows through estuaries. Are such 

 flows beneficial or detrimental? In 

 addition to the consequences for fish- 

 eries and wildlife, what are the effects 

 of regulating streamflows to the 

 ocean on sediment deposits in es- 

 tuaries and harbors and on nourish- 

 ment of beaches? 



In summary, probability estimates 

 of water supply are limited only by 

 hydrologic understanding, and solu- 

 tions appear to be close at hand. 

 Projections of water usage are heavily 

 dependent on projections of new 

 technology. Little effort has been 

 devoted to this latter problem and, 

 therefore, current projections of use 

 are quite uncertain. 



Water Movement and Storage in Plants and Soils 



Since only five feet of soil can 

 generally store fully ten inches of 

 precipitation and since evaporation 

 from soil and foliage returns to the 

 air about 70 percent of our precipi- 

 tation, these two factors represent a 

 significant portion of the hydrologic 

 cycle and a determinant of our water 

 resources. (See Figure VII-2) Further, 

 and less often noted, the relations of 

 precipitation, evaporation, and stor- 

 age will determine the escape of 

 soluble substances such as nitrate 

 from the region of roots and into 

 groundwater and streams. 



Because the plant roots are inter- 

 twined among the soil particles and 

 water flows readily from one to the 

 other, plant and soil — and, for that 

 matter, the atmosphere as well — - 



must be analyzed as a continuous 

 system. Then the components can 

 be examined in order of their impact 

 on the system, and the results used 

 to improve our understanding and 

 ability to predict the functioning of 

 the entire system outdoors. Fortu- 

 nately, our ability to cope with the 

 entire system has been advanced 

 materially in recent years. 



Total Evaporation 



Essentially, the soil-plant-water 

 problem is to measure the extraction 

 from the soil, conduction to the 

 leaves, and then evaporation from 

 the leaves. Some water may short- 

 circuit this path and be evaporated 

 from the soil or leach beyond the 



roots, but a lot — often most — 

 takes the route of soil to plant to air. 



Evaporation from the Canopy — 

 Recently, research has greatly im- 

 proved our understanding of how 

 water gets from the canopy of foliage 

 to the atmosphere above. When 

 evaporation from the canopy strata is 

 viewed as a factor in an energy 

 budget and evaporation and convec- 

 tion are set proportional to tempera- 

 ture and humidity differences, the 

 evaporation (and the temperature and 

 humidity of the air within the canopy 

 microclimate) can be calculated from 

 the weather above and below the 

 canopy, the profiles of radiation and 

 ventilation, the distribution of foliage 

 area, and the boundary layer and 

 stomatal resistance of the foliage. In 



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