PART VII — WATER RESOURCES, FORESTRY, AND AGRICULTURE 



the probability that a proposed reser- 

 voir will be adequate against any of 

 these ten alternative futures. 



Streamflow is inherently more vari- 

 able than precipitation and it is fair 

 to assume that we know the statisti- 

 cal parameters of precipitation with 

 greater accuracy than those for 

 streamflow. It follows that the 

 stochastic generation of precipitation 

 data should be a more certain process 

 than stochastic generation of stream- 

 flow data. Stochastically generated 

 precipitation data can be converted 

 to streamflow by deterministic simu- 

 lation models, although the process 

 would be substantially more expen- 

 sive than direct stochastic generation 

 of flow data, since deterministic 

 simulation is inherently more com- 

 plex and time-consuming. Preliminary 

 work on stochastic generation of 

 rainfall has recently begun, but fur- 

 ther research should be encouraged. 



The Relevance of Climate — In 

 addition to the stochastic properties 

 of future streamflow, a number of 

 other issues arise before the planner 

 can be content with his projections 

 of future water supply. The first of 

 these is the question of long-term 

 climatic trends. An abundance of 

 data demonstrates the existence of 

 such trends in terms of geologic time 

 and in terms of periods as short as 

 a few hundred years. However, no 

 sound basis exists for predicting the 

 existence of a trend and its conse- 

 quences over the next century. Cli- 

 matic trends could alter the water- 

 supply outlook in arid and semi-arid 

 regions, since the hydrologic balance 

 is sensitive to small changes in pre- 

 cipitation input or evapotranspiration 

 outgo. Techniques that could iden- 

 tify causes and project trends, even 

 in an approximate fashion, would be 

 extremely valuable to the water- 

 resource planner. 



The Relevance of Human Activ- 

 ity — In addition to natural climatic 

 trends, future water supplies may 

 be affected by man-induced changes, 

 both intentional and inadvertent. 



Intentional changes include those 

 brought about by land-management 

 practices, vegetation management, de- 

 salinating of brackish or saline wa- 

 ters, or effective reclamation of waste 

 water. The question that confronts 

 the planner is "Will any of these 

 become practically useful and if so 

 when?" The issue is the evaluation 

 of probable rates of technological 

 advance. It will be seen that similar 

 questions arise in the discussion of 

 water usage. 



Inadvertent changes in water sup- 

 ply may be brought about by urban- 

 ization, which increases surface runoff 

 and decreases infiltration to ground- 

 water. If one can make reasonable 

 projections of future urban growth, 

 deterministic hydrologic models can 

 project the alterations in streamflow 

 and accretion to groundwater. More 

 subtle are the effects of air pollution, 

 urbanization, and changes in land 

 use and vegetative cover as they 

 may affect climate. These possibilities 

 underline the importance of research 

 on climatic change. 



Estimates of Water Use 



The problem of predicting future 

 water use is far more complex than 

 that of predicting water supply, if 

 only because of the much larger 

 number of components that must 

 enter the forecast. It is convenient 

 to divide the discussion of water 

 use into the requirements for the 

 several purposes to which water is 

 most commonly applied. Before each 

 of these purposes is discussed, how- 

 ever, two general topics should be 

 noted. 



General Considerations — First, the 

 distinction between diversion and 

 consumption should be underlined. 

 For many purposes, large quantities 

 of water are diverted for use but only 

 a small fraction of the diverted water 

 is consumed; the rest is returned 

 to the environment — sometimes de- 

 graded in quality. (See Figure VII-1) 

 An outstanding example is the use of 



water for cooling in industry and 

 power generation, which actually con- 

 sumes very little water; most of the 

 water used is returned to a stream 

 or to the groundwater substantially 

 warmer than when originally diverted. 



Because of the re-use aspects, dis- 

 cussion of diversion requirements is 

 confusing. Here we will consider 

 only consumptive use. Consumptive 

 use is defined as that portion of the 

 water which is evaporated or com- 

 bined in the product so that it is no 

 longer available for re-use in the 

 original source system. 



A second topic which deserves 

 consideration on a general basis is 

 that of population forecasting. For 

 nearly all water uses, estimates of 

 population and its geographic dis- 

 tribution are fundamental. If prob- 

 ability estimates of future water use 

 are to be derived, they must begin 

 with estimates of probable future 

 population. Research has been done 

 on the variance of population esti- 

 mates as indicated by statistical eval- 

 uation of historic predictions. A 

 more fundamental study might ex- 

 plore the uncertainties in each of 

 the factors involved in population 

 forecasting. 



The most difficult problem is the 

 forecasting of local population by 

 county or city units. Factors that do 

 not enter national population fore- 

 casting are involved in predictions 

 of the distribution of population. 

 Not the least of the factors that may 

 affect future distributions is govern- 

 ment policy concerning desirable 

 population distribution. Some re- 

 search on the optimal size of popu- 

 lation concentrations may be useful. 

 Is there a city size at which the 

 unit cost of infrastructure is mini- 

 mized? What are the advantages of 

 population dispersal against increased 

 growth of major metropolitan cen- 

 ters? 



Domestic Water Use — The ques- 

 tion of domestic water requirements 

 depends largely on two issues. One 



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