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<br />I <br /> <br />I <br />I <br /> <br />I <br />I <br /> <br />Another assumption of what constitutes water requirements is "full water require- <br />ments". As explained in Chapter III, for each month this is either the calculated <br />minimum water requirement or the historical diversion, whichever is larger. This <br />means that full water requirements over the growing season are greater than <br />minimum calculated water requirements. Thus, minimum water requirements were <br />used for reservoir demands and reservoir sizing in the operation studies. <br /> <br />I <br /> <br />Irrigation firmness was used as an index for measuring the degree of assurance of <br />obtaining the minimum water requirement. Irrigation firmness is defined as the <br />number of months during the irrigation season (April through October, seven <br />months) of the operation period (1;4 years) when irrigation demands are met fully. <br />During the months when irrigation demands are not fully met, the shortage may be <br />I percent to 100 percent. Firmness is expressed as a percentage as given in the <br />following equation: <br /> <br />I <br />I <br />I <br /> <br />I ' t' F' (%) Number months irrigation demand fully met 100 <br />rnga IOn Irmness = x <br />7 x 44 <br /> <br />I <br /> <br />For example, a reservoir which could always supply all of the water needed would <br />have a firmness of 100 percent. Most of the water in such a reservoir would <br />seldom be needed. In most years, capacity would exceed the need. Such a <br />reservoir would not be economically sound, in view of its oversizing. Eighty-five to <br />90 percent firmness is typically considered to be an acceptable degree in <br />formulating plans for an irrigation project. <br /> <br />I <br /> <br />I <br /> <br />I <br />I <br /> <br />Another factor is directly related to the need for some carryover storage capacity <br />at Cactus Park in order to meet minimum water requirements with an 85 or 90 <br />pecent degree of firmness. All of the water in the reservoir would not be used <br />each year, but it would be refilled the following spring. Normally, water would be <br />carried over from one year to another, in order to meet crop irrigation require- <br />ments in the years of unusually low stream flows and low rainfall in the growing <br />season. The amount of storage capacity needed to provide for carryover, while <br />maintaining firmness on the order of 85 or 90 percent, depends upon the reliability <br />of the source or sources of water supply. The more reliable the source, the smaller <br />the storage capacity for a given firmness. <br /> <br />I <br />I <br /> <br />I <br />I <br />I <br /> <br />IV-4 <br />