and Union Gap) yet at Kiona, about 70 miles 

 downstream, tbe river flow (with no natunuL 

 tributaries at this time of year) will be 

 around 2,000 c.f.s. This 2,000 c.f.s. Is 

 made up almost entirely of Irrigation re- 

 turn flows. These return flows continue 

 into the river bed frcm ground water deple- 

 tion after the irrigation season has ended 

 in September. Table 28 lists the 19^9-53 

 average river flows at ftirker and Kiona 

 together with their difference and the avBi> 

 age monthly precipitation at Prosser. Since 

 this is an arid area (average yearly preci- 

 pitation at Prosser is 7 '5^ inches), the 

 difference in flow between Parker and Kiona 

 is made up largely of irrigation return 

 flows. 



Figure 6l is a plot of these flow 

 differences and the average monthly preci- 

 pitation. Irrigation return flows continue 

 through March and Increase abruptly with 

 the ccnmencement of the irrigation season 

 in April. Maximum return flows 6a^^ in May 

 when irrigation diversions axe ^gb, air 

 temperatures relatively low and consumptive 

 use is low. Return flows drop to around 



1,300 c.f.s. in July and August whan air 

 temperatures are high and consumptive use 

 Is greatest. 



From the table of water quality com- 

 parisons in 1953-54 for the Yaiima River 

 at Kiona (table 22) , it will be noted that 

 the time of highest water quality (lowest 

 mineral constitiients ) Is in April, about 

 the time when irrigation coonences. The 

 average monthly flow at Kiona in April is 

 about the same as for the preceding months 

 of January, February and Mca*ch and is less 

 than that in the succeeding mcxiths of May, 

 June and July. (Water quality constituents 

 are usually inversely proportional to the 

 flow. ) If it would be assumed that the 

 water quality constituents in April are 

 representative of those that would be pre- 

 sent in the absence of large return flows 

 and summertime food processing, a compari- 

 son can be made between these April values 

 and the high constituent values in Septem- 

 ber. 



Table 29 shows the comparison of the 

 April and September 1953-5^ constituent 



1.000 



OEC JAN FEB. MAR APR MAY JUN JUL AUG SEP 



MONTH 



RAINFALL - INCHES AT PROSSER 



YAKIMA RIVER PARKER TO KIONA 



IRRIGATION RETURN FLOWS AND PRECIPITATION 



FIG. 61 



89 



