effective ground cover, improve soil infiltration 

 characteristics, and stabilize waterways will insure 

 both efficient water conservation and help avoid 

 excessive soil loss. Sediment control is best effected 

 at its source : the watershed. 



Unstable stream channels are another important 

 source of sediment. Rectification of stream chan- 

 nels and stabilization of streambanks will minimize 

 sediment production from this source. 



Once sediment occurs in streams, it can be re- 

 moved where impoundments are used. The con- 

 struction of sediment traps has proved to be very 

 effective for this purpose, if properly designed. 

 Another possibility is the construction of desilting 

 works at diversion points. Chemicals have also 

 been used for flocculating colloidal sediments. 



Phytotoxic Substances 



The control of phytotoxic substances in irriga- 

 tion water is difficult. Where these materials origi- 

 nate from industrial or municipal sources, control 

 should be focused at the point of origin. Once they 

 are present in irrigation water, removal is not eco- 

 nomically feasible. Substances such as insecticides 

 and herbicides can be hazardous if misused. Con- 

 trol of the use of these materials can prevent their 

 becoming a problem in irrigation agriculture. There 

 are good indications that eliminating tail water and 

 other surface returns from irrigation provides an 

 excellent means for reducing and controlling pesti- 

 cide residues in return flows (73). 



Monitoring and measuring 



Certain principles relating to monitoring and 

 measuring water quality are common to all agricul- 

 tural uses. These will be discussed later. There are, 

 however, certain factors peculiar to irrigation 

 water sources which should be pointed out. 



Sampling of water for analysis on a daily or 

 periodic basis will depend upon numerous varia- 

 bles, including: sources and adequacy of water 

 supply, crops grown, discharges of water quality 

 deterrents into the stream above the points of diver- 

 sion, and geographic location of irrigated areas 

 with respect to sources of supply. 



Monitoring of water supplies on a scheduled or 

 unscheduled basis provides information for daily 

 or weekly irrigation unit operational purposes and 

 checks on changes in water quality resulting from 

 upstream changes. The frequency of sampling and 

 analysis for operational purposes will be dependent 

 upon previously obtained and correlated data with 



the timing of additional samples and parameters 

 to be evaluated, based upon conditions peculiar to 

 each geographic area. If significant changes in con- 

 centrations or constituents are noted from water 

 samples taken, increased frequency of sampling 

 and additional monitoring points may be desirable. 

 Possibility of industrial wastes upstream should be 

 evaluated. 



Historical water quality data should be reviewed 

 when considering the type, location, and frequency 

 of sampling. If such data are not available, a sys- 

 tematic water quality sampling program to provide 

 background information is often desirable to evalu- 

 ate changes in water quality which may occur with 

 time. 



Water for a given area may be obtained from 

 one or more sources; i.e., ground water, one or 

 more surface reservoirs, unregulated streamflows, 

 and return flows from other uses. The ground 

 water source may be directly used by pumping 

 from the formation, or by ground water discharges 

 to a river upstream of the point of irrigation diver- 

 sion. As quality variations usually occur very 

 slowly in underground supply sources, sampling of 

 the source can be at intervals of several months 

 and often can be safely taken on a yearly basis. 

 In instances where ground water discharges up- 

 stream from the point of diversion form a signifi- 

 cant portion of the total supply, depletion of the 

 ground water discharge may affect the quality of 

 water in the stream. Under such conditions, pru- 

 dent management would provide for regular as- 

 sessment of sources and the quality implications of 

 changes in available supplies. 



Unregulated streams can be expected to have 

 the largest fluctuations in water quality throughout 

 the year. Monitoring programs for such streams 

 generally will require sampling at more frequent 

 intervals. During periods of flood flows, monitor- 

 ing of suspended solids is important. The fre- 

 quency of sampling can be reduced as the percent- 

 age of the stream system regulated by reservoirs is 

 increased. 



Operation of reservoirs can improve the quantity 

 and quality of water at downstream locations. At 

 locations where water temperatures may be an 

 important variable to monitor, vertical profiles of 

 reservoir water temperature should be obtained 

 before releases; and, where possible, water should 

 be released from that segment of reservoir having 

 the most favorable temperature. With reservoirs 

 on two or more streams supplying water to the 

 same lands, adequate data should be obtained to 

 provide for either operational blending of supplies 

 or to indicate that direct delivery will not have any 

 adverse effects on lands or crops. 



177 



