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WSP05034
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Last modified
1/26/2010 2:16:40 PM
Creation date
10/12/2006 12:48:09 AM
Metadata
Fields
Template:
Water Supply Protection
File Number
8273.100
Description
Colorado River Basin Salinity Control - Federal Agencies - Bureau of Reclamation
Basin
Colorado Mainstem
Water Division
5
Date
7/1/1987
Author
BOR
Title
Monitoring and Evaluation of Salinity Control Projects - Interim Guide for the Colorado River Basin Salinity Control Program
Water Supply Pro - Doc Type
Report/Study
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<br />Z8GO <br /> <br />locations where tbe effect of the salinity control features <br />will be large relative to tbe existing salt load. <br /> <br />For verification of point -source salinity control projects, <br />samples may consist, of the difference in salt loads between <br />an upstream and downstream site computed over a short <br />period of time. Because the sample period can be much <br />shorter than a year, it may be practical,to collect a large <br />number of samples. However, the resolution of the Hest <br />~proves only from a 5 percent to a 4 percent detectable <br />difference as the sample size within each period increases <br />from 15 to 25. Therefore, 15 samples each during the <br />preproject and postproject periods should be the maxi- <br />mum necessary to provide an adequate comparison. <br /> <br />A similar analysis can be used to determine the number of <br />discharge measurements required to detect significant <br />seepage losses from a canal using the inflow-outflow <br />method. Assuming the measurement accuracy is 5 per- <br />cent, the minimum detectable seepage loss between two <br />measured cross sections is related to the number of <br />measurements as follows: <br /> <br />For any study requiring a large number of samples, t-tests <br />should be conducted regularly during data collection to <br />determine if the measured differences are significant. If <br />the differences are larger or the standard errors are <br />smaller than anticipated, fewer samples may be needed. <br /> <br />Surface Water <br />A fundamental assumption of sampling is that the sample <br />represents a derIDed population. Therefore, a repre- <br />sentative water sample can be obtained only if the source <br />population (stream reach or aquifer) is adequately <br />described. For surface_water sources this is not a difficult <br />problem. The sampled reach can be located on a map and <br />the streamflow at the time of sampling can be measured or <br />computed. If the sample is taken at an existing streamflow- <br />gaging station, streamflow can be computed from the stage <br />read from the recorder chart or staff gage. To compute <br />streamflow, the stage discharge relationship and the' <br />streambed datum correction must be known. If the <br />sample is not taken at a gaging station, streamflow should <br />be determined directly using standard procedures (Rantz <br />and others, 1982). <br /> <br />15 <br />10 <br />5 <br />2 <br /> <br />2 <br />3 <br />7 <br />30 <br /> <br />Samples for chemical analysis should be collected using a <br />depth integrating sampler, such as the US DH-48 (Guy <br />and Norman, 1970). This type of sampler accumulates <br />water along a vertical column from the water surface to <br />the streambed. A composite sample from several verticals <br />normally must be collected in order to represent the <br />average water quality in a channel cross section. The com- <br />monly used method for collecting a composite sample is <br />the eqnal-width increment (EWI) method (U.S. Geologi- <br />cal Survey, 19n, chapter 3), also called the eqnal-transit- <br />rate (ETR) method (Guy and Norman, 1970). This <br />method should be used except in the following <br />circumstances: <br /> <br />Detectable <br />seepage loss <br />(percent inflow) <br /> <br />Required <br />number of <br />samples <br /> <br />In this table, each sample is a paired inflow-outflow <br />measurement. As seel?age loss becomes small, the number ' <br />of measurements reqwred to detect the loss becomes very <br />large, This illustrates the major disadvantage of inflow-out- <br />flow studies on large canals with low seepage rates, <br /> <br />1. At streamflows less than l' cubic feet per second or <br />depths less than 0.5 feet, samples may be collected by <br />hand dipping a bottle in the centroid of flow, <br /> <br />30 <br /> <br />31 <br /> <br />
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