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Tl.GO Other items which should be considered in mass-balance hydrosalinity budgets inclnde: 1, Canals flowing throngh the study area require both an inflow and outflow measurement, but a single chemical analysis may be adequate if specific conductance is con- stant along the canal, ' 2. Crop and phreatophyte consumptive use is an impor- tant outflow and should be estimated as accurately as pos- sible, 3. In areas with significant inflow or outflow of regional ground water, the mass-balance approach is not applicable since determination of such flow is difficult. Other methods, e.g., geochemical modeling, would have to be used for the analysis. Prediction of project effects is made by computing the mass-balance hydrosalinity budget for existing conditions and then nnposing the changes expected to occur becaus~ of salinity control implementation. These changes could m- clude a decrease in flow from a saline point source, such as a well or spring, or a decrease in return flow caused by canal or ditch lining or a reduction in onfarm deep per- colation, Usually, simplifying assumptions must be made concerning other conditions that could affect dissolved- solids loading in the project area. Several common as- sumptions are: . The dissolved-solids concentration in return flow generaIly is assumed to be unaffected by a decrease in return flow volume for project areas overlying , saline geologic formations, such as the Mancos shale, · Phreatophyte consumptive use can be assumed to decrease in proportion to the seepage reduction' and depth to ground water, · Cana1 diversions mayor may not remain the same as apolitical consideration, Federal agencies can- not presume what water users will do with their ap- propriatious, The water saved by canal lining is as- sumed tll be wasted directly into surface channels, left in storl/ge, or delivered to the farm. If it is delivered, it cou1d reduce an irrigation shortage, return to the stream as tailwater, or increase deep percolation. The Bureau of Reclamation's Technical Guidelines for Salinity Control Invesagations (1978) can be consulted for a more detailed discussion of assumptions used in hydrosalinity analyses, The use of ,all assumptions must be adequately defended in reporting the hydrosalinity study. Some assumptions can be tested during project verification, . The average annnal crop requirement is usuaIly held constant, but consumptive use can be assumed to increase if the project will reduce an existing irrigation shortage. Verification of Project Effects The verification stage is required to evaluate the actnal ef- fects of a project. Of most concern is the reductioD in dis- solved-solids loading. Statistical tests may be applied to determine the significance of the difference between , preproject and postproject loading. Preproject data must be available from the prediction (planning) stl/ge. There- fore, the design of a verification data-collection system must ~ considered far enough in advance of project con- struction to provide adequate and statisticaIly reliable baseline data. Verification sampling should also be used to test the impacts of specific project features, such as the reduction in canal seepage due to lining, or to test the validity of assumptions, such as no increase in the dis- solved,solids concentration of return flow after project im- plementation. Verifying the salinity control project effects includes (1) determining that project features are functioning as 8 9 -