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r fll.60 Ponding tests and seepage meter studies are described in a U.S. Bureau of Reclamation (1961) technical report, Inflow-outflow studies utilize discharge measurements that are described in a U.S. Geological Survey Water-Supply Paper (Rantz and others, 1982). . determined, but geohydrological analysis is often inade- quate. The hydraulic conditions under which canal seepage occurs must be specifically determined for test reac~es and at least qualitatively estimated for the remamder of the canaI system, Basically, two conditions may exist. If the water table inter- sects t!'e ~ prism, the pore water in the canaI-bank material will be under positive pressure (greater than at' mospheric), and the see~age rate will. be controlled by the rate of wate! movement m the phreatic aquifer, If the pore water In the canal-bank material and beneath the canal prism is at or below atmospheric pressure the ' seepage rate. will be controlled by the hydraulic ~roperties of the material and the gradient conditions beneath the canal" Generally, seepage will be higher for this free ' drainage condition than for the water table' condition T~ansects of piezometers or wells can be nsed to det~r- mme the geohydrologic condition, If the geohydro~o~c co?dition is not the same during the s:c:page test as It IS durmg normal operation of the canal, direct use of tes~ results to estimate annual seepage loss are not appropnate. Other conditions that might differ be- twe~n testing and o~eration include water temperature, ~ediment concentral1on in the water, and biological growth In the wate~ and along the canaI banks. In these cases, s~epage es~ates need to be adjusted to account for the differences m geohydrologic or other conditions. When cana1s and laterals are lined or replaced by pipe seepage losses may be estimated by assuming conveyan'ce l~es of 0,07 cubic,foot per sqnare-foot per day for canal lining and no seepage loss from pipelines. Several methods are ,C011lD1only used to measure canaI seepage, These include: ' 1, Ponding tests, 2, Inflow-outflow studies, and 3. Seepage meter studies. Ponding tests and inflow-outflow studies measure seepage from the entire canal prism in a defined reach. Measure' ments from selected reaches are extrapolated throughout the canal system based on reach characteristics, Seepage estimates from ponding tests generally yield the most precise results, Seepage meters measure seepage through a small area. These measurements must be extrapolated to the entire canal prism and then throughout the canaI system. Al- though seepage meters can provide the most accurate point-measurement, l:Xtrapolation errors substantially in- crease the uncertainty of the estimated seepage from a defined reach. ' Inflow-outflow studies can be conducted over large reaches, therefore requiring the least extrapolation of the three methods. However, the uncertainty of an inflow-out- flow measurement is proportionate to the total flow in the canal, and the error in measurement can be'much greater than seepage in a low-permeability reach. To extrapolate seepage measurements throughout the project area, the canaI system must be adequately charac- terized. Reaches should be classified by soil type, con- veyance properties (mean flow, wetted perimeter, and slope), and geohydrologic setting. In saIinity-control studies, soil type and conveyance properties generally are t i I i i' Onfarm Ditch Seepage , Onfarm ditch seepage is generally the smallest part of the total o~arm water loss, but it can be significant. Becanse of the tune and cost required to run seepage studies data from e~ting studies normally are used to establish ' preproJect seepage losses from earthen ditches, If 14 15