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<br />CC 16 .:8 <br /> <br />The Rio Grande channel has changed with time (National Biological Service digital data, <br />1935,1989; Bureau of Reclamation, 1955, 1975, 1992). Documenting any changes that also may <br />affect seepage is essential. <br /> <br />Canals <br /> <br />The system of canals in the Rio Grande inner valley is composed of main canals, laterals, <br />and ditches. The main canals move water along the valley from the Angostura diversion for <br />distribution to the irrigated areas upstream from the diversion at Isleta. In general, laterals <br />distribute the water from the main canals to more localized irrigated areas, and ditches distribute <br />the water from the main canals or laterals to irrigated fields. For the purpose of the following <br />discussions, the tenn canals refers to all of these categories, unless they are specifically identified. <br /> <br />In the same manner as described for the Rio Grande, the rate of movement of water <br />between canals and the alluvium in the inner valley is controlled by the wetted perimeter, <br />thickness of the canal bed, hydraulic conductivity across the canal bed, and the difference in <br />hydraulic head across the canal bed. Gould and Hanson (1994) describe the canals from <br />Albuquerque to Isleta Pueblo to typically contain silt loam at the bottom (the lowest most <br />surface-water sediment interface) that grades into sandy material at 1 to 3 feet below the canal <br />bottom. The effective bed of the canal that would restrict the vertical movement of water would <br />be about 1 to 3 feet thick. Because the silt in the canal bottoms grade into coarser material, 1 foot <br />is probably a reasonable estimate for the average effective thickness of canal beds in the <br />Albuquerque area. If the water table is at or above the bottom of the canal bed (about 1 foot <br />below the canal bottom or higher), the hydraulic-head difference is the difference between the <br />canal stage and the hydraulic head just beneath the canal bed (about 1 foot below the canal <br />bottom). If the water table is below the canal bed, as it is in most of the Albuquerque area, the <br />hydraulic-head difference is the difference between the canal stage and the elevation of the <br />bottom of the canal bed. The canal system has the effect of spreading water over the inner valley. <br />Because many interior drains no longer function as drains in the Albuquerque area, much of the <br />canal seepage may now be retained in the aquifer system rather than be intercepted by the drains <br />and returned to the river. <br /> <br />The digital data bases described earlier also include the distribution of canals in the <br />Albuquerque area. The length of any particular canal reach can be estimated from these data, <br />but not the canal dimensions, However, the 1972 and 1992 digital data from the Bureau of <br />Reclamation include information on whether a canal isa main canal, lateral, or ditch, which may <br />indicate some of the dimensions. The hydraulic conductivity of canal beds in the Albuquerque <br />area can be estimated using canal seepage rates detennined by Gould and Hansen (1994). These <br />tests indicated that bed hydraulic conductivity below the normal operating level of canals is <br />relatively consistent because of silt sealing. Based on the seepage rate (an average of 0.40 foot <br />per day) determined for the Atrisco Ditch at the normal operation water surface, the water depth <br />(1.8 feet), and an assumed thickness of 1 foot for the canal bed (the water table is below the <br />bottom of the canal bed), the estimated hydraulic conductivity across the canal bed is 0.14 foot <br />per day [0.40/ (1.8 + 1)]. On the basis of this information, Kernodle and others (1995, p. 20, 110) <br />assumed canal-bed hydraulic conductivity to be 0.15 foot per day and assumed bed thickness to <br />be 1 foot in their ground-water-flow model. The consistency of canal-bed hydraulic conductivity <br />in the range of normal canal operation indicates that this component is sufficiently known. <br />However, seepage from canals is dependent on the total hydraulic conductance of the bed <br />(which includes the wetted perimeter) and on the head difference between the canal and the <br />aquifer. To better quantify hydraulic relations between the canals and the aquifer system, the <br />essential information needed is canal widths, elevations of canal bottoms, normal operating <br />stage, proportion of time a canal contains water, and whether a canal is cement lined or unlined. <br /> <br />12 <br />