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C 16..:J The main canals and laterals gel)erally are operated continuously during the irrigating season; however, many ditches contain water only while fields along the ditches are being irrigated. Itis essential to document changes to the canal system, such as new canals, abandonment, or change in operation. Drains Drains ' remove water from the aquifer system only if the water table is above.the level of the drain bottom (the lowest most surface-water/sediment interface). The rate of water removal by drains is controlled by the wetted perimeter of the drain, the thickness and hydraulic conductivity of the drain bed, and the difference between the drain stage and the adjacent water"table elevation. As described previously, many interior drains in the Albuquerque area no longer function as drains because the water table remains below the drain bottom. However, some drains receive stonnwaterandcanal-return water (water in canals not diverted for irrigation flow into either another canal, wasteway, or drain), which, like canals, allows the movement of water from the drain to the aquifer. In addition, some drains are used to convey canal-return water to a point where the water can be diverted to another canal, such as from the Albuquerque Riverside Drain to the Arenal Main Canal (fig. 3B).The previously discussed digital data bases contain the distribution of drains, from which the length of drain reaches can be estimated; but not their dimensions. Information on the horizontal hydraUlic conductivity of soils with various textures was used in a drain design analysis for parts of the Albuquerque Basin (Willis; 1993). Hydraulic conductivities for 22 soil textures were estimated and ranged from 0,2 foot per day for a silty clay to 65feetper day for a gravelly coarse sand. Kernodle and others (1995, p. 110) assumed drain- bed hydraulic conductivity to be 1 fOot per day and assumed drain-bed thickness to be 1 foot in theirground.water~flow model. Hydraulic conductivities of beds of drains used to convey irrigation water probably are similar to those of canals. The modified simulation using the Kernodle and others (1995) model described in the "Rio Grande" section illustrates the operation of the riverside drains, Although seepage from the river and canals was reduced about 5 percent from the original simulation, the total simulated effect on flow in the river, which includes drain and canal seepage, was changed less than one- half ofl percent. The reduction in simulated seepage from the Rio Grande was almost completely compensated for by a reduction in seepage to the riverside drains; which return water to the river. This shows that the riverside drains have a buffering effect on seepage from the Rio Grande--that is, excess seepage from the river is captured by the drains and returned to the river. The essential information needed related to drains includes depth, width, and bottom elevation, which along with the elevation of the water table would. provide an estimate of wetted perimeter; the area in the inner valley where the drains are nonfunctional; and the reaches of drains that convey irrigation, water. The buffering effect of the riverside drains on river seepage shows that changes in the drain system may have a significant effect on the amount of river seepage that recharges the aquifer system. Therefore, documenting changes to the drain system is essen tial. Santa. Fe Group Movement of water in. the Santa Fe Group aquifer system is controlled by the lithologic composition oUhe Santa Fe Group and adjacent deposits, and the location and characteristics of internal or' bounding faults. . Because the majority of ground-water withdrawal in the Albuquerque area is yielded from the Santa Fe Group, the stresses from those withdrawals must 13