| OCR Text |
Show uniform restriction of drawdown across the aquifer decrees the degree to which the aquifer may be exploited or mined. For the Salt Lake County aquifer an additional concern was that the vertical head gradient from the deep confined aquifer to the shallow unconfined aquifer not be reversed. This implies a need to control drawdowns at critical nodes where the shallow aquifer has water of poor quality. The maximum aquifer drawdown typically occurs at the pumping wells. Constraints to restrict drawdown thus need to be specified only at the nodes considered for pumping. Further, the drawdown from all background wells is a constant that can be independently estimated through aquifer simulation. Our concern then is with restricting the incremental drawdown due to the wells considered optimizable. The drawdown constraint at each node ( ij) where drawdown control is of interest may then be stated as : A na E X VQka * DRij ( 4.4) » - l k- 1 where DRjj is the permissible drawdown at node ( i j) The, permissible drawdown at each node was also varied parametrically to assess its effect on the optimal aquifer yield. The maximum drawdowns occur at the well nodes themselves and not at the other nodes of interest. Consequently, this constraint was specified only for the well nodes k. Further from considerations of parity of use across the 10 agencies, the permissible drawdowns were specified as a constant level DR below the 1982 level. This is stated as Hk > H^ 982 - DR k= l NW ( 4.5) ( 4) Water rights maintenance: Unfortunately, ground water rights are not usually stated in an explicit, quantifiable form. An adverse effect by an agency's pumping activities on the ability of another agency to pump water is construed as a violation of a water right. The adverse effect may be specified in terms of either an excessive drawdown or the abstraction of a certain amount of flow. It is often hard to arrive at criteria that honor a seniority or priority use structure, that are at the same time equitable and also lead to an efficient use of the groundwater resource from a regional perspective. The criteria adopted in our study represents an attempt at equity and efficiency. Other suitable criteria could however be used. To preserve ground water rights for each agency, flows ( net exchanges of water) across the geographical boundary of each water agency are restricted to predetermined levels. Flow is permitted across sections of each boundary, allowing exchange of water between each water agency. The sum of these exchanges across each boundary is maintained at predetermined levels. The predetermined levels may be set based on ( 1) the net exchange that would occur if there were no pumping, ( 2) the net exchange that would occur with the average historical pumping, ( 3) the net exchange that would occur if each agency exploited all of its existing water rights to the maximum. For the Salt Lake County application the 1982 pumping rates as simulated to steady state by Waddell et al ( 1986) were used to specify the permissible net exchange across each boundary. This constraint is derived in a manner similar to the corresponding derivation in 95 |
