AQUIFER SYSTEMATICS
The net benefit to the many water users of a given aquifer lies in access to the volume of water produced from the transi- tional period when aquifer storage contributes to basin yield and from the benefit gained from distributed wells that access water offset from distant sources without bearing the cost of transport. Hydrogeologic information also benefits planning by alerting users to situations where established pumping trends must decline before reaching sustainable, though low- er, water levels.
Historical Benchmarks
Hydrogeologists have sought to communicate these prin- ciples for generations. Theis (1940, p. 277) wrote that “recharge is governed (1) by the rate at which water is made available… by the flow of streams, or (2) by the rate at which water can move vertically downward through the soil to the water table.” Theis also discussed the availability of “rejected” recharge, which is added to groundwater where space is made available in formerly waterlogged areas. For a time, our science applied analytical models (Theis 1941; Glover 1974) to calculate sur- face water depletion rates. These days, numerical models such as MODFLOW (Harbaugh, 2005) represent those governing factors laid out by Theis.
Franke and Reilly (1987) reported on the effects of alterna- tive boundary conditions for the generalized flow domain of aquifers and surface water. They showed that aquifers are “open” systems, with new water added to the aquifer under the stress of development. It is revealing to see in their experiments the large portion of well production supported by newly-induced water, rather than from the baseline of “old” recharge moving through the aquifer. They show clearly that pre-development “natural” recharge is unaltered by develop- ment processes, either in advancing or retarding an eventual balance. Knowing recharge is of no help to the core questions (items a–d, above) of concern.
Where the attractive simplicity of the policy to “pump the recharge” persists as guidance for planning (such as in the Western Water Policy Review Advisory Commission 1998; or in the European Commission 2009), then wells and streams might incidentally sustain, but they also run the risk either of going dry (where PWL is limited), or of beneficial water resources potentially going unused (where abundant boundary sources are available). Thus, to simply pump the recharge does not control potentially undesirable outcomes.
The time required to approach a new balance with bound- ary sources might range across many orders of magnitude for various settings. As a rule, wells in Quaternary alluvium near mainstem rivers can be assumed to stabilize quickly, whereas wells in thick basins hundreds of kilometers from surface water (e.g., Thar Desert, High Plains) can be expected to drawdown for centuries and are likely to reach a PWL limit that requires declining (but not zero) future system yields. Between those extremes on the spectrum of distance, effective planning seeks some forward-looking calculations. Historical success in this effort has been seen by way of the approaches outlined below.
Hydrogeologic Procedure for Policy Support
The area of influence of a well-field expands at a rate depending on aquifer diffusivity (the ratio of transmissivity to specific yield), time, and the boundary locations encountered.
Six parameters suffice for addressing the hydrologic condi- tions of concern. Existing information or serviceable estimates are widely available on the following:
www.aipg.org
1. the map layout of interrelated water features, 2. the available surface supply for potential capture, 3. the vertical conductance of aquifers below streambeds, 4. targeted pumping rates from wells or wellfields, 5. aquifer diffusivity, and 6. practical pumping water levels (PWL) in wells
The scope of these general elements to be examined is out- lined here. The technical approach is to be fitted to specific cases. The first task is to inventory the surface-water avail- able to capture.
• Begin with mapping the aquifer, wellfield and stream reaches, springs, wetlands, and evapotranspiration (ET) areas in a region centered on the wellfield.
• Then, estimate the base flow, ET, and the rejected recharge amounts available for each surface water feature. Allow for past and committed depletions.
• Categorize each surface water feature as water-limited or vertical-conductance limited to avoid overstating supplies.
• Project the wellfield pumping rate to define the stress to be balanced. Pumping rate may grow, remain steady, or decline with time.
• Accumulate the availability of capturable water at the water features in radial order of distance from the wellfield to identify the set of nearby features that prospectively balance the pumping rate.
Where the groundwater conditions are part of the question, then the aquifer hydraulic properties must be investigated:
• Aquifer transmissivity and specific yield (i.e., the aquifer diffusivity).
• Calculated or simulated time for the radius of influence to enclose the maximum radius of capturable supply.
• Similarly, calculate or simulate the drawdown pattern. [These tasks can be done numerically or very simply in accordance with Cooper-Jacob (1946).]
• Plot the contours of drawdown inside the radius enclos- ing the offsetting sources.
• Estimate the practical PWL for wells in terms of drawdown below static (non-pumping) water levels. Reduce pumping-rate targets if the expected drawdown exceeds constraints on PWL.
• Calculate the time for the system to approach balance as the cone progressively deepens and depletes the set of offsetting sources. The time to approach balance is orders of magnitude longer than the time for the cone to reach the features.
This second sequence of ground-water tasks is best done with numerical models, but the Theis (1941) and Glover (1974) papers give analytical formulae. At this point in the technical approach, the planning questions (a–d, above) can be addressed.
Finally, hydrologists might assist the planners to manage other basin-wide objectives such as downstream obligations, waters required for habitat or ecological needs, salt manage- ment or other issues, and to design a policy for managing the overall impact of depletion (i.e., water replacement, augmen- tation, control levels, retirement of rights, reduced pumping).
Oct.Nov.Dec 2021 • TPG 9
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