These comments are directed to water professionals (real or wanna-be) and not to hoi polloi (not intended as a derogatory term). Some faithful readers might get a sense of déjà vu when they read this post. I feel obliged to repeat that December 2008 missive.
Today there is a lot of chatter about water budgets and how similar they are to checking accounts. You know the story: outflows = withdrawals and inflows = deposits.
Of course, like your checking account, water outflows should not exceed water inflows, lest your hydrologic system becomes depleted. It's a good analogy, easily understood. I first heard it 45 years ago in Marty Fogel's Watershed Management class at the University of Arizona. I use it in my classes. But I don't teach it as a water management tool.
Caveats
1) Few 'checkbook hydrologists' talk about the stocks, only the flows. If you don't know how much water you have in your system (analogous to the checking balance) you cannot accurately assess how depleted your system is. If you see your outflows exceeding inflows, you know you're going to be in trouble if that keeps up, but you don't know when. 10 months? 10 years? 100 years?
2) The stock assessment is generally more difficult for groundwater systems than surface water systems. People lament higher outflows relatively to inflows but do not know how much is stored in the system. So what's the % of depletion?
3) Inflows, outflows and stocks do (more often than not) vary with time. So your systems are transient, not steady state, which means you need to keep measuring and calcuating.
4) Steady-state budgets are especially dangerous as a tool to manage a groundwater system. Reason: as you develop (start pumping) the groundwater, the budget becomes invalid. Discharge (outlfow) and recharge (inflow) change, and the increase in discharge might cause an increase in recharge (by harvesting 'rejected recharge' in Figure 1 below from Theis [1940]). How about that - increase withdrawals from your checking account to increase deposits!
5) Declining water levels in an aquifer - whether a confined or unconfined one - do not necessarily signify a state of 'groundwater mining'. What declining water levels signify is that the aquifer is seeking a new equilibrium and some water is coming from storage.
John D. Bredehoeft ('The Water Budget Myth Revisited: Why Hydrogeologists Model', Ground Water, Volume 40, Issue 4, pages 340–345, July 2002) said it best:
Within the ground water community, the idea persists that if one can estimate the recharge to a ground water system, one then can determine the size of a sustainable development. Theis addressed this idea in 1940 and showed it to be wrong-yet the myth continues. The size of a sustainable ground water development usually depends on how much of the discharge from the system can be “captured” by the development. Capture is independent of the recharge; it depends on the dynamic response of the aquifer system to the development. Ground water models were created to study the response dynamics of ground water systems; it is one of the principal reasons hydrogeologists model.
Download Theis_source_of_water (original published paper)
Download Theis-1940 (better copy - open-file report)
Another great resource: Another excellent resource is the U.S. Geological Survey's Circular 1186, Sustainability of Ground-Water Resources, by W.M. Alley, T. E. Reillly, and O.L. Franke.
Upshot: water budgets are useful for illustrative purposes, but otherwise (water resources devlopment and management) must be used with great caution, especially for groundwater systems (don't even think about it).
”It is our responsibilities, not ourselves, that we should take seriously.” - Peter Ustinov (thanks to Faruck Morcos)
Recent Comments