Is groundwater “recharge” a useful concept?

Garth van der Kamp, Andrew Ireson, Alan Barr

Dans les comptes rendus d’articles de la conférence: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: General Hydrogeology II

ABSTRACT: Groundwater recharge is generally seen as an important limiting factor in estimations of sustainable groundwater yields and as a useful concept in describing groundwater flow. Yet further examination of the standard definitions of firechargefl and fidischargefl show that they involve ambiguity, and are used to denote some quite different concepts. Groundwater textbooks present varying definitions of figroundwater rechargefl. Recharge is generally defined in terms of: a) flow into, plus additions to, the total volume of groundwater beneath the water table, or b) groundwater flow into an aquifer. The choice of definitions tends to depend on the context: whether the focus is on the subsurface portion of the hydrologic cycle or on estimates of sustainable groundwater withdrawals. The concept of groundwater recharge can be clearly and unambiguously defined for situations where the groundwater table is far enough below the ground surface so that there is no upward flux of water at any time from the water table back to evapotranspiration. In such cases disturbance of the water table is decoupled from evapotranspiration and net infiltration. Changes of the water table elevation by pumping do not change the recharge flux to the water table through the vadose zone. This concept of groundwater recharge is illustrated by the case of a site in Saskatchewan, Canada (BOREAS/Fluxnet fiOld Jack Pinefl) where the depth to the water table varies from 6 to 8 m below the ground surface in a surficial sand and the pine tree roots penetrate only to about 3 m depth. Long-term precipitation and eddy-correlation records for the site give a reliable record of net infiltration which equals groundwater recharge over the long-term because there is no surface runoff. The site water balance is illustrated in Figure 1 and the long-term precipitation, evapotranspiration and net infiltration (2002-2009, Barr et al, 2012, Table 3) are 527, 306 and 221 mm/year respectively. Over this 8 year period the change of soil moisture storage is estimated to be - 5 mm/year and the net recharge was 216 mm/year. Figure 1. Ten-day variation at the Old jack Pine site of the effective precipitation (rain and snowmelt - evapotranspiration), and depth to the water table. Winter snow accumulation is assumed to infiltrate over a 10-day melt period in April of each year. The years 2001 to 2003 were exceptionally dry. For the case described above groundwater management in terms of predicting the consequences of groundwater withdrawals would be fairly straightforward because the long-term groundwater recharge rate will not change if the water table is lowered (but changes of land-use and vegetation can of course change the net infiltration). For much of Canada the case is not nearly so simple because the water table is shallow and the groundwater interacts with the infiltration and evapotranspiration fluxes across the ground surface. For all such cases the water table rises and falls in response to changing downward and upward fluxes of water through the vadose zone and the concept of firechargefl and fidischargefl hardly seem applicable - are in fact difficult to define. Minor changes of the upward or downward fluxes to the saturated zone commonly lead to large fluctuations of the water table height and thus of the volume of water below the water table. If recharge and discharge are defined as changes in the volume of groundwater (below the water table) together with the flux away from the water table then such locations would have to be considered to be subject to frequent reversals from recharge to discharge conditions.

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