The mechanism of natural ground‐water recharge and discharge: 2. Laboratory column experiments and field measurements

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<jats:p>A mathematical model has previously been used to simulate one‐dimensional, vertical, unsteady, unsaturated flow above a recharging ground‐water flow system. Results from this model are given further credence by their successful correlation with the results from a laboratory column experiment using a tensiometer‐transducer system. The process of ground‐water recharge involves the entry into the saturated zone of water made available at the water‐table surface, together with the associated flow away from the water table within the saturated zone. Ground‐water recharge rates are controlled by the regional ground‐water flow system. Infiltration to the water table through the unsaturated zone leads to water‐table rises that provide the source of replenishment which allow prevailing rates of ground‐water recharge to continue. On the Canadian prairies, cases of recharge‐sustaining infiltration to the water table are isolated in time and space. Only spring snowmelt, and on occasion heavy summer rains, are effective. A large percentage of the snowmelt, however, is made available at the surface prior to the thawing of the frost wedge and therefore does not infiltrate. Effective infiltration is depression focused, and certain portions of ground‐water recharge areas may never receive direct infiltration to the water table. Observation well hydrographs of water table fluctuations alone are not sufficient to monitor recharge phenomena; measurements of the hydraulic gradient above and below the water table are necessary. It is erroneous to map a sand or gravel plain as an effective recharge area without first investigating the influence of the unsaturated hydrologic properties of the soil.</jats:p>

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