Browsing by Author "Ackroyd, Earl A."

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    Effects of Induced Streambed Infiltration on Water Levels in Well During Aquifer Tests
    (Water Resources Research Center, University of Minnesota, 1966-06) Ackroyd, Earl A.; Walton, William C.
    The aquifer test, involving a production well and several observation wells near a stream, is one of the effects of induced streambed infiltration on water levels in aquifers. In presently available analytical treatments of flow problems associated with induced infiltration, recharge by the influent seepage of surface water is commonly simulated by use of a hypothetical image well. The assumption is made that water levels in the aquifer will behave the same whether recharge occurs over an area or through an image well. Electric analog computers, in which the streambed is simulated as an area of recharge in accordance with natural conditions instead of as a recharging image well, were used to appraise the accuracy of estimated effects of induced infiltration on water levels based on the image well theory. Electric analog computers for two aquifer test sites for which field data are available were constructed. The selected aquifer tests involve the two extreme aquifer-stream situations, i.e. the case where the cone of depression spreads beneath and beyond the entire streambed and the case where the cone of depression spreads only part way beneath the streambed. The analog computers consist of analog models and excitation-response apparatus. The analog models are regular arrays of resistors and capacitors and are scaled-down versions of aquifer-stream situations. The excitation-response apparatus consists of a power supply, waveform generator, pulse generator and an oscilloscope. It is concluded that, during induced infiltration aquifer tests, the image-well theory closely describes drawdowns on the land sides of streams with a high degree of accuracy whether the cone of depression spreads beneath and beyond or only part way beneath the streambed. Drawdowns beneath or beyond the streambed and the streambed areas of infiltration based on the image-well theory are not those which are observed in the field. However, the streambed infiltration rates per foot of head loss based on hypothetical drawdowns beneath streambeds and streambed areas of infiltration computed with the image-well theory seem to be empirically correct.
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    RI-06 Ground-Water Contribution to Streamflow and Its Relation to Basin Characteristics in Minnesota
    (Minnesota Geological Survey, 1967) Ackroyd, Earl A.; Walton, William C.; Hills, David L.
    Estimates of annual ground-water contribution to streamflow in 38 drainage basins of Minnesota by use of standard streamflow hydrograph separation methods permit determination of relations between ground-water runoff and such basin characteristics as geologic environment, precipitation and temperature, and percentage of lake and wetland cover. Generalized conclusions derived from analysis of the data are that ground-water runoff is (1) least from glaciated basins that have surficial lake bed sediments or gray-drift ground moraine immediately underlain by relatively impermeable bedrock and (2) greatest from glaciated basins that have surficial depos its immediately underlain by permeable bedrock or that have thick surficial loess deposits immediately underlain by permeable bedrock. Ground-water runoff is much greater from glaciated basins having red drift than from basins having gray drift. The rate of ground-water increases as annual precipitation increases. Lakes and wetlands sustain and regulate streamflow during rainless periods; if they were absent sustained streamflow from northern parts of the state would be greatly reduced. Recharge to aquifers in the state is difficult to ascertain. Because many aquifers are deeply buried by glacial materials of varying characteristics, not all ground-water runoff can be diverted into cones of depression, for there is some lateral as well as vertical movement of water in surficial deposits. Data on ground-water runoff can be useful in estimating the rate of recharge to aquifers and in evaluating the potential yields of ground-water reservoirs. However, no simple re- 1ation exists between ground-water runoff and ground-water recharge or the potential yields of aquifers. Studies of basin characteristics were handicapped because of a lack of detailed geologic information. Collection and study of data intended to describe the dimensions and water -yielding properties of unconsolidated deposits in the basins of the state are urgently needed to support hydrologic studies involving ground-water development and management decisions.