Relationships in Regional Groundwater Discharge to Streams: An Analysis by Numerical Simulation

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Baseflow components of streamflow were determined for 32 selected basins in the Plains Regions of Alberta. Plots of baseflow per unit area vs. total basin surface (q(A) curves), show a rapid increase in baseflow rate with initial increase in basin area, followed by a reduced rate of increase at larger basin areas, thereby suggesting a functional dependence of baseflow rate on basin size. The empirically determined baseflow relationships were studied by numerical simulations of groundwater flow in two-dimensional vertical sections of type environments of the Alberta Plains. These models show that the baseflow relationships develop according to the theory of regional gravity flow of groundwater. The types of discharge relationships indicate greater baseflow in regional discharge areas (areas of low topography and including local recharge regions) than in regional recharge areas (areas of high topography and including local discharge regions). An asymptotic discharge-drainage area relationship was obtained in this analysis, which showed a high rate of increase of specific discharge with initial increase in basin area from the headwaters downgradient in the regional setting. At and above a basin area of 10 000 km2, the rate of increase of specific discharge approaches a maximum value asymptotically with further increase in basin area. The asymptotic relationship signifies the loss of water from high to low elevation in a flow pattern that consists of all local, intermediate and regional systems. Several hydrogeological environmental parameters act to modify the shape and nature of the asymptotic relationship; namely, depth to width ratio, regional slope, and geology of the basin. Low depth to width ratio and low regional slope convert the asymptotic relationship to a stationary discharge-drainage area relationship which gives a constant specific discharge for the entire range of basin area, and identifies a flow pattern of wholly local systems. Contrarily, high depth to width ratio, high regional slope, and the presence of aquifers in a basin transform the asymptotic relationship into a linear discharge-drainage area relationship which describes a linear increase in specific discharge with basin area. Inverse discharge-drainage area relationships or mirror images of all three above relationships are formed when specific discharge is plotted against basin area which increases from low elevation upgradient towards the headwaters. The discharge-drainage area relationships uncovered in this study demonstrate quantitatively that sub-basins at low topographic elevations possess more water than those of equivalent sizes at high topography. For example, a basin with an area of 5000 m2 at a low topographic elevation, discharges naturally at 1.4 × 10-2m3s-1m-2, whereas a similar basin at a high elevation would discharge at 7.8 × 10-3m3s-1m-2.



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