Comparative solute-discharge hysteresis analysis for an urbanized and a ‘control basin’ in the Georgia (USA) Piedmont

Solute-discharge hysteresis (C/Q) relationships were investigated in an urbanized (Peachtree Creek) and a less-urbanized ‘control’ basin (Sweetwater Creek, SWC) within the Atlanta metropolitan region of the Georgia Piedmont. C/Q hysteresis loops for the nine parameters (pH, specific conductance, dissolved silica, calcium, magnesium, sodium, bicarbonate, sulfate and chloride) representative of urban storm flow were almost exclusively characterized by clockwise rotation and concave curvature (‘C3’ type hysteresis of [Evans and Davies, 1998]). The most straight forward interpretation of this hysteresis is that it is the result of twocomponent mixing in which the concentration of a given component in ‘pre-event’ water (ground water) is greater than ‘event’ water (storm runoff). Simple binary mixing calculations confirm that C3 hysteresis loops result where the more dilute storm runoff dominates both the rising and falling limbs of the storm hydrograph. The two-component mixing model is consistent with the very short recession (24-48 h) periods for this urban stream and the high regression constants (r2 values>0.85) for many pairs of the geochemical parameters. The hysteresis dynamics for the less-urbanized control basin (SWC) are more complicated; however, the dominance of anticlockwise-concave C/Q loops (‘A3’ type of [Evans and Davies, 1998]) for two of the three storm events suggests that three-component mixing may be occurring, particularly during lower flow events. A3 hysteresis loops can be produced where event and soil water dominate the rising limb and soil and ground water dominate the falling limb of the hydrograph. Three-component mixing is consistent with the low regression coefficients (r2<0.45) for most pairs of geochemical parameters and with the characteristically long recession periods (4 days or more) associated with SWC that might allow more time for soil water to contribute to stream flow.