Abstract
Abstract. This study compares baseflow estimates using chemical mass balance, local minimum methods, and recursive digital filters in the upper reaches of the Barwon River, southeast Australia. During the early stages of high-discharge events, the chemical mass balance overestimates groundwater inflows, probably due to flushing of saline water from wetlands and marshes, soils, or the unsaturated zone. Overall, however, estimates of baseflow from the local minimum and recursive digital filters are higher than those based on chemical mass balance using Cl calculated from continuous electrical conductivity measurements. Between 2001 and 2011, the baseflow contribution to the upper Barwon River calculated using chemical mass balance is between 12 and 25% of the annual discharge with a net baseflow contribution of 16% of total discharge. Recursive digital filters predict higher baseflow contributions of 19 to 52% of discharge annually with a net baseflow contribution between 2001 and 2011 of 35% of total discharge. These estimates are similar to those from the local minimum method (16 to 45% of annual discharge and 26% of total discharge). These differences most probably reflect how the different techniques characterise baseflow. The local minimum and recursive digital filters probably aggregate much of the water from delayed sources as baseflow. However, as many delayed transient water stores (such as bank return flow, floodplain storage, or interflow) are likely to be geochemically similar to surface runoff, chemical mass balance calculations aggregate them with the surface runoff component. The difference between the estimates is greatest following periods of high discharge in winter, implying that these transient stores of water feed the river for several weeks to months at that time. Cl vs. discharge variations during individual flow events also demonstrate that inflows of high-salinity older water occurs on the rising limbs of hydrographs followed by inflows of low-salinity water from the transient stores as discharge falls. The joint use of complementary techniques allows a better understanding of the different components of water that contribute to river flow, which is important for the management and protection of water resources.
Highlights
Documenting the sources of water in rivers and streams is critical to our understanding of hydrological processes and for the management of groundwater and surface water resources (e.g. Yu and Schwartz, 1999; Uhlenbrook et al, 2002; Eckhardt, 2005; Gonzales et al, 2009; Kirchner, 2009; Sanford et al, 2012)
Estimates of baseflow from the local minimum and recursive digital filters are higher than those based on chemical mass balance using Cl calculated from continuous electrical conductivity measurements
The baseflow fluxes estimated from the chemical mass balance using the initial assumptions of groundwater and surface water Cl concentrations are maxima, and uncertainties in the groundwater and surface water chemistry cannot explain the differences in the results from the two sets of techniques
Summary
Documenting the sources of water in rivers and streams is critical to our understanding of hydrological processes and for the management of groundwater and surface water resources (e.g. Yu and Schwartz, 1999; Uhlenbrook et al, 2002; Eckhardt, 2005; Gonzales et al, 2009; Kirchner, 2009; Sanford et al, 2012). Documenting the sources of water in rivers and streams is critical to our understanding of hydrological processes and for the management of groundwater and surface water resources If rivers receive substantial groundwater inflows, groundwater extraction may significantly reduce river flow during periods of low rainfall with consequent impacts on riverine ecosystems or the utility of surface water resources. Managing surface water and groundwater resources requires a sound knowledge of the likely quantities of groundwater that rivers receive. Understanding the relative contributions of groundwater and surface water to river discharge is important to assessing potential impacts of climate change and for flood forecasting (Winter, 1999, 2000). While it is well understood that groundwater and surface water systems interact, it is difficult to robustly measure. Cartwright et al.: Contrasts between estimates of baseflow the fluxes of groundwater to gaining streams (Winter, 1999, 2000; Sophocleous, 2002)
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