Can seasonal groundwater level rises explain non-linear increases in baseflow?

Groundwater discharge is a major contributor to stream base flow and quantifying this flux is of considerable importance to water resource management. It can be estimated using tracers, water balance studies and numerical modelling, but insufficient use is made of groundwater level observations and water table mapping. We illustrate a method that makes empirical use of groundwater data by generating a relationship between groundwater levels in a bore near a river and unregulated stream discharge. An exponential function can be fitted to the outer envelope (i.e. 5^th percentile) of the data.
Q_gw = e^(h-h0)/A +Q₀ 
Where Q_gw is the groundwater discharge to the stream, h is the groundwater level measured in the monitoring bore and h₀, A and Q₀ require calibration. In a perennial, gaining river reach in southeastern Australia (Gellibrand River), the relationship at lower groundwater levels can be adequately defined by a linear relationship that corresponds to low flows (typically 0.1 - 0.6 m³ s^-1) that occurred over the summer period in a catchment which shows a strongly seasonal flow response. However, the relationship becomes increasingly non-linear as groundwater levels rise. We investigate whether the non-linear part of the relationship is due to increased groundwater discharge into small tributary streams draining the valley slopes or coincides with the onset of interflow. We use interpolated monitoring bore records to construct monthly water table maps using the kriging with external drift method. The intersection of the water table surfaces with a digital elevation model of the ground surface was analysed for seasonal high (spring) and low (autumn) periods of the water table. We found that the marginal change in the intersecting surfaces between the low and high periods did not show non-linear increases and this indicates that interflow is the more likely driver of non-linear increases in baseflow in the winter-spring period.