An evaluation of analytical streambank flux methods and connections to end-member mixing models: a comparison of a new method and traditional methods

Abstract. In this paper, a novel method for estimating gross gains and losses between streams and groundwater is developed and evaluated against two traditional approaches. These three streambank flux estimation methods are distinct in their assumptions on the spatial distribution of the inflowing and outflowing fluxes along the stream. The two traditional methods assume that the fluxes are independent and in a specific sequence, while the third and newly derived method assumes that both fluxes occur simultaneously and uniformly throughout the stream. The analytic expressions in connection to the underlying assumptions are investigated to evaluate the individual and mutual dynamics of the streambank flux estimation methods and to understand the causes for the different performances. The results show that the three methods produce significantly different results and that the mean absolute normalized error can have up to an order of magnitude difference between the methods. These differences between the streambank flux methods are entirely due to the assumptions of the streambank flux spatial dynamics of the methods, and the performances for a particular approach strongly decrease if its assumptions are not fulfilled. An assessment of the three methods through numerical simulations, representing a variety of streambank flux dynamics, show that the method introduced, considering simultaneous stream gains and losses, presents overall the highest performance. These streambank flux methods can also be used in conjunction with other end-member mixing models to acquire even more hydrologic information as both require the same type of input data.