On the relation between dynamic storage and runoff: A discussion on thresholds, efficiency, and function

[1] Unit hydrograph theory as derived by Nash (1957), Dooge (1959), and Wooding (1965) provides a foundation for evaluating the function controlling catchment runoff. Past literature has often investigated this function converting catchment storage to runoff with probabilistic or statistical approaches. A geophysically based framework is missing. The objective of the research reported here is to evaluate this function explicitly, using empirical nested measurements of surface storage and runoff in a Canadian Prairie catchment during the 2006 spring snowmelt. Variation in the value of this function, K at the basin scale, or k at the sub-basin scale, was found to embody the hydrological processes acting upon a catchment. It is the relative difference between the runoff rate and the change in storage that indicates the hydrological function of a sub-basin, and k is indicative of the efficiency of this functioning. Field results show that previous assumptions of instantaneously responding reservoirs within catchments are not applicable in this landscape. Storage thresholds control the value of the transfer function by influencing when runoff production occurs. In doing so, storage thresholds play a crucial role in the efficiency with which a catchment can transfer water to its outlet. The transfer function and storage threshold, by dictating runoff, can be considered hydrological signatures. Applying these signatures within a geophysically based framework has proven to be successful in predicting the efficiency of runoff production in this particular catchment and is proposed as an approach to evaluate the interrelationships between catchment stores and runoff production. Furthermore, if hydrological processes or physioclimatic predictors can be related to these hydrological signatures in a dimensionally consistent framework, this framework could be tested as a bridging mechanism between hydrological processes at the sub-basin scale and prediction of runoff at the basin scale.