Catchment‐scale estimates of flow path partitioning and water storage based on transit time and runoff modelling

Abstract Tracer‐derived mean transit times (MTT) and rainfall–runoff modelling were used to explore stream flow generation in 14 Scottish catchments. Both approaches conceptualise the partitioning, storage, and release of water at the catchment scale. The study catchments were predominantly upland and ranged from 0.5 to 1800 km 2 . Lumped convolution integral models using tracer input–output relationships generally provided well‐constrained MTT estimates using a gamma function as the transit time distribution. These ranged from 60 days to >10 years and are mainly controlled by catchment soil cover and drainage density. The HBV model was calibrated using upper and lower storage layers to conceptualise rapidly responding near‐surface flow paths and slower groundwater contributions to runoff. Calibrated parameters that regulate groundwater recharge and partitioning between the two storages were reasonably well‐identified and correlations with MTTs. The most clearly identified parameters and those with the strongest correlations with MTT and landscape controls (particularly soil cover) were the recession coefficients which control the release of water from the upper and lower storage layers. There was also strong correlation between the ‘dynamic’ storage estimated by HBV and the ‘total’ catchment storage inferred by tracer damping, although the latter was usually two orders of magnitude greater. This is explained by the different storages estimated: while the ‘total’ storage inferred by tracers also includes the passive storage involved in mixing, the model estimates ‘dynamic’ storage from water balance considerations. The former can be interpreted as relating to total porosity, whereas the latter rather corresponds to the drainable porosity. As MTTs for Scottish the uplands can be estimated from catchment characteristics, landscape analysis can be used to constrain sensitive model parameters when modelling in ungauged basins. Furthermore, the ‘dynamic’ storage inferred by HBV may also be used to provide a first approximation of minimum total catchment storage. Copyright © 2011 John Wiley & Sons, Ltd.