Isotopic approach to linking landscape and catchment storage across multiple spatial scales

Abstract

Catchment heterogeneity leads to uncertainty about estimating timescales of storage and difficulty in extrapolating to larger scales. A heterogeneity-independent storage index (SI) was employed to build a connection with catchment characteristics, especially for landscapes, over six humid, forestry catchments. The SI is derived from the amplitudes of tracer cycles in precipitation and ephemeral/intermittent streamflow. The SI was significantly correlated with landscape metric (R2 = 0.81 and p = 0.014), specifically the fractal dimension index (FRAC) indicating the shape complexity of the landscape. Meanwhile, the topographic wetness index (TWI) (R2 = 0.77, p = 0.021) showed a close connection with the storage index as well. Close correlations suggested that the FRAC and TWI regulate the timescale of storage over the range of spatial scales, in particular the young water fraction in streams, because the SI is a heterogeneity-independent metric derived from seasonal tracer cycle amplitudes, which has a close relation to young water fraction. This also indicated the SI can be treated as an alternative travel-time metric and a reflection of the hydrological nature. Moreover, the findings implied the possibility of hydrological modeling over multiple spatial scales. The linkage between SI and landscape and topography provides insight into the mechanism of streamflow generation. Based on ephemeral streams, the crucial control of landscape and topography on the timescale of storage contributes to available water assessment and sustainable ecosystem management.