Identifying intermittent river sections with similar hydrology using remotely sensed metrics

As the human population grows and the demand for freshwater intensifies, river systems previously overlooked for water production are increasingly being earmarked for development. Many of these rivers are intermittent or in remote locations, and most are insufficiently instrumented (i.e., few to no gauging stations) and poorly understood, even though sustainable water resource development and the effective management of these systems relies on a thorough understanding of their hydrology. Remote sensing of surface water has been posed as a viable method for describing the morphology, resilience and fragmentation of river hydrology, much like metrics generated from discharge data. The low cost and increasing frequency and quality of spectral images means that this approach has a great potential to characterise hydrology at fine scales across large areas. However, the approach remains largely untested with previous research only examining small river sections. This study aimed to characterise a hydrologically diverse 400 km reach of an intermittent lowland river using newly developed, spatially derived, ecohydrological metrics. We used multidimensional clustering to identify river sections with similar hydrology, examined their spatial arrangement along the river and compared their characteristics with discharge data from four gauging stations. Metric clustering revealed four hydrological types (zones) that spanned a continuum from highly intermittent to highly persistent. Spatially, zones were at the scale of pool-run/ riffle geomorphic units that alternated along the study area. Density maps of hydrological zones revealed that the mid-section of the study reach had higher persistence and longitudinal connectivity, a finding that aligned with a pre-existing groundwater discharge map generated from field sampling of environmental tracers. Limited gauging station coverage constrained comparisons, but available stations in Persistent or Refuge zones exhibited similar hydrograph responses. The ability to map hydrology continuously along the length of a river is a significant advance compared to a gauged approach, which can only classify hydrology at a single point (i.e., the gauge). Continuous classification increases our ability to describe spatial patterns in hydrology, which could markedly affect how a river is managed.