Extended Strahler Ordering to Distinguish Mapped River Channels From Overland Flow Pathways and Consistently Compare Digital Networks

ABSTRACT Digital networks are virtual representations of freshwater systems that provide important inputs to, and mapping of, river classifications, simulation models, and quantitative data analyses for policy, planning, and management. Strahler order has often been used to characterize network configuration and as a proxy indicator of river channel size and distance from headwaters. We propose and demonstrate an extension to standard Strahler ordering that uses non‐positive numbers to indicate relative positions of overland flow pathways. Non‐positive extended Strahler orders are calculated by subtracting the initially calculated standard Strahler order of the most downstream overland flow segment from: (a) the initially calculated standard Strahler order calculated for that segment; and (b) the initially calculated standard Strahler order for all upstream connected overland flow segments. The original meaning of Strahler order in terms of network configuration and relation to headwaters is preserved by applying standard Strahler ordering to assign positive numbers to network segments representing river channels. Extended Strahler ordering allows consistent comparisons between networks with different resolutions, and overland flow pathways to be distinguished from river channels within networks. A method for determining river channel headwater and sinkhole locations from cartographical maps is demonstrated because these locations are required to identify river channels within networks. The number of network segments assigned to each extended Strahler order was calculated for a case study catchment. Results demonstrated that positive Strahler orders calculated for a digital network using the extended Strahler ordering method corresponded well with those derived from cartographical maps regardless of network resolution. Extended Strahler ordering therefore provides consistency in the meaning of each Strahler order when overland flow pathways are included within a network, which is desirable if the network is being used for out‐of‐channel routing, and feasible if high‐resolution digital elevation data are available to generate detailed networks. The source of the cartographical and elevation data should be communicated when describing the origins of extended Strahler orders.