Using Equivalent Horton-Strahler Ratios to Predict Extreme Events in Colombian Andes Catchments

The frequency and severity of extreme hydrometeorological events in the Colombian Andes have increased due to the combined effects of climate change and climate variability, with the El Niño-Southern Oscillation (ENSO) being the main contributor. To address this issue and improve hydrologic and hydraulic infrastructure designs, it is necessary to develop better tools for accurately predicting the impact of these events. Hydrological scaling and similarity, based on relatively simple mathematical laws, can synthetically translate the high heterogeneity of hydrological processes into equations that are particularly useful in ungauged catchments, a widespread problem in the Colombian Andes. This research proposes the use of specific hydrological scaling tools, including the Geomorphological Instantaneous Unit Hydrograph (GIUH) and the equivalent Horton-Strahler (H-S) ratios, to calculate peak flows. These ratios express the self-similarity of channels and basins, independent of the threshold area for channel initiation, which the classical bifurcation ratio (R_B), length ratio (R_L), and area ratio (R_A) depend on. Using digital elevation model (DEM) data from NASA's ALOS-PALSAR mission, which provides terrain elevation at a resolution of 12.5m x 12.5m, we analyzed regional patterns of extreme event scaling on various slopes of the Andes Mountains (Colombia) using the GIUH/equivalent H-S theory. With this DEM data, we developed a methodology for automatically extracting the equivalent H-S (R_Be, R_Le, R_Ae) in several catchments of the Western, Central, and Eastern ranges that compose the Colombian Andes, while simultaneously validating the self-similarity assumption of their channel networks. Our results highlight the importance of the equivalent H-S ratios as self-similarity indices and regional indicators of the intrinsic relationship between geomorphology and hydrology in the Colombian Andes and their usefulness for hydrological design engineering purposes.