χ-maps and Hypsometric Analysis for River Basin Management and Prioritization: The Case of Bohol River Basins, Central Philippines
Topographic indices represent and simplify complex surficial processes fundamental to characterizing landform dynamics and to prioritizing river basin conservation and management goals. A method of characterizing landform dynamics integrating the steady-state river channel elevation and the hypsometric analysis is presented. The chi (χ) metric, a proxy for the steady-state river channel elevation, gages the stability of drainage divides while hypsometric analysis quantifies the stages of basin
... eological development. Using a 30m SRTM DEM and the TopoToolbox tool in MATLAB, χ - values along stream networks are computed. At the channel heads of opposing stream networks of a divide section, equal χ – values indicate a stable divide while across difference in χ - values suggest unstable divide with a potential to migrate from low χ – values towards the high χ – values side of the divide. To classify the degree of potential divide mobility, the quantity called mean chi difference (χmd) is proposed. The features of the aggressive and victim river streams are visually differentiated using their elevation profiles, map-view arrangement and pathways to discharge points. Hypsometric analysis examines the erosional stages of basins indicated by the hypsometric integrals (HI) and hypsometric curves. A basin and its subbasins show different levels of geologic development that the disaggregation of large basin into small hydrologic units enables the identification of areas of different erosional stages. The prioritization of subbasins considers the intersection of highly mobile divides and highly erosional areas. Over the study area, nine subbasins are identified which are all located at the headwaters of major basins in the island. A considerable earthquake-triggered landslide has been found in one of the identified subbasins. The study presents a new approach in the initial characterization of landforms in order to facilitate the identification and prioritization of highly erodible areas for high consideration especially at the local or village level.