Topographic control on meander-driven hyporheic exchange in the igneous rock dominated headwater stream

Abstract

The hyporheic zone (HZ) is a transition zone which connects rivers and aquifers. This area is a mixture of groundwater and surface water. Surface water moves into the surrounding alluvium and returns to the river again, and this movement of water is called lateral hyporheic exchange (LHE). Hyporheic exchange is important to the biogeochemical activity and nutrient cycling. To better understand the characteristics of hyporheic exchange, in this study, topographic controls on meandering induced lateral hyporheic exchange are fully investigated. This study aims to understand how the topographic metrics such as stream depth, stream width, stream length, slope and sinuosity regulate lateral hyporheic exchange in the representative headwater catchments of Hong Kong. Using the functional predictive model for estimating the hyporheic exchange based on the stream topography, a comparison between the two headwater catchments of distinctive topography in Hong Kong is made and discussed. The applicability of the prediction model in small scale networks in Hong Kong is analyzed. Studies on high resolution hyporheic exchange over headwater catchments (within kilometres) are limited due to the lack of high resolution geomorphic and hydrogeologic data. In this work, the Digital Elevation Model (DEM) data with a resolution of 5 meters is used to generate the topographic metrics of two study areas namely high-relief Tai Po headwater catchment and low-relief Yuen Long headwater catchment. The time series of river water geochemistry and discharge dataset is obtained from Hong Kong Environmental Protection Department (HKEPD) and Water Supplies Department (WSD). The software ArcGIS is used for the visualisation of data. In Hong Kong where the headwater streams are dominated by igneous rocks, the topography plays an important role in the lateral hyporheic exchange flow. Baseflow has a significant influence on the characteristics of hyporheic exchange. The lateral hyporheic exchange model proposed by Cardenas (2009a) is more applicable in high-relief area where the spatial distribution of hyporheic exchange flux is obvious. The predictive function of residence time is not applicable in the rivers where the channel slope is very small and baseflow accounts for a relatively large proportion to the river runoff.