Evaluation of hyporheic zone residence time distribution

Abstract

Hyporheic zone (HZ) is the interstitial space immediately below or adjacent to a stream at which there is an active exchange of stream and groundwater. It is the ecotone between stream and shallow groundwater ecosystems, providing unique and important ecological, hydrological and biogeochemical functions. Residence time (RT) is the duration a water molecule or a solute remains within the HZ and is directly related to the proper functioning of HZ. However, in-situ determination of RTs involves extensive and time consuming procedures. This project aims to derive equations to relate the distribution of RTs with its governing factors. It first examines the residence times of HZ in idealized streams using numerical models. More specifically, it evaluates the influence of various factors (e.g., streambed hydraulic conductivity, stream sinuosity and stream width) on HZ residence times using MODFLOW. To properly specify the hydraulic head distributions in models with stream meanders, it employs a conformal mapping which transfers the original meanders to the rectangular grids in MODFLOW. Modeling results indicate that streambed hydraulic conductivity, as expected, strongly influences the HZ residence time distribution. The impact of stream sinuosity is also significant, whereas that of stream width is minimal. This project further derives equations to relate residence time distributions with each of its governing factors (e.g., streambed hydraulic conductivity and stream sinuosity). The equations, detected and derived using MATLAB and Symbolic Regression in Eureqa, are of various forms, which demonstrate the different impacts of the factors. They also allow researchers to efficiently approximate the residence times and thus the HZ’s functional level of any stream reach.