Impacts of different rainfall patterns on hyporheic zone under transient conditions

Abstract

The hyporheic zone (HZ), the region beneath or alongside a streambed, can play a vital role in stream ecology. Several previous studies have investigated the influential factors on the HZ in the steady state. However, the exchange between surface water and groundwater in the HZ can be dynamic and transient, during a transient event such as a storm. Therefore, this study investigates the changes of the HZ under the transient conditions of a storm, and examines the impacts of different rainfall patterns (i.e., intensity and duration) on the HZ. A two-dimensional groundwater-stream model is developed with a domain of 10-meter long and 2-meter deep. The streambed consists of a series of dunes that induce hyporheic exchanges. Brinkman-Darcy and Navier-Stokes equations are respectively employed for the subsurface and stream water, and the velocity and the pressure are coupled at the interface (i.e., the streambed). To compare the results from different rainfall patterns, the influential duration (IT) and the influential depth (ID) are proposed and evaluated. IT is the time required for the HZ to return to its intial stage, once it starts to change. ID is the maximum increment in the depth of the HZ. To accurately detect the region of the HZ in different situations, the moving split-window analysis method is used. The region of the HZ is found to vary significantly under different rainfall intensities. Rainfall intensity displays logarithmic relationships with both the IT and ID with high coefficients of determination (r2=0.98). The derived relationships can be used to predict the influrence of a rainfall event on the HZ. However, the influence of rainfall duration on the HZ depends on other factors such as groundwater response. Rainfall duration displays positive realionships with the IT and ID, but only between certain lower and upper thresholds of rainfall duration. If rainfall duration is shorter than the lower threshold value or longer than the upper value, the IT and ID will have little change with rainfall duration.