Laboratory investigation of the riparian groundwater flow system evolution during a single rainfall event

Abstract: [Objective] Riparian zones serve as transitional interfaces between terrestrial ecosystems and surface water bodies. The groundwater flow processes in these zones exerts a dominant influence on river water quality and riparian ecological processes. However, the influence of a single rainfall event on the transient evolution of groundwater flow system patterns remains insufficiently understood. [Methods] Laboratory sandbox experiments and numerical simulations were conducted to investigate the evolution pattern of groundwater flow systems within a riparian slope under the combined influence of lateral recharge and rainfall infiltration recharge. The experiments were designed to replicate the dynamic interaction between hillslope subsurface flow and vertical infiltration, while the numerical model reproduced the transient redistribution of hydraulic heads and flow directions in variably saturated media. Laboratory sandbox experiments were carried out to simulate the response of riparian groundwater levels to combined lateral runoff and rainfall infiltration recharge, whereas numerical simulations reproduced the dynamic evolution of groundwater flow directions and the developmental patterns of the groundwater flow system. [Results] A single rainfall event is a key external disturbance that triggers changes in riparian groundwater flow system evolution. Such alterations in the flow system can cause abrupt shifts in the internal riparian environment, thereby significantly affecting the migration, transformation, and the attenuation of contaminants.The results demonstrate that, before rainfall occurs, under the condition of lateral groundwater recharge alone, a single regional groundwater flow system developed, directed from the left recharge boundary toward the right-side stream outlet. At the onset of rainfall, the internal groundwater flow field underwent rapid and pronounced reorganization: multiple local flow systems emerged in the shallow zone due to enhanced infiltration and capillary effects. As rainfall continued, these localized flow systems gradually merged into a codirectional regional flow system draining from both the upper and lateral boundaries toward the stream. The relative magnitude between rainfall infiltration and lateral inflow strongly governed the number, scale, and direction of the transient local flow systems. [Conclusion] High-resolution characterization of short-term evolution process of groundwater flow system in riparian zones during rainfall events reveals event-scale hydrological responses. This evolution process provides mechanistic insights essential for improving pollutant attenuation predictions and guiding targeted riparian buffer management strategies.