**3. Results**

## *3.1. Variation and Hysteresis of Groundwater Table in the Riparian Hyporheic Zone under Tidal Effect*

The river stage fluctuated twice a day and was of irregular semidiurnal characteristics (Figure 3). During the monitoring period, the mean tidal duration was about 12.2 h. The maximum level was 1.196 m a.s.l., and the minimum level was 0.850 m a.s.l., which contributed to a fluctuation range of 34.6 cm.

The groundwater level in any riparian well fluctuated twice a day, just like the river water stage (Figure 3). Regardless of the river channel or the riparian wells, the dominant oscillation period of the water level time series was 12.2 h. The fluctuation range of the water level gradually decreased upon increasing the distance from the shore on any monitoring day, as shown in Table 1. Over the 3 day study period, the fluctuation range of the daily mean river stage declined from 22.7 to 12.9 cm, while that of the daily mean groundwater level in each riparian well changed a little. Furthermore, the groundwater level in the riparian hyporheic zone exhibited a slow response to the rise and fall movement of the river stage. The delayed response time extended from 90 to 180 min, and the correlation coefficient decreased from 0.75 to 0.52, when the distance increased from 0.71 to 3.85 m away from the shore.

**Table 1.** Fluctuations and the response relationship of the river stage and the groundwater level



During most of the monitoring period, the river stage was higher than the groundwater level and, hence, the river water continued to penetrate into the riparian hyporheic zone under such a hydraulic gradient. As a result, the groundwater level showed an overall upward trend (Figure 3). The daily mean groundwater levels in different riparian wells showed upward trends with the increase of 3.1–3.9 cm over the monitoring period (Figure 4a). Since the temperature of the river water was lower than that of the groundwater during the study period, the infiltration of the river water to the groundwater contributed to the drop in groundwater temperature. Although the river water temperature increased, the groundwater temperature in the riparian hyporheic zone declined (Figure 4b). Moreover, the decline range had a negative correlation with the distance from the shore. In view of the variations in water level and temperature of the groundwater at WB4, the migration range of water moisture and heat in the riparian hyporheic zone could reach 4 m away from the shore during the study period.
