Lag response of groundwater to changes in water and sediment characteristics in the lower Yellow River, China

The process of water exchange between rivers and groundwater is critical for the circulation of material and energy; however, this process is strongly affected by dam-regulated water and sediment. The lower Yellow River seeps downward throughout its coarse and suspended (perched) riverbed and feeds the surrounding floodplain groundwater. The Xiaolangdi Dam (XLD) and water-sediment regulation scheme (WSRS) have altered the exchange processes between river and groundwater along the lower Yellow River. However, few researchers have examined the response of this river-fed groundwater system to the operation of the XLD and implementation of the WSRS. As such, in this study, we analyzed the responses of groundwater to changes in flow and sediment characteristics based on statistical analysis using historical data, extremely randomized tree models, and distributed lag non-linear models. We found that, since the XLD began operating in 2002, the groundwater level near the lower Yellow River substantially decreased, and the groundwater level in June significantly decreased (at the 0.05 level). The highest groundwater level in the pre-dam period occurred in August, whereas the highest level in the post-dam period occurred in October. Changes in the annual groundwater level and in the mid- and post-WSRS seasons along the lower Yellow River were mainly determined by the water level and incoming sediment coefficient in the river, whereas those in the pre-WSRS season were mainly determined by the water level and suspended sediment concentration. Regarding the changes in groundwater level between the mid- and post-WSRS seasons, we found that suspended sediment concentration was the dominant factor in the pre-dam period, whereas the combined effect of water and sediment was the dominant factor in post-dam period. The response of the monthly groundwater level to changes in water and sediment characteristics lagged by 2-5 months. These results are essential to understanding the interaction between river water and groundwater.