A document by Chengji Shen. Click on the document to view its contents.

The Qiangtang Basin is a large endorheic basin as the inner part of the Tibetan Plateau and has been thought to be a dry region in contrast with its wet surrounding outer region that feeds all the major Asian rivers. Combing surface hydrological data with modelling and satellite data between 2002 and 2017, our study reveals that an enormous amount of water of 54.52±15.36 km is unaccounted for annually in the Qiangtang Basin. The amount of this missing water is comparable to the total annual discharge of the Yellow River. Data from the Gravity Recovery and Climate Experiment (GRACE) show no increase of the local terrestrial water storage. Thus the missing water must have flowed out of the basin through underground passages. Interpreting this result with recent seismic and geological studies of Tibet, we suggest that a significant amount of meteoric water in the Qiangtang Basin have leaked out by way of groundwater flow through deep normal faults and tensional fractures along the nearly N-S rift valleys that are oriented sub-normal to and cross the surficial hydrological divide on the southern margin of the basin. Cross-basin groundwater outflow of such a magnitude defies the traditional view of basin-scale water cycle and leads to a very different picture from the previous hydrological view of the Qiangtang Basin. The finding calls for major rethinking of the water balance in Tibet and the nearby regions.

Colloids are often in forms of inorganic and organic particles whose particle sizes (dp) are ranging from 1.0 nm to 10.0 μm. On the one hand, their transport processes in the hyporheic zone such as deposition, resuspension, clogging and release are substantially affected by hyporheic exchange. On the other hand, the existence of colloids can modify the hydraulic structure of the hyporheic zone due to clogging. Despite the general acknowledgement that particle size plays an important role in contaminant adsorption and clogging, it remains unclear how the particle size of colloids changes during their transport processes in the hyporheic zone. This study aims to investigate the variation of the particle size of colloids in the overlying water and the effects of settlement and convection-diffusion via laboratory experiment and numerical simulation. The results show that both settlement and convection-diffusion play roles in the exchange of colloids between the stream and the streambed. More specifically, settlement is the dominant factor affecting the exchange process of large-sized particles (dp > 3.06 μm) in the hyporheic zone as the high settling velocity dominates the outflux of colloids in the overlying water; the exchange process of small-sized particles (dp < 1.10 μm) is more affected by convection-diffusion and some of them can be released from the streambed to the overlying water; while the exchange process of middle-sized particles (1.10 μm < dp < 3.06 μm) is affected by both convection-diffusion and settlement. Thus, this study may provide important insights into the variation of the particle size of colloids in the overlying water and the effects of settlement and convection-diffusion.

In this study, both laboratory experiments and numerical simulations were conducted to investigate the effect of density-driven flow on the transport of high-concentration pollutants in the hyporheic zone. The results show that the density gradient can change the flow of pore water and the strong density-driven flow can lead to an unstable flow, which increases the effect of preferential flow and thus causes the appearance of solute fingers in the hyporheic zone. Notably, these solute fingers become more obvious with the increase of depth. The appearance of solute fingers depends on the relative strength of the pumping exchange and density gradient, which are represented by the dimensionless number M* and N* respectively. Finger flows appear near the interface when M* is less than 0.5 N*. This study may contribute to better understanding the transport and destination of solutes and thus may provide some insights into the assessment on pollution incidents.

Hyporheic exchange plays a critical role in driving the transport of colloids in the hyporheic zone, but it remains unexplored how the particle size distribution of colloids varies during their transport process. This study aims to investigate the particle size variations and effects of factors on different sized colloids via laboratory experiment and simulation. The results show that both settlement and convection-diffusion play a role in the exchange of colloids between the stream and the streambed. Settlement is the dominant factor determining the exchange of large-sized particles due to their high settling velocity; the transport of small-sized particles is affected more by convection and diffusion, and part of them can be released from the streambed to the stream; while the exchange of middle-sized particles is affected by both convection-diffusion and settlement. As the colloids can affect the environment and eco-system of hyporheic zone in both positive and negative ways, the knowledge of how size variations affect the transport patterns of colloids is significant to future studies.

Colloids exist widely in rivers which can act as contaminants or carries of contaminants. Hyporheic exchange drives colloids to transport into the hyporheic zone. However, the variation of the particle size of colloids has seldom been considered in previous transport theories of colloids. This study aims to investigate the variation of the particle size of colloids and functions for different sized particles via laboratory experiments and simulations. The results show that the settlement and convection-diffusion of colloids play a dominant role in the exchange of colloidal particles between the stream and the streambed. Large particles can settle into the streambed more rapidly as settlement domains the process, which however can hardly be detected in the overlying water during the later period of the experiment; the exchange process of small-sized particles is affected more by convection and diffusion, and a retarded trace release can be monitored from the streambed to the stream; while for middle-sized particles, mass transfer coefficient and settling velocity affect exchange process together. Because the variation of particle size (affecting specific surface area, surface electrochemical characteristics) can have a substantial effect on the contaminants carriers (colloids) in the hyporheic zone, the knowledge of it should be taken into account in future studies.