Nonlinear and abnormal relationship between turbidity and suspended solids concentration in mountainous rivers: A case study of the Lai Chi Wo river in Hong Kong, China.

Suspended solids concentration (SSC) in a river is closely relevant to river water turbidity. Investigation of their relationship in this study is accompanied by observed turbidity and SSC values, which were obtained from the testing results of water samples and monitored conditions in streamflow. The water samples were collected from two observation stations with a broad range of sediment concentrations in the Lai Chi Wo catchment in Hong Kong, China. We classified the target rainfall events into single-peak event type and dual-peak event type for a distinguished discussion of the relationship between SSC and turbidity in this study. At a finer classification, each event is separated into defined processes for the analysis, where two main processes refer to the periods that SSC rises from a normal state to a peak state first and the followed periods that SSC recesses to ordinary status gradually. It is advised by the analysis results that the estimation of SSC through turbidity values should be based on the same rainfall types for the upstream station. However, the results show that the classification of rainfall types does not need to take downstream areas into consideration. Furthermore, current research implies that the individual established connections between SSC and turbidity value at different stages (particularly referring to the rising period and recessing period) could be applied to estimate SSC at the same station via continuous turbidity values for both this and other ungauged stations with similar topographical features in the future. Meanwhile, this research approach provides new insight exploring various behaviors of sediments at different stages during an integral rainfall event. A comparison of distinguished performances of sediment during corresponding stages in a rainfall event makes contributions to diverse relationship between SSC and turbidity in the mountainous river.

Spatiotemporal variations of river water turbidity in responding to rainstorm-streamflow processes and farming activities in a mountainous catchment, Lai Chi Wo, Hong Kong, China.

River turbidity is an important factor in evaluating environmental water quality, and turbidity dynamics can reflect water sediment changes. During rainfall periods, specifically in mountainous areas, river turbidity varies dramatically, and knowledge of spatiotemporal turbidity variations in association with rainfall features and farming activities is valuable for soil erosion prevention and catchment management. However, due to the difficulties in collecting reliable field turbidity data during rainstorms at a fine temporal scale, our understanding of the features of turbidity variations in mountainous rivers is still vague. This study conducted field measurements of hydrological and environmental variables in a mountainous river, the Lai Chi Wo river, in Hong Kong, China. The study results revealed that variations of turbidity graphs during rainstorms closely match variations of streamflow hydrographs, and the occurrence of the turbidity peaks and water level peaks are almost at the same time. Moreover, the study disclosed that the increasing rates of the turbidity values are closely related to the rainfall intensity at temporal scales of 15 and 20 min, and the impact of farming activities on river turbidity changes is largely dependent on rainfall intensity. In the study area, when the rainfall intensity is larger than 35 mm/hr at a time interval of 15 min, the surface runoff over the farmland would result in higher river water turbidity downstream than that upstream. The study results would enrich our understanding of river water turbidity dynamics at minute scales and be valuable for further exploration of the river water environment in association with turbidity.

Distribution and availability of heavy metals in soils near electroplating factories.

Comprehensive understanding of the influence of soil properties on the potential availability of heavy metals could facilitate soil environment management. This study investigated the distribution of heavy metals and their potential availability in paddy and vegetable fields around electroplating factory outlets and irrigated with its wastewater. The potential availability was assessed using secondary phase fraction (SPF) of heavy metals, including acid-soluble, reducible, and oxidizable fractions resulting from BCR sequential extraction procedure. In total, 94 of topsoil samples (0-20 cm) were collected. Total and SPF concentrations of heavy metals as well as soil physicochemical properties were determined. Multivariable statistical analyses (i.e., principal component analysis (PCA) and redundancy analysis (RDA)) were employed. Results showed that total and SPF concentrations of heavy metals in soil decreased (P<0.05) as sampling distance away from the electroplating factories increased, suggesting that sampling distance was the major parameter that affected gradient variations of both total and potential availability of soil heavy metals. According to PCA, soil samples distributed on the PCA axis representing anthropogenic effect, illustrating that the variation of soil properties resulted from irrigation with electroplating wastewater. RDA and stepwise regression indicated that soil Mn oxides, amorphous Fe oxides, silt content, and pH could explain 68.8% and 43.5% of the variation of SPF concentration in paddy and vegetable garden soil, respectively, suggesting they were the most important factors influencing the potential availability of heavy metals in soils. The potential availability of heavy metals in soil was positively correlated with soil Mn oxides but negatively associated with soil amorphous Fe oxides, indicating that Mn oxides enhanced the potential availability of heavy metals while amorphous Fe oxides reduced the potential availability.

Exploration of severities of rainfall and runoff extremes in ungauged catchments: A case study of Lai Chi Wo in Hong Kong, China.

Exploration for estimating rainfall and runoff extremes in ungauged catchments is challenging since there are no field measurements of rainfall and streamflow for confirming study results. This study proposed a systematic approach to tackle the challenge, and the approach includes field survey, rainfall data collection, frequency analysis, installation of equipment in the study area, and numerical modeling. The approach was then applied to the Lai Chi Wo (LCW) catchment in Hong Kong, China, in order to evaluate the severity of a rainstorm and flood event occurred on 11 May 2014. With the collection of rainfall data from the rain gauges near the catchment, the proxy rainfall dataset for LCW was developed. Since the time of concentration of the catchment is about 30 to 40min, this study derived rainfall intensity duration frequency (IDF) curves for 9 different durations (5min, 10min, 15min, 20min, 30min, 45min, 1h, 1.5h, and 1day) and 7 different return periods (2, 3, 5, 10, 20, 50 and 100years). Further, a hydrological model, TOPMODEL, was used to simulate streamflow process; to calibrate the model parameters, a rain gauge was set up in the catchment and a water level sensor was installed at a control cross-section of the LCW river in January 2015, and the recorded rainfall and runoff data were used to calibrate the model parameters. Using the proxy rainfall data, this study obtained the simulated streamflow for the catchment, and then derived the streamflow peaks for 7 different return periods (2, 3, 5, 10, 20, 50 and 100years). Since the time of concentration of the catchment is less than 1h, this study derived that the return periods of the rainstorm on 11 May 2014 for the durations of 30min and 1h are 2.4 and 1.9years, respectively; the return period of the daily rainfall is 9.6years. The return period of the peak flood of the event is 7.0years, and this value is between the return periods of the rainfall for the durations of the time of concentration and 1day. This study revealed that the severities of rainfall and runoff extremes are not consistent but rationally related, and the 1- and 3-day antecedent rainfalls can considerably influence flood peak severity. Overall, to achieve rational prediction of ungauged basin hydrological processes, it is fundamental to install measurement equipment and to record rainfall and streamflow data. Even though the period of the recorded data in the ungauged catchment is short, the observations are necessary for evaluating the proxy data quality, and calibrating and validating the numerical model.