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.

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.

Integrating the social, hydrological and ecological dimensions of freshwater health: The Freshwater Health Index.

Degradation of freshwater ecosystems and the services they provide is a primary cause of increasing water insecurity, raising the need for integrated solutions to freshwater management. While methods for characterizing the multi-faceted challenges of managing freshwater ecosystems abound, they tend to emphasize either social or ecological dimensions and fall short of being truly integrative. This paper suggests that management for sustainability of freshwater systems needs to consider the linkages between human water uses, freshwater ecosystems and governance. We present a conceptualization of freshwater resources as part of an integrated social-ecological system and propose a set of corresponding indicators to monitor freshwater ecosystem health and to highlight priorities for management. We demonstrate an application of this new framework -the Freshwater Health Index (FHI) - in the Dongjiang River Basin in southern China, where stakeholders are addressing multiple and conflicting freshwater demands. By combining empirical and modeled datasets with surveys to gauge stakeholders' preferences and elicit expert information about governance mechanisms, the FHI helps stakeholders understand the status of freshwater ecosystems in their basin, how ecosystems are being manipulated to enhance or decrease water-related services, and how well the existing water resource management regime is equipped to govern these dynamics over time. This framework helps to operationalize a truly integrated approach to water resource management by recognizing the interplay between governance, stakeholders, freshwater ecosystems and the services they provide.

Can centralized sanctioning promote trust in social dilemmas? A two-level trust game with incomplete information.

The problem of trust is a paradigmatic social dilemma. Previous literature has paid much academic attention on effects of peer punishment and altruistic third-party punishment on trust and human cooperation in dyadic interactions. However, the effects of centralized sanctioning institutions on decentralized reciprocity in hierarchical interactions remain to be further explored. This paper presents a formal two-level trust game with incomplete information which adds an authority as a strategic purposive actor into the traditional trust game. This model allows scholars to examine the problem of trust in more complex game theoretic configurations. The analysis demonstrates how the centralized institutions might change the dynamics of reciprocity between the trustor and the trustee. Findings suggest that the sequential equilibria of the newly proposed two-level model simultaneously include the risk of placing trust for the trustor and the temptation of short-term defection for the trustee. Moreover, they have shown that even a slight uncertainty about the type of the newly introduced authority might facilitate the establishment of trust and reciprocity in social dilemmas.

Joint effects of asymmetric payoff and reciprocity mechanisms on collective cooperation in water sharing interactions: a game theoretic perspective.

Common-pool resource (CPR) dilemmas distinguish themselves from general public good problems by encompassing both social and physical features. This paper examines how a physical mechanism, namely asymmetric payoff; and a social mechanism, reciprocity; simultaneously affect collective cooperation in theoretical water sharing interactions. We present an iterative N-person game theoretic model to investigate the joint effects of these two mechanisms in a linear fully connected river system under three information assumptions. From a simple evolutionary perspective, this paper quantitatively addresses the conditions for Nash Equilibrium in which collective cooperation might be established. The results suggest that direct reciprocity increases every actor's motivation to contribute to the collective good of the river system. Meanwhile, various upstream and downstream actors manifest individual disparities as a result of the direct reciprocity and asymmetric payoff mechanisms. More specifically, the downstream actors are less willing to cooperate unless there is a high probability that long-term interactions are ensured; however, a greater level of asymmetries is likely to increase upstream actors' incentives to cooperate even though the interactions could quickly end. The upstream actors also display weak sensitivity to an increase in the total number of actors, which generally results in a reduction in the other actors' motivation for cooperation. It is also shown that the indirect reciprocity mechanism relaxes the overall conditions for cooperative Nash Equilibrium.