Assessing the floodwater utilization potential in a reservoir-controlled watershed: A novel method considering engineering regulations and an empirical case from China
Introduction
Water scarcity is one of the most serious challenges in the progress of global urbanization (Anandhi and Kannan, 2018; Dessu et al., 2014; Greve et al., 2018). China is also facing severe water scarcity due to population increases, rapid economic growth, and the uneven temporal distribution of water resources (Qin et al., 2020; Wang et al., 2019a, Wang et al., 2019b). More than 60% of the annual rainfall is concentrated in the flood season, which leads to the floodwater, that generated by the concentrated precipitation, is not only the control object of flood control but also the main source of freshwater for humans (Ding et al., 2015; Jiang et al., 2015; Lu et al., 2021; Wang et al., 2021). Thus, floodwater utilization is considered the main approach to the alleviation of such a water shortage (Lu et al., 2021; Zhang et al., 2018). But the potential of floodwater to be exploited is still unclear in most basins of China, which impeding the utilization of floodwater.
Floodwater utilization aims to transfer floodwater into ordinary water resources by both structural and nonstructural measures without decreasing flood control standards or damaging the ecological environment of rivers (Liu et al., 2011a, Liu et al., 2011b; Ye et al., 2019). Multipurpose reservoirs are the main measure used to solve the uneven temporal distribution of water resources and are undoubtedly considered important structural measures to implement flood control and water conservation (Liu et al., 2011a, Liu et al., 2011b; Wallington and Cai, 2020). Thus, the optimal operation of multipurpose reservoirs as nonstructural measures is vital to floodwater utilization. Many studies have investigated approaches to utilize floodwater through reservoir operation, i.e., multistage design of the flood-limited water level (FLWL) (Lu et al., 2021; Ning et al., 2021) and dynamic control of the FLWL (Gong et al., 2020; Li et al., 2010; Yun and Singh, 2008).
Most of these papers on floodwater utilization focus on how to exploit floodwater, while few studies focus on how much floodwater can still be exploited in a basin, especially in a reservoir-controlled basin. Floodwater utilization potential (FUP), defined as the current amount of unexploited floodwater that can be exploited through structural and nonstructural measures without increasing flood risk and destroying river ecology (Ye et al., 2019), can be used to quantify floodwater in a reservoir-controlled basin. Similar to the assessment of other unconventional water resource utilization potentials (Hussain et al., 2019; Jamrah et al., 2008), FUP assessment can provide the potential under current conditions (engineering conditions, ecological water demand downstream, etc.) and further the direction for future floodwater utilization. Therefore, the factors that affect the FUP should be determined first, and the relationships between the FUP and various factors should be clarified. Ye et al. (2019) proposed an assessment approach based on the hydrograph of the outflow in a basin and identified the FUP in the Hai River basin. However, the assessment method based on the measured discharge at the outlet of a basin with a long history of ignoring the flood regulation process. There are two drawbacks in this assessment method, one is the assessment result cannot represent the current FUP once the engineering condition changes during the observation period, the other is the influencing factors mentioned in the paper, especially the ability to regulate and utilize floodwater (ARUF) and ecological water demand downstream, has not been explicitly considered in it. Those drawbacks may reduce the guiding significance of the assessment results to floodwater utilization.
To address these drawbacks, we clarify the influencing factors of the FUP in a reservoir-controlled basin by analyzing the process of flood retention and utilization and establish an FUP evaluation method based on a reservoir simulation model that can explicitly consider the influencing factors, especially the ARUF. Specifically, floodwater utilization involves both flood control and water conservation, and the time scales of these two regulatory processes are different, i.e., flood control processes are often simulated on an hourly or daily scale (Croley and Raja Rao, 1979; Delaney et al., 2020; Ding et al., 2017; Krzysztofowicz and Duckstein, 1979; Zhao et al., 2014), while water conservation processes are usually simulated on a monthly scale (Dessu et al., 2014; Nalbantis and Koutsoyiannis, 1997; Wang et al., 2020). In this study, a time-varying reservoir operation simulation model, which can represent the time scale difference between flood control and water conservation, was established to evaluate the FUP under different engineering and water demand levels. The application results of the Longjiaoshan Reservoir (LR), a case study in China, showed that the model could evaluate the FUP under different floodwater utilization statuses and support the exploitation of the FUP. Our objective is to provide a practical method for FUP assessment and provide further empirical evidence implicating floodwater utilization.
Section snippets
Theoretical derivation
Floodwater utilization consists of two processes: flood flow impoundment and water utilization (Ding et al., 2017; Liu et al., 2011a, Liu et al., 2011b). Floodwater conserved by flood control projects during the flood season will be consumed in the following non-flood season, and a conceptual model was established to present reservoir operation during these two processes (Fig. 1). Fig. 1 shows the storage and water release of the reservoir in the flood season and non-flood season for n years. V
Results and discussion
The assessment of the FUP and the relationship between the FUP and its influencing factors are the basis to implement floodwater utilization. The ARUF, water demand, and ecological water demand downstream are the main influencing factors of the FUP according to Section 2.1. An assessment method based on a time-varying reservoir simulation model was developed to assess the FUP in the reservoir-controlled watershed under different water demands and engineering conditions. The method was applied
Conclusions
This study demonstrates that the floodwater utilization potential of a reservoir-controlled basin is influenced not only by engineering ability and ecological water downstream but also by water demand. The FUP assessment method can reflect the impacts of all the influencing factors via a time-varying reservoir simulation model. Thus, the method can evaluate the FUP under different floodwater utilization statuses and lead to the exploitation of the FUP. The case study of the LR, where the
Declaration of Competing Interest
None.
Acknowledgments
This study was financially supported by the National Key R & D Program of China (No. 2018YFC0408005) and Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. KYCX20_0475). Fundamental Research Funds for the Central Universities of China (No. B200203062).
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