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Pressure on river water resources may increase continuously under global climate change and the increasing intensity of human activity, and the identification of river water sources is needed to maintain water resource sustainability. In this study, river water sources were studied using hydrochemical tracers (i.e. the major ions) and end‐member mixing analysis (EMMA) during two hydrological years, in the subtropical monsoon basin of the Xiangjiang River in the south‐central China. Various water types including precipitation, river water, and mountain creek water, were collected in the basin at a high frequency. The results of EMMA showed that the average contributions from the direct input of precipitation, near‐surface runoff (represented by mountain creek water), and groundwater (represented by river water sampled during the base flow period) were 28 ± 24%, 49 ± 24% and 23 ± 16%, respectively. The contributions from these water sources to the river water varied greatly within the year, which may indicate a shift in the major flow pathways from direct precipitation input and near‐surface flow pathways in the spring rainy months (e.g. from March to May), to a deeper flow pathway in the dry or rainless months (e.g. January, February, July and December). The shift of the major flow pathways may be due to the factors such as the seasonality of precipitation, evapotranspiration and soil wetness conditions. Groundwater and near‐subsurface runoff dominated the river water and the direct input of precipitation was negligible during the dry or rainless months, and consequently, the river water chemistry was less diluted and more influenced by water‐soil/rock interactions and anthropogenic pollution during these months. The creek water contribution to the river water was relatively high in summer, especially in June and July 2021, which may indicate the substantial inter‐seasonal carryover of precipitation that contributed to the river water. With the projected increased frequency of extreme weather events such as seasonal drought, streamflow in the subtropical monsoon regions may experience greater recharge by deeper flow pathways, with the enhanced role of base flow in regulating streamflow, and there may be an increased risk to the water resource security under the condition of the decreased river dilution capacity and increased anthropogenic pollution. [ABSTRACT FROM AUTHOR]

Risk analysis of urban flood and drought can provide useful guidance for urban rainwater management. Based on an analysis of urban climate characteristics in 2,264 Chinese cities from 1958 to 2017, this study evaluated urban flood and drought risks. The results demonstrated that the annual average values of precipitation, aridity index, frequency and intensity of extreme precipitation and extreme drought events differed significantly in these cities. The values of the above six climatic indicators in the cities ranged from 9.29–2639.30 mm, 0.47–54.73, 1.08–8.79 time, 7.82–107.25 mm, 0.76–2.99 time, and 10.30–131.19 days, respectively. The geographical patterns of urban precipitation, aridity index, intensity and frequency of extreme precipitation and drought events in China fit well to the Hu‐Huanyong Line that was created in 1940s to identify the pattern of population distribution. Extreme precipitation in most cities has upward trends, except for those around the Hu‐Huanyong Line. The extreme drought events had upward trends in the cities east of the Hu‐Huanyong Line, but there were downward trends in the cities west of the line. The risk assessment indicated that 3.80% cities were facing serious flood and 6.01% cities were facing serious drought risks, which are located in the coast of southern China and northwestern China, respectively, and other 90.19% cities were facing different types of drought and flood risks in terms of their intensity and frequency. [ABSTRACT FROM AUTHOR]

Climate change and human activities are two major driving forces affecting the hydrologic cycle, which further influence the stationarity of the hydrologic regime. Hydrological drought is a substantial negative deviation from the normal hydrologic conditions affected by these two phenomena. In this study, we propose a framework for quantifying the effects of climate change and human activities on hydrological drought. First, trend analysis and change‐point test are performed to determine variations of hydrological variables. After that, the fixed runoff threshold level method (TLM) and the standardized runoff index (SRI) are used to verify whether the traditional assessment methods for hydrological drought are applicable in a changing environment. Finally, two improved drought assessment methods, the variable TLM and the SRI based on parameter transplantation are employed to quantify the impacts of climate change and human activities on hydrological drought based on the reconstructed natural runoff series obtained using the variable infiltration capacity hydrologic model. The results of a case study on the typical semiarid Laohahe basin in North China show that the stationarity of the hydrological processes in the basin is destroyed by human activities (an obvious change‐point for runoff series is identified in 1979). The traditional hydrological drought assessment methods can no longer be applied to the period of 1980–2015. In contrast, the proposed separation framework is able to quantify the contributions of climate change and human activities to hydrological drought during the above period. Their ranges of contributions to hydrological drought calculated by the variable TLM method are 20.6–41.2% and 58.8–79.4%, and the results determined by the SRI based on parameter transplantation method are 15.3–45.3% and 54.7–84.7%, respectively. It is concluded that human activities have a dominant effect on hydrological drought in the study region. The novelty of the study is twofold. First, the proposed method is demonstrated to be efficient in quantifying the effects of climate change and human activities on hydrological drought. Second, the findings of this study can be used for hydrological drought assessment and water resource management in water‐stressed regions under nonstationary conditions. [ABSTRACT FROM AUTHOR]