Intermittent rivers in semiarid and arid regions, constituting over half of the world\'s rivers, alternate the carbon cycle interactions among the biosphere, hydrosphere, and atmosphere. Inadequate quantification of flow duration and river water surface area, along with overlooked CO2 emissions from dry riverbeds, result in notable inaccuracies in global carbon cycle assessments. High-resolution remote sensing images combined with intensive field measurements and hydrological modelling were used to estimate and extract the flow duration, river water surface area and dry riverbed area of Huangfuchuan, an intermittent river watershed that acts as a major tributary of the Yellow River in semiarid Northwest China. CO2 emission rates and partial pressures in water and air across the watershed were in-situ measured. In 2018, the flow duration of Huangfuchuan increased from less than 5 days in the first-order tributary to 150 days in the sixth-order mainstream. River water surface area estimated by remote sensing extraction plus the hydrodynamic model simulation varied from 3.9 to 88.6 km2 under 5 %-95 % discharge frequencies. CO2 emissions from the water-air interface and dry riverbed in 2018 were estimated at 582.3 × 103 and 355.2 × 103 ton, respectively. The estimated total annual emission (937.5 × 103 ton) aligns closely with the range of emissions (67.3 × 103-1377.2 × 103 ton) calculated for the water-air interface alone, derived using DEM river length and hydraulic geometry method. This similarity can be attributed to the overestimation of flow duration and flow velocity, as well as the over- or under-estimation of river water surface area and slope. The new method proposed in this study has large potential to be applied in estimating CO2 emissions from data-scarce intermittent rivers located in mountainous regions and provides a standardized solution in the estimation of CO2 emission. Results of this research reveal the spatiotemporal distribution of CO2 emissions along an intermittent river system and highlight the substantial role of dry riverbed in carbon cycle.

Dominant functions usually vary greatly in different reaches of mountainous rivers and are influenced by different adjacent land uses. Assessing river health based on dominant functions is of great practical value to river management. To reveal the health status of different reaches in Beijing\'s northern mountainous rivers, 60 investigated plots (river length 38.1 km) were surveyed in 2016 in the Huaijiu River, which is a typical mountainous river in northern Beijing, and a hierarchy-comprehensive analysis method was employed. Based on the degree of human influences, the Huaijiu River could be classified into six types, including natural reaches, near-natural reaches, artificial bank plant reaches, artificial bank ornamental plant reaches, artificial bank sparse plant dry-stone reaches and artificial bank masonry reaches. The river health assessment index system was established based on flood control, landscape, hydrology and water quality, and ecological functions. The analytic hierarchy process (AHP) was used to determine the weights of the function layer and indicator layer. The assessment results showed that healthy, subhealthy, slightly damaged, damaged and severely damaged plots accounted for 20.0%, 26.7%, 26.7%, 15.0% and 11.6% of the total plots, respectively. In summary, all plots in natural reaches, artificial bank plant reaches and artificial bank ornamental plant reaches were either healthy, subhealthy or slightly damaged. Plots in artificial bank masonry reaches were either subhealthy, slightly damaged, damaged or severely damaged, accounting for 9.1%, 27.3%, 27.3% and 36.4% of the total plots, respectively. The study proposed a method to assess mountainous river health based on dominant functions, which is a multiobjective approach and is not based solely on natural river functions. The assessment method is appropriate for the socioeconomic development and management of river basins.

As an important part of the riparian zone, bars are an important barrier for the interception of phosphorus (P) originated from leaching and runoff. The spatial variation in P as well as the influence of factors on the said spatial variation in mountainous river (Lingshan River) bars was investigated. A total of 100 soil samples were collected from 11 sampling sites. Soil total phosphorus (TP) and soil available P were determined to explore the spatial variation of soil P in mountainous river bars. One-way analysis of variance, Pearson\'s correlation analyses, stepwise multiple linear regressions and curve fitting were used to explore the dominant factors affecting the spatial variation of soil P in mountainous river bars. Affected by erosion effect of flowing water, the TP of the bar soils decreased in the longitudinal direction, the TP and available P of the bar soils increased in the cross-sectional direction and the variation in TP between the surface and deep soils firstly increased and then decreased as the height of the bar above the water surface increased. The stronger the erosion effect of flowing water, the more P releases to the water, which may cause the spatial variation of soil P in mountainous river bars, and the results of this study facilitated control of non-point source pollution in mountainous river and restoration of the ecosystems in mountainous river bars.

Watershed ecological security is strongly associated with the aquatic ecological status of the upper mountainous area. The present study aimed to assess the watershed ecological security status of the mountainous area under the PSFR (Pressure-State-Function-Response) assessment framework. An evaluation index system was established according to the watershed characteristics, which included four project hierarchical layers, i.e., aquatic ecological pressure, aquatic ecological state, ecological function, and social response, 11 component layers and 23 evaluation indexes. This index system was applied to evaluate the watershed ecological security status of the mountainous area (35 sub-watersheds) in the Taizi River Basin, Liaoning Province. Our results showed that the aquatic ecological status of the study area could be classified into three groups: insecure, general secure and secure, no very insecure and very secure status. Nine sub-watersheds were at the insecure ecological status, accounting for 25.7% of the total sub-watersheds, whereas 22 sub-watersheds were at the general secure state, representing 62.9% of the study area. In contrast, only four sub-watersheds were grouped at the secure status. Furthermore, agricultural activity was identified as the most significant factor responsible for the aquatic ecological security of mountainous area in the Taizi River Basin. Habitat degradation, including water quality deterioration and habitat loss, significantly reduced the ecological functions of the Taizi River Basin, and decreases in rare and peculiar species and biodiversity also posed a threat to the ecological integrity of the study region. Our results could be applied to diagnose the major factors affecting aquatic ecological security, and provide information for effective ecological restoration.
河流山区段上游地区的水生态状况直接影响着流域整个水系生态系统的安全.本研究以生态安全评估模型PSFR(压力-状态-功能-响应,Pressure-State-Function-Response)框架为基础,选取辽宁太子河上游山区段为研究区域,结合太子河流域山区段河流的特征,构建了包括4个方案层、11个要素层和23个指标层的评估指标体系,从水生态压力、水生态状态、生态功能和社会响应4个方面,对太子河流域山区段进行河流生态安全评估.评估结果显示,在5种安全等级标准下,研究区内35个子流域的水生态安全状态有3种:不安全状态、基本安全状态和较安全状态,未出现极不安全和非常安全状态.其中,处于不安全状态的子流域有9个,比例为25.7%;处于基本安全状态的子流域有22个,占总流域的62.9%;处于较安全状态的子流域有4个,只占11.4%.表明太子河流域山区段水生态安全的主要压力来自于农业活动.流域栖息地环境质量下降、水质恶化及生境面积的衰退,导致河流生态系统的生态功能受损,河流水体内珍稀和特有物种减少、生物多样性降低,从而威胁河流生态系统的完整性,影响整体水生态安全.本研究结果能诊断流域内影响河流水生态安全的主要因子,为河流生态修复提供科学决策依据.