Abstract: To describe the characteristics of intra-storm temporal distributions of rainfall, minute resolution rainfall data were collected from 14 weather stations in China covering nearly 40 years. A total of 10 256 storms with precipitation amounts greater than 10 mm were analyzed. Based on a dynamic K-means cluster method and the time to peak index, the storms were divided into four hyetograph types: Rainfall concentrated early in the storm (Type I); rainfall concentrated in the middle of the storm (Type Ⅱ); rainfall concentrated at the end of the storm (Type Ⅲ); and rainfall distributed uniformly over time (Type IV). Type I storms occurred most frequently (47.1%), followed by Type Ⅱ (21.2%), Type Ⅲ (15.3%), and Type IV (16.4%). In summer, Type I storms had a normalized occurring frequency of 52.2%, suggesting that Type I storms were the most prevalent storm type during the summer. In winter, all four storm types occurred with similar frequencies. Type I storms were characterized by shorter durations and higher intensities, whereas Type IV storms had longer durations and lower intensities. The average duration, intensity, and the maximum 30 min intensity for Type I storms were 19.2 h, 2.8 mm/h, and 20.3 mm/h, respectively, whereas that for Type IV storms were 30.0 h, 1.4 mm/h, and 8.6 mm/h.
Abstract: It is important to study the drought-flood variation in the arid Northwestern China for regional development and disaster prevention. This work reconstructs the Regional Comprehensive Drought-flood Index (RCDI) series in arid Northwestern China and relevant provinces, based on the drought-flood grades series of 19 sites in the arid Northwestern China. The RCDI series shows that the drought is a major hazard during the last 539 years. It is also found that the drought happened more frequently in the western and high altitude areas than the eastern and low altitude areas of arid Northwestern China. The paper analyzes the fluctuation characteristics of the RCDI series by Empirical Mode Decomposition (EMD) method. The results show that the variations of drought-flood in arid Northwestern China have six quasi-periodic fluctuations which are 2.5a, 7.5a, 13.1a, 25.7a, 77.0a, and 134.8a, respectively. The results also show that the trend of the regional drought severity decreased in the last 539 years.
Abstract: In the traditional models for areal evapotranspiration estimation, net radiation and its composition elements were generated by simulation of foreign empirical formulas or by interpolation of site observation data, which was one of the key reasons influencing the simulation accuracy of areal evapotranspiration. Using the developed models fitted by the meteorological observation data and the land surface albedo derived by remote sensing data, we achieved distributed modeling results of meteorological elements such as net radiation, air temperature, vapor pressure etc in the Yellow River basin. The heterogeneity of land surface such as topography and land cover diversity was considered in these simulations. With confirmation that the complementary relationship between actual evapotranspiration and potential evapotranspiration is available on a basin-wide scale, distributed simulation of evapotranspiration in the Yellow River basin was generated by coupling the distributed modeling results of the above elements with the advection-aridity model which was founded on the complementary relationship theory. Comparison shows that the spatial distribution of the simulated annual evapotranspiration has a good consistency with the isograms which was derived by water balance and sub-region tests showed that the minimum relative error is 1.14%, the maximum is 26.80% and the whole river basin average is 1.50%. Furthermore, the simulated evapotranspiration indicts the details more fine. The integrated distributed model for areal evapotranspiration simulation was founded on the complementary relationship theory, which takes into account the feedback of regional evapotranspiration upon the near-surface layer atmosphere, and it is easy to use for only taking DEM data and conventional meteorological data as input.
Abstract: This study aims to evaluate the rainfall estimation accuracy and hydrological potential of the updated Version 7 Tropical Rainfall Measuring Mission (TRMM) multi-satellite precipitation product (TRMM 3B42V7, post-real-time estimates). First, the TRMM 3B42V7 precipitation estimates were compared and validated against the ground rain gauge observations from Jan. 2003 to Dec. 2008. Then, the TRMM 3B42V7 precipitation product was applied to drive the Gridded Xinanjiang model for streamflow simulation and the SCEM-UA algorithm was utilized to analyze and diagnose the model parameters uncertainty. At daily scale, 3B42V7 shows a better performance with lower mean error and bias (only about 6.68% underestimation). However, in terms of the absolute error sense, its data accuracy was not performed as expected (approximately 57.76% of absolute bias). At monthly scale, the TRMM 3B42V7 estimates have been improved dramatically by the month-to-month gauge adjustments during the data processing algorithm. The simulating accuracy of the daily streamflow driven by TRMM 3B42V7 is not satisfactory in the common sense for missing some flood events, but it can still capture the diurnal variation of the streamflow. By contrast, the TRMM-driven streamflow simulation performs better at monthly scale relative to the daily scale. Furthermore, it is capable to represent the seasonal and inter-annual variations of the streamflow. The daily and monthly 95% confidence intervals contain most of the measured streamflow process. This work was anticipated to provide the insights to the hydrologic user community when applying the TRMM satellite precipitation products for water resources simulation and management, especially over ungauged or data-scarce basins.
Abstract: In the present work, spatial and temporal changes of hydrological regime in the middle reaches of the Yangtze River after the impoundment of the Three Gorges Project were quantitatively evaluated, which provides a scientific evidence for the ecological protection of the middle reaches of the Yangtze River and regional water resources management. The range of variability approach was used to analyze average daily data for discharge and sediment concentration from five hydrological stations on the stem stream of the Yangtze River. Thus, river sections with the maximum water and sediment variation degree in the middle reach of Yangtze River and hydrological index categories and their corresponding ecological effects after the impounding of Three Gorges Project were quantitatively evaluated. The results showed that the changes of sediment concentration downstream were far greater than that of discharge after the impounding of the Three Gorges Project, which had a large scale decline compared with that of before the impounding except for Chenglingji station. Decrease range of sediment concentrations in Yichang station attained to one order of magnitude. The regulations of Dongting Lake on sediment concentration were obvious. Variation of discharge decreased with the increase of the distance from the dam, with a decrease of flow obvious between July and November. These changes in hydrological regime will affect both the fish-spawning habitat downstream and the growth of the aquatic organisms in the retarding area and the surrounding vegetation.
Abstract: This paper characterizes the temporal and spatial patterns evident in annual runoff and sediment load data from selected hydrological stations in the Poyang Lake region over the period 1956—2008. The Mann-Kendall test was used to analyze for trend, interannual variability was characterized, and change point years were identified. The main causes of the observed patterns in runoff and sediment load were investigated among the variables catchment rainfall and runoff, reservoir sediment trapping, soil erosion and soil conservation works. The results indicate that overall lake inflows and outflows display a slightly increasing trend. Inflowing sediment load to the lake displayed a significant decreasing trend, with a change point occurring in 1998—1999. Outflowing sediment load from the lake for the 10 years beginning in 2002 displayed a decreasing trend. Reduced rainfall within the catchment of Poyang Lake had a significant influence on the observed pattern of decreasing lake inflow and outflow, while trapping of sediment within catchment reservoirs was the primary reason for reducing the sediment load to the lake. The reduced sediment outflow from the lake was mainly influenced by the reduced sediment inflow and sand mining within the lake.
Abstract: It is necessary to analyse the relationship between channel evolution and variation in sediment-laden floods based on the energy dissipation principle, in order to investigate the bed-forming mechanisms of an alluvial river. As an example of the braided reach in the Lower Yellow River, the characteristics of channel scouring and silting in the reach under various sediment-laden floods were presented, using a two-dimensional numerical model and the analysis of observed data. It is founded from this investigation that the relevance among bankfull area, geomorphic coefficient and characteristic values of a sediment-laden flood can reflect the restricting mechanisms in relation of the river energy; the channel scouring intensity changes periodically with various sediment-laden floods, and the accumulative scouring effect for the main channel can correlate strongly with the process of water and sediment transport; and only a flood process that can shape the most appropriate cross-sectional geometry of an alluvial river can attain both the best efficiency for sediment transport and the maximum efficiency of the main channel scouring. These results also indicate that a flood process with the maximum comprehensive factor Φmax can obtain the best accumulative channel scouring efficiency, and therefore, the discharge corresponding to Φmax can be considered as a control indicator for overbank floods in Lower Yellow River.
Abstract: Bank failure is a typical process of lateral channel migration. As a huge sediment source, failed bank soil may directly cause the increase of sediment concentration and considerable channel evolution in a short time. Based on the failure mode of composite river bank, a series of experimental tests in flume was conducted to investigate the processes of disintegration and transportation of a failed bank block at the toe of concave bank, and the interaction between the failed soil, bank stability and bed evolution. Experimental results indicate that a part of disintegrated bank block will be transported to convex bank or downstream, and the amount of the residual part depends on the hydraulic scour force and the properties of bank material. Larger flow shear stresses occur in a local near-bank zone with violent turbulence and higher velocity, especially around the failed block. The shear stress at the near-bank end of the failed block is smaller, as compared with the condition without failed block. The larger the size of the failed block is, the greater the influence on shear stress is. Although the failed block cannot prevent the non-cohesive bank soil from collapsing, the processes of bank failure and bed deposition can be reduced. This reduction is more obvious for a larger failed cohesive bank block. Furthermore, the reduction degree in the rate of bed deposition caused by the failed bank block is much higher than that of the occurrence of bank failure.
Abstract: To obtain more effectively root-zone soil moisture over gullies of the Loess Plateau, this study used an exponential filter to estimate root-zone (20—80 cm) soil moisture at transect and gully scales based on surface observations (0—20 cm). The results showed that the approach reliably retrieved root-zone soil moisture whereas larger errors were observed at deeper layers. If the initial root-zone soil moisture was available, the estimation accuracy was improved to some extent. However, note that significant accuracy improvement was only observed at the first several points in soil moisture time series. The optimal characteristic time length T (Topt) increased with depth, and it generally increased by one factor compared to that in the 20—40 cm with an increase of 20 cm depth. Furthermore, as the Topt was exchanged to calculate root-zone soil moisture between transects and scales, the estimation accuracy only changed negligibly. This indicated the exponential filter method was non-sensitive to Topt. However, it is worth noting that strong spatial heterogeneity of soil properties exists at region scale on the Loess Plateau, the sensitivity of the exponential filter to Topt should be investigated at larger scales.
Abstract: 2-D water age model by operator-splitting algorithm is developed. The governing equation is splitted into three or two successive initial value problems. In any arbitrary triangle calculation grid, the operators are calculated by different algorithm on the basis of their characteristics. In convection problem, the method of characteristics is used. For the dispersion and propagation problem, a semi-implicit finite element algorithm is employed in the calculation. The model validation is tested by two different ways, including establishing a water transfer physical model and analytic solutions based on the symmetry characteristic of age model. The computed results well agree with the two tests, including velocity, concentration and age. This study can provide technical support for the age model validation.
Abstract: To investigate the air-water structural characteristic of the self-aerated developing zone in the open channel flow, the bubble size and quantity changes with the self-aeration development were evaluated experimentally, using a double-tip conductivity probe. The results showed that the distribution range of bubble chord length and mean chord length decrease gradually with reduced air concentration in the same flow cross-section; within the self-aerated zone, small-sized bubbles dominate quantitatively while large-sized bubbles have a large portion in terms of air content. It was believed from analysis that from the area at high air concentration near the water surface to the one at low air concentration at the bottom of the self-aerated zone, the development process for the air-water structure of self-aerated flow is predominant surface deformation-coexistence of surface deformation and individual bubbles-individual bubbles, which will be helpful for further study on formation mechanism of self-aeration in open channel flows.
Abstract: In order to study the hydrodynamic characteristics of saltwater intrusion in the Modaomen waterway, a three dimensional baroclinic numerical model with large scale and high-resolution grids, covering the entire upstream river network, the Pearl River estuary and its adjacent sea, was set up based on the Finite Volume Coastal Ocean Model (FVCOM) and then validated and calibrated against measured data. Numerical experiment of saltwater intrusion in the Modaomen waterway was carried out with this model. According to simulated results and measured data, the mechanisms of stratification, mixing and salt transportation during spring, moderate and neap tide were analyzed. Results show that the salt transportation in the Modaomen waterway is dominated by bottom high-salinity water and its net mass is landward during neap tide. However, it is dominated by surface low-salinity water and the net mass is seaward during flood tide. The balance of salt transportation will appear within moderate tide. As a result, the saltwater intrusion in the Modaomen waterway reaches its peak and nadir during the coming moderate tide after neap and flood tide respectively.
Abstract: A new non-parametric method combining a seasonal-trend decomposition procedure loess model, a frequency analysis and a geometrically distributed block Bootstrap method was established to calculate the reference nutrient levels of a shallow lake situated in the east China which is influenced by human activities. Firstly, the best period in which to make observations to assess the reference nutrient levels and their response matter in the lake was calculated out by using a trend-seasonal decomposition model. Then the reference levels of total nitrogen (TN), total phosphorus (TP) and chlorophylla (Chla) were obtained by frequency analysis. Finally, the confidence intervals were calculated using the geometrically distributed block Bootstrap method. This method overcomes the deficiencies of former methods. The reference levels in Taihu Lake, as one typical shallow lake in the east China, were worked out. The results of TN, TP and Chla in Taihu Lake were 0.78 mg/L, 0.030 mg/L and 2.63 μg/L. The 95% confidence intervals were 0.57~0.83 mg/L, 0.025~0.046 mg/L and 1.86~2.65 μg/L respectivly. The reference nutrient levels of other shallow lakes influenced by human activities in east China can also be obtained using this method.
Abstract: To understand the temporal and spatial variation of heat conditions in a large reservoir of a cold region, a prototype observation in the Fengman reservoir was performed to investigate the water temperature and ice conditions from June 2010 to August 2011. Based on the data, the annual heat structural variation of the water temperature in the reservoir was elaborated, the effect of the flood process on the layered water temperature structure was analyzed, and the buoyancy flow on the surface layer in the early period of the thawing season was observed. The vertical water temperature structures, 75 ~800 m in front of the Fengman dam, are almost the same in June and July, and the slight difference between the discharge temperature and the average water temperature in the water intake elevation indicates that the three-dimensional effect in front of the dam is non-existent. Influenced by the deep water intake, the temperature of the discharge is stable, while in contrast with the natural water temperature, the mean discharge temperature can be 10.8℃ lower from May to July and 7.9℃ higher from October to December. In addition, the ice condition observation results indicate that the Fengman reservoir is ice covered during the entire winter and that the maximum value of ice thickness reaches 0.7 m; meanwhile, the distribution of the ice thickness is uneven during the icebound season.
Abstract: This paper examines the behavior of floodwater in a flood detention basin and uses quantitative methods to establish the benefits and losses of floodwater utilization. The Dahuangpu Wa flood detention basin was taken to demonstrate semi-quantified functions of benefit and loss. A hydraulic mathematical model was applied to reveal the temporal and spatial conversion during three floods events, with specified return periods, using local hydrological and land-use information. The study indicates that critical water depth is the key parameter for optimum floodwater utilization. The inflow area, benefit area, average water depth and maximum water depth are non-linear and proportional to the flood inflow. Floodwater utilization is strongly linked to topographic features and land-use change.
Abstract: The completion of the Three Gorges Dam project would result in significant improvements in the flood control of the middle and lower reaches of the Yangtze River. However, additional flood control measures would be needed to address population and economic growth along with a more prominent role of reservoirs in attenuating floods. These measures were addressed herein by considering scenarios post dam construction, and using as input the historic and extremely large floods of 1954 and 1998. The locations of the storage areas and flows between the rivers and lakes within the basin were considered along with other key factors affecting flood control. In addition, new issues were considered, such as chemical runoff, sediment transport and subsequent river evolution, reservoir regulation, and alteration of the probability of flood storage. The results showed that the construction of the Three Gorges Dam would improve flood control for the 100-year and more frequent event. However, the results showed that flood controls would not greatly affect the flooding resulting from larger flood. It was also observed that the flood risk was transferred to the reservoirs (or lakes). Therefore, to mitigate flooding from extremely large floods, more effort needs to be put on increasing flood control measures in the reservoirs using both structural and non-structural approaches.
Abstract: Cities are suffering from extremely heavy rain and urban waterlogging disaster in recent years, due to the change in the natural hydrologic processes impacted by global climate change and rapid urbanization. The development and application of precipitation observation and forecasting, calculation methods for runoff generation and flow concentration, and urban stormwater simulation techniques are reviewed in the context of hydrological processing and modeling. The features, applicability and limitations of these techniques are discussed. The shortcomings and gaps of approaches on urban stormwater modeling, such as the lack of knowledge and the data to understand the response mechanism of the rainfall-runoff processes in the urban areas, are pointed out. The conceptual framework and flowchart for urban stormwater modeling is presented. Viewing from the prospective of rainwater utilization, essential principles and strategies on the urban stormwater management are presented, and the techniques and measures are also discussed. Then, the main future development direction and challenges in urban stormwater modeling and management are highlighted. Further research into the high spatio-temporal resolution observation and forecasting of urban rainfall are required to improve storm-term rainfall prediction. The performance of urban stormwater modeling technologies remain poorly quantified, which need a multisource information-fusion-based approach to the efficient estimation of urban runoff. The mechanism also remains poorly understood, which need much more research to understand the rainfall-runoff response relationship in the urban areas. In addition, urban stormwater is now increasingly regarded as a resource. The methods of harvesting, utilization and management of urban stormwater also need to pay more attention to, which imposes a requirement to ensure that stormwater management systems are adaptable and resilient to changes.