Abstract: Applying precipitation indices series of quality-controlled daily data from over 2 300 stations, the characteristics of long-term precipitation variations over the past 60 years for mainland China are analyzed. The main conclusions are as follows: ① No significant long-term trends of annual and seasonal precipitation percentage anomalies have been found for mainland China as a whole, though a relatively significant decrease and increase have been seen for the seasonal precipitation percentage anomalies for autumn and winter respectively; ② Decrease in annual and summer precipitation mainly occurred in central and southern Northeast China, North China, western Central China and Southwest China. Increase mainly occurred in southeastern coastal region, the mid and lower reaches of the Yangtze River, the Qinghai-Tibetan Plateau and Northwest China; ③ The spatial patterns of the annual and seasonal precipitation trends were relatively stable. The decrease areas migrated from the Loess Plateau and North China toward northeast and southwest after 1990s, the increase areas in Northwest China and the Qinghai-Tibetan Plateau did not change much or marginally expanded, and the increase areas in northern Northeast China and the mid and lower reaches of the Yangtze River obviously shrank. In the eastern monsoon region, the areas with either increasing or decreasing trends reduced for the last two decades; ④ Significant increases in annual amount and frequency of rainstorms, 3.18 mm/10 a and 0.03 d/10 a respectively, have been observed for the past 60 years, though the intensity of rainstorms remained unchanged. The increase mostly appeared in the Pearl River basin and the rivers of southeastern region. The Haihe River and the rivers of Southwest China witnessed a remarkable decrease in annual amount, frequency and intensity of rainstorms; ⑤ Proportions of the stations with increase (decrease) annual maximum 1 d, continuous maximum 3 d and continuous maximum 5 d precipitation were 59.2% (40.8%), 54.4% (45.6%) and 51.5% (48.5%) respectively, and those with statistically significant increase (decrease) are 5.1% (1.2%), 4.2% (2.5%) and 3.7% (2.8%) respectively, indicating an increasing frequency and intensity of extreme intense precipitation events, and a tendency toward shorter duration for single extreme intense precipitation events.
Abstract: The hydrological partitioning is not only governed by the relative magnitude of precipitation and potential evapotranspiration (PET), but also by their seasonal distribution. Based on the daily observations from 743 national meteorological stations during 1956 to 2010, the FAO-Penman equation was employed to calculate daily PET. To construct the seasonality indices, the interpolated 10-km grid daily precipitation and PET were analyzed. The results indicate that the daily PET shows an overall decreasing trend over 82.8% of the China, while the decreasing rate is not large. And the other seasonal indices of PET also have no significant trend. As for precipitation, the seasonality index has a significant decreasing trend across the China with a largest rate of 6.4% per 10 years in northwestern regions and the decreasing trend is more severe in northern basins. The centroid and duration of precipitation regime have not shown obvious changing pattern. The changes in precipitation and PET seasonality can lead to decreasing runoff, which should be noted in the future research on the attribution of change in runoff.
Abstract: Appropriate hydrological models for any given watersheds can be determined through the simulation performance of these models; however, well understanding the runoff generation characteristics of a given watershed can significantly reduce the workload to select the appropriate models from a large number of conceptual models for this watershed, also can solve the problem of hydrologically similar ungauged watersheds. In this study, we selected six conceptual hydrological models and applied them to three semi-humid and semi-arid watersheds to explore the relationships between model structures and watershed characteristics, and to analyze the impact of precipitation and topography on model simulation results. The results show that the topography and vegetation of watershed have important impacts on runoff generation and concentration. Due to the phenomenon of partial area runoff generation in such watersheds, the impact of river slope is greater than that of watershed average slope. The underlying surface conditions have a larger effect than precipitation conditions when the catchments have similar climate. Therefore, we can choose the appropriate hydrological models to get better predictions according to known runoff characteristics of catchments. Models with both saturation-excess and infiltration-excess mechanisms can be better used in the semi-humid and semi-arid catchments.
Abstract: The Variable Infiltration Capacity(VIC) model and the Mann-Kendall test are employed to evaluate the temporal-spatial variations of hydro-meteorological variables in the Yellow River Basin(YRB) over the past 60 years. The relationships between those variables and the East Asian Summer Monsoon (EASM) are also examined. The results show that the air-temperature has increased remarkably in the YRB since the mid-1980s; while rainfall, ET, runoff, and soil moisture have decreased in the YRB, especially in the middle reach region. Those hydro-meteorological variables in the YRB are closely related to EASM in general, although the relationships significantly vary in space. Overall, EASM shows negative relationships with air-temperature in the YRB, especially in the Ningxia-Inner Mongolia reach and inner regions. Positive relationships, however, exist between the EASM and other hydro-meteorological variables, especially in the Hekouzhen—Sanmenxia reach region. Besides, the results indicate that the runoff is more sensitive to the variation of EASM than rainfall does in the Loess Plateau. Understanding the relationships between EASM and hydro-meteorological variables has important implications for the water resources management in the YRB.
Abstract: In order to research the reaction of the runoff of Haihe River basin towards the change of land use, Fuping basin, Kuangmenkou basin and Jiehepu basin were regarded as the research areas and the data of hydro meteorology from 1970 to 2011 were combined to analyze the space-time transfer characteristics of different land use. Moreover, four scenarios of land use were set in combination of SWAT (Soil and Water Assessment Tool) model and influence on runoff by the change of land use types was evaluated. Result showed that this model could better simulate monthly flow process of the whole year as well as in flood seasons. The runoff had experienced a gradual decrease for many years and it fluctuated from 1980s to the middle of 1990s. The transformation in time and space among different types of land use presented the reversibility and major changes included an increase in forest, a decrease in grassland and a little increase in agricultural land. The increase of forest and the decrease of grassland would cause a decrease of runoff in flood season and the maximum monthly peak discharge. The coefficient in flood season decreased with the increase of forest area. Therefore, it was very important to properly plan land use pattern so as to control watershed hydrological events.
Abstract: To study the rainfall spatial distribution laws as well as the formation mechanism in China's Tianshan Mountains area, this thesis built the mountain area rainfall estimation model and analyzed the rainfall causes with the partial least squares(PLS) and GIS technology, based on the DEM in the research areas as well as the data information from the meteorological stations. The result shows that the rainfall in the Tianshan Mountain area is showing an obvious longitudinal and latitudinal zonality, with more rainfall in the west section than that in the east, and more in the north slope (windward slope) than that in the south slope (leeward slope). We can find a linear increase of the rainfall in the research areas with an altitude of 4 000 m below, and then a rapid decrease, with the second maximum belt at around 5 500 m. The rainfall and the slope are positively associated when the slope is less than 50°. When the topography lifts, the rainfall will increase with the drops of temperature and the rise of humidity, which is the requirement for the rainfall formation in the mountain areas. Generally, the method of PLS can effectively solve the multiple correlation issues between the rainfall and various factors, the regression effect of the model being obvious. This model has certain adaptability for the mountain area rainfall simulation.
Abstract: The paper is intended to show the patterns of the changes both in the cross section and the local flow due to a peculiar kind of scour, bottom-block scour, or named as bottom tearing scour and bottom ripping up, which is often found to occur in the forms of bottom-block uplifting and moving on the Yellow River, instead of commonly seen sediment particles' transport. Based on the data measured at the Longmen and Tongguan hydrometric stations and the lab experiments by the authors, the bottom-block scour is found to follow a four-staged pattern: Starting from pre-scour, to channel bed elevation keeping relatively stable, bed rapid declining due to mud-layered block uplift, and continual scour followed by back-silting. The bottom-block scour is found to tend to make a steeper and clockwise rating curve loop while non-bottom-block scour tends to make a mild-sloped and anticlockwise one. When the bottom-block scour starts, the local water level begins with a sudden rising and is followed by a rapid fall. Intensively turbulent flow is found not at the place where the bottom-block scour happens, but downstream of it. The paper is of significance in predicting and handling the risk of river training works failure due to the bottom-block scour.
Abstract: The braided reach of the Lower Yellow River (LYR) is experiencing severe channel degradation, and significant bank retreat processes have occurred owing to the operation of the Xiaolangdi reservoir. Estimation of bank retreat processes at typical sections can not only help to understand the characteristics of fluvial processes, but also offer parameters relevant to river regulation and planning. In this study, the variation in bankfull widths at typical sections of the braided reach has been investigated based on the observed cross-sectional profiles during the period from 1999 to 2013, with the maximum multi-year average bank retreat rate of 215 m/a being obtained. Several factors influencing bank retreat have been presented, which reveals that the composition and mechanical properties of the bank soil, and the elevation difference between floodplain and main channel can influence the processes of bank retreat, but the incoming flow and sediment regime is the dominant factor in controlling bank erosion. Empirical relationships between the accumulated bank retreat width and the previous 5-year average fluvial erosion intensity were developed at typical hydrometric sections and sedimentation sections, with the minimum correlation coefficients of above 0.85. Therefore, the calculated processes of bank retreat agree well with the measurements at these typical sections, and the proposed empirical relations can be used to estimate bank retreat processes in the braided reach of the LYR.
Abstract: Interflow output constitutes an important part of the hydrologic process of red soil slopes. In order to study interflow output characteristics on red soil slopes, a large-size field soil infiltration device (Lysimeter) was used to conduct a natural rainfall-interflow production observation experiment over grassland and bare land and at different depths (30cm, 60cm). The results indicated that: ① The retardation time and trailing time of the interflow production process both extended at deeper soil layers, where the compatibility with the rainfall process weakened. The interflow yield, peak value, and production duration of grassland were all larger than those of bare land, and their differences became more obvious under abrupt rainfalls. ② The interflow yield presented a significant positive correlation with the amount of rainfall (R2=0.694 2—0.877 0), but no apparent relationship with the intensity of rainfall. In the case of abrupt and extreme rainfalls, the interflow production process showed a rapid rise of flood peak, while in the case of uniform rainfall the interflow production process was relatively gentle. In the cases of abrupt, extreme, and uniform rainfall, the ratios of interflow yield to surface runoff of bare land were 13.82%, 91.73%, and 159.04%, respectively, while those of grassland were 118.95%, 312.11%, and 368.33%, respectively. ③ With the increasing antecedent soil moisture content, the retardation time of interflow production was shortened, and its peak value and flow increased.
Abstract: Based on the long term hydrological and bathymetric analysis, the morphological evolution in the estuary features the significantly large riverbed change, that is, the great erosion in runoff-dominated periods and the great sedimentation in tide-dominated period in the upper reach of the Qiangtang River estuary. This morphdynamic characteristics results in the large variation of tide in the upstream section and therefore affects saltwater intrusion. With plentiful runoff during wet seasons, the riverbed suffers great erosion, leading to the increase of river convey volume and enhancement of tide. This results in the increase of saltwater intrusion in falls with strong tide. On the contrary, with small runoff during wet seasons, the river convey volume is small. Therefore, it results in weakening saltwater intrusion in falls. The results indicated that the runoff variation has direct and indirect impacts on saltwater intrusion in the Qiantang River estuary. The direct impacts are the common characteristics of tidal estuary, i.e. saltwater intrusion increases as runoff decreases. The indirect impact is that the large erosion caused by the large runoff results in the increase of tide, and therefore indirectly leading to intensified saltwater intrusion. This feature results from the characteristics of the large riverbed change and its great response of tide strength in the Qiantang River estuary.
Abstract: To evaluate natural colloid transport potentials into groundwater, via fracture flow, a field-scale study was undertaken on a large sloping farmland plot (1 500 m2) in central Sichuan in the summer of 2013. Results indicated that colloid response times varied between 30—90 min; this depending mainly on the preceding soil moisture content and the intensity of rainfall. Colloid flushing occurred prior to peak fracture flow; with colloid concentration peaks appearing earlier than maximum discharge peaks. Peak colloid concentrations were 1—2 orders of magnitude larger than background colloid concentrations. Maximum rainfall intensity and patterns of rainfall dominated fracture flow and colloid concentration. Processes at the air-water interface are suggested to be the main driving factor controlling colloid release and transport during initial responses to rainfall. At these times, the flushing of colloids from soil mesopores and macropores surfaces, driven by the mixing of event rainwater and pre-event mobile soil water, is suggested to be the major mechanism of colloid mobilization and subsequent transport. It is submitted that colloid-facilitated transport could be a very important pathway for the migration of contaminants, particularly those that show large adsorption affinities to soil particles (e.g. phosphorus and hydrophobic pesticides), in the vast purple soil region of Sichuan.
Abstract: Though Series-Mixing-Cell-Model (SMCM) has been widely used in modeling solute transport and reactions, it's mainly for saturated conditions. Therefore, SMCM was extended to simulate salt movement during leaching process of initial unsaturated soils, by employing Kostiakov equation and piston assumption to describe water movement during infiltration. An analytic solution of the extended model was derived and analysis showed that, for given soil types, initial water and salt content profiles, simulation results would be influenced by layer thickness, and there was a quantitative relationship between optimized layer thickness, saturated hydraulic conductivity and solute diffusion coefficient. When layer thickness was chosen appropriately, results of this extended model were close to those of HYDRUS-1D. Further, the extended model was validated by experiment data, which showed that this model has advantages such as a small number of parameters which could be easily acquired and reasonably accurate modeling results.
Abstract: Study of a coordinated optimal operation of complex flood control system which consists of multi reservoirs, river channels and flood storage areas, is one of the most complex tasks in flood disaster mitigation. This paper introduces a Multi-objective Hierarchy Optimization Operation Model (MoHOOM), which sets maximum multi reservoirs system safety and minimum flood storage areas system losses as two objective functions and uses safe river channel discharge capacity as a constraint. The MoHOOM theoretical background is established by decomposition and coordination approach of large scale system theory and particle swarm optimization, the optimal solution is derived for each level derived by a third-order hierarchical decomposition and coordination structure. The MoHOOM is applied to optimal operation of complex flood control system in middle reaches of Huaihe River basin of China. The results show that the MoHOOM can efficiently calculate outflow hydrograph of multi reservoirs system and flood diversion hydrograph of flood storage areas system. Meanwhile, under the same initial calculation conditions, optimal operation reduces the safety indicator by 0.37 and 0.01 in Nianyushan and Meishan reservoir respectively; lowers the peak flow upon safe discharge capacity by 100 m3/s and 720 m3/s at Jiangjiaji and Runheji channel respectively; leads to 12.56 million Yuan losses in flood storage area, when compared to actual operation. Optimal operation reduces the safety indicator by 0.24 and 0.21 in Nianyushan and Meishan reservoir respectively; lowers the peak flow upon safe discharge capacity by 750 m3/s at Jiangjiaji channel; reduces losses by 3.42 million Yuan at flood storage area, when compared to rule-based operation. The MoHOOM is in favor of excavating multi reservoirs flood control capacities, reduces unnecessary losses in flood storage area under condition of ensuring safe discharge capacity in river channels, and ensures global optimization to coordinated operation of complex flood control system.
Abstract: Variations in long-term river discharge and tides caused by upstream structures can have a significant influence on estuarine geomorphological features. We used the ECOMSED model to simulate dynamic flow processes for a range of river discharges and tides in the Yangtze River estuary so that we could determine the influence of the Three Gorges Project (TGP) impoundment on stagnation points in the estuary. The results show that, as a result of changes in flows and tides, there were obvious spatial and temporal variations in the extent of major stagnation points in different parts of the estuary. The model demonstrated that there was significant movement in the stagnation points in the North Branch during flood periods. Because of the strong force of the rising tide, the stagnation points changed dramatically with changes in tides. Overall, the ranges of the variations in the South Branch channels were less than those in the North Branch channels. The range of the variations because of changes in river discharge decreased in the order: North Channel > South Passage > North Passage. The range of the variations because of changes in tides decreased in the order: South Passage > North Channel > North Passage. Results confirm that the geomorphological conditions are important for the stagnation points. Since the TGP impoundment, the decrease in the range of river discharge has resulted in a decrease in the extent of the stagnation points in all channels, which indicates that the area of the Turbidity Maximum Zone (TMZ) is decreasing. The TGP impoundment has therefore resulted in changes in the TMZ and related topographic features.
Abstract: A linked simulation-optimization model (SWT-NPTSGA) using the niched Pareto tabu search combined with a genetic algorithm (NPTSGA), is developed for deriving multiple objective management strategies for coastal aquifers, involving seawater intrusion control, mitigation measures, and groundwater resources optimization. The superiority of the NPTSGA-based optimization approach lies in its ability to find the appropriate balance between diversity and convergence of solutions with high computational efficiency. Density-dependent groundwater flow and solute transport simulator SEAWAT is used to generate input-output patterns of groundwater extraction rates and salinity levels. The performance of the presented simulation-optimization model is evaluated through a synthetic example application. The main advantage of the developed model is that it can make a tradeoff among maximization of total pumping rate, minimization of total artificial recharge and minimization of the extent of seawater intrusion. The seawater intrusion mitigation measures by artificial recharge can increase groundwater supply and reduce the area of seawater intrusion. The optimization results show potential feasibility of the proposed methodology in solving multiple objective optimization models for managing coastal aquifers.
Abstract: More scientific and comprehensive risk management concept is in constant development simultaneously with the rising level of Chinese dam safety management. From two aspects, which are the progress in the study along with the laws and regulations, a comparative analysis of Chinese dam safety management and risk management is carried out, and several problems concerning current safety management method are pointed out including the difficulty in calculating the accident probability, lack of attention to the consequences of the accident, and the evaluation criteria and the management measures are not comprehensive enough. In order to promote the transformation of management mode and to improve dam management level in China, a few thoughts and suggestions are put forward. These suggestions include that quantitative analysis of accident probability and consequences in future research and practical operation should be more valued, and risk criteria should be built in time along with more comprehensive risk control measures.
Abstract: Wetland hydrology is the driving force of wetland formation, "water stage-duration-frequency" (S, D, F) are the characteristic indexes of wetland hydrology. Aimed to promote the development of the theories and methods for determining (S, D, F) thresholds, the agreed conclusions derived from (S, D, F) threshold studies were summarized, which include: The principle of determining S threshold is "saturate to the earth's surface"; the principle of determining D threshold is "the lag time from the onset of saturation to the formation of anaerobic condition"; and F threshold should be≥50%. Through the analysis of these agreed conclusions, the results suggested: ① There are scientific problems in using the principle of determining D threshold; ② The present theories and methods for determining (S, D, F) threshold were built for the wetlands of "normal condition", and can not be used to determine the (S, D, F) threshold of "abnormal condition". In order to get (S, D, F) thresholds scientifically, new scientific theories and methods for determining (S, D, F) threshold should be built, which should include: The (S, D, F) threshold theories and methods for wetlands of both "normal condition" and "abnormal condition", and the theories and methods of scientific testing for (S, D, F) thresholds.