Abstract: As a key state variable, soil moisture influences hydrological, meteorological and ecological processes. The objective of this study is to study the spatiotemporal characteristics of surface soil moisture across China using multi-satellite observations and uncover the driving forces of these characteristics. We applied an ensemble average approach to derive daily surface soil moisture with a spatial resolution of 25 km across China from 2015 to 2016 from five satellites, including SMAP, SMOS, AMSR2, FY3B, and FY3C. Uncertainty in these satellite products and their differences were further quantified by intercomparison. The spatial distribution of surface soil moisture and its connection with the spatial distribution of hydroclimatic zones were also analyzed. The correlations of soil moisture with precipitation and actual evapotranspiration were studied, too. The results show that spatial distribution of soil moisture across China corresponds well with the distribution of hydroclimatic zones:soil moisture generally increases from the northwest of China to the southeast and northeast of China. Clear seasonality appears in most areas of China with higher values in summer and lower values in winter. However, the amplitude and shape of seasonality differ in different parts of China. Temporal variabilities of soil moisture in over 60% of China are strongly controlled by the synchronous and antecedent precipitations. In over 87.5% of China, soil moisture and actual evapotranspiration show significant positive correlations and mutual dependence.
Abstract: To study changes in water vapor transport during continuous precipitation, stable isotope data, NCEP/NCAR data and Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) backward trajectory results during 22 continuous precipitation events in Beijing are analyzed. The results indicate that there are five different vapor transport types for the 22 continuous precipitations:westerly vapor transport, remote oceanic vapor transport, vicinal oceanic vapor transport, remote continental vapor transport, and vicinal continental vapor transport. Vicinal oceanic vapor transport and westerly vapor transport are predominate vapor transport types, contributing to 60.8% of the total precipitation. Variations of precipitation δ18O signals in different vapor transport types are mainly caused by differences in isotopic components in the source regions and the upstream rainout processes along the vapor transport pathways. Changes in deuterium excess reveal the influence of re-evaporation during precipitation and the humidity conditions in the vapor source areas. Fourteen continuous precipitation events are controlled by different vapor transport types at different stages, and the variations in precipitation δ18O indicate the changes in vapor transport types. These results show that isotope signals in precipitation can provide useful information for identifying vapor transport types.
Abstract: The relationship between the water levels in Dongting Lake and Chenglingji station was investigated using long-term hydrology data record and the hydraulics theory. The mechanisms of the changing water level relationship between the two regions were discussed, and the critical condition in the changing process was analyzed. Furthermore, the impact of the Three Georges Project on the water level linkage was examined. Theoretical analysis showed that the relationship between the water levels in Dongting Lake and Chenglingji station agreed with the monotonic exponential function under fixed discharge. Considering distance and morphology of the lake bed, an empirical model was established to estimate the water level in Dongting Lake. Based on the empirical model, the water level variations of the Dongting Lake was estimated all throughout the ranges of inflow discharges and water level stages at the lake outlet by extrapolation from the observed data. Based on these calculations, rating curves of the water level relationships between Dongting Lake and Chenglingji station were established with lake inflow as parameters. Therefore, it was found that the combination of inflow discharge and water level at Chenglingji station can be divided into three types, as follows:no-effect zone, influencing zone, and determining zone in the coordinate plane. Based on the definition of critical condition, the threshold lines between the three zones were quantitatively determined. The water level estimation methods in different parts of the lake under the conditions of the three zones were proposed separately. Finally, the impact of the Three Georges Project on the critical condition and threshold lines were evaluated along with the observed data. The hydraulic linkage between Dongting Lake and Chenglingji station slightly changed after dam impoundment.
Abstract: Flood compensative regulation is the fundamental principle of reservoir flood-control planning and real-time operation, while the solution to the flood compensative regulation mathematical model is heavily restricted by the modeling methods and the aftereffect. In a basic flood control system consisting of a reservoir and a flood control station, the objective function transforms from minimizing the flood control volume to minimizing the cumulative water storage. The transformation is based on the stage division of operation and the analysis of outflow limits upon which a linear programming model of Reservoir Flood Compensative Regulation (RFCR-LP) has been established. This model couples the operational decisions and downstream routing methods, which avoids the aftereffect accompanying conventional methods for a multi-stage decision. The RFCR-LP is a simple structure that is easy to solve, and it unifies the optimal mathematical model between flood-control planning and real-time operation, which forms a multi-function and full-cycle model for reservoir flood-control compensative regulation. The proposed model promotes an efficient solution for complex system modeling. The feasibility, flexibility and stability of the RFCR-LP has been verified by the case study.
Abstract: Soil water is central to our understanding of water cycle, groundwater recharge and vegetation restoration in the critical zone of the Loess Plateau. To ascertain the spatial variability of soil water for the entire profile in the critical zone of the Loess Plateau, we obtained soil samples from the top of soil profile to bedrock in five typical sites (Yangling, Changwu, Fuxian, Ansai and Shenmu) based on soil core drilling and analyzed soil water vertical distribution, variation character and spatial structure in the entire soil profiles by classic statistics combining with geo-statistics. Results indicated that mean soil water content in the entire profile decreased at south-north direction in the critical zone of the Loess Plateau. Variabilities of soil water in the five sites all were moderate. Soil water variation in each site became weak gradually and difference in soil water content between the sites decreased as soil depth increased from 40 m to 200 m. Geo-statistics analysis demonstrated that semi-variogram of soil water content in the sites could be fitted well by theory model except Yangling site and exponential model could describe spatial variation structure in the most profiles. These results can benefit the understanding of soil water status and distribution regime in ultra-deep soil profile and are valuable for soil water resources estimation and vegetation restoration in the Loess Plateau.
Abstract: The saturated permeability, ks is a key parameter in solving geotechnical engineering problems such as stability analyses of slopes and earth-retaining structures during rainfall. Precisely determining this quantity for soils in the field is difficult. Hence, this paper suggests a simple approach for estimating the saturated permeability of loess in the field using a new, modified, double-ring infiltrometer. We use this device, which can perform infiltration tests accurately and effectively, to study the infiltration characteristics of loess. The results of these tests show that water infiltration into uniform, dry loess can be considered to divide the soil into a saturated, wetting, and dry-soil zone. Moreover, the depth of the saturated zone is approximately equal to that of the wetting zone, and the degree of saturation in the wetting zone is greater than 70%. The soil-moisture profile derived from the infiltration tests is similar to that given by the Green-Ampt model. The field tests also indicate the double-ring infiltrometer method based on the conventional Green-Ampt model (Specification of soil test, SL237-042-1999) provided larger ks values of the loess. To determine the saturated permeability more reliably, we suggest a modified approach herein, which is based on the Green-Ampt model and employs the soil-water characteristic curves (SWCC) along with the main wetting path. The concept of the modified approach is to redefine the meanings of the key parameters of the Green-Ampt model (the matric suction head at the wetting front, the wetted depth, and the infiltration rate). We take the matric suction head at the wetting front to be that corresponding to the initial soil moisture content; its value can either be measured with tensiometers in the experimental field or else determined by the SWCC along with the main wetting path. The wetted depth during the infiltration test period is taken to be that observed by the soil moisture sensors. In the Green-Ampt model, we also adopt the average infiltration rate in the early infiltration stage rather than the steady-state infiltration rate. From the results of our infiltration tests and laboratory permeability tests, we find this approach to be a suitable method for determining the saturated permeability of loess in the field.
Abstract: Predicting soil water characteristic curve (SWCC) from particle size distribution was an easy, rapid and economical method. However, the traditional estimate of water content in this method didn't consider the adsorbed water films, which led to an underestimate of water content in the dry range of the SWCC. In this study, a modified method was developed to take the film water into account when estimating water content, and subsequently the SWCC was predicted by combining water content with the matric suction which estimated by the method in HP model and MV model. The modified model was tested on seventeen soil samples, selected from the UNSODA database. The results illustrated that the modified model significantly improved the model performance in the dry range of the SWCC and accurately estimated the SWCC of sand. Furthermore, the modified model was also applicable for loam and clay by introducing a parameter β.
Abstract: The Jingjiang Reach is undergoing considerable channel degradation, and significant bank retreat processes have occurred at local sites due to the operation of the Three Gorges Project (TGP). By investigating the adjustments in planform and cross-sectional geometries based on the measured topographic and hydrological data after the TGP operation, it is discovered that the process of bankline migration was remarkable in the Jingjiang Reach, with the average annual bank-erosion rate of 15.0 m/a and the cumulative bank-erosion length of 42.3 km; 59.2% of the bank-erosion regions were located at the left bank, and 68.7% of them were located in the Lower Jingjiang Reach (LJR); and severe bank retreat processes also occurred at convex banks, with 35.5% of the bank-erosion regions in the LJR. In addition, the variation in cross-sectional profiles indicates that 21% of all the cross-sections underwent obvious bank retreat processes, of which 74% occurred in the LJR. In summary, the bank-erosion regions were mainly located at the left bank and the bank retreat intensity was much higher in the LJR. Various factors influencing bank retreat in the Jingjiang Reach have been presented, and the results indicate that the incoming flow-sediment regime is a dominant control factor. Finally, empirical relationships were developed between the section-scale bankfull widths at typical sections with severe bank retreat and the previous five-year average fluvial erosion intensity during flood seasons, with the correlation coefficients of being higher than 0.85. Therefore, the proposed empirical relations can better consider the effect of the altered flow and sediment regime on the variation in bankfull widths at typical sections.
Abstract: The bank erosion of sediment through the exchange of momentum and energy transfer within the debris flow affects the unstable motion of the debris flow. Many models have been established to study the impact of erosion on debris flow initiation and motion, but most of the models were based on bed erosion. This paper analyzed the process of unstable motion of debris flows through an experimental flume to contrast bank erosion-dominated conditions and bed erosion-only conditions. The experiments showed that bank erosion enhanced the formation and propagation of debris flows. The height and velocity of the debris flows fluctuated along the path under both conditions. However, bank erosion made the motion fluctuation more obvious. Bank erosion usually occurred at the body of the debris flow and the particles moved more rapidly in the body than in the head, resulting in rapid growth of the head by the contribution of particles from the body. The height and additional gradient of the bank erosion-dominated debris flows were higher than those of the bed erosion-only debris flows. So the velocity of debris flow for bank erosion-dominated conditions was higher than that for bed erosion-only conditions.
Abstract: In order to address the issue of unstable numerical solution due to transition between supercritical and subcritical flows in local region in river-network modeling, the real flow status were firstly divided into four types including subcritical flow, supercritical flow, trans-critical flow, and shock, and then an adaptable model was introduced by increasing internal boundaries according to water balance and characteristics theory in the traditional Preissmann model. The final developed model preserves the advantages of the Preissmann model, which only need two points for each numerical template and can be easily integrated with mature river-network model. Numerical results for classic cases show that the introduced model is able to deal with gradual transition between subcritical flow and supercritical flow. A case study in Shiting river shows that the introduced model can converge to constant flow state using both single-river or river-network pattern, when frequent transition on supercritical and subcritical flow appears; this indicates the model can keep stable during complex flow modeling.
Abstract: The experimental formulas for the frictional resistance in open channel 1-D steady flow are based on one-to-one relationships among the hydraulic parameters. These formulas of frictional losses in steady flow cannot simulate the frictional losses in unsteady flow accurately because the hydraulic parameters reach their peaks at different times and their relationships among themselves are more complex. Therefore, using dimensional analysis, an improved differential structure formula has been developed. The experiment done in this study shows that the improved formula which takes the temporal and spatial variations of hydraulic parameters into account has higher accuracy than that of Chezy formula, and be of practical value.
Abstract: Vortices convecting with the local flow play an important role in mass transfer in a turbulent open-channel flow. To capture the movements of vortices throughout their life spans, two time-resolved particle image velocimetry systems are placed side by side in the streamwise direction to achieve a wider field of view. This experimental setup is used to measure the two-dimensional velocity fields in the streamwise-wall-normal plane of uniform open-channel flows with friction Reynolds number ranging from 490 to 870. By tracking the spanwise vortices in the measured velocity fields, their lifetime, displacements, convection velocities, maximum strengths, and rates of change in strength are obtained. The vortices are found to lift away from the wall while convecting with the local flow in the streamwise direction. For the case of the lowest Reynolds number, the maximum lifetime and vertical displacement are approximately 10 eddy turnover times and 0.4 water depth, respectively. As the Reynolds number increases, the maximum strength of the vortices increases at a smaller speed than the rate of change in strength, causing a shortening of their lifetime. Because the dimensionless convection velocities remain unchanged in magnitude, the vortex displacements shorten accordingly. The investigation reveals the lifetime to be the critical parameter for characterizing the movement of vortices and yields a preliminary estimate of the trend of variation in the movement of vortices under different flow conditions.
Abstract: Most previous numerical studies merely focused on Internal Solitary Wave (ISW) forces acting on a circular cylinder, and only a few studies have been conducted on ISW forces acting on a square cylinder. A 3-D numerical wave flume is used to investigate the generation and propagation of ISWs. The ISW forces acting on circular and square cylinders are examined through a Large-Eddy Simulation (LES) approach. ISW force behaviors in the circular and square cylinders are studied at various ISW amplitudes. Results show that the forces acting on the circular and square cylinders increase with increasing amplitudes. At the same amplitude, the ISW forces exerted on the square cylinder are larger than those exerted on the circular cylinder because of the more uneven pressure distribution and larger differential pressure between the front and rear of the square cylinder.
Abstract: We propose a new method for reconstructing complex surface velocity field in weir flow which is notoriously difficult to measure due to its sharp drop and high fluctuation. The new method is able to map the curved flow surface by two coplanar cameras and achieves measurement of two-dimensional velocity field through large-scale particle image velocimetry (LSPIV). The measured two-dimensional velocity field is projected onto the three-dimensional flow surface based on complex linear mapping relationship between the object space and image space. Application of the method to weir flow measurement yields satisfactory results, showing that the longitudinal velocity turns in trend from increase to decrease at the middle point of nappe, while the vertical velocity increases all the way along the nappe and plays a dominant role in the lower half of the nappe.
Abstract: We develop a numerical model that encompasses wave induced current as well as advective and diffusion models to examine the effects of Stokes drift on pollutant transport within the surf zone on a plane beach. We also carried out dye diffusion experiments in the surf zone, and modelled wave-induced currents by applying the concept of radiation stress. We based wave propagation on the equation of wave energy conservation, while the advective diffusion model, including Stokes drift, was used to describe pollutant transport in the surf zone. We then used the alternative direction implicit finite difference method to differentiate between models, and evaluated the validity of these numerical approaches using a Gaussian pulse with a known exact solution. The numerical results of this study are in close agreement with analytical solutions. Finally, we applied our model to simulate pollutant transport within the surf zone on a plane beach. Results show that numerical modeling including Stokes drift more closely agrees with experiments than do results that exclude this drift. Thus, in general, we show that a pollutant will be more obviously transported shoreward in addition to the extent of expected drift. We therefore suggest that Stokes drift plays an important role inpollutant drift within thesurf zone, especially in ashoreward direction.
Abstract: To study the characteristics of wave motions over reef islands under the effect of tidal current, laboratory experiments were performed in a wave-current flume to investigate the wave transformation and wave-induced setup in the presence of tidal current. A series of tidal flowrates were tested, and three flow regimes (shoreward, seaward and no-flow) were compared. Results show that the seaward flow moves the breaking point to the offshore side while the shoreward flow moves it to the coastal side. The turbulence in the surf zone associated with the seaward flow is stronger than that with the shoreward flow. Incident wave energy around the breaking point transferred from primary waves to their higher harmonics, and this type of transfer is more evident with the seaward flow. The tidal current has insignificant effect on wave reflection, transmission and energy dissipation. The shoreward flow decreased the wave-induced setup on the reef flat while the seaward flow increased it, and there is strong linear relationship between the maximum wave setup and the tidal flowrate. This study may provide theoretical guidance for construction and maintenance projects around those reef islands.
Abstract: In order to promote water resources big data and its applications in water resources management, it is necessary to conduct a review of the researches in this field and recommend future research directions. This paper firstly introduces the background and development of water resources big data and explains its concept and connotation. Based on these, the research framework of water resources big data has been developed, and meanwhile data storage and sharing, data analysis platform, data mining algorithms, data visualization, and its applications have been analyzed respectively. Finally, the future explorations about water resources big data have been recommended from the following aspects:exploring data integrated methods, developing data mining algorithms, establishing the data security mechanisms, and developing the integrated decision-making platform of water resources big data.
Abstract: Hydrological experiment plays an important role in hydrology research. In order to promote the development of hydrological experiment and its application in hydrology research, it is necessary to carry out a periodic comb on the relevant work and look forward the future holds. In this paper, the concept and connotation of hydrological experiment are expounded, and some common methods as well as the development of hydrological experiment both at home and abroad are reviewed systematically. Meanwhile, recent research progresses about hydrological effects of vegetation, rainfall-runoff relationship, water movement in unsaturated zone, and impacts of climate change and human activities on hydrologic cycle based on hydrological experiment are summarized. Based on all above, the existing insufficiencies in current hydrological experiment are analyzed and possible future directions of development are pointed out. Firstly, the traditional experiments mainly focused on the surface hydrology rather than explore the interior of the hydrological cycle. It is suggested that the researchers should not only focus on the surface hydrology, but also extend the experimental basin to the critical zone, and pay more attentions on the study of inter-basin supply, runoff and drainage. Secondly, interdisciplinary observations and experiments will become a foreseeable trend of the future hydrological experiments. Thirdly, the operational specifications and standards of current hydrological experiment are not sufficient, and more attentions should be paid to the construction of hydrological experiment station network and the specifications of hydrological experiments, which will make it possible for data sharing and comparison analysis.