Overland Flow Velocity Research Articles

A thermal infrared imaging observation system for the measurement of overland flow velocities under controlled laboratory conditions

Flow velocity is an important physical parameter in characterizing the hydraulics of water flow. The accurate measurement of overland flow velocities has great significance for understanding and modeling sediment transport and soil erosion. In this study, a Thermal Infrared Imaging Observation System (TIIOS) based on thermal infrared imaging techniques and computer vision recognition technology was established to measure overland flow velocities under controlled laboratory conditions. TIIOS has three subsystems including thermal tracer control, image acquisition and transmission, and image storage and processing. It can be used to dynamically monitor changes in shallow flow velocity by means of automatic control of thermal tracer, instantaneous image acquisition, image correction, noise removal and centroid determination. The observation precision was high with a standard deviation of 0.004 m·s−1 for flow velocity, a temporal resolution of 1/9th s, and a spatial resolution of 2 mm. The new system achieved higher accuracy compared to the traditional dye tracing and salt tracing techniques in observing the dynamic transport process of thermal tracer movement. The relative errors of the centroid velocity measured by the TIIOS ranged from −9.83 % to 6.40 %; whereas the relative errors of centroid velocity measured by salt tracer ranged from −21.36 % to 17.22 %. Measurements by the established TIIOS under different hydraulic conditions demonstrate that the relative errors of the centroid velocity are all smaller than those of the leading-edge velocity. This observational system provided a reliable way to measure shallow flow velocity for better understanding the mechanisms of water erosion processes and showed its potential to be used in field experiments under natural conditions.

Implementation of a GPU-enhanced multiclass soil erosion model based on the 2D shallow water equations in the software Iber

We present the implementation of a new fully distributed multiclass soil erosion module. The model is based on a 2D finite volume solver (Iber+) for the 2D shallow water equations that computes the overland flow water depths and velocities. From these, the model evaluates the transport of sediment particles due to bed load and suspended load, including rainfall-driven and runoff-driven erosion processes, and using well-established physically-based formulations. The evolution of the mass of sediment particles in the soil layer is computed from a mass conservation equation for each sediment class. The solver is implemented using High Performance Computing techniques that take advantage of the computational capabilities of standard Graphical Processing Units, achieving speed-ups of two orders of magnitude relative to a sequential implementation on the CPU. We show the application and validation of the model at different spatial scales, ranging from laboratory experiments to meso-scale catchments.

Effect of gravel coverage on the hydrodynamic characteristics of overland flow on the Loess Plateau in China

In the Loess Plateau of China, the presence of gravel mulching is widespread, and investigating the dynamic changes in hydrodynamic properties induced by gravel coverage is crucial for optimizing soil and water conservation in this region. In this study, the effect of gravel coverage on the hydrodynamic parameters characterizing the overland flow was investigated through indoor artificially simulated scouring experiments, considering ten different gravel coverages, four slope gradients, and nine flow discharges. The results demonstrated significant differences in hydrodynamic parameters under various experimental conditions (P < 0.01). The flow depth exhibited a linear increase with increasing gravel coverage. Compared to flow velocity observed on non-gravel-covered slope, the reduction percentage of flow velocity ranged from 13.62 % to 72.4 % on gravel-covered slopes. Furthermore, a prediction model was developed to quantify the impact of gravel coverage on the mean flow velocity of overland flow. The model achieved a high R2 of 0.858 and a low RME of 13.61 %. The experiments revealed that the overland flow was distributed in the “virtual laminar”-subcritical and transitional-supercritical zones, with mutual restrictions between the influence of gravel coverage and slope gradient on the flow regime. Finally, it was observed that the Darcy–Weisbach resistance coefficient increased with an increase in gravel coverage. These findings provide a solid theoretical basis for optimizing soil erosion prediction models based on the hydrodynamic characteristics of overland flow, and offer guidance for the rational allocation of soil and water conservation measures for gravel-covered slopes.

Characteristics of the Sediment Transport Process in Vegetation Hillslopes under Different Flow Rates

Vegetation filter strips (VFSs) have always been an important measure to control agricultural soil erosion, especially in mountainous and hilly areas with more sloping farmland. To investigate the mechanism of the sediment-trapping process by VFSs, a series of tests were conducted with four gradients of flow rate, 7.5–45 L min−1 m−1, and two different sediment concentrations of 40 and 120 g L−1. The whole process of overland flow was monitored, and sediment and particle size samples from the inflow and outflow were collected and measured. The results showed that the changes in sediment concentration did not significantly affect the corresponding coefficients in the power function relationship between overland flow rate and velocity. Using the Reynolds number alone cannot effectively indicate the flow pattern of overland flow on vegetation hillslopes. The peak particle size and linear function were effective in describing the relationship between sediment particle composition and delivery rate during the sediment-trapping process by VFSs. During the sediment-trapping process, the sediment-trapping capacity of VFSs continued to decrease. The increase in sediment discharge was accompanied by a higher proportion of coarse particles. Under the same flow rate conditions, when the sediment concentration was higher, the coarse particles and their proportion also increased faster. Therefore, using only a certain particle size threshold to distinguish suspended and transported sediment may lead to inaccurate estimation of the sediment-trapping performance of VFSs. This study deepened the understanding of the mechanism of water–sediment processes on vegetation hillslopes and promoted the widespread and efficient application of VFSs management technology.

A Case Study on GIS Based Method to Develop Geographically Distributed Synthetic Unit Hydrograph using Geo-Spatial Tools in Ungauged Sub-watershed of Ambika River Basin, Gujarat

Abstract In this work, using Snyder, SCS, and the geographically distributed approaches, synthetic unit hydrographs are developed for the recurring flood-impacted ungauged sub-watershed in the Ambika river basin of Gujarat. Due to the scarcity of observed rainfall-runoff data, enforcement of effective flood control measures have not been implemented in this area. Hence the districts in the Ambika river basin regularly experience unexpected floods during the monsoon season, resulting in loss of life and property. The unit hydrograph of a watershed is essential for developing a flood hydrograph, which is useful for flood forecasting. The majority of Indian watersheds are not gauged, and there is not enough rainfall-runoff data to develop a unit hydrograph. However, there are several approaches available that may be used to develop synthetic unit hydrographs using different regionally based empirical equations, and some of them are mentioned above. Using geospatial tools is one of the GIS-based methods for obtaining synthetic unit hydrographs that solely take into account the digital elevation model (DEM) which eliminates the use of regional empirical constants. The geographically distributed synthetic unit hydrograph (GDSUH) approach is employed here on the idea of distributed time-area unit hydrograph and spatial distribution of overland and channel flow velocity. The cumulative travel time map has been divided into isochrones to develop a time-area curve and the resulting unit hydrograph. In this technique, a pure translation flow process is assumed. This approach is most appropriate for watersheds of smaller size. Hence, in this present study, a smaller, unmeasured sub-watershed with an area of 46 km2 is chosen, and the synthetic unit hydrographs are derived using each of the three approaches. The findings from this study were analyzed to determine the most appropriate method for this region. It was observed that the GDSUH is more suitable in comparison to the other two conventional synthetic unit hydrograph approaches. In smaller and un-gauged watersheds, this method gives a conceptual model for forecasting stream flow hydrographs that will capture the geographically distributed nature of the rainfall-runoff process. Additionally, it has the advantages of being simple, quick, inexpensive, and time-saving, especially for smaller regions that lack rainfall-runoff data.

How a photovoltaic panel impacts rainfall-runoff and soil erosion processes on slopes at the plot scale

Photovoltaic (PV) power plants are fast growing worldwide due to the environmental benefit of solar power generation and the development of photovoltaic technology. However, the impacts of PV panels on rainfall-runoff and soil erosion processes in hillslope are not well understood. This study quantitatively investigated these impacts on a plot-scale slope through rainfall simulation experiments. A bare plot with in-situ loess soil in the Chinese Loess Plateau was divided to two 4 m × 1 m slopes (i.e., a test slope with a PV panel above its middle and a control slope without cover) as the study site in which 1-hr artificial rainfall with varying intensities (30 mm hr-1 to 100 mm hr-1) were conducted. The result showed that runoff volume, peak flow discharge rate and overland flow velocity were not remarkably different between the panel slope and the control slope, although the soil surface under the PV panel was rougher than the surface of the control slope. However, the slope with the PV panel produced 27 %–63 % less sediment flux at the outlet than the control slope, especially under heavy rainfall. This was attributed to the weakened splash erosion on the slope section under the PV panel due to the rainfall interception by the panel, which indicated that the key impact of the PV panel was preventing soil detachment by raindrop impacts. The PV panel delayed runoff start time under rainfall with heavy rainfall intensities (80 and 100 mm hr-1) due to the overland flow attenuation of the depression beneath the lower edge of the PV panel. These findings implied that PV panels on hillslopes may have the potential to retain soil organic matter in top soil layers and to improve soil structure (e.g., soil sealing control and soil aggregate protection), which may benefit to hillslope soil conservation and vegetation restoration in long term.

A Generalized Multistep Dynamic (GMD) TOPMODEL

AbstractThere is a lack of Ordinary Differential Equation (ODE) formulations in numerical hydrology, contributing to the lack of application of canned adaptive timestep solvers; hence the continued dominance of fixed (e.g., Euler) timestep techniques despite their fundamental problems. In this paper, we reformulate Dynamic‐TOPMODEL into a constraint‐handling ODE form and use MATLAB's advanced adaptive ODE‐solvers to solve the resulting system of equations. For wider applicability, but based on existing research and/or first principles, we developed Generalized Multistep Dynamic TOPMODEL which includes: iso‐basin spatial discretization, diffusion wave routing, depth‐dependent overland flow velocity, relaxing the assumption of water‐table parallelism to the ground surface, a power‐law hydraulic conductivity profile, new unsaturated zone flux, and a reference frame adjustment. To demonstrate the model we calibrate it to a peat catchment case study, for which we also test sensitivity to spatial discretization. Our results suggest that (a) a five‐fold improvement in model runtime can result from adaptive timestepping; (b) the additional iso‐basin discretization layer, as a way to further constrain spatial information where needed, also improves performance; and (c) the common‐practice arbitrary Topographic Index (TI) discretization substantially alters calibrated parameters. More objective and physically constrained (e.g., top‐down) approaches to TI classification may be needed.

Quantification of the effects on the flow velocity caused by gramineous plants in the loess plateau in North-Western China

Accurate prediction of the mean velocity of overland flow is the premise and foundation for establishing a soil erosion model, but it is difficult to accurately estimate the mean flow velocity with the presence of vegetation. To explore the variation law of the mean velocity of overland flow under the influence of gramineous plants typical in the Loess Plateau in North-Western China, indoor scouring tests with ten levels of vegetation coverage (9.42 %–94.25 %), seven unit discharges (0.278–1.667 L·m−1·s−1), and five slope gradients (4°–12°) were performed. The results showed that the mean flow velocity initially increased and then decreased with an increase in vegetation coverage, and the critical cover was affected by the unit discharge. For a slope of 4°, the mean flow velocity with a vegetation coverage of 94.25 % was only 21.6 %–32.0 % of that on a bare slope, indicating that vegetation can effectively reduce flow velocity. For each experiment conducted, with an increase in vegetation coverage, the overland flow gradually moved from laminar flow to transitional flow. Based on the principle of equivalent roughness and Manning's equation, a prediction model was also established in order to predict more accurately the mean velocity associated with overland flow, and it has been validated against the experimental results demonstrating a satisfactory agreement with the measured values (adj.R2 = 0.879, NSE = 0.867). These results provide further insights regarding the influence that the vegetation can have on the flow velocity and contribute to develop a better management of these environmental areas.

Investigating the effects of herbaceous root systems on the soil detachment process at the species level

Plant root growth significantly affect soil detachment process, whereas the mechanism of how roots affect the soil detachment process by overland flow at species level is not fully understood. This study was conducted to investigate the soil detachment rate responds to plant-induce soil properties and root traits at species level. Two typical herbaceous plants, Bothriochloa ischcemum (Linn.). Keng (BI; fibrous root system) and Artemisia vestita Wall. ex Bess (AG; tap root system), from the Loess Plateau were studies for one year under six planted densities of 5 plants m−2, 10 plants m−2, 15 plants m−2, 20 plants m−2, 25 plants m−2, and 30 plants m−2. In total, 24 steel tanks were planted, and two plots were used as bare soil controls. Their soil detachment rates were tested under a constant overland flow (1.5 l s−1) on a 26.2 % slope. The results showed that soil detachment rate under the six planted densities ranged from 0.034 kg m2 s−1 to 0.112 kg m2 s−1 for BI and was ranged from 0.053 kg m2 s−1 to 0.132 kg m2 s−1 for AG, which all greatly reduced soil detachment rate and were 68.17 % to 92.33 % and 69.20 % to 87.27 % less than that of the control. In general, BI was more effective in reducing soil detachment rate than AG, achieving a mean soil detachment rate that was 23.75 % lower. With increasing plant density, soil detachment rate decreased as a power function. The overland flow hydraulic characteristics, soil properties and root traits influenced by plant density were positively or negatively correlated with soil detachment rate. Specifically, soil detachment rate decreased with velocity, bulk density, root length density, and increased with shear stress and Darcy–Weisbach friction factor as power or exponential functions. On this basis, the soil detachment rate (Dr) can be satisfactorily estimated by overland flow velocity (v), soil bulk density (BD) and root length density (RLD) as a power function (Dr = 63.03v0.174 × BD−20.712 × RLD−0.233R2 = 0.65; NSE = 0.60; p < 0.01).

Experimental study on the correction factor of surface overland flow velocity

The correction factor (α) of surface overland flow velocity is a significant parameter for determining the mean flow velocity in the study of soil erosion. A series of experiments with six types of surface roughness, five types of rainfall intensity, ten types of grass cover density, and three types of grass-shrub communities were conducted to quantify the influence of surface roughness, rainfall intensity, and grass cover on α. These results confirmed that for smooth surfaces, there is a negative correlation between α and the slope gradient in a large slope gradient range of 0.087–0.4226. The low critical value of Re (Rel) that resulting the influence of Re on α is not significant may be less than 170. There was a significant negative correlation between α and roughness (p < 0.001). In addition, whether on a smooth or rough surface, the rainfall increased α. When the grass cover density (C) was smaller than 3.45%, α increased with an increase in C; when C was larger than 3.45%, α showed the opposite trend as C increased. This is because as C increased, us did not continue to decrease but increased slightly. In addition, the mean flow velocity decreased significantly. This leads to a decrease in α. The rainfall intensity, roughness, and grass cover density had significant effects on α. Finally, the prediction equation of α quantifying the influence of rainfall and roughness on α was obtained with a high R2 (0.84) and low RRMSE (14%). h is a significant parameter for predicting α under grass-cover conditions. The prediction equation of α quantifying the influence of grass cover on α was also obtained with a high R2 (0.87) and low RRMSE (13%). These equations should be further validated when applied beyond the scope of the conditions under which they were developed.

Effects of shrub-grass cover on the hillslope overland flow and soil erosion under simulated rainfall

Vegetation plays a vital role in regulating hydrological cycle and controlling soil erosion at multiple spatial and temporal scales. Establishing shrub-grass community is one of the widely adopted practices to increase rainfall infiltration and reduce soil erosion in water-limited and highland regions. To understand the effects of such vegetation communities on soil erosion and overland flow under different rainfall regimes at the hillslope scale, we conducted rainfall simulation experiments by setting up parallel plots at fixed slope of 15° including unvegetated (coverage 0%), shrub only (coverage 50%), grass only (coverage 50%), and shrub-grass covered (coverages 25%, 50%, 75%, and 100%) and constant rainfall intensities of 30, 60, and 90 mm h−1 rainfalls lasting 60 min each after the initiation of overland flow. Two native species Lespedeza bicolor and Carex giraldiana, distributed in the soil sampling region were planted on the plots to achieve designed coverages. We found that the overland flow and sediment load from vegetated slopes were reduced by 9%–58% and 27%–98%, respectively, compared with unvegetated slopes while the infiltration rate increased by over 45%. Shrub-grass community reduced the overland flow and sediment yield more significantly than shrub only and grass only treatments with the same coverage of 50% under three rainfall intensities. In addition, the overland flow rate linearly decreased while the mean sediment yield exponentially reduced against the increase in shrub-grass community coverage. Hydrodynamically, shrub-grass communities not only increased the critical hydrodynamic forces for the initiating soil erosion but also increased the resistance coefficient leading to reduce overland flow velocity, stream power, and thus soil erosion from the vegetative slope even under extreme rainfalls. Our research highlights the importance of developing the shrub-grass communities to reduce the quantity and energy of overland flow and control soil erosion on the hillslopes in water-limited and highland regions.

Overland flow velocity and soil properties in established semi‐natural woodland and wood pasture in an upland catchment

AbstractManagement of upland land‐use has considerable potential for mitigating flood risk by increasing topsoil storage and slowing overland flow. Recent work has highlighted the potential for vegetation to impact the velocity of saturation‐excess overland flow. Woodland creation is widely proposed for Natural Flood Management (NFM), but data on saturation‐excess overland flow in woodland habitats is lacking. Here we measure soil properties and overland flow velocities in established broadleaf woodland and wood pasture with an understorey dominated by either grass or bracken. We show that wood pasture dominated by bracken has overland flow velocity 12–20% lower than established broadleaf woodland and 19–27% lower than grass‐dominated wood pasture. Established woodland soils exhibited eight times higher saturated hydraulic conductivity than bracken‐dominated wood pasture and 80 times higher than grass‐dominated wood pasture. We conclude that upland habitats can be managed to reduce flood risk, first by storing storm water in the soil and then by reducing overland flow velocity through rough surface vegetation. These factors combine to reduce floods by delaying the onset of overland flow runoff and slowing its delivery to streams. It is clear than Manning's n is far from constant in these shallow overland flows, the development of overland flow datasets is, therefore, also beneficial for improving the theory and practice of hillslope rainfall‐runoff modelling.

Importance of grass stolons in mitigating runoff and sediment yield under simulated rainstorms

The influence of grass cover on surface runoff and erosion processes on hillslopes differs with plant morphology and grass components (e.g., leaves, stems and roots). However, the importance of grass components, especially stolons, in reducing runoff and sediment yield is not fully understood. Thirty field simulation experiments with rainfall intensities of 90 mm h−1 were performed to study the effects of three grass species (Trifolium repens, Cynodon dactylon and Eremochloa ophiuroides) and their aboveground and belowground components on runoff, hydrodynamic parameters and sediment reductions during rainstorms. To examine the effects of different grass components on soil erosion, three management treatments, intact grass plants (IG), grass stems and roots (SR) and only grass roots (OR), were applied to each grass species. The results showed that the three grass species effectively reduced runoff and soil erosion, and the erosion reduction was greater than the runoff reduction. The highest reduction in runoff (78.2%) and sediment yield (97.9%) was observed for E. ophiuroides, followed by the T. repens and C. dactylon plots. The overland flow velocity and runoff energy increased with the sequential removal of grass aboveground components and led to increasing runoff and erosion. The effects of grass components on runoff and erosion reduction varied with grass species and plant morphology. The aboveground components had a greater impact on runoff reduction, and the roots primarily contributed to sediment reduction in T. repens and C. dactylon, which have erect stems. However, the contribution rates of grass stems to runoff and erosion reduction were highest for E. ophiuroides with well-developed stolons at 41.51% and 61.04%, respectively, which were much higher than the leaves and roots. This finding indicated that grass stolons played an important role in reducing runoff and sediment with efficiencies of 32.44% and 59.79%, respectively. These results help guide the selection of grass species with the best traits for erosion control and highlight the importance of grass stolons in preventing water erosion caused by extreme rainstorms.

A theoretically-based overland flow resistance law for upland grassland habitats

Sediments conveyed from interrill areas to rills are transported by a thin flow (overland flow) and the knowledge of overland flow resistance is useful to evaluate overland flow velocity and sediment transport capacity.The aim of this paper was to verify the applicability of a theoretical overland flow resistance law, based on a power-velocity distribution, using field measurements for four upland grassland types used in different management strategies (Hay Meadows, Low-density Grazing, Rushes and Rank Grassland). Particular attention was deserved to the effects of vegetation growth cycles on the surface roughness and the corresponding reduction of overland flow velocity.The relationship between the parameter Γ of the power velocity profile, the flow Froude number and the Reynolds number, obtained by dimensional analysis, was calibrated using the available data. A specific calibration for each upland grassland type and taking into account the temporal phases of the vegetation growth was also carried out.The theoretical approach, checked by the available field measurements, allowed to establish that a) the proposed theoretical flow resistance law, which considers also the effect of the flow Reynolds number, allows an accurate estimate of the Darcy–Weisbach friction factor, b) the theoretical overland flow resistance law considers the temporal variability of vegetation characteristics, c) this flow resistance law can be specifically calibrated for taking into account for each upland grassland type the effect of the vegetation temporal variability and d) a low effect of the seasonal variation of roughness in grassland environments is detected.The interrill soil erosion processes are strongly related to hillslope hydraulics and the proposed overland flow resistance law is useful to model soil erosion processes.

Performance evaluation of spatially distributed, CN-based rainfall-runoff model configurations for implementation in spatial land use optimization analyses

Due to their large computational burden, spatially distributed hydrological models are unsuitable for iterative spatial optimization analyses, which need to take into account location-specific runoff water accumulation and re-infiltration processes. Therefore, a computationally efficient, raster-based and spatially-explicit RR-model was conceptualized. For this purpose, the SCS-CN approach was implemented in a spatially distributed manner, adjusting CN values to antecedent soil moisture conditions (AMC) and routing runoff to the catchment outlet. To take into account spatial interaction along the overland flow paths, the SCS-CN approach was extended with a re-infiltration algorithm. Several model configurations were evaluated, based on combinations of two AMC correction methods and three re-infiltration algorithms. These model configurations were applied in three catchments in Flanders, Belgium. The results indicate a poor performance of the default SCS-CN method, which was, however, considerably improved by the implementation of AMC correction and re-infiltration. Though calibration of the RR-model configurations was limited, several model configurations achieved model performances adequate for use in spatial optimization analyses. The most performant RR-model configuration (NSE of 0.57, 0.56 and 0.64 for the three catchments) implements a λ of 0.05, the AMC correction method proposed by Neitsch et al. (2011), and the re-infiltration method of Van Loo (2018), calculating overland flow velocity with Manning’s n adjusted to seasonal vegetation cover variations. As this model allows for spatial interaction through re-infiltration, but with limited computation times, it is well applicable for iterative spatial optimization analyses, although a more extensive calibration and validation exercise on catchments with a wider range of characteristics is required to provide a general assessment of its applicability and accuracy.

Using Quinine as a Fluorescent Tracer to Estimate Overland Flow Velocities on Bare Soil: Proof of Concept under Controlled Laboratory Conditions

This study presents a tracer technique based on the fluorescent properties of quinine to help on the visualization of shallow flows and allow a quantitative measurement of overland flow velocities. Laboratory experiments were conducted to compare the traditional dye tracer and thermal tracer techniques with this novel fluorescent (quinine) tracer by injecting a quinine solution and the other tracers into shallow flowing surface water. The leading-edge tracer velocities, estimated using videos of the experiments with the quinine tracer were compared with the velocities obtained by using thermograms and real imaging videos of the dye tracers. The results show that the quinine tracer can be used to estimate both overland and rill flow velocities, since measurements are similar to those resulting from using other commonly used tracers. The main advantage of using the quinine tracer is the higher visibility of the injected tracer under ultraviolet A (UVA) light for low luminosity conditions. In addition, smaller amounts of quinine tracer are needed than for dye tracers, which lead to smaller disturbances in the flow. It requires a simple experimental setup and is non-toxic to the environment.

Improved flood forecasting using geomorphic unit hydrograph based on spatially distributed velocity field

Abstract This paper presents an energy model for determining the overland flow velocity in order to improve the low accuracy problem in flow concentration simulation. It furnishes a novel idea for studying flow concentration in ungauged basins. The model can be widely applied in analysis of spatial velocity field, extraction of instantaneous geomorphic unit hydrograph and development of distributed hydrological model. A distributed flood-forecasting model is constructed for Lianyuan Basin in Hunan Province of China. In the proposed method, gravitational potential energy is transformed into kinetic energy via an analysis of energy distribution of water particles in the basin. Based on the kinetic energy equation, the overland flow velocity simulating the geomorphic unit hydrograph is computed. Rainfall-runoff simulation is then performed by integrating with runoff yield and concentration model. Results indicate that the model based on energy conversion leads to more accurate results. The model has the following advantages: firstly, the spatial distribution of the velocity field is appropriate; secondly, the model has only one parameter, which is easily determined; and finally, flow velocity results can be used for the computation of river network flow concentration.

Correction factor for measuring mean overland flow velocities on stony surfaces under rainfall using dye tracer

Dye tracing is a common and simple method for measuring surface velocity of overland flow. However, a correction factor is needed to convert the velocity of the dye front to mean velocity. Selecting the appropriate correction factor for particular conditions is complex and its behavior has not been well studied in sheet flow under rainfall on actively eroding surfaces. A series of simulated rainfalls were conducted on a 2 m by 6 m plot with gravelly sandy loam soil. The plot was progressively eroded to examine a wide range of hydraulic conditions. A total of 178 velocities (maximum and mean) were measured using electrolyte and dye tracers over three travel distances (1.65, 3.5, and 5.8 m), and three slope gradients (5%, 12% and 20%). Discharge, sediment yield, random roughness, rock cover, and a range of hydraulic variables were also determined. Mean flow velocity was derived from the centroid of the electrolyte breakthrough curve. Our findings suggest that the velocity correction factor is a dynamic, site specific property. A linear model was proposed to estimate the correction factor based on readily available predictor variables, those being travel distance, unit discharge, and maximum velocity. The results will benefit further investigations of overland flow hydraulics on semiarid rangelands by providing more comprehensive correction factors to determine mean velocity.

Vegetation cover effects on sediment concentration and overland flow under artificial rainfall intensity

Soil erosion depends on a number of factors including rainfall intensity, density of plant cover, and area cover. The objective of this study is to investigate the impact of these factors on flow velocity, overland flow regimes, sediment concentration, and absolute soil detachment. The soil used in this study was sandy remolded agricultural soil. The soil is packed in a tray of 1 m2 fixed on a slope of 3%; five different intensities were simulated under different vegetation cover (density and area). The results indicated that the overland flow velocity with vegetation cover was best described by polynomial function. The mean flow velocity varied from 0.021 to 1.244 m/s. Overland flow regime is subcritical and laminar. However, there are significant relationships between the vegetation cover density and sediment concentration and absolute soil detachment. The sediment concentration ranged from 1.38 to 5.65 kg/m3 whereas the absolute soil detachment ranged from 0.021?10-3 to 1.244?10-3 kg/m2/s. Finally, the vegetation cover presented a good protector to soil sediment from erosion.

A New Runoff Routing Scheme for Xin’anjiang Model and Its Routing Parameters Estimation Based on Geographical Information

The Xin’anjiang model is a conceptual hydrological model, which has an essential application in humid and semi-humid regions. In the model, the parameters estimation of runoff routing has always been a significant problem in hydrology. The quantitative relationship between parameters of the lag-and-route method and catchment characteristics has not been well studied. In addition, channels in Muskingum method of the Xin’anjiang model are assumed to be virtual channels. Therefore, its parameters need to be estimated by observed flow data. In this paper, a new routing scheme for the Xin’anjiang model is proposed, adopting isochrones method for overland flow and the grid-to-grid Muskingum–Cunge–Todini (MCT) method for channel routing, so that the routing parameters can be estimated according to the geographic information. For the new routing scheme the average overland flow velocity can be determined through the land cover and overland slope, and the channel routing parameters can be determined through channel geometric characteristic, stream order and channel gradient. The improved model was applied at a 90 m grid scale to a nested watershed located in Anhui province, China. The parent Tunxi watershed, with a drainage area of 2692 km2, contains four internal points with available observed streamflow data, allowing us to evaluate the model’s ability to simulate the hydrologic processes within the watershed. Calibration and verification of the improved model were carried out for hourly time scales using hourly streamflow data from 1982 to 2005. Model performance was assessed by comparing simulated and observed flows at the watershed outlet and interior gauging stations. The performance of both original and new runoff routing schemes were tested and compared at hourly scale. Similar and satisfactory performances were achieved at the outlet both in the new runoff routing scheme using the estimated routing parameters and in the original runoff routing scheme using the calibrated routing parameters, with averaged Nash-Sutcliffe efficiency (NSE) of 0.92 and 0.93, respectively. Moreover, the new runoff routing scheme is also able to reproduce promising hydrographs at internal gauges in study catchment with the mean NSE ranging from 0.84 to 0.88. These results indicate that the parameter estimation approach is efficient and the developed model can satisfactorily simulate not only the streamflow at the parent watershed outlet, but also the flood hydrograph at the interior gauging points without model recalibration. This study can provide some guidance for the application of the Xin’anjiang model in ungauged areas.