Comparison of the influences of vegetation stem parameters on hydraulic variables and sediment transport capacity

The hydraulic variables(Hv)and sediment transport capacity(Tc)of overland flow have changed immensely due to large-scale revegetation.However,research comparing the influences of stem pa-rameters(diameter,cover,and arrangement)on Hv and Tc is limited.The objectives of this study were to explore and compare the influences of stem parameters on Hv and Tc.Data on three treatment groups with varying stem diameters,covers,arrangements,flow discharges and slopes were collected in this study.With increasing stem diameter,Hv and Tc increased;Hv included the Reynolds and Froude numbers,flow velocity(v),shear stress(τ),stream power(Ω),and unit stream power(ω).However,the trend of the Darcy-Weisbach friction coefficient(f)was opposite.Hv and Tc were significantly influenced by stem diameter and cover.The effect of stem diameter on Hv and Tc was greater than the effects of cover and arrangement.Stem cover as a variable could not be used to adequately estimate the Tc when there were various stem diameters.A new exponential equation involving stem cover and stem diameter was demonstrated to be an appropriate predictor of Tc.Stem diameter and arrangement had no obvious influence on the relationship between Tc and v,and v as an indicator could explain the effect of stem parameters on Hv and Tc.This result could illustrate why the variables,including v,were able to predict Tc under vegetation stem and litter cover.

Effects of sediment characteristics on the sediment transport capacity of overland flow

The transport of sediments is a crucial part of soil erosion.Accurately calculating the sediment transport capacity is key to the construction of soil erosion process models.Research on Tc has focused mainly on the dynamics of a single particle of sediment and hydraulic variables.There have been few studies of the impact of soil aggregates on the Tc.To clarify how sediment characteristics,including those for single particles and aggregates,affect the Tc of overland flow with no raindrop import,flume experiments were implemented at slope gradients varying from 5.24%to 26.80%and flow discharges ranging from 0.68 to 5.41×10^(-3)m^(2)s^(-1).The experimental materials were five typical soils in China.The results indicated that the correlation between the measured Tc and sediment mechanical composition indexes of the five soils was indistinctive in this study.The sediment settling velocity with aggregates has a significant corre-lation with the measured Tc.New equations,including for the sediment settling velocity with aggregatesωud75,were established to calculate the Tc.The empirical equation that includedωud75,slope gradient and unit discharge performed greatly in predicting Tc(R^(2)=0.93,NSE=0.90).ωud75 can effectively improve the calculation accuracy of Tc.The new equation including flow and sediment properties obtained through dimensional analysis performed well in predicting Tc(R^(2)=0.99,NSE=0.91),and the calculation accuracy was better than that of the empirical model derived in this study.These findings indicate that the sediment settling velocity is an important variable in the equation for predicting sediment transport capacity of overland flow.

Comparison and quantitative assessment of two regional soil erosion survey approaches

Regional soil erosion surveys are the first step of soil conservation planning.Grid and sampling ap-proaches are commonly used for soil erosion surveys at the regional scale.With the grid approach,the region is divided into grids(e.g.,1 km × 1 km),and the soil erosion rate of each grid was calculated.With the sampling approach,a small portion of the area(typically 4%or less)was taken and then the soil erosion rate was calculated.Based on the above calculation,both approaches evaluated the soil erosion area(which is the area where the soil erosion rate is greater than the soil loss tolerance(t))and its distribution in the whole region.The purpose of this paper is to compare the assessment results of the two approaches and analyse their practicality using Yunyang County,Chongqing,China(area is 3636 km^(2)).The soil erosion rate,percentage of soil erosion area(PSEA,which is the ratio of the area where the soil erosion rate is greater than the soil loss tolerance(t)to the area of the total study region)and its spatial distribution were compared between the two approaches.The results showed that the grid approach overestimated the average soil erosion rate and PSEA overall for all 33 sample units and the whole region in comparison to the sampling approach.The sampling approach can yield a reasonable spatial distribution of the soil erosion rate in the whole region.In addition,the soil erosion regions were more clustered than those using the grid approach,which can be more suitable for soil conservation planning.Therefore,the sampling approach is an efficient and practical approach in regional soil erosion surveys.The results can provide insights into regional soil erosion surveys.

The assessment of soil loss by water erosion in China

Soil erosion is a major environmental problem in China.Planning for soil erosion control requires accurate soil erosion rate and spatial distribution information.The aim of this article is to present the methods and results of the national soil erosion survey of China completed in 2011.A multi-stage,unequal probability,systematic area sampling method was employed.A total of 32,948 sample units,which were either 0.2-3 km2 small catchments or 1 km2 grids,were investigated on site.Soil erosion rates were calculated with the Chinese Soil Loss Equation in 10 m by 10 m grids for each sample unit,along with the area of soil loss exceeding the soil loss tolerance and the proportion of area in excess of soil loss tolerance relative to the total land area of the sample units.Maps were created by using a spatial interpolation method at national,river basin,and provincial scales.Results showed that the calculated average soil erosion rate was 5 t ha-1 yr-1 in China,and was 18.2 t ha-1 yr-1 for sloped,cultivated cropland.Intensive soil erosion occurred on cropland,overgrazing grassland,and sparsely forested land.The proportions of soil loss tolerance exceedance areas of sample units were interpolated through the country in 250 m grids.The national average ratio was 13.5%,which represents the area of land in China that requires the implementation of soil conservation practices.These survey results and the maps provide the basic information for national conservation planning and policymaking.

A fast and simple algorithm for calculating flow accumulation matrices from raster digital elevation

Calculating the flow accumulation matrix is an essential step for many hydrological and topographical analyses.This study gives an overview of the existing algorithms for flow accumulation calculations for singleflow direction matrices.A fast and simple algorithm for calculating flow accumulation matrices is proposed in this study.The algorithm identifies three types of cells in a flow direction matrix: source cells,intersection cells,and interior cells.It traverses all source cells and traces the downstream interior cells of each source cell until an intersection cell is encountered.An intersection cell is treated as an interior cell when its last drainage path is traced and the tracing continues with its downstream cells.Experiments are conducted on thirty datasets with a resolution of 3 m.Compared with the existing algorithms for flow accumulation calculation,the proposed algorithm is easy to implement,runs much faster than existing algorithms,and generally requires less memory space.

Quantifying spatial distribution of interrill and rill erosion in a loess at different slopes using structure from motion(SfM)photogrammetry

The spatial distribution of interrill and rill erosion is essential for unravelling soil erosion principles and the application of soil and water conservation practices.To quantify interrill and rill erosion and their spatial development,four 30-min rainfalls at 90 mm h^(-1)intensity were consecutively simulated on runoff plots packed with a loess at six slopes of 10°,15°,20°,25°,30°and 35°.The soil surface was measured using the structure from motion(SfM)photogrammetry upon each simulation run,and the runoff and sediment samples were collected and measured at every 10 min.Rills did not develop until the third simulation run.During the initial two runs,the lower third section was more severely eroded than the upper and middle thirds along the slope direction,yet the interrill erosion was statistically uniform from left to right.Rills tended to emerge by both sidewalls and in the lower portion in the third run.The corresponding rill erosion increased with slope from 10°to 20°and then decreased for the slopes steeper,which was consistent with the slope trend of the sediment yield directly measured.The rills expanded substantially primarily via head retreat and to a lesser extent via sideward erosion after receiving another 30-min rainfall.Rill erosion contributed 69.3%of the total erosion loss,and shifted the critical slope corresponding to the maximum loss from 20°to 25°.These findings demonstrate the significance of rill erosion not only in total soil loss but also in its relation to slope,as well as the effectiveness of SfM photogrammetry in quantifying interrill and rill erosion.

Efficient Priority-Flood depression filling in raster digital elevation models

Depressions in raster digital elevation models(DEM)present a challenge for extracting hydrological networks.They are commonly filled before subsequent algorithms are further applied.Among existing algorithms for filling depressions,the Priority-Flood algorithm runs the fastest.In this study,we propose an improved variant over the fastest existing sequential variant of the Priority-Flood algorithm for filling depressions in floating-point DEMs.The proposed variant introduces a series of improvements and greatly reduces the number of cells that need to be processed by the priority queue(PQ),the key data structure used in the algorithm.The proposed variant is evaluated based on statistics from 30 experiments.On average,our proposed variant reduces the number of cells processed by the PQ by around 70%.The speed-up ratios of our proposed variant over the existing fastest variant of the Priority-Flood algorithm range from 31%to 52%,with an average of 45%.The proposed variant can be used to fill depressions in large DEMs in much less time and in the parallel implementation of the Priority-Flood algorithm to further reduce the running time for processing huge DEMs that cannot be dealt with easily on single computers.