X. C. Zhang

Flow Detachment of Soils under Different Land Uses in the Loess Plateau of China

Land use has a significant effect on soil properties, and therefore on soil erodibility. In many cases, land use controls soil erosion to a greater extent than does climate change or other system properties, yet only a few studies have quantified the effect of land use on soil detachment rates. This study was conducted to evaluate the effect of land use on soil detachment of natural, undisturbed soil samples, which were taken from fields under five typical land uses in the Loess Plateau of China. Flow discharges ranged from 0.25 to 2.0 L s-1 and slope gradients varied from 8.8% to 46.6%. The results indicated that soil detachment rates were significantly influenced by land uses. Cropland soil was most easily detached, followed by grassland, shrub land, wasteland, and woodland soils. The average ratios of detachment rates of cropland soil to grassland, shrub land, wasteland, and woodland soils were 2.05, 2.76, 3.23, and 13.32, respectively. Detachment rates were shown to be a power function of flow discharge and slope gradient (R2 > 0.93, NSE > 0.87). Both critical shear stress and erodibility were affected by land use. The different land uses in this study resulted in erodibility ranging from 0.0021 to 0.164 s m-1 and critical shear stress ranging from 2.08 to 6.30 Pa. No superiority was found if shear stress was replaced by stream power for detachment prediction, and either one could simulate rill detachment rates well. The result indicated that soil detachment rates in rill erosion could be well simulated by hydraulic parameters of stream power, slope, and runoff density and by soil properties of clay content, bulk density, aggregate median diameter, and soil strength. Further studies are needed to quantify the potential effects of plant root systems on detachment rates and to investigate the potential temporal variability of soil detachment rates.

EFFECTS OF SURFACE TREATMENT ON SURFACE SEALING, RUNOFF, AND INTERRILL EROSION

Surface treatment influences the nature and extent of seal/crust formation, which affects runoff and erosion. This study evaluated the effects and longevity of soil amendments, tillage, and screen cover on runoff and interrill erosion on a Cecil sandy loam (clayey, kaolinitic, thermic Typic Kanhapludult) under natural rainfall conditions. Six field plots (3.5 ×0.9 m) with a slope of 0.06-m m–1 were used in the study. Three treatments (control, screen cover, crust-breaking shallow tillage) were studied in duplicate in the first two-month period, and another three (control, anionic polyacrylamide (PAM), and phosphogypsum) in a subsequent five-month period. Total runoff and soil loss were 69, 61, and 47 mm and 5.3, 1.6, and 4.3 Mg ha–1 for the control, screen cover, and tillage treatments, respectively. Compared with control, screen cover reduced soil loss significantly but not runoff, while tillage reduced runoff more than soil loss. Total runoff and soil loss for the control, gypsum, and PAM treatments were 146, 48, and 81 mm and 3.1, 2.6, and 2.5 Mg ha–1, respectively. Runoff was reduced by 67% and 44% for the gypsum and PAM relative to control and soil loss by 16% and 19%, showing gypsum and PAM were more effective in reducing runoff than soil loss. Runoff was significantly reduced in the gypsum and PAM treatments in the five months following surface application. Results showed screen cover and tillage temporally reduced or delayed seal/crust formation, while the effects of gypsum and PAM were more persistent. Combined with earlier findings, it appears that a combination of physical and chemical treatments is the best practice for controlling surface sealing and reducing runoff and erosion on this soil.

COUPLING MIXING ZONE CONCEPT WITH CONVECTION-DIFFUSION EQUATION TO PREDICT CHEMICAL TRANSFER TO SURFACE RUNOFF

Modeling chemical transfer from soil solution to surface runoff is essential for developing a surface water quality model that can be used to assess pollution potentials of agricultural chemicals. Chemical transfer to runoff can be modeled as a two-rate process. A fast rate subprocess, which prevails at early stages of rainfall, causes an exponential depletion of chemicals from the mixing zone. A slow rate subprocess, which becomes significant under poor drainage conditions, transports chemicals into the mixing zone from the soil below. The two-rate process can be described by coupling the mixing zone concept with the convection-diffusion equation (CDE). We evaluated this coupling approach by comparing predicted results with measured bromide concentration data. A finite element scheme was developed to solve the CDE in conjunction with a near-surface boundary condition derived from a complete and uniform mixing theory. Overall results showed that without a calibration the coupling approach satisfactorily predicted bromide concentrations in both surface runoff and soil solution under the zero infiltration conditions. The proposed model adequately reproduced measured data for restricted infiltration conditions by introducing a mechanical dispersion coefficient (Dh). The fitted Dh is within the range reported in the literature for the repacked soil conditions. The coupling approach, while allowing for direct use of the mixing theory under free infiltration conditions, refines the theory for use under poorly drained conditions.

Effects of sediment size on transport capacity of overland flow on steep slopes

Abstract Sediment transport capacity is a key concept in determining rates of detachment and deposition in process-based erosion models, yet limited studies have been conducted on steep slopes. We investigated the effects of sediment size on transport capacity of overland flow in a flume. Unit flow discharge ranged from 0.66 to 5.26 × 10-3 m2 s-1, and slope gradient varied from 8.7 to 42.3%. Five sediment size classes (median diameter, d 50, of 0.10, 0.22, 0.41, 0.69 and 1.16 mm) were used. Sediment size was inversely related to transport capacity. The ratios of average transport capacity of the finest class to those of the 0.22, 0.41, 0.69 and 1.16 mm classes were 1.09, 1.30, 1.55 and 1.92, respectively. Sediment transport capacity increased as a power function of flow discharge and slope gradient (R2 = 0.98), shear stress (R2 = 0.95), stream power (R2 = 0.94), or unit stream power (R2 = 0.76). Transport capacity generally decreased as a power function of sediment size (exponent = −0.35). Shear stress and stream power predicted transport capacity better than unit stream power on steep slopes when transport capacity was <7 kg m-1 s-1. Sediment transport capacity increased linearly with mean flow velocity. Critical or threshold velocity increased as a power function of sediment size (R2 = 0.93). Further studies with fine soil particles are needed to quantify the effects of sediment size on transport capacity of overland flow on steep slopes. Citation Zhang, G.-H., Wang, L.-L., Tang, K.-M., Luo, R.-T. & Zhang, X.C. (2011) Effects of sediment size on transport capacity of overland flow on steep slopes. Hydrol. Sci. J. 56(7), 1289–1299.

ASSESSMENT AND IMPROVEMENT OF CLIGEN NON-PRECIPITATION PARAMETERS FOR THE LOESS PLATEAU OF CHINA

Stochastic weather generators are often used to generate daily weather input for hydrologic and crop models. The objective of this study was to evaluate and improve the ability of the Climate Generator (CLIGEN v5.22564) model to generate non-precipitation parameters, including dewpoint temperature(Tdp), daily maximum (Tmax) and minimum (Tmin) air temperatures, solar radiation (SR), and wind velocity (u) at 12 meteorological stations located in the Loess Plateau of China. We used daily weather data to evaluate the model and to improve SR and u simulation. The results showed that CLIGEN reproduced daily Tmax and, Tmin reasonably well. The t- and F-tests showed that neither means nor standard deviations of measured data were significantly different from those of the CLIGEN-generated data at P = 0.01 for all stations. Means and distributions of daily Tdp were reproduced very well; however, standard deviations were less well reproduced with significant differences at P = 0.01 for 4 out of 12 stations for the F-test. Mean and standard deviation for daily SR were much better reproduced by our modified CLIGEN at all stations, although distributions were slightly worsened. Daily u was reproduced well after fixing a unit conversion error with an absolute relative error (RE) of 0.17% for the means and 0.71% for the standard deviation. Mean of same-day temperature range (Tmax1-Tmin1) and one-day lag temperature ranges for both (Tmax1-Tmin2) and (Tmax2-Tmin1) of the CLIGEN-generated data were reproduced well with the absolute RE close to zero. However, compared with the measured data, standard deviations of Tmax1-Tmin1 were consistently underestimated with the RE of -38.7%, and those of Tmax1-Tmin2 and Tmax2-Tmin1 were consistently overestimated with the respective RE values being 55.8% and 19.6% for all stations. Seasonal serial correlations of SR and cross correlation between temperatures and SR were much better reproduced by the modified model. Specifically, compared to CLIGEN (v5.111), Tmin and Tdp were improved considerably in v5.22564, as well as means of Tmax1-Tmin2 and Tmax2-Tmin1, but standard deviations of Tmax1-Tmin1 were worsened. Standard deviations of Tmax1-Tmin2 and Tmax2-Tmin1 generated by v5.111 and v5.22564 were similar. Furthermore, generation of SR and u was similar in both versions, but was significantly improved in our modified v5.22564. Due to the improvement in SR generation, seasonal serial correlations of SR and cross correlation between temperatures and SR were also improved. Overall results showed that non-precipitation variables were much better generated by the modified version of 5.22564 than the previous versions in which Tmax, Tmin and Tdp were generated independently.

Applicability of WEPP Sediment Transport Equation to Steep Slopes

This study was carried out to evaluate the transport capacity equations of the Yalin equation and the WEPP model for steep slopes, and to recommend the best-fitting exponent value of shear stress in the WEPP transport capacity equation. The transport capacity was measured in a 5 m long, 0.4 m wide hydraulic flume, and the diameter of test sediment varied from 20 to 2000 µm with a median diameter of 280 µm. Flow discharge ranged from 0.625 × 10-3 to 5 × 10-3 m2 s-1, and slope gradient ranged from 8.8% to 46.6%. An averaged dimensionless critical shear stress of 0.052 was used for Tc calculation in the Yalin equation. The relationship between transport coefficient Kt and shear stress was graphically determined for the transport capacity equation of the WEPP model. The transport capacities predicted by the Yalin equation and the WEPP model were compared with the measured Tc to qualify their suitability on steep slopes. The results showed that the Yalin equation overestimated the measured Tc by 109%. The error increased as a power function of shear stress (r2 = 0.97). The transport capacity of the WEPP model underestimated the measured Tc by 65%. The absolute error increased as a linear function of shear stress (r2 = 0.91). Paired t-tests showed that the transport capacities calculated using the Yalin and WEPP equations were different significantly from measured Tc at the 0.05 level. Sediment transport coefficient Kt, calculated with the WEPP equation using the measured Tc of this study, varied with shear stress. However, Kt converged to a steady value of 0.053 when the best-fitting exponent of 2 for shear stress was used (r2 = 0.98). Thus, Ktt2 is considered when estimating Tc for steep slopes. More studies are needed to further evaluate the WEPP Tc equation as well as this new equation using various soils on steep slopes.

Evaluation of cesium-137 conversion models and parameter sensitivity for erosion estimation.

The Cs technique has been widely used to provide soil redistribution estimates since the 1970s. However, most Cs-conversion models remain theoretical and largely unvalidated. Our objectives were to validate the four widely used conversion models, examine model parameter sensitivity, and evaluate the potential of using kriging to improve soil redistribution estimation. Soil loss was measured from a 1.6-ha plot since 1978. Winter wheat ( L.) was grown primarily under conventional tillage. Soil samples in a 10-m grid were taken from the plot to estimate the Cs inventory. Soil redistribution rates were estimated using four models and were further interpolated using ordinary kriging. The parameter sensitivity analyses at the 95% confidence limits showed that reference inventory had the most impact on estimated water erosion, followed by particle size correction for erosion and tillage depth, with minimal impacts from mass depth, bulk density, and particle size correction for deposition. Compared with the measured water erosion, the relative errors of the mean net water erosion estimates across the entire plot without and with kriging were 28 and -17% for the proportional model (PM), 141 and 106% for the simplified mass balance model, 133 and 100% for the improved mass balance model (MBM2), and 109% for the extended MBM2 with tillage erosion (MBM3). Results indicated that the PM performed better than the mass balance models under the study conditions and that kriging improved mean soil redistribution estimates. However, the full potential of the MBM2 and MBM3 needs to be further evaluated under conditions where loss of newly deposited Cs exists.

ASSESSING THE IMPACT OF CLIMATE CHANGE ON SOIL WATER BALANCE IN THE LOESS PLATEAU OF CHINA

Soil water balance has response to climate change and evaluation of soil water change is one of the most important items of climate change impact assessment. GCM outputs under three scenarios were statistically downscaled during 2010~2039 to simulate the potential change of soil water balance in Wangdonggou watershed on the Loess Plateau with WEPP model. GCM predicted a 1.8 to 17.5% increase in annual precipitation, 0.5 to 0.9 °C rises in maximum temperature, 2.0 to 2.3 °C rise in minimum temperature for the region. Plant transpiration will mainly change from April to June and soil evaporation mainly changed during July to September. Percent increases under climate changes, as averaged for each emissions scenario and slope, ranged from -5 to19% for crop transpiration, -4 to 4% for soil moisture, -7 to 7% for soil evaporation, 6.5 to 44.1% for wheat grain yield, 26.3 to 41.7% for maize yield. Climate change will affect soil water balance significantly and some countermeasures are necessary.

Quantifying Sediment Provenance Using Multiple Composite Fingerprints in a Small Watershed in Oklahoma.

Quantitative information on sediment provenance is needed for improved calibration and validation of process-based soil erosion models. However, sediment source data are often limited due to difficulties in directly measuring source contributions at a watershed scale. Our objectives in this study were to estimate sediment source contributions in a 15-km watershed using analytical solutions to a three end-member mixing model using multiple composite fingerprints and to compare the results with those estimated with a single radionuclide, Cs. Surface soil samples were collected from 23 croplands, 19 rangelands, and 26 gully banks in the watershed, and 31 geochemical elements were analyzed for each sample. The elements served as tracers and were screened using statistical tests and range checks. The mean concentrations of all the nonconflict tracer pairs were used in the mixing model to calculate source contributions for the three sources. Results showed that although source contributions were strongly influenced by topography and land use, gully or subsoil erosion was found to be the main source of fine sediment in most subwatersheds. This study demonstrates that estimated source contributions may vary substantially among different composite fingerprints and that the use of multiple composite fingerprints greatly improves accuracy while reducing uncertainty. The source contributions estimated using multiple composite fingerprints agreed well with those estimated with Cs, with a correlation coefficient of 0.69 for gully contributions. This good agreement increases our confidence in using the multiple composite fingerprint method to identify sediment provenance in relatively small watersheds.

Temporal variability in rill erodibility for two types of grasslands

The temporal variability in rill erodibility (Kr) and its influencing factors are not fully quantified in grasslands. This study was conducted to detect temporal variation and quantify the potential factors causing changes in rill erodibility by using natural, undisturbed soil samples collected from two grasslands and one bare soil near Beijing, China. Sampling was at ~20-day intervals from April to October 2011. Soil detachment capacity by concentrated flow was measured in a hydraulic flume with the fixed bed under six different flow shear stresses to determine rill erodibility. Root mass density was measured to analyse potential effects on temporal variability in rill erodibility. Mean rill erodibility of bare soil was 13.2 and 19.6 times greater than under switchgrass (Panicum virgatum) and smooth bromegrass (Bromus inermis). The temporal variability in rill erodibility under grasslands differed significantly from that of bare soil. Distinctive temporal variation patterns were found throughout the growing season. Rill erodibility declined as root density increased, and the rill erodibility of grassland could be well estimated from the measured erodibility of bare soil and root density (R2 ≥ 0.92). The results of this study aid understanding of soil erosion mechanisms and development of process-based erosion models to simulate the seasonal variation in soil detachment by concentrated flow for grassland.