Fenli Zheng

Effect of Vegetation Changes on Soil Erosion on the Loess Plateau

Vegetation is one of the key factors affecting soil erosion on the Loess Plateau. The effects of vegetation destruction and vegetation restoration on soil erosion were quantified using data from long-term field runoff plots established on the eastern slope of the Ziwuling secondary forest region, China and a field survey. The results showed that before the secondary vegetation restoration period (before about 1866-1872), soil erosion in the Ziwuling region of the Loess Plateau was similar to the current erosion conditions in neighboring regions, where the soil erosion rate now is 8 000 to 10 000 t km(-2) year(-1). After the secondary vegetation restoration, soil erosion was very low; influences of rainfall and slope gradient on soil erosion were small; the vegetation effect on soil erosion was predominant; shallow gully and gully erosion ceased; and sediment deposition occurred in shallow gully and gully channels. In modern times when human activities destroyed secondary forests, soil erosion increased markedly, and erosion rates in the deforested lands reached 10 000 to 24 000 t km(-2) year(-1), which was 797 to 1682 times greater than those in the forested land prior to deforestation. Rainfall intensity and landform. greatly affected the soil erosion process after deforestation. These results showed that accelerated erosion caused by vegetation destruction played a key role in soil degradation and eco-environmental deterioration in deforested regions.

SWAT-based runoff and sediment simulation in a small watershed, the loessial hilly-gullied region of China: capabilities and challenges

Abstract Model calibration and validation are necessary before applying it for scenario assessment and watershed management. This study presented the methodology of evaluating Soil and Water Assessment Tool (SWAT) and tested the feasibility of SWAT on runoff and sediment load simulation in the Zhifanggou watershed located in hilly-gullied region of China. Daily runoff and sediment event data from 1998–2008 were used in this study; data from 1998–2003 were used for calibration and 2004–2008 for validation. The evaluation statistics for the daily runoff simulation showed that the model results were acceptable, but the model underestimated the runoff for high-flow events. For sediment load simulation, the SWAT performed well in capturing the trend of sediment load, while the model tended to underestimate sediment load during both the calibration and validation periods. The disparity between observed and simulated data most likely resulted from limitations of the existing SCS-CN and MUSLE methods in the model. This study indicated that the modification of SWAT components is needed to take rainfall intensity and its duration into account to enhance the model performance on peak flow and sediment load simulation during heavy rainfall season.

Aggregate Characteristics During Natural Revegetation on the Loess Plateau

Field investigations and laboratory analysis were conducted to study the characteristics of soil water-stable aggregates during vegetation rehabilitation in typical grassland soils of the hilly-gullied loess area. The relationship between water-stable aggregates and other soil properties was analyzed using canonical correlation analysis and principal component analysis. The results show that during the natural revegetation, the aggregates > 5 mm dominated and constituted between 50% and 80% of the total soil water-stable aggregates in most of the soil layers. The 2-5 mm aggregate class was the second main component. The mean value of water-stable aggregates > 5 mm within the 0-2 m soil profile under different plant communities decreased in the following order: Stipa grandis > Stipa bungeana Trin. > Artemisia sacrorum Ledeb. > Thymus mongolicus Ronn. > Hierochloe odorata (L.) Beauv. Clay, organic matter, and total N were the key factors that influenced the water stability of the aggregates. Total N and organic matter were the main factors that affected the water stability of the aggregates > 5 mm and 0.5-1 mm in size. The contents of Fe(2)O(3), Al(2)O(3), and physical clay (< 0.01 mm) were the main factors which affected the water stability of the 1-2 and 0.25-0.5 mm aggregates.

Comparison of soil erodibility factors in USLE, RUSLE2, EPIC and Dg models based on a Chinese soil erodibility database

Abstract Soil erodibility (K-value) is a key parameter in erosion prediction and is important for conservation planning in the face of a rising need for protecting the limited land resources. This study investigated the predictive capability of the K-value estimated by Universal Soil Loss Equation (USLE), Revised Universal Soil Loss Equation (RUSLE), Erosion Productivity Impact Calculator (EPIC) and Dg models for different soil regions using a Chinese soil erodibility database covering 51 natural runoff plots. Model performance was evaluated using R 2 (coefficient of determination), relative error (RE), Nash–Sutcliff efficiency (NSE) and P value (Mann–Whitney U test) statistics. The results showed that the existing four models overestimated almost all the K-values for the Chinese erodibility database, with most observed values concentrated in the range of 0.015–0.035. Without calibration, only the USLE and Dg models could be reliable and directly applied for the black soil region and the loess soil region, respectively. The Dg–OM model (R 2=0.67, n=32) was established by the non-linear best fitting techniques of multiple regression. In the Dg–OM model, K-values accounted for the vibration in a combination of the D g (geometric mean diameter) and OM (soil organic matter). NSE, R 2 and the average RE was 0.94, 0.67 and 9.5% for the Dg–OM models calibration based on the Chinese erodibility database; similar results were found for the validation process, with NSE of 0.93, R 2 of 0.66 and average RE of 6.5%. The model performances showed that the Dg–OM model reached ‘good’ satisfactory level. Compared with the four existing erodibility models, the Dg–OM model permitted the best parameterization and accuracy, and was proved to be suitable for estimating soil erodibility values in China.

The land use changes and its relationship with topographic factors in the Jing river catchment on the Loess Plateau of China

A series of soil conservation measures have been carried out to reduce soil loss on the Loess Plateau of China since 1950s, and the biologic measures were implemented according to topographic factors such as slope and elevation; therefore, the changes in topographic factors of land use can indicate the effects of the biologic measures. The objectives of this study were to (i) analyze the land use changes in the Jing River catchment during 1986-2000 and to (ii) examine the effects of biologic measures through relating land use changes with topographic factors. During 1986-2000, the dominant land use types were farmland and grassland (88% of the whole catchment). Compared with 1986, farmland and forest decreased while grassland and construction land increased with little changes in water and unused land. Three main conversion types occurred, i.e. the mutual conversion between forest and grassland, the mutual conversion between farmland and grassland, and farmland converted to other types. The elevation of farmland, forest, construction land and water increased, while that of grassland and unused land decreased. The mean slope gradient of each land use type changed little except for unused land. The above results suggested farmland has greatly decreased on tableland region due to the increase in construction land, forest has moved to gully region while grassland has increased despite elevation and slope. The land use in the Jing River catchment during 1986-2000 was changing to a more reasonable spatial pattern.

Effects of Rainstorm Patterns on Runoff and Sediment Yield Processes

Abstract Varying rainfall intensity during rainfall events is a common phenomenon, but little information is available concerning its effects on soil erosion processes. Five simulated rainstorm patterns (even, rising, falling, rising-falling, and falling-rising patterns), each with a different rainfall intensity pattern but with the same average rainfall intensity and total rainfall amount, were designed to quantify how rainstorm patterns affect runoff and sediment yield processes. The five rainstorm patterns were subjected to two slope gradients (5° and 10°) and two surface treatments that entailed the presence or absence of raindrop impact, in which the absence was simulated by placing nylon nets over soil pans. Results showed not only the total runoff but also each stage of runoff rate for a given rainfall intensity exhibited no significant differences among rainstorm patterns. However, soil loss from varying-intensity rainstorms was 1.13 to 5.17 times greater than that from even-intensity rainstorm, and the rising pattern was associated with the greatest soil loss. Moreover, each stage of soil loss by unit runoff for a given rainfall intensity was significantly different among rainstorm patterns. The variation in sediment regime and runoff hydraulic characteristic explained those. For sediment regime, in the presence of raindrop impact, for the rising pattern, soil detachment capacity was the dominant factor controlling soil loss, and for the other four patterns, runoff transport capacity was an important control on soil transfer, but it played different roles in soil loss under different patterns; in the absence of raindrop impact, the differences in sediment regime mainly reflected in transport capacity among storm patterns. With respect to flow hydraulic characteristics, the key hydraulic parameter (the parameter that had the greatest influence on soil loss) from varying-intensity rainstorms increased on average by 4.70% to 70.53% relative to those from even-intensity rainstorms.

Response of soil detachment rate to the hydraulic parameters of concentrated flow on steep loessial slopes on the Loess Plateau of China

Using hydraulic parameters is essential for describing soil detachment and developing physically based erosion prediction models. Many hydraulic parameters have been used, but the one that performs the best for describing soil detachment on steep slopes when the lateral expansion (widening) of rills is not limited has not been identified. An indoor concentrated flow scouring experiment was performed on steep loessial slopes to investigate soil detachment rates for different flow rates and slope gradients. The experiments were conducted on a slope-adjustable plot (5 m length, 1 m width, 0.5 m depth). Sixteen combinations of four flow rates (10, 15, 20 and 25 L min–1) and four slope gradients (17.6%, 26.8%, 36.4% and 46.6%) were investigated. The individual and combined effects of slope gradient and flow hydraulic parameters on soil detachment rate were analyzed. The results indicated that soil detachment rate increased with flow rate and slope gradient. Soil detachment rate varied linearly and exponentially with flow rate and slope gradient, respectively. Multivariate, non-linear regression analysis indicated that flow depth exerted the greatest influence on the soil detachment rate, followed by unit discharge per unit width, slope gradient, and flow rate in this study. Shear stress and stream power could efficiently describe the soil detachment rate using a power equation. However, the unit stream power and unit energy of the water-carrying section changed linearly with soil detachment rate. Stream power was an optimal hydraulic parameter for describing soil detachment. These findings improve our understanding of concentrated flow erosion on steep loessial slopes.

Impact of rainfall pattern on interrill erosion process

The impact of rainfall pattern on the interrill erosion process is not fully understood despite its importance. Systematic rainfall simulation experiments involving various rainfall intensities, stages, intensity sequences, and surface cover conditions were conducted in this study to investigate their effects on the interrill erosion process. Five rainfall patterns designed with the same total kinetic energy/precipitation (increasing, decreasing, rising–falling, falling–rising and constant patterns) were randomly delivered to a pre-wet clay loam soil surface at a 10° slope gradient. Significant differences in soil losses were observed among the different rainfall patterns and stages, but there was no obvious difference in runoff. Kinetic energy flux (KEr) was a governing factor for interrill erosion, and constant rainfall pattern (CST) produced nine times greater soil loss than runs with no KEr. Varied-intensity patterns had a profound effect on raindrop-induced sediment transport processes; path analysis results indicated that said effect was complex, interactive and intensity-dependent. Low hydraulic parameter thresholds further indicated that KEr was the dominant factor in detaching soil particles, while overland flow mainly contributed to transporting the pre-detached particles. This study not only sheds light on the mechanism of interrill sediment transport capacity and detachability, but also may provide a useful database for developing event-based interrill erosion prediction models. Copyright

Effects of rainfall regime and its character indices on soil loss at loessial hillslope with ephemeral gully

Understanding the relationship between hillslope soil loss with ephemeral gully and rainfall regime is important for soil loss prediction and erosion control. Based on 12-year field observation data, this paper quantified the rainfall regime impacts on soil loss at loessial hillslope with ephemeral gully. According to three rainfall parameters including precipitation (P), rainfall duration (t), and maximum 30-minute rainfall intensity (I30), 115 rainfall events were classified by using K-mean clustering method and Discriminant Analysis. The results showed that 115 rainfall events could be divided into three rainfall regimes. Rainfall Regime 1 (RR1) had large I30 values with low precipitation and short duration, while the three rainfall parameters of Rainfall Regime 3 (RR3) were inversely different compared with those of RR1; for Rainfall Regime 2 (RR2), the precipitation, duration and I30 values were all between those of RR1 and RR3. Compared with RR2 and RR3, RR1 was the dominant rainfall regime for causing soil loss at the loessial hillslope with ephemeral gully, especially for causing extreme soil loss events. PI30 (Product of P and I30) was selected as the key index of rainfall characteristics to fit soil loss equations. Two sets of linear regression equations between soil loss and PI30 with and without rainfall regime classification were fitted. Compared with the equation without rainfall regime classification, the cross validation results of the equations with rainfall regime classification was satisfactory. These results indicated that rainfall regime classification could not only depict rainfall characteristics precisely, but also improve soil loss equation prediction accuracy at loessial hillslope with ephemeral gully.

Effects of slaking and mechanical breakdown on disaggregation and splash erosion

&NA; The contributions of different mechanisms of aggregate breakdown to splash erosion are still obscure. This study was designed to investigate the effects of various mechanisms of soil disaggregation on splash erosion. Loamy clay, clay loam and sandy loam soil types were used in this research. Soil aggregate stability was determined by the Le Bissonnais method. Deionized water was used to simulate the combined effect of slaking and mechanical disaggregation, whereas alcohol was used to estimate the contribution of mechanical breakdown only. Simulated rain with an intensity of 60 mm hour−1 was applied at five heights (0.5, 1, 1.5, 2 and 2.5 m) to achieve different amounts of rainfall kinetic energy. The results indicated that the rate of splash erosion increased with the increase in rainfall kinetic energy in tests with both deionized water and alcohol. The rates of splash erosion for three types of soil followed the order of loamy clay soil < clay loam soil < sandy loam soil, but the mean weight diameter (MWD) of disintegrated aggregates followed the reverse order. The rates of splash erosion from the effects of slaking and mechanical breakdown increased with an increase in rainfall kinetic energy. The contributions of slaking and mechanical breakdown to splash erosion decreased for the former, whereas it increased for the latter as rainfall kinetic energy increased. The slaking effect contributed more than 50% of splash erosion. The rates of contribution of slaking and mechanical breakdown to splash erosion depended on rainfall kinetic energy and soil type. HighlightsContributions of different mechanisms of aggregate breakdown to splash erosion remain obscure.Alcohol was used to simulate the effect of mechanical breakdown only.Slaking contributed more than 50% of splash erosion.Contributions by mechanisms of aggregate breakdown depend on rainfall kinetic energy and soil type.