Impacts of precipitation variation and soil and water conservation measures on runoff and sediment yield in the Loess Plateau Gully Region,China

The Loess Plateau of China has experienced a lengthy drought and severe soil erosion.Changes in precipitation and land use largely determine the dynamics of runoff and sediment yield in this region. Trend and mutation analyses were performed on hydrological data(1981–2012) from the Yanwachuan watershed in the Loess Plateau Gully Region to study the evolution characteristics of runoff and sediment yield. A time-series contrasting method also was used to evaluate the effects of precipitation and soil and water conservation(SWC) on runoff and sediment yield. Annual sediment yield declined markedly from 1981 to 2012 although there was no significant change in annual precipitation and annual runoff. Change points of annual runoff and annual sediment yield occurred in 1996 and 1997,respectively. Compared with that in the baseline period(1981–1996), annual runoff and annual sediment yield in the change period(1997–2012)decreased by 17.0% and 76.0%, respectively, but annual precipitation increased by 6.3%. Runoff decreased in the flood season and normal season, but increased in the dry season, while sediment yield significantly declined in the whole study period. The SWC measures contributed significantly to the reduction of annual runoff(137.9%) and annual sediment yield(135%) and were more important than precipitation. Biological measures(forestland and grassland) accounted for 61.04% of total runoff reduction, while engineering measures(terraces and dams) accounted for 102.84% of total sediment yield reduction. Furthermore, SWC measures had positive ecological effects. This study provides a scientific basis for soil erosion control on the Loess Plateau.     

川中紫色丘陵区径流泥沙SWAT模型的模拟应用分析

针对川中丘陵区紫色土坡耕地严重水土流失,选取典型代表李子溪流域为研究区,构建了其SWAT的模型数据库,包括地形、土壤、气象和土地利用数据库。并利用赵家祠水文站1970-1979年的实测径流和泥沙资料,对该流域的SWAT模型参数进行率定再采用1980-1986年的实测资料,对模型的适用性进行验证,同时用相对误差Re和Nash确定性系数Ens评价模拟效果。结果显示,径流和泥沙模拟相对误差均在±15%范围以内,Nash确定性系数均大于等于0.70,说明SWAT模型对李子溪流域年、月径流和年泥沙量的模拟精度较高。同时模拟值与实测值和降雨量的变化趋于一致。可见,用SWAT模型模拟和预测雨量较为丰沛、土壤侵蚀较严重的紫色丘陵地区的产流产沙是实用、可行的。     

RELATION BETWEEN PRECIPITATIONAND SEDIMENT TRANSPORT IN THE DASHA RIVER WATERSHED

The study on sediment production and its relationship with climatic and hydrological factors in watershed is a major environment issue of concern in the international community. Based on the observational records covering the period from 1954 to 1999, the characteristics of precipitation changing over the Dasha River Watershed in Anhui Province and its relation to sediment yield were studied using tendency analysis and correlation analysis.Results showed that the precipitation of the Dasha River Watershed has high variability. In those 46 years, 34% of spring rainfall, 58% of summer rainfall and 30% of annual rainfall will be considered anomaly. The gray correlation analysis shows that sediment discharge correlates most closely with the frequency of the rainstorm with a daily precipitation above 100mm, secondly with the frequency of the rainstorm with a daily precipitation of 50-100mm, and thirdly with the number of rainy days. Their correlation coefficients are 0.98,0.90 and 0.85 respectively. In addition,the paper suggests the major countermeasures and methods for controlling of soil and water losses in this area.     

The suitability of using leaf area index to quantify soil loss under vegetation cover

Soil erosion by water under forest cover is a serious problem in southern China. A comparative study was carried out on the use of leaf area index (LAI) and vegetation fractional coverage (VFC) in quantifying soil loss under vegetation cover. Five types of vegetation with varied LAI and VFC under field conditions were exposed to two rainfall rates (40 mm h⁻¹ and 54 mm h⁻¹) using a portable rainfall simulator. Runoff rate, sediment concentration and soil loss rate were measured at relatively runoff stable state. Significant negative exponential relationship (p < 0.05, R² = 0.83) and linear relationship (p < 0.05, R² = 0.84) were obtained between LAI and sediment concentration, while no significant relationship existed between VFC and sediment concentration. The mechanism by which vegetation canopy prevents soil loss was by reducing rainfall kinetic energy and sediment concentration. LAI could better quantify such a role than VFC. However, neither LAI nor VFC could explain runoff rate or soil loss rate. Caution must be taken when using LAI to quantify the role of certain vegetation in soil and water conservation.     

基于GIS和USLE的东圳库区土壤侵蚀量预测研究

运用GIS、遥感技术与USLE模型相结合预测东圳库区流域土壤侵蚀量,利用GIS平台建立东圳库区基础地理数据库,以10×10m为栅格单元对库区进行离散化,在GIS空间分析功能支持下,结合USLE土壤侵蚀预测模型,根据合适的USLE因子算法生成栅格图层,通过图层运算,预测了东圳库区土壤侵蚀量,进而标识库区土壤侵蚀严重区域,为库区非点源污染控制及水土流失治理提供定量依据,并通过建立实用的泥沙输移比模型结合水文监测数据,对预测结果进行验证。根据USLE方程,采用改进后的SDR方程,预测的渡里流域年输沙量与流域实际监测得到的年输沙量仅相差14.08%,说明本文采用USLE方程模拟流域土壤侵蚀结果可信,可定量评价、拟定水土保持措施方案。     

Soil detachment and transport under the combined action of rainfall and runoff energy on shallow overland flow

Rainfall and runoff energy results in soil erosion. This paper presents new the concepts of rainfall and runoff energy and analyzes the relationship of rainfall energy and runoff energy with sediment transport based on the conversion theory of kinetic and potential energy using artificial rainfall and mechanical calculation. The results show that the ratio of sediment detachment in sloping fallow overland flow increases with the slope gradient, rainfall energy and runoff energy, while the sediment detachment ratio under raindrop impact are significantly higher than those under no raindrop impact. The sediment concentration increases with the slope gradient and rainfall energy; when the slope gradient and rainfall energy are constant, the sediment concentration decreases as the runoff energy increases. Rainfall disturbance coefficients have a logarithmic correlation with the rate of rainfall energy and runoff energy. On the same slope gradient, when the rainfall energy is constant, the disturbance coefficient decreases as the runoff energy increases, while when the runoff energy is constant, the disturbance coefficient increases as the rainfall energy increases. Rainfall energy results in sediment detachment, and runoff energy is the transportation for erosion sediment. This showed that rainfall energy and runoff energy are important in the sediment detachment and transportation of shallow overland flow.     

Influence of vegetation cover and other sources of variability on sediment and runoff response in a burned forest in South Korea

Post-fire field measurements of sediment and run off yield were undertaken in natural rainfall event-basis during five rainy months in Korea on a total of 15 small plots: four replica burned unseeded plots, six replica burned seeded plots, and five replica unburned plots. The main aim was to evaluate the effects of vegetation recovery and spatial distribution patterns on sediment and runoff response between and within the treatment replica erosion plots. Six-years after the wildfire, total sediment and runoff yield in the burned unseeded plots with 20%-30% vegetation cover was still 120.8 and 20.6 times higher than in the unburned treatment plots with 100% ground cover, 8.3 and 6.7 times higher than in the burned seeded plots with 70%-80% vegetation cover, while only 1.6 and 2.0 times higher than in the burned seeded plots with 50%-60% vegetation cover, respectively. The differences in sediment and runoff yield between the treatment plots was proportional to total vegetation cover, distance of bare soil to vegetation cover, magnitude of rainfall characteristics and changes in soil properties, but not slope gradient. Three out of the six within-treatment pairs of two replica plots showed large differences in sediment and runoff yield of up to 6.0 and 4.2 times and mean CV of up to 99.1% and 62.2%, respectively. This was due to differences in the spatial distribution patterns of surface cover features, including aggregation of vegetation and litter covers, the distance of bare soil exposed to vegetation cover closer to the plot sediment collector and micro topographic mounds and sinks between pairs of replica plots. Small differences in sediment and runoff of only 0.9-1.4 folds and mean CV of 8.6%-25% were observed where the within-treatment pairs of replica plots had similar slope, total surface cover components and comparable spatial distribution pattern of vegetation and bare soil exposed surface covers. The results indicated that post-fire hillslopes undergoing effective vegetation recovery have the potential to reduce sediment and runoff production nearer to unburned levels within 6-years after burning while wildfire impacts could last more than 6-years on burned unseeded ridge slopes undergoing slow vegetation recovery.     

Effect of storm pattern on soil erosion in damaged rangeland; field rainfall simulation approach

Raindrop size,rainfall intensity and runoff discharge affect the detachment and transportation of soil particles.Among these three factors,the rainfall intensity seems to be more important because it can change other two factors.Storm patterns can be determined by changing the rainfall intensity during the storm.Therefore,the objective of this research is to test the influence of storm pattern on runoff,soil erosion and sediment concentration on a rangeland soil slope under field rainfall simulation.Four storm rainfall intensity patterns were selected for examining the effects of variations in storm event characteristics on soil erosion processes.The selected storm patterns were:I(45,55 and 70 mm h-1);II(45,70 and 55 mm h-1);III:(70,55 and 45 mm h-1);and IV(55,45 and 70 mm h1).The last pattern is a new one instead of the uniform pattern which has been sufficiently studied in previous researches.The experiments were conducted in field plots(in Kojour watershed,Mazandaran Province,Iran)with an area of one square meter and an constant slope gradient of 18%,surrounded by galvanised sheets.Following the nonuniform prioritization of the storm patterns for the studied variables,time to runoff(I>II>IV>III),runoff volume(III>IV>II>I),sediment concentration(IV>III>I>II)and soil erosion(III>IV>II>I)),it can be generally inferred that each pattern has specific effect on soil erosion processes during a storm.The results of the general linear model(GLM)test indicated that the effects of storm pattern on time to runoff,total runoff volume,runoff coefficient and soil erosion were significant at a level of 99%.The Duncan test showed that the storm patterns can be divided into three groups of III,IV;II;I(for time to runoff),I,II;IV,III(for runoff coefficient),and I;II;IV,III(for runoff volume and soil erosion).     

Mechanisms involved in triggering debris flows within a cohesive gravel soil mass on a slope: a case in SW China

The triggering mechanisms of debris flows were explored in the field using artificial rainfall experiments in two gullies, Dawazi Gully and Aizi Gully, in Yunnan and Sichuan Provinces, China, respectively. The soils at both sites are bare, loose and cohesive gravel-dominated. The results of a direct shear test, rheological test and back-analysis using soil mass stability calculations indicate that the mechanisms responsible for triggering debris flows involved the decreases in static and dynamic resistance of the soil. The triggering processes can be divided into 7 stages: rainfall infiltration, generation of excess runoff, high pore water pressure, surface erosion, soil creep, soil slipping, debris flow triggering and debris flow increment. In addition, two critical steps are evident: (i) During the process of the soil mass changing from a static to a mobile state, its cohesion decreased sharply (e.g., the cohesion of the soil mass in Dawazi Gully decreased from 0.520 to 0.090 kPa, a decrease of 83%). This would have reduced the soil strength and the kinetic energy during slipping, eventually triggered the debris flow. (ii) When the soil mass began to slip, the velocity and the volume increment of the debris flow fluctuated as a result of the interaction of soil resistance and the sliding force. The displaced soil mass from the source area of the slope resulted in the deposition of a volume of soil more than 7 - 8 times greater than that in the source area.     

Dynamic monitoring of soil erosion for upper stream of Miyun Reservoir in the last 30 years

The Revised Universal Soil Loss Equation(RUSLE) was applied to assess the spatial distribution and dynamic properties of soil loss with geographic information system(GIS) and remote sensing(RS) technologies.To improve the accuracy of soil-erosion estimates,a new C-factor estimation model was developed based on land cover and time series normalized difference vegetation index(NDVI) datasets.The new C-factor was then applied in the RUSLE to integrate rainfall,soil,vegetation,and topography data of different periods,and thus monitor the distribution of soil erosion patterns and their dynamics during a 30-year period of the upstream watershed of Miyun Reservoir(UWMR),China.The results showed that the new C-factor estimation method,which considers land cover status and dynamics,and explicitly incorporates within-land cover variability,was more rational,quantitative,and reliable.An average annual soil loss in UWMR of 25.68,21.04,and 16.80 t ha-1a-1was estimated for 1990,2000 and 2010,respectively,corroborated by comparing spatial and temporal variation in sediment yield.Between 2000 and 2010,a 1.38% average annual increase was observed in the area of lands that lost less than 5 t ha-1a-1,while during 1990-2000 such lands only increased on average by 0.46%.Areas that classified as severe,very severe and extremely severe accounted for 5.68% of the total UWMR in 2010,and primarily occurred in dry areas or grasslands of sloping fields.The reason for the change in rate of soil loss is explained by an increased appreciation of soil conservation by developers and planners.Moreover,we recommend that UWMR watershed adopt further conservation measures such as terraced plowing of dry land,afforestation,or grassland enclosures as part of a concerted effort to reduce on-going soil erosion.     

Impacts of land cover changes on runoff and sediment in the Cedar Creek Watershed,St. Joseph River,Indiana, United States

The relation between runoff and sediment and land cover is investigated in the Cedar Creek Watershed （CCW）, located in Northeastern Indiana, United States. The major land cover types in this watershed are cultivated land, woodland and pasture /Conservation Reserve Program （CRP）, which account for approximate 90 % of the total area in the region. Moreover, land use was changed tremendously from aooo to 9004, even without regarding the effect of the crop rotation system （corn ＆ soybean）. At least 49 % of land cover types were changed into other types in this period. The land cover types, ranking by changing area from high to low series, are rye, soybean, corn, woodland and pasture/CRP. The CCW is divided into 21 subwatersheds, and soil and water loss in each sub-watershed is computed by using Soil and Water Assessment Tool （SWAT）. The results indicate that the variations in runoff and sediment have positive relation to the area of crops （especially corn and soybean）; sediment is more sensitive to land cover changes than runoff; more heavy rainfall does not always mean more runoff because the combination of different land cover types always modify runoff coefficient; and rye, soybean and corn are the key land cover types, which affected the variation in runoff and sediment in the CCW.     

六盘水市土壤侵蚀时空特征及影响因素分析

六盘水市是我国生态地位极其重要,水土流失又较为严重的城市。近些年,六盘水市实施了一系列生态工程,为了定量分析六盘水市土壤侵蚀状况及其影响因素,本文基于RUSLE模型,利用降雨数据、遥感影像数据、土地利用数据等,对贵州省六盘水市1990-2015年土壤侵蚀模数和土壤侵蚀量进行定量模拟,分析其时空动态变化,利用地理探测器定量分析影响因素,并进行空间相关性分析,结果表明： ① 六盘水市土壤侵蚀以微度和中度侵蚀为主。土壤侵蚀严重地区主要分布在北盘江流域与南盘江流域交界处以及喀斯特山地地区,煤矿开采使植被覆盖等抑制土壤侵蚀因子减少作用,使局部地区土壤侵蚀程度加剧。② 1990-2010年平均土壤侵蚀模数整体为下降趋势,2010-2015年为上升趋势。其中2000年平均土壤侵蚀模数最大,2010年平均土壤侵蚀模数最小。该变化由降雨可蚀性因子和植被覆盖度因子综合影响所致。③ 植被覆盖度因子和多年平均降雨量因子是影响区域土壤侵蚀空间分布的重要因素。未利用土地、植被覆盖度小于0.3、坡度在25°以上和降雨量在1543~1593 mm之间的地区为高风险侵蚀区域。④ 植被覆盖度与土壤侵蚀在空间上全部呈负相关性,降雨因子与土壤侵蚀在空间上存在负相关性和正相关性。⑤ 土壤侵蚀改善区域大多分布在生态工程区域内,生态工程建设能够改善土壤侵蚀情况,不同生态工程保护侧重点不同导致土壤侵蚀改善程度不同。退耕还林还草工程对六盘水市土壤侵蚀的改善具有重要意义,六盘水市更宜退耕还林。     

Effects of soil conservation practices on soil erosion and the size selectivity of eroded sediment on cultivated slopes

Soil conservation practices can greatly affect the soil erosion process, but limited information is available about its influence on the particle size distribution(PSD) of eroded sediment, especially under natural rainfall. In this study, the runoff, sediment yields, and effective/ultimate PSD were measured under two conventional tillage practices, downhill ridge tillage(DT) and plat tillage(PT) and three soil conservation practices, contour ridge tillage(CT), mulching with downhill ridge tillage(MDT), and mulching with contour ridge tillage(MCT) during 21 natural rainfall events in the lower Jinsha River. The results showed that(1) soil conservation practices had a significant effect on soil erosion. The conventional tillage of DT caused highest runoff depth(0.58 to 29.13 mm) and sediment yield(0.01 to 3.19 t hm-2). Compared with DT, the annual runoff depths and sediment yields of CT, MDT and MCT decreased by 12.24%-49.75% and 40.79%-88.30%, respectively.(2) Soil conservation practices can reduce the decomposition of aggregates in sediments. The ratios of effective and ultimate particle size(E/U) of siltand sand-sized particles of DT and PT plots were close to 1, indicating that they were transported as primary particles, however, values lower/greater than 1 subject to CT, MDT and MCT plots indicated they were transported as aggregates. The ratios of E/U of claysized particles were all less than 1 independently of tillage practices.(3) The sediments of soil conservation practices were more selective than those of conventional tillage practices. For CT, MDT and MCT plots, the average enrichment ratios(ERs) of clay, silt and sand were 1.99, 1.93 and 0.42, respectively, with enrichment of clay and silt and depletion of sand in sediments. However, the compositions of the eroded sediments of DT and PT plots were similar to that of the original soil. These findings support the use of both effective and ultimate particle size distributions for studying the size selectivity of eroded sediment, and provide a scientific basis for revealing the erosion mechanism in the purple soil area of China.     

Identifying the driving factors of sediment delivery ratio on individual flood events in a long-term monitoring headwater basin

The Sediment Delivery Ratio (SDR) has multi-fold environmental implications both in evaluating the soil and water losses and the effectiveness of conservation measures in watersheds. Various factors, including hydrological regime and watershed properties, may influence the SDR at interannual timescales. However, the effect of certain important dynamic factors, such as rainfall peak distribution, runoff erosion power and sediment bulk density, on the sediment delivery ratio of single flood events (SDRe) has received little attention. The Qiaogou headwater basin is in the hilly-gully region of the Chinese Loess Plateau, and it encompasses a 0.45 km2 catchment. Three large-scale field runoff plots at different geomorphological positions were chosen to obtain the observation data, and the 20-year period between 1986 and 2005 is presented. The results showed that the SDRe of the Qiaogou headwaters varied from 0.49 to 2.77. Among the numerous influential factors, rainfall and runoff were the driving factors causing slope erosion and sediment transport. The rainfall erosivity had a significant positive relationship with the sediment transport modulus (R²=0.85, P<0.01) but had no significant relationship with SDRe. The rainfall peak coefficient was significantly positively correlated with the SDRe (R²=0.64, P<0.05), indicating the influence of rainfall energy distribution on the SDRe. The runoff erosion power index was not only significantly related to the sediment transport modulus (R²=0.84, P<0.01) but also significantly related to the SDRe (R²=0.57, P<0.01). In addition, the relative bulk density was significantly related to the SDRe, indicating that hyper-concentrated flow characteristics contributed to more transported sediment in the catchment. Thus, the rainfall peak coefficient, runoff erosion power and sediment relative bulk density could be used as dynamic indexes to predict the SDRe in the hilly areas of the Chinese Loess Plateau.     

Synergistic effects of vegetation layers of maize and potato intercropping on soil erosion on sloping land in Yunnan Province,China

Intercropping, as an overyielding system, can decrease soil erosion on sloping land through the presence of dense canopy covers. However, the structure mechanism in canopy is still unclear. We conducted a two-year field experiment on runoff plots, exploring whether the interaction between vegetation layers reduce soil erosion in maize and potato intercropping systems. The maize, potato, and weed layers in the intercropping system were removed by a single layer, two layers and three layers, respectively(total of 8 treatments including all layers removed as the control). Then, throughfall, runoff and sediment were measured at the plot and row scale on a weekly basis. Based on the difference between each treatment and the control, we calculated and found a relative reduction of runoff and sediment by any combination of the two vegetation layers greater than the sum of each single layer. In 2016 and 2017, the highest relative reduction of runoff reached 15.65% and 46.73%, respectively. Sediment loss decreased by 33.96% and 42.77%, respectively. Moreover, runoffand sediment reduced by the combination of all vegetation layers(no layers removed) was also larger than the sum of that by each single layer. In 2016 and 2017, the highest relative reduction of runoff reached 7.32% and 3.48%, respectively. So, there were synergistic effects among multi-level(two or three layers) vegetation layers in terms of decreasing soil erosion on sloping land. Maize redistributes more throughfall at the maize intra-specific row and the maize and potato inter-specific, which is favorable for the synergistic effect of reducing soil erosion. This finding shows an important mechanism of maize and potato intercropping for soil and water conservation, and may promote the application of diverse cropping systems for sustainable agriculture in mountainous areas.     

VARIATION IN FLOW AND SEDIMENT OF DASHA RIVER AND INFLUENCE OF HUMAN ACTIVITIES ON IT IN SOUTHWEST REGION OF ANHUI PROVINCE

1INTRODUCTION Soil and water loss in the Dabie mountainous area is one of the most severe environmental problems in Anhui Province and the Changjiang (Yangtze) River basin as well.Itnotonlyresultsinenvironmentaldegradation,but alsorestrictstheproductionoflocalindustryand agricul- ture and hinders the sustainable economic and social de- velopment of the area, which has done great harm to the people's life and production there (XU, 2000; ZHANG and SHI, 2001). Since the 1980s, especial…     

Debris flows introduced in landslide deposits under rainfall conditions: The case of Wenjiagou gully

Debris flows often occur in landslide deposits during heavy rainstorms. Debris flows are initiated by surface water runoff and unsaturated seepage under rainfall conditions. A physical model based on an infinitely long, uniform and void-rich sediment layer was applied to analyze the triggering of debris-flow introduced in landslide deposits. To determine the initiation condition for rainfall-induced debris flows, we conducted a surface water runoff and saturated-unsaturated seepage numerical program to model rainfall infiltration and runoff on a slope. This program was combined with physical modeling and stability analysis to make certain the initiation condition for rainfall-introduced debris flows. Taking the landslide deposits at Wenjiagou gully as an example, the initiation conditions for debris flow were computed. The results show that increase height of surface-water runoff and the decrease of saturated sediment shear strength of are the main reasons for triggering debris-flows under heavy rainfall conditions. The debris-flow triggering is affected by the depth of surface-water runoff, the slope saturation and shear strength of the sediment.     

草地覆盖度变化对生态系统防风固沙服务的影响分析——以内蒙古典型草原区为例

锡林郭勒盟为我国北方典型草原区,草地的退化与恢复影响生态系统的防风固沙服务功能。为了便于制定区域生态恢复的防治措施,实现草地生态系统的可持续发展与防治土壤风蚀危害,本文基于气象、遥感数据,运用RWEQ(Revised Wind Erosion Equation)模型,结合锡林郭勒盟的草地覆盖度变化对20世纪90年代以来的防风固沙服务功能的时空变化趋势进行了定量评估,并分析了草地覆盖度变化对防风固沙服务功能的影响。研究表明:锡林郭勒盟土壤风蚀以微度和轻度侵蚀为主,主要集中在植被盖度较高、降水相对较多,风场强度相对较低的东部、中部和南部地区;中度以上区域主要集中在西部的荒漠草原区与浑善达克沙地区,且侵蚀面积随侵蚀强度的增加而递减;防风固沙服务功能量的分布趋势与土壤风蚀模数的分布趋势基本一致;防风固沙服务功能保有率的分布特征与植被盖度的分布特征基本一致,表现为由西北到东南逐渐增加的趋势;在气候暖干化背景下,受京津风沙源治理工程实施的影响,以微度和轻度侵蚀为主的草地覆盖度减小区转为以微度和轻度为主的覆盖度增加区,轻度和中度以上侵蚀为主的草地覆盖度减小区转为基本持衡区;草地覆盖度的降低与增加对土壤风蚀的加剧和抑制作用明显,大部分地区的防风固沙服务功能保有率的下降(提升)与风蚀季节草地覆盖度的减小与提升呈显著正相关(r0.6,p0.05)。     

Modelling the Effects of Land-use Change on Runoff and Sediment Yield in the Weicheng River Watershed,Southwest China

As a major sediment area in the upper Yangtze River, Jialing River basin experienced substantial land-use changes, many water conservancy projects were constructed from the 1980 s onward to promote water and soil conservation. The water and sediment yield at the watershed outlet was strongly affected by these water conservation works, including ponds and reservoirs, which should be considered in the modelling. In this study, based on the observed data of the Weicheng River catchment, the relationships between precipitation, runoff, vegetation, topography and sediment yield were analyzed, a distributed runoff and sediment yield model(WSTD-SED) was developed, and the hydrological processes of different land-use scenarios were simulated by using the model. The main results are summarized as follows: 1) there is an alternating characteristic in river channels and reservoirs in the Jialing River hilly area, with scour occurring in wet years and deposit occurring in dry years. 2) Most of the sediment deposited in river channels and reservoirs is carried off by the largest flood in the year. 3) The model yielded plausible results for runoff and sediment yield dynamics without the need of calibration, and the WSTD-SED model could be usedto obtain qualitative estimates on the effects of land use change scenarios. 4) The modelling results suggest that a 10% increase in cropland(dry land) reforestation results in a 0.7% decrease in runoff and 1.5% decrease in sediment yield.     

A review on rill erosion process and its influencing factors

Rills are frequently observed on slope farmlands and rill erosion significantly contributes to sediment yields. This paper focuses on reviewing the various factors affecting rill erosion processes and the threshold conditions of rill initiation. Six factors, including rainfall, runoff, soil, topography, vegetation and tillage system, are discussed. Rill initiation and network are explored. Runoff erosivity and soil erodibility are recognized as two direct factors affecting rill erosion and other types of factors may have indirect influences on rill erosion through increasing or decreasing the effects of the direct factors. Certain conditions are necessary for rill initiation and the critical conditions are different with different factors. Future studies should be focused on 1) the dynamic changes of rill networks; 2) the combined effect of multiple factors; and 3) the relationships of threshold values with other related factors.