Influences of conservation measures on runoff and sediment yield in different intra-event-based flood regimes in the Chabagou watershed.

The Loess Plateau in China has suffered severe soil erosion. To control soil erosion, extensive conservation measures aimed at redistributing rainfall, hindering flow velocity and intercepting sediment were implemented on the Loess Plateau. To accurately evaluate the combined effect of conservation measures in the Chabagou watershed, this study classified intra-event-based floods into four regimes via cluster and discriminant analyses. Regime A was characterized by short flood duration and low erosive energy, regime B was characterized by short flood duration and high erosive energy, regime C was characterized by long flood duration and low erosive energy, and regime D was characterized by long flood duration and high erosive energy. The results indicated that peak discharge (qp), runoff depth (H), mean discharge (qm), and runoff erosion power (E) decreased by 75.2%, 56.0%, 68.0% and 89.2%, respectively, in response to conservation measures. Moreover, area-specific sediment yield (SSY), average suspended sediment concentration (SCE), and maximum suspended sediment concentration (MSCE) decreased by 69.2%, 33.3% and 11.9%, respectively, due to conservation measures. The nonlinear regression analysis revealed a power function relationship between SSY and E in both the baseline (1961-1969) and measurement period (1971-1990) in all regimes. Conservation measures reduced sediment yield by not only reducing the runoff amount and soil erosion energy but also transforming the flood regime, for example, transforming a high-sediment-yield regime into a low-sediment-yield regime. Moreover, conservation measures altered the SSY-E relationship in regime A, whereas no obvious difference in regime B or C/D was observed between the measurement period and the baseline period. This study provides a better understanding of the mechanism of runoff regulation and the sediment yield reduction under comprehensive conservation measures in a small watershed on the Chinese Loess Plateau.

Dynamic change of vegetation and its response to climate and topographic factors in the Xijiang River basin, China.

Vegetation plays an important role in the energy exchange, water cycle, carbon cycle, biogeochemical cycle, and maintenance of surface ecosystems. In recent years, regional vegetation cover has changed significantly. This study used statistical analyses, including the Mann-Kendall trend test, the Hurst exponent, and Pettitt test, to analyze the characteristics of temporal and spatial variation of vegetation coverage in the Xijiang River basin from 2000 to 2013. The results showed that vegetation coverage of 98.76% of the Xijiang River basin is weakly variable (Cv < 0.1). The area with significantly increased vegetation accounts for 43.45% of the total area (p < = 0.05). A total of 19.47% of vegetation coverage in the Xijiang River basin had significant change-points from 2004 to 2008 (p < = 0.05), and the area of concave change-points accounted for 25.99% of the total area of point increased the vegetation coverage. At an altitude of 500-2000 m, the altitude has an inhibitory effect on vegetation coverage. When the slope is less than 35 degrees, the slope has a promoting effect on vegetation coverage. Rich precipitation resources are the main source of soil water supply, and higher temperature provides better thermal energy resources, which may have a significant impact on vegetation growth in the future and cause time lag effects of climatic factors on vegetation coverage. The vegetation coverage and the area affected by the precipitation and temperature (time lag factors) accounted for 32.99% and 31.47% of the total watershed, respectively. The correlation between climatic factors, topographic factors, and vegetation coverage increased over time. The results from this study will help to further deepen the understanding of vegetation cover and its influencing factors, and provide a scientific basis for ecological restoration projects such as vegetation restoration in the Xijiang River basin of China.

[Characteristics of Hydrogen and Oxygen Isotopes in Different Water Bodies in Hilly and Gully Regions of the Loess Plateau].

Studying the isotope characteristics and water body transformation relationships among different water bodies in the hilly and gully region of the Loess Plateau can provide a theoretical foundation for evaluating regional climate, ecology, and water resources. In this study, daily and monthly averaged δD and δ18O in precipitation, river water, and shallow groundwater were measured in 2017 in the Jiuyuangou watershed, which has a good ecological condition. The compositional relationship between the stable hydrogen and oxygen isotopes in difference water bodies was explored, the influence factors and spatio-temporal variation of δ18O in precipitation and river water were analyzed, and the stable isotope conversion ratios between different water bodies in the study area were calculated using the two-terminal mixed model. The main conclusions of the study are as follows:the d-excess of river water showed an increasing trend with elevation during the observation period; the δ18O of river water was enriched with increasing distance from the river source and decreased with increasing altitude; temperature, wind speed, and relative humidity had significant effects on the hydrogen and oxygen isotopes of precipitation; precipitation and shallow groundwater replenish the river during the non-flood period, the proportions of which were 46% and 54%, respectively; and during the flood season, the shallow groundwater is replenished by river water and precipitation, the proportions were 60% and 40%, respectively. These results indicate that there is a good conversion relationship between "precipitation-river-shallow groundwater" in the study area. The implementation of soil and water conservation measures has had some influence on the conversion of different water bodies in small watersheds. The results provide a basis for the establishment of water cycle models for hilly and gully regions of the Loess Plateau.

[Comparison of soil organic matter, bulk density and clay content in small watersheds under different ecological managements of Loess Plateau, China]. / 黄土高原不同生态治理小流域土壤有机质、容重及黏粒含量的对比.

To explore the effects of small watersheds with different ecological managements on soil properties, the spatial differences of soil organic matter (SOM), bulk density (BD), and clay content (CC) in the four facets, including slope aspect, slope position, zone, and soil layer, were analyzed between Yangjiagou (YJG, artificial Robinia pseudoacacia forest watershed) and Dongzhuanggou (DZG, closed grassland watershed). The results showed that SOM, BD and CC were 12.78 g·kg-1, 1.24 g·cm-3, 19.2% for YJG and 11.13 g·kg-1, 1.21 g·cm-3, 18.2% for DZG, respectively. The values for YJG were slightly higher than those for DZG, but the difference was insignificant. All indices in the east slope were bigger than those in the west slope. Across different slope positions, the variation of BD was small, SOM and CC showed increasing trends from top to bottom. BD and CC declined downward the watershed, whereas SOM changed in an opposite trend. From the soil surface down to 60 cm soil depth, BD and CC increased and SOM decreased. The spatial sensitivity followed CC > SOM > BD, and the effects of the spatial factors can be ordered as soil layer > zone > slope aspect > slope position. There were significant differences in CC of the upper reaches, BD and CC of the middle reaches between the two basins. The sensitivity of each index to slope position, zone and soil layer in YJG was lower than that in DZG.

[Research progress on the effects of freeze-thaw on soil physical and chemical properties and wind and water erosion]. / å†»èžä½œç”¨å¯¹åœŸå£¤ç†åŒ–æ€§è´¨åŠé£Žæ°´èš€å½±å“ç ”ç©¶è¿›å±•.

Freeze-thaw erosion, one of the main types of soil erosion, is widely distributed in China. The distribution and harm of the combined erosion of freeze-thaw and other forces were greater than freeze-thaw erosion. We reviewed related research progress of the effects of freeze-thaw on soil phy-sical and chemical properties, wind erosion, and water erosion, based on literatures from China and abroad. Under the condition of freeze-thaw, soil water was transported and soil structure was dama-ged. Soil porosity, bulk density, shear strength, aggregate stability and organic matter were all changed. The change tendency and amplitude were related to soil texture and the degree of freeze-thaw. The occurrence and process of soil wind erosion and water erosion were influenced by the condition of freeze-thaw. Soil erodibility and erosion intensity increased as a result of the changes of soil physical and chemical properties. At present, the research on freeze-thaw mainly based on indoor simulation, which was quite different from the actual freeze-thaw process in the field. The conclusions obtained were not unified or even contrary due to different test conditions. Therefore, through combining indoor simulation and field survey, to strengthen the research of soil erosion mechanism of freeze-thaw conditions was the focus of the future research, which was of great significance for forecasting and preventing of soil erosion in the periods of thawing and the regions of seasonal freeze-thaw.

Effect of vegetation utilization on runoff and sediment production on grain-for-green slopes in the wind-water erosion crisscross region.

To effectively utilize the vegetation on grain-for-green slopes in the wind-water erosion crisscross region, it is necessary to determine the reasonable vegetation utilization intensity. We set up runoff plots on slopes which were not cultivated and were closed for many years in the Liudaogou catchment, a typical catchment in wind-water erosion crisscross region of the Loess Plateau. With artificial simulated rainfall experiments, the characteristics of runoff and sediment yield on slopes (10°, 20° and 30°) under different utilization intensity of vegetation were studied to select the reasonable utilization intensity. The results showed that the runoff rate in the process of rainfall simulation could be divided into two periods: rapidly increasing in the initial period and slow increasing or quasi-steady state in the middle and late periods. The variation of erosion rate during the rainfall was dependent on the slope. The utilization intensity had a significant effect on the runoff yield, which increased with the increasing utilization intensities. The slope gradient had a significant effect on the sediment yield, with the variation of sediment yield with slope gradient being: 20°>30°>10°. Compared with the unused (natural) plots, the relative runoff and sediment increased with increasing utilization intensities. Predicted based on the rainfall data, annual soil erosion amount on the slope would be basically lower than the tolerance level of soil loss when the vegetation cover on the slope surface reached 25% in 15 years after abandoning reclamation. More attention should be paid to the restoration and management of vegetation on the slope of 20° in this area.

Growth, Morphological, and Physiological Responses to Drought Stress in Bothriochloa ischaemum.

Water shortage in the arid-semiarid regions of China seriously hampers ecosystem construction. Therefore, elucidation of the mechanisms by which vegetation in that area responds to drought stress may enable us to improve utilization of limited water resources and thus contend with the problem of drought and water shortage. We studied Bothriochloa ischaemum, a native grass species, conducted potting control tests to compare several indicators of B. ischaemum grown under three different moisture conditions (80%, 60%, 40% Field capacity represent sufficient water supply, mild water stress, and serious water stress, respectively). Plant response parameters measured included biomass accumulation, root morphology, transient water use efficiency (WUE), stable carbon isotope ratio (δ13C), and stable carbon isotope discrimination (Δ13C) of various plant organs and their interrelationships. B. ischaemum had the greatest WUE under mild drought stress. However, serious drought stress resulted in considerable decline in overall biomass but substantial increase in root-to-shoot ratio and fine-root biomass. Coarse-root biomass dropped appreciably, indicating that serious drought stress leads to allocation non-uniformity of the carbon "sink." δ13C and Δ13C of stem correlated considerably with root morphology, suggesting the feasibility of characterizing WUE, biomass, and root morphology of B. ischaemum via the stable carbon isotope approach. Our evaluation of 21 drought resistance indicators of B. ischaemum showed that under a given moisture treatment gradient one can isolate an optimal indicator to express growth, morphology, and physiology, to improve the accuracy of depicting plant drought resistance and simplify the drought resistance indicator system. This study elucidates the response mechanism of B. ischaemum to drought stress and provides theoretical support to screening of drought-resistant plants across the arid-semiarid regions of China.

Spatial scale effect on sediment dynamics in basin-wide floods within a typical agro-watershed: A case study in the hilly loess region of the Chinese Loess Plateau.

Scale issues, which have been extensively studied in the domain of soil erosion, are considerably significant in geomorphologic processes and hydrologic modelling. However, relatively scarce efforts have been made to quantify the spatial scale effect on event-based sediment dynamics in basin-wide floods. To address this issue, sediment-runoff yield data of 44 basin-wide flood events were collected from gauging stations at the Chabagou river basin, a typical agro-basin (unmanaged) in the hilly loess region of the Chinese Loess Plateau. Thus, the spatial scale effect on event-based sediment dynamics was investigated in the basin system across three different spatial scales from sublateral to basin outlet. Results showed that the event-based suspended sediment concentration, as well as the intra- and inter-scale flow-sediment relationships remained spatially constant. Hence, almost all the sediment-laden flows can reach at the detachment-limited maximum concentration across scales, specifically for hyperconcentrated flows. Consequently, limited influence was exerted by upstream sediment-laden flow on downstream sediment output, particularly for major sediment-producing events. However, flood peak discharge instead of total flood runoff amount can better interpret the dynamics of sediment yield across scales. As a composite parameter, the proposed stream energy factor combines flood runoff depth and flood peak discharge, thereby showing more advantages to describe the event-based inter-scale flow-sediment relationship than other flow-related variables. Overall, this study demonstrates the process-specific characteristics of soil erosion by water flows in the basin system. Therefore, event-based sediment control should be oriented by the process to cut off the connectivity of hyperconcentrated flows and redistribute the erosive energy of flowing water in terms of temporality and spatiality. Furthermore, evaluation of soil conservation benefits should be based on the process of runoff regulation to comprehensively assess the efficiency of anti-erosion strategies in sediment control at the basin scale.

[Carbon sequestration in soil particle-sized fractions during reversion of desertification at Mu Us Sand land.] / æ¯›ä¹Œç´ æ²™åœ°æ²™æ¼ åŒ–é€†è½¬è¿‡ç¨‹åœŸå£¤é¢—ç²’å›ºç¢³æ•ˆåº”.

The aim of this study was to investigate the effects of carbon sequestration in soil particle-sized fractions during reversion of desertification at Mu Us Sand Land, soil samples were collected from quicksand land, semifixed sand and fixed sand lands that were established by the shrub for 20-55 year-old and the arbor for 20-50 year-old at sand control region of Yulin in Northern Shaanxi Province. The dynamics and sequestration rate of soil organic carbon (SOC) associated with sand, silt and clay were measured by physical fractionation method. The results indicated that, compared with quicksand area, the carbon content in total SOC and all soil particle-sized fractions at bothsand-fixing sand forest lands showed a significant increasing trend, and the maximum carbon content was observed in the top layer of soils. From quicksand to fixed sand land with 55-year-old shrub and 50-year-old arbor, the annual sequestration rate of carbon stock in 0-5 cm soil depth was same in silt by 0.05 Mg·hm-2·a-1. The increase rate of carbon sequestration in sand was 0.05 and 0.08 Mg·hm-2·a-1, and in clay was 0.02 and 0.03 Mg·hm-2·a-1 at shrubs and arbors land, respectively. The increase rate of carbon sequestration in 0-20 cm soil layer for all the soil particles was averagely 2.1 times as that of 0-5 cm. At the annual increase rate of carbon, the stock of carbon in sand, silt and clay at the two fixed sand lands were increased by 6.7, 18.1 and 4.4 times after 50-55 year-old reversion of quicksand land to fixed sand. In addition, the average percentages that contributed to accumulation of total SOC by different particles in 0-20 cm soil were in the order of silt carbon (39.7%)≈sand carbon (34.6%) > clay carbon (25.6%). Generally, the soil particle-sized fractions had great carbon sequestration potential during reversion of desertification in Mu Us Sand Land, and the slit and sand were the main fractions for carbon sequestration at both fixed sand lands.

[Response characteristics of soil water use patterns by different plants to precipitation in rocky mountainou areas.] / 土石山区不同植物土壤水分利用方式对降雨的响应特征.

Water-use characteristics of plants are important for vegetation restoration in shallow earth-rock mountain area. In this study, soil and plant samples of Platycladus orientalis and corn were collected after rainfall events in Yingwugou watershed of Dan River to analyze the signatures of oxygen isotopes and the response of water use patterns to precipitation using stable isotope technology. The results showed that there were different response characteristics of the soil water utilization to precipitation between P. orientalis and corn. The root of P. orientalis mainly used the soil moisture from 10-30 cm layer, while corn mainly used that in the depth of 0-20 cm. The water absorption depth (WAP) of P. orientalis root decreased from 20-30 cm to 10-20 cm, while that of corn altered from 10-20 cm to 0-20 cm, when precipitation decreased from 29 mm to 8 mm. The WAP of P. orientalis gradually changed from deep to shallow soil, while the main WAP of corn increased from 10-20 cm to 0-20 cm, whenprecipitation decreased. The response of P. orientalis and corn to precipitation was very obvious.

[Vegetation succession on retired croplands during their recovery processes in Dan-Han River Watershed of Shaanxi Province, Northwest China].

Taking naturally recovering plant communities on the croplands having been retired for different years and distributed on the southern and northern slopes in the Dan-Han River watershed of Shaanxi Province as test objects, an investigation was conducted on their community species composition, diversity characteristics, and community polar ordination. In the study area, the vegetation succession process on the retired croplands followed the stages of annual herb community-->perennial herb community-->shrub-herb community-->arbor-shrub-herb community. The formation period for perennial herb community, shrub-herb community, and arbor-shrub-herb community was about 2-5 a, 7-10 a, and more than 30 a respectively. On the southern and northern slopes, dif ferent types of retired croplands had the same vegetation succession stages, and had small discrepancies in species diversity indices. With increasing retired years, the species diversity indices on the southern slope increased after an initial decrease, while those on the northern slope had a fluctuated increase first, and slightly decreased by the end of the 30 a.

Using rare earth element tracers and neutron activation analysis to study rill erosion process.

Spatially averaged soil erosion data provide little information on the process of rill erosion. The dynamically varied data on the temporal and spatial distributions in the rill erosion process are needed to better understand the erosion process and reveal its innate characteristics. The objectives of this study were to examine the feasibility and effectiveness of rare earth element (REE) tracers and the neutron activation analysis (NAA) method on the study of the rill erosion process and to reveal quantitatively the relationships and characteristics of temporal and spatial distributions of sediment yield in rill erosion. Four REEs were used to study the changeable process of rill erosion at 4 slope positions. Four water inflow rates were applied to a 0.3 x 5 m soil bed at 3 slopes of 10.5%, 15.8% and 21.2% in scouring experiments. All of the runoff was collected in the experiment. Each sample was air-dried and well mixed. Then 20 g of each sample was sieved through 100-mesh and about a 50 mg sample was weighed for analysis of the four elemental compositions by NAA. Results indicate that the REE tracers and NAA method can be used to not only quantitatively determine soil erosion amounts on different slope segments, but also to reveal the changeable process of rill erosion amount. All of the relative errors of the experimental results were less than 25%, which is considered satisfactory on the study of rill erosion process.

Using Cesium-137 technique to study the characteristics of different aspect of soil erosion in the Wind-water Erosion Crisscross Region on Loess Plateau of China.

The most serious soil erosion on Loess Plateau exists in the Wind-water Erosion Crisscross Region. In the past 20 years, the types and intensity of soil erosion and its temporal and spatial distribution were studied, but studies on the difference of soil erosion between slope aspects and slope positions in this area have no report. However, it is very important to analyze and evaluate quantitatively the characteristics of different aspects and positions of soil loss for the prevention and treatment of soil erosion in this area. The spatial pattern of net soil loss on 4 downslope transects in four aspects (east, west, south and north) on a typical Mao (round loess mound) in Liudaogou catchment in Wind-water Erosion Crisscross Region was measured in 2000 using the resident cesium-137 deficit technique. The purposes of this investigation were undertaken to determine whether or not 137Cs measurement would give a useful indication of the extent of soil loss and their characteristics from cultivated hillsides in different slope aspect and slope position in the study area. The results showed that the difference of soil erosion in different aspect was significant and the erosion rate was in this order: north > east > south > west. Compared with other areas, the difference of erosion rate between north hillside and south hillside was on the contrary, and the possible explanations could be the effect of wind erosion. Also, the percentage of wind erosion was estimated to be at least larger than 18% of total soil loss by comparing the difference of erosion amount in south hillside and north hillside. The erosion rates on different slope positions in all aspects were also different, the highest net soil loss occurred in the lower slope position, and the upper and middle slope positions were slight. The general trend of net soil loss on sloping surface was to increase in fluctuation with increasing downslope distance.