•We investigate impacts of climate variability and human activity on annual flow in the midstream of the Yellow River.•Annual streamflow shows significant decreases from 0.10mm/yr to 1.61mm/yr at ...most stations.•Decreasing precipitation and increasing temperature are detected for the whole study area.•Climate variability has a greater effect on the streamflow decrease in the Beiluo and Yan Rivers.•Human activities accounted for more of the streamflow reduction in the majority of tributaries.
The middle reaches of the Yellow River basin (MRYRB) contribute significantly to the total streamflow and sediment discharge of the Yellow River. Significant changes in streamflow have been detected; these changes result in part from large number of soil and water conservation measures implemented over the past six decades in this area. This study investigates streamflow variations and evaluates the impacts of climate variability and human activity on the mean annual flow in the MRYRB. The non-parametric Mann–Kendall test and Pettitt’s test are applied to characterize the trends and abrupt changes of hydro-climatic variables in the MRYRB. The analysis was performed on streamflow data taken over the period from the 1950s to 2010 at 18 hydrological stations and on precipitation, temperature and potential evapotranspiration (PET) data from 43 climate stations. We find that 16 of these stations recorded significant decreases in annual streamflow, with reduction rates ranging from 0.10mm/yr to 1.61mm/yr over the study period. Precipitation at all of the stations also had negative trends, with changes ranging from −4.7mm/yr to −0.19mm/yr. Temperature increased significantly at most stations, while PET showed a mixed of upward and downward trend. Abrupt changes in streamflow at mainstream stations occurred when large reservoirs were built, while breakpoints of streamflow at tributary stations were mainly driven by the implementation of soil and water conservation measures. We used both Budyko’s curve (a simple water balance model) and linear regression to evaluate the potential impacts of climate variability and human activities on mean annual streamflow. Climate variability has a greater effect on the streamflow reduction in the Beiluo River and Yan River, while human activities accounted for more of the streamflow changes in other tributaries, especially in the northern catchments. In general, human activities, including soil and water conservation projects, the operation of dams and reservoirs, and water consumption, are found to be the dominant factors responsible for the significant decline in the annual streamflow in the MRYRB over the last six decades.
The increasing seriousness of salinization aggravates the food, population and environmental issues. Ameliorating the salt-resistance of plants especially the crops is the most effective measure to ...solve the worldwide problem. The salinity can cause damage to plants mainly from two aspects: hyperosmotic and hyperionic stresses leading to the restrain of growth and photosynthesis. To the adverse effects, the plants derive corresponding strategies including: ion regulation and compartmentalization, biosynthesis of compatible solutes, induction of antioxidant enzymes and plant hormones. With the development of molecular biology, our understanding of the molecular and physiology knowledge is becoming clearness. The complex signal transduction underlying the salt resistance is being illuminated brighter and clearer. The SOS pathway is the central of the cell signaling in salt stress. The accumulation of the compatible solutes and the activation of the antioxidant system are the effective measures for plants to enhance the salt resistance. How to make full use of our understanding to improve the output of crops is a huge challenge for us, yet the application of the genetic engineering makes this possible. In this review, we will discuss the influence of the salt stress and the response of the plants in detail expecting to provide a particular account for the plant resistance in molecular, physiological and transgenic fields.
In recent decades, extreme climatic events have been a major issue worldwide. Regional assessments on various climates and geographic regions are needed for understanding uncertainties in extreme ...events' responses to global warming. The objective of this study was to assess the annual and decadal trends in 12 extreme temperature and 10 extreme precipitation indices in terms of intensity, frequency, and duration over the Loess Plateau during 1960–2013. The results indicated that the regionally averaged trends in temperature extremes were consistent with global warming. The occurrence of warm extremes, including summer days (SU), tropical nights (TR), warm days (TX90), and nights (TN90) and a warm spell duration indicator (WSDI), increased by 2.76 (P<0.01), 1.24 (P<0.01), 2.60 (P=0.0003), 3.41 (P<0.01), and 0.68 (P=0.0041) days/decade during the period of 1960–2013, particularly, sharp increases in these indices occurred in 1985–2000. Over the same period, the occurrence of cold extremes, including frost days (FD), ice days (ID), cold days (TX10) and nights (TN10), and a cold spell duration indicator (CSDI) exhibited decreases of −3.22 (P<0.01), −2.21 (P=0.0028), −2.71 (P=0.0028), −4.31 (P<0.01), and −0.69 (P=0.0951) days/decade, respectively. Moreover, extreme warm events in most regions tended to increase while cold indices tended to decrease in the Loess Plateau, but the trend magnitudes of cold extremes were greater than those of warm extremes. The growing season (GSL) in the Loess Plateau was lengthened at a rate of 3.16days/decade (P<0.01). Diurnal temperature range (DTR) declined at a rate of −0.06°C /decade (P=0.0931). Regarding the precipitation indices, the annual total precipitation (PRCPTOT) showed no obvious trends (P=0.7828). The regionally averaged daily rainfall intensity (SDII) exhibited significant decreases (−0.14mm/day/decade, P=0.0158), whereas consecutive dry days (CDD) significantly increased (1.96days/decade, P=0.0001) during 1960–2013. Most of stations with significant changes in SDII and CDD occurred in central and southeastern Loess Plateau. However, the changes in days of erosive rainfall, heavy rain, rainstorm, maximum 5-day precipitation, and very-wet-day and extremely wet-day precipitation were not significant. Large-scale atmospheric circulation indices, such as the Western Pacific Subtropical High Intensity Index (WPSHII) and Arctic Oscillation (AO), strongly influences warm/cold extremes and contributes significantly to climate changes in the Loess Plateau. The enhanced geopotential height over the Eurasian continent and increase in water vapor divergence in the rainy season have contributed to the changes of the rapid warming and consecutive drying in the Loess Plateau.
•Extreme temperature indices exhibited trends consistent with global warming.•Warm-event indices significantly increased, whereas cold-event indices decreased.•The growing season was lengthened in the Loess Plateau.•The daily rainfall intensity exhibited a significant decrease over time.
Aims
Since the 1970s, extensive croplands were converted to forest and pasture lands to control severe soil erosion on the Loess Plateau of China. We quantify the direct and indirect effects of ...vegetation restoration on runoff and sediment yield on hillslopes in the field to improve environmental governance.
Methods
An artificial rainfall experiment at a rainfall intensity of 120 mm h
−1
and a slope gradient of 22° were used to distinguish the effects of vegetation restoration on runoff and sediment yield.
Results
Compared to the farmland slopes, vegetation restoration directly prolonged the time-to-runoff by 140%, reduced the runoff rate by 20%, and increased the soil infiltration capacity by 15%. Vegetation restoration indirectly delayed the time-to-runoff by 120%, reduced the runoff rate and sediment yield rate by 50% and 94%, respectively, and increased the soil infiltration capacity by 58% on the hillslopes with vegetation restoration.
Conclusions
The direct effects of vegetation restoration on runoff and sediment yield were lower than its indirect impacts. Vegetation cover, decreases in soil bulk density, and increases in belowground root biomasses and > 0.25 mm aggregate stability were the primary causes of runoff and sediment yield reduction on the slopes with vegetation restoration.
•Soil hydrological properties are improved after 20 years of vegetation restoration.•Shrubland and forest have higher KS than grassland and cropland in the Loess Plateau.•Soil porosity and bulk ...density explain 93.8% of soil hydrological properties variation.•Locally-developed PTF of KS using bulk density, Clay and Silt shows good performance.
Soil hydrological properties play a key role in soil hydrological processes. However, the effect of long-term vegetation restoration on soil hydrological properties and the corresponding influencing mechanisms remains poorly understood. Here, three soil hydrological properties including saturated water-holding capacity (SWHC), field capacity (FC) and saturated hydraulic conductivity (Ks), as well as several basic soil properties in the Zhifanggou watershed of the Loess plateau were investigated. The variations in SWHC, FC and Ks under different vegetation restoration types and their dominant influencing factors were analyzed. Moreover, we collected available Ks data from peer-reviewed publications to determine the land use with the largest Ks across the entire Loess Plateau. The results showed that SWHC FC and Ks were increased after 20 years of vegetation restoration. The higher Ks was found in shrubland and forest in the whole Loess Plateau. Compared with cropland, Ks in shrubland was increased by 87.10% at 0–20 cm, 48.89% at 20–40 cm, and 18.37% at 40–100 cm, respectively, indicating that the impact of revegetation on Ks were most obvious in the upper soil layer. Bulk density (BD), total porosity (TP), capillary porosity (CP), noncapillary porosity (NCP) and soil organic matter (SOM) had a significant effect on SWHC, FC and Ks for different land-use types (P < 0.01). Soil porosity (i.e., TP, CP and NCP) and BD, soil chemical properties (i.e., SOM and pH), and soil particle composition explained 93.8%, 59.2%, and 13.4% of the total variance in soil hydrological properties (i.e., SWHC, FC and Ks), respectively. This indicates that soil porosity and BD are the dominant factors affecting soil hydrological properties. Moreover, soil particle composition played an important role in regulating Ks, with the contribution of 38.6%. The established pedotransfer function (PTF) of Ks using BD, clay and silt content had a better performance than two existing PTFs. This research provides a more systematic and comprehensive understanding of the soil hydrological effect of vegetation restoration in the Loess Plateau.
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•A connectivity index was coupled with a soil erosion model to estimate sediment yield.•Afforestation led to evident decreases in both the connectivity and the sediment delivery ...ratio.•The modeled sediment yield showed a 60.7% reduction due to changes in land use and check dams.
The hydrological connectivity on the Loess Plateau has been greatly altered by soil and water conservation measures in the past six decades, leading to significant reduction in sediment load. Nevertheless, how these measures affect sediment transport at the catchment scale remains unclear. This study coupled an index of connectivity (IC) with the Revised Universal Soil Loss Equation (RUSLE) model to estimate the sediment yield in the Yanhe catchment (7,725 km2). The model was used to assess the impacts of land use changes and check dam construction on sediment delivery. The land use maps showed increasing grassland (10.3%) and decreasing arable land (12.3%) from 1990 to 2010. The ranges of IC between the two land use scenarios showed significant changes with an average of 0.3 in 1990 and −0.5 in 2010. Land use changes resulted in average sediment delivery ratio decreasing from 0.39 in 1990 to 0.30 in 2010. The model was calibrated by annual sediment load at a gauging station, covering 5,852 km2 of the catchment and by sedimentation rates behind check dams in a representative sub-catchment. Measured and simulated sediment load showed consistent agreement, suggesting acceptable model performance. An approximately 60.7% reduction in sediment yield was attributed to land use changes and increasing check dams′ construction from 1990 to 2010. This study provided a good example of combining the IC with a soil erosion model to identify spatial patterns and temporal variations in sediment yield due to soil conservation measures.
Investigation of the variations in runoff, sediment load, and their dynamic relation is conducive to understanding hydrological regime changes and supporting channel regulation and fluvial ...management. This study is undertaken in the Xihanshui catchment, which is known for its high sediment‐laden in the Jialing River of the Yangtze River basin, southern China, to evaluate the change characteristics of runoff, sediment load, and their relationship at multi‐temporal scales from 1966 to 2016. The results showed that runoff changed significantly for more months, whereas the significant changes in monthly sediment load occurred from April to September. The contributions of runoff in summer and autumn and sediment load in summer to their annual value changes were greater. Annual runoff and sediment load in the Xihanshui catchment both exhibited significant decreasing trends (p < 0.05) with a significant mutation in 1993 (p < 0.05). The average annual runoff in the change period (1994–2016) decreased by 49.58% and annual sediment load displayed a substantial decline with a reduction of 77.77% in comparison with the reference period (1966–1993) due to climate change and intensive human activity. The power functions were satisfactory to describe annual and extreme monthly runoff–sediment relationships, whereas the monthly runoff–sediment relationship and extreme monthly sediment‐runoff relationship were changeable. Spatially, annual runoff–sediment relationship alteration could be partly attributed to sediment load changes in the upstream area and runoff variations in the downstream region. Three quantitative methods revealed that the main driver for significant reductions of annual runoff and sediment load is the human activity dominated by soil and water conservation measures, while climate change only contributed 22.73%–38.99% (mean 32.07%) to the total runoff reduction and 3.39%–35.56% (mean 17.32%) to the total decrease in sediment load.
The monthly runoff–sediment relationships were changeable and annual runoff–sediment relationship could be generally expressed as a power function for the study period, while extreme monthly runoff–sediment and sediment‐runoff relationships had different performances during P1 and P2. The main driver for the significant reductions of annual runoff and sediment load is the human activity dominated by soil and water conservation measures, as the contributions of climate change were only 22.73%–38.99% (mean 32.07%) and 3.39%–35.56% (mean 17.32%), respectively.
The Loess Plateau suffers from severe soil erosion that leads to a series of ecological and economic problems such as reduced land productivity, exacerbated rural poverty, decreased biodiversity and ...sedimentation of the riverbed in the lower reaches of the Yellow River. Soil erosion models are commonly used on the Loess Plateau to help target sustainable land management strategies to control soil erosion. In this study, we compared eleven soil erosion models that were previously used on the Loess Plateau. We studied their prediction accuracy, process representation, data and calibration requirements, and potential application in scenario studies. The selected models consisted of a broad range of model types, structures and scales. The comparison showed that process-based and empirical models did not necessarily yield more accurate results over one another for the Loess Plateau. Among the process-based models, Si’ model, WEPP and MMF had the highest prediction accuracy. However, some of the selected models were tested with total sediment load while others were tested with suspended sediment load (i.e. bedload is not included), which is subject to several drawbacks. Research questions that each of the models can address on the Loess Plateau were suggested. Further improvement of soil erosion models for the Loess Plateau should concentrate on enhancing the quality of data for model implementation and testing, incorporating key processes into process-based models according to their aims and scales, comparing models that address the same research questions, and implementing internal and spatial model testing.
Under the background of global climate change, variations in streamflow and sediment discharge in the Yellow River would continue with intensified human activities and changes in the ecological ...environment. To harness the Yellow River, analyzing the degrees of change and the reasons for the streamflow and sediment discharge, as well as predicting the future trends, are urgently needed.
In this paper, the non-parametric Mann–Kendall test, Sen's estimator of slope, Pettitt's test and wavelet transform were applied to detect the trends, the magnitude of the trends and the abrupt changes and periodic variation in streamflow and suspended sediment discharge at eight hydrological stations from 1950 to 2013 along the mainstream of the Yellow River.
Over the past 64years (1950–2013), the spatial distribution of the annual average streamflow was of the “M” type, and the annual average suspended sediment discharge had a parabolic curve shape along the mainstream of the Yellow River. The temporal variation showed a distinct decreasing trend in streamflow since the 1990s and in suspended sediment discharge since the 1980s. Many cycles of oscillations occurred in the streamflow and suspended sediment discharge variation, leading to an alternate change in the wet/dry periods and the high/low sediment discharge periods.
In various regions of the Yellow River Basin since the 1970s, climate changes contributed reductions of 17.0–45.0% and 12.2–50.3% for streamflow and suspended sediment discharge, respectively, whereas human activities contributed reductions of 55.0–83.0% and 49.7–87.8%. Therefore, human activities were the major causes of the significant decline in the streamflow and the suspended sediment discharge of the Yellow River over the past six decades. Reasonable and possible measures should be taken to achieve water resources optimization and configuration and to promote watershed management and sustainability in the regions of the Yellow River Basin.
•The streamflow was mainly from above LZ and sediment discharge was TDG–TG section.•The hydrological time series exhibited a significant decreasing trend.•The transition years of hydrological time series were detected in 1985 and 1979.•Many times cycle oscillations led to the alternate change in hydrological series.•Human activities reduction rates were more than 50% for hydrological series.
Variation of streamflow and sediment load in the Yellow River basin has received considerable attention due to its drastic reduction during the past several decades. This paper presents a detailed ...investigation on the changes of streamflow and sediment load from 1952 to 2011 using monthly observations at four gauging stations along the Yellow River. The results show significant decreasing trends for both streamflow and sediment load at all four gauging stations over the past 60 years. The wavelet transform demonstrated discontinuous periodicities from 1969 to 1973 and after 1986 due to the construction of large reservoirs and implementation of numerous soil and water conservations practices. The sediment rating curves with the power-law function was applied to investigate the relationship between discharge and sediment load. The results indicate distinct variations of the relationship between streamflow and sediment and implied significant hydro-morphological changes within different periods. The reducing sediment supply from the source region and the increased erosive power of the river are detected at Lanzhou station, while the decrease of the transport capacity at Toudaoguai is caused by severe siltation. Significant changes in the relationship between streamflow and sediment load are found at Huayuankou and Gaocun stations, which are largely induced by evident sediment income and trapping effects of large reservoirs. It is estimated that numerous reservoirs have strongly altered the regime and magnitude of streamflow and trapped large amount of sediment, leading to severe siltation and evident reduction of their total volumes. A decrease in precipitation, incoming water from the upper reaches, soil and water conservation measures as well as water consumption contribute most to the significant reduction of streamflow. The decrease of sediment load mainly resulted from various soil and water conservation measures and trapping in reservoirs from 1986 to 2011.
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