Hanjiang River Valley Research Articles

Spatiotemporal variation and driving forces of NDVI from 1982 to 2015 in the Qinba Mountains, China.

The spatiotemporal variation and driving force of the Normalized Difference Vegetation Index (NDVI) are helpful to ecological environment protection and natural resource management. Using the Sen and Mann-Kendall methods, Hurt index, and the Geodetector, this study investigated the temporal and spatial changes and driving forces of NDVI during 1982-2015. The results showed that (1) From 1982 to 2015, the high vegetation coverage was mainly distributed in the Qinling Mountains and the Daba Mountains, while the low vegetation coverage was in high altitude areas in the west, low altitude in the east, and the Hanjiang River valley. (2) NDVI in the Qinba Mountains increased continuously accounting for 81.1%, with 68% showing slow growth. In the future, only 37.8% of the vegetation will have significant change. The area of vegetation increase will be greater than the area of decrease. (3) NDVI increased firstly and then decreased with the increase of altitude, reaching the maximum value at 1100m. NDVI showed a trend of fluctuating growth. It reached the maximum value of 0.86 in 2015. (4) Through the Geodetector, the main factors affecting NDVI were natural factors mainly including rainfall, soil type, and digital elevation model (DEM), while human activities, including population density, had little influence on NDVI. Natural environment factors and human activities together had a greater impact on the spatial distribution of NDVI. This study could provide help for the sustainable development of the natural environment in the Qinba Mountains.

Sedimentary record and luminescence chronology of palaeoflood events along the Gold Gorge of the upper Hanjiang River, middle Yangtze River basin, China

Palaeoflood slackwater deposits (SWDs) along the river banks have important implications for the reconstruction of the past hydro-climatic events. Two palaeoflood SWD beds were identified in the Holocene loess-soil sequences on the cliff river banks along the Gold Gorge of the upper Hanjiang River by field investigation and laboratory analysis. They have recorded two palaeoflood events which were dated by optically stimulated luminescence to 3.2–2.8 ka and 2.1–1.8 ka, respectively. The reliability of the ages obtained for the two events are further confirmed by the presence of archaeological remains and good regional pedostratigraphic correlation. The peak discharges of two palaeoflood events at the studied sites were estimated to be 16,560–17,930 m3/s. A correlation with the palaeoflood events identified in the other reaches shows that great floods occurred frequently during the episodes of 3200–2800 and 2000–1700 a BP along the upper Hanjiang River valley during the last 4000 years. These phases of palaeoflood events in central China are well correlated with the climatic variability identified by δ18O record in the stalagmites from the middle Yangtze River Basin and show apparent global linkages. Palaeoflood studies in a watershed scale also imply that strengthened human activities during the Shang dynasty (BCE 1600–1100) and Han dynasty (BCE206–CE265) may have caused accelerated soil erosion along the upper Hanjiang River valley.

Loess deposits of the upper Hanjiang River valley, south of Qinling Mountains, China: Implication for the pedogenic dynamics controlled by paleomonsoon climate evolution

Aeolian deposits in the Chinese Loess Plateau (CLP) provide a detailed archive for reconstructing the pedogenic intensity as well as the East Asian monsoon climate change. However, study on the loess in the upper Hanjiang River valley, south of Qinling Mountains has seldom been comprehensively reported. Located at the transition zone between temperate and subtropical monsoon climate, the study area is more sensitive to the climate change. In this paper, three loess-paleosol profiles at the first terrace of the upper Hanjiang River were studied in detail. High-resolution investigations, including field observations, optically stimulated luminescence (OSL) dating, and measurements of magnetic susceptibility (MS), grain-size (GS), color variation, loss-on-ignition (LOI) and chemical elements were carried out. The results show that the stratigraphic sequences, in order from the top to the bottom, are topsoil (TS), recent loess (L0), paleosol (S0), transitional loess (Lt), Malan loess (L1) and fluvial deposits (T1-al). The pedogenic intensity varies significantly in different layers and presents such a tendency of S0 > L0 > Lt > L1. This indicates four distinct stages in the paleoclimate evolution: a cold-dry period (55.0–11.5 ka B.P.); a phase of gradual transition to warm-wet (11.5–8.5 ka B.P.); the maximum warm-wet period (8.5–3.0 ka B.P.); and a phase of gradually shifting to cool-dry (3.0–0.0 ka B.P.). The climate change trends are similar with the loess records from the CLP and the stalagmite and peat records in southern China. But the paleosol development in the study is probably a better indicator of the strength of summer monsoon climate change during the mid-Holocene Climatic Optimum. This study also provides basic data for exploring the pedogenesis and climate differences in the East Asian monsoon climate zones.

Geochemical characterization of loess-paleosol sequences: Comparison between the upper reaches of the Hanjiang and Weihe river valleys, China

This paper aims to compare the geochemical characteristics of loess-paleosol sequences in the upper reaches of the Hanjiang and Weihe river valleys, which are located in the semi-humid temperate zone and humid subtropical zone, respectively. The Mituosi (MTS) profile in the upper reaches of the Hanjiang River valley and the Yaohecun (YHC) profile in the Weihe River valley were selected for this comparative research. The stratigraphic characteristics, composition, chemical weathering intensity, leaching rates of Ca and Na, mobility of major elements, and transport features of Na and Fe were analyzed with respect to depth and compared between the two profiles. This study reached the following conclusions. (1) The composition of the loess-paleosol sequences in two regions are quite similar to the average composition of the upper continental crust (UCC), indicating that the loess in the two regions came from multiple sources and was mixed well. Therefore, the loess in the two regions is considered aeolian loess. (2) Compared with the loess-paleosol sequence in the Weihe River valley, the loess-paleosol sequence in the upper reaches of the Hanjiang River valley features a darker color; a higher chemical index of alteration (CIA) value; higher leaching rates of Na and Ca; higher migration ratio (relative to K) of Al, Si, Mg, and Na; and lower migration ratio of Fe and Ca. This evidence indicates that the loess-paleosol sequence in the humid subtropical environment experienced stronger chemical weathering intensity than the loess-paleosol sequence in the semi-humid temperate zone. (3) Both the YHC profile and MTS profile record a period of climate deterioration at 6000–5000 a BP. The period punctuated the mid-Holocene Climatic Optimum (8500–3100 a BP) in the study area.

Late Pleistocene and Holocene extreme hydrological event records from slackwater flood deposits of theAnkang east reach in the upperHanjiangRiver valley,China

This paper reports an investigation into palaeoflooding along the upper reaches of the Hanjiang River valley, China. Based on the sedimentary evidence of the palaeohydrological regime, two bedsets of palaeoflood slackwater deposits (SWDs) were identified interbedded within the Holocene loess–soil sequence along the riverbanks of the Ankang east reach. Optical Stimulated Luminescence (OSL) dating and stratigraphical correlation with previously dated sites were used to reconstruct the chronology of the palaeoflood events. The results show that the palaeoflood events represented bySWD1 occurred between 13 000 and 12 400 a, coincident with the climatic transition from the Bølling–Allerød (BL+AL) stage to the Younger Dryas (YD) event. The palaeoflood events recorded bySWD2 were dated to 1000–800 a, corresponding to the later stages of the North Song Dynasty (AD960–1127) and the subsequent South Song Dynasty (AD1127–1279), which was a time of climatic decline according to historical documents. Palaeoflood discharges were estimated using the step‐backwater method, and the peak discharges were estimated to be in the range 35 200–47 400 m3 s−1. These results are of significance to our understanding of the regional hydrological response to global climatic change, the utilization of water resources, hydraulic engineering, flood control and disaster reduction.

Investigating extreme flood response to Holocene palaeoclimate in the Chinese monsoonal zone: A palaeoflood case study from the Hanjiang River

Palaeoflood events recorded by slackwater deposits (SWDs) were investigated extensively by sedimentological criteria of palaeohydrology along the upper Hanjiang River valley. Modern flood SWDs were collected for comparison with palaeoflood SWD in the same reaches. Three typical palaeoflood SWDs were observed within Holocene loess-soil blanket on the first river terrace land. The grain size distributions of palaeoflood SWDs are similar to modern flood SWDs, whereas they are different from eolian loess and soil. Palaeoflood SWD lies in three major pedo-stratigraphic boundaries (TS/L0, L0/S0, and S0/Lt) in the Holocene loess-soil profiles. The chronology of three palaeoflood episodes was established by OSL dating and pedo-stratigraphic correlation with the well-dated Holocene loess-soil profiles in the upper Hanjiang River basin. Holocene palaeoflood events were dated to 9500–8500, 3200–2800, and 1800–1700 a B.P., respectively. Palaeoflood discharges were estimated by the palaeoflood model (i.e., slope–area method and step-backwater method). The highest discharges are 51,680–53,950m3s−1 at the 11,500-time scale in the Xunyang reach of the upper Hanjiang River valley. Holocene extraordinary hydroclimatic events in the Hanjiang River often result from abnormal atmospheric circulations from Southwest monsoons in the Chinese monsoonal zone. These results provide a regional expression of extreme flood response to Holocene palaeoclimate to understand the effects of global climatic variations on the river system dynamics.

Late Pleistocene and Holocene palaeoflood events recorded by slackwater deposits in the upper Hanjiang River valley, China

Summary Slackwater palaeoflood deposits (SWDs) were identified in a bedrock gorge in the upper reaches of the Hanjiang River of central China. The Hanjiang River is the longest tributary of the Yangtze River, one of the most flood-prone rivers in China, and the main source of water for the South-to-North Water Transfer Project (SNWTP). Three main loess-soil profiles with late Pleistocene and Holocene palaeoflood SWD bedsets were found. Palaeoflood SWDs identified interbedded in the loess–soil sequence of late Pleistocene and Holocene age within the cliff riverbanks were studied by field observations and laboratory analysis, including particle−size distribution and Optically Stimulated Luminescence (OSL) dating. At least eight extreme flood events documented by palaeoflood SWDs occurred in the Wufeng reaches of the upper Hanjiang River. The discharge estimation associated with palaeoflood SWDs, indicates that the minimum flood peak discharges of these flood episodes range from 42,220 to 63,400 m3/s. The SWDs were OSL dated to between 12,600–12,400, 4200–4000, 3200–2800 and 1900–1700 a BP and these dates were corroborated with pottery remains retrieved from the profiles and dated by archaeological methods. These periods of increased flood magnitude coincide with contemporaneous global climatic events dated to 12,500, 4200, 3100 and 1900 a BP worldwide. These findings are of great significance in understanding the interactions between hydrological systems and climatic change in monsoonal zones.

Paleofloods recorded by slackwater deposits in the upper reaches of the Hanjiang River valley, middle Yangtze River basin, China

A paleo-hydrological study was carried out along the upper-reach gorges of the Hanjiang River that drains from the Qinling and Dabashan Mountains. Two bedsets of paleoflood slackwater deposits were found embedded in Holocene loess soil sequences within riverbank cliffs. All of these were studied by field observations and laboratory analyses, including magnetic susceptibility, particle size distribution and optically stimulated luminescence (OSL) dating. These analyses provided a complete catalog of Holocene flood events on a watershed scale that is important for understanding the regional hydrological responses to monsoonal climatic change. The results show that these hydro-climatic events were dated to 9000–8500 and 3200–2800 a B.P. The paleoflood peak discharges were estimated to be 44,350–49,860 m3/s by using the HEC-RAS model and were verified by different approaches. These results have extended the flood data sequence of the Hanjiang River to a 10,000-year time-scale. The additional data allowed us to refine the flood frequency-peak discharge curve and potentially increase its accuracy. These results will play an important role in mitigating flood hazards and will be useful for hydraulic engineering and the development of water resources in the Hanjiang River basin.

Concentration of heavy metals in the modern flood slackwater deposits along the upper Hanjiang River valley, China

Modern flood slackwater deposits (SWD) were systematically collected after the great flood events which occurred on 19 July, 2010 and on 19 September, 2011 along the upper Hanjiang River valley. Concentrations of heavy metals (Co, Cr, Cu, Ni, Pb, Zn, Ba, Mn and V) in the fresh flood SWD were determined. Enrichment Factor (EF) and Index of Geoaccumulation (Igeo) were employed for heavy metal assessment. These results show that Cu, Ni, Pb, Zn and Mn are defined as only minor enrichment and unpolluted in the upper Hanjiang River basin, whereas Co, Cr, Ba and V are defined as between moderate enrichment and moderately severe enrichment, and moderately polluted in the downstream of Ankang city. Principal component analysis (PCA) and correlation analysis confirm the results of EF and Igeo. Cluster analysis (CA) identifies that Ankang city is the boundary between higher and lower heavy metals in the modern flood SWD. Therefore, we can conclude that the source of Co, Cr, Ba and V is mainly from anthropogenic inputs, but the source of Cu, Ni, Pb, Zn and Mn is mixed from natural and anthropogenic inputs. Concentration of heavy metals in the downstream of Ankang city is higher than that in the upstream. These results would help to establish non-point sources of heavy metal pollution and provide important information for water conservation strategy during the great flood periods in the upper Hanjiang River basin.

Holocene paleofloods related to climatic events in the upper reaches of the Hanjiang River valley, middle Yangtze River basin, China

Holocene slackwater deposits along the river channels were used to study the magnitude and frequency of the paleofloods that occurred prior to gaged and historical data sets all over the world. Paleoflood hydrological investigations were carried out in the upper reaches of the Hanjiang River, China. Four bedsets of paleoflood slackwater deposits were found interbedded in the Holocene loess–soil sequence within the cliff riverbanks. They were studied by field observations and laboratory analysis including magnetic susceptibility, particle size distribution and optically stimulated luminescence (OSL) dating. These provide a complete catalog of Holocene flood events at a watershed scale that is very important in understanding the regional hydrological response to monsoonal climatic change. Four episodes of Holocene overbank flooding have been recorded over the basin. These hydroclimatic events were dated to 8500–8400, 4200–4000, 3200–2800, and 1800–1700YBP by using the OSL method in combination with pedostratigraphic correlations with the previously studied Holocene pedostratigraphy in the Hanjiang River basin. The peak discharges of these flood episodes vary in a range from 26,610 to 47,900m3s−1 as estimated by using the slope–area method of streamflow measurement. These flood events are therefore considered a regional expression of known climatic events and demonstrate Holocene climatic instability. The study provides a solid reference that magnitudes and recurrence frequencies of floods are highly sensitive to climate change in the monsoonal zone in other parts of the world.

Extraordinary hydro-climatic events during the period AD 200−300 recorded by slackwater deposits in the upper Hanjiang River valley, China

The Hanjiang River, a major tributary of the Yangtze River in China, is noted for the current national South-to-North Water Diversion project. Palaeo-hydrological investigations were carried out along the upper reach gorges of the Hanjiang River that drains the Qinling and the Dabashan Mountains. A set of palaeoflood slackwater deposit beds (SWDs) was identified in Holocene pedo-stratigraphy of the riverbanks along the valley. These SWDs are interbedded in the eolian loess-soil profiles in the cliffy riverbanks and they thin out toward the upper slopes. The palaeoflood SWDs were differentiated from eolian loess and soil by the sedimentary criteria and analytical results. The minimum flood peak discharges were estimated to be 65,400−65,830m3s−1 by using palaeo-hydrological methods. They are about twice of the largest gauged flood (34,300m3s−1) that has ever been measured. They represent the largest flood events in the upper reaches of the Hanjiang River over the Holocene. These extraordinary flood events were dated to 1810−1710 a (AD 200−300) with the optically stimulated luminescence method and checked by archaeological dating of the human remains retrieved from the profiles. This indicates that the extraordinary floods occurred during the dynasties of Eastern Han to Western Jin (AD 25−316) in Chinese history, during which severe droughts and floods were recorded in documents. In the reconstructed 2000-year temperature time series based on high-resolution climatic proxies from tree-rings, stalagmites, ice-cores and lake sediments from over the world, these extraordinary palaeoflood events are correlated with an increased climatic variability characterized by cooling and drying during the period AD 150−350. This result is important for understanding the effects of global change on river system dynamics.

Sedimentological study of the stratigraphy at the site of Homo erectus yunxianensis in the upper Hanjiang River valley, China

Geomorphological and sedimentological investigations were carried out in the upper Hanjiang River valley. The sediments in the profile at the site of Homo erectus yunxianensis were sampled systematically. Magnetic susceptibility, loss-on-ignition, particle size distribution and geochemical elements were analyzed in the laboratory. The results show that the medium sand and gravel beds in the lower part of the profile are fluvial deposits of the third terrace land of the Hanjiang River, the upper part is a blanket of aeolian loess and palaeosol deposited from dust storms and dust falls. According to stratigraphical correlation and palaeomagnetic dating, the carbonate nodules encasing the fossils of H. erectus yunxianensis and other mammalian faunas were formed during 800–785 ka. The palaeosol overlying on the fossil layer is defined as the S8 that was developed during the MIS-19, and the aeolian loess below the fossil layer is defined as L9 that was deposited during MIS-20. These loess and palaeosol are similar to those over the Chinese Loess Plateau and correlatable with those in the Luochuan loess profile. They are very different from the fluvial deposits in the Hanjiang River valley. These indicate that H. erectus yunxianensis lived at a climatic transition from glacial to interglacial. These results are very important in understanding the environment of H. erectus yunxianensis in the upper Hanjiang River valley.

Comparative study of the modern flood slackwater deposits in the upper reaches of Hanjiang and Weihe River Valleys, China

Modern flood slackwater deposits (SWD) were investigated and sampled in the upper reaches of Hanjiang River and the Weihe River valleys, China. Magnetic susceptibility and grain-size distribution were analyzed in the laboratory. The results show that the magnetic susceptibility values of the modern flood SWD vary between 20 × 10 −8 m 3 kg −1 and 60 × 10 −8 m 3 kg −1 , very close to that of the Malan Loess over the Weihe River basin . These sediments were newly deposited and uninfluenced by weathering and pedogenesis . The grain-size distribution of the modern flood SWD is dominated by silt, with sand and clay, indicating that these sediments were sourced from the suspended sediment load of floodwater. However, the grain-size distribution of the modern flood SWD has more sand-sized sediments in the upper reaches of Hanjiang River valley than that in the Weihe River valley. The modern flood SWD are defined as sandy silt and silty sand in the upper reaches of Hanjiang River valley, whereas the modern flood SWD are defined as silt, sandy silt and clayey silt in the Weihe River valley. The modern flood SWD have different sediment sources and transporting forces. The modern flood SWD are sourced mainly from the Qinling and the Bashan bedrock mountains and the related residual deposits, slopewash and mud-rock flow deposits in the upper reaches of Hanjiang River valley. During rainstorms , torrential tributary rivers can bring much coarse deposits into the mainstream of the Hanjiang River. However, the sources of the modern flood SWD are the eolian loess and the related surface soil in the Weihe River valley. Comparative study shows different sedimentary features between the modern flood SWD in the upper reaches of Hanjiang River valley and that in the Weihe River valley. These results give insight into the different sediment sources and transporting force of the modern flood SWD, which provide a basic reference to mitigate flood disasters, and to soil and water conservation.