Table of Content

    15 September 2000, Volume 55 Issue 5 Previous Issue    Next Issue
    Causes of Sand-stormy Weather in Northern China and Contral Measures
    YE Du zheng, CHOU Ji fan, LIU Ji yuan, ZHANG Zeng xiang, WANG Yi mou, ZHOU Zi jiang, JU Hong bo, HUANG Hong qian
    2000, 55 (5):  513-521.  doi: 10.11821/xb200005001
    Abstract ( )   PDF (472KB) ( )   Save
    In northern China, the number of days with sand stormy weather has been decreasing in the past 40 years, but in the spring of 2000, an unprecedented heavy sand stormy weather with high frequency took place, which exerts adverse effect on traffic, environmental management and people’s daily life and work. Especially, it brought direct damage to Beijing, Tianjin and their neighbouring areas, which has aroused even more extensive concern of personages of various circles. Therefore, the reasons and the rational suggestions are proposed in this paper. The sand stormy weather is the result of the special geographic environment and weather conditions. Changes in the number of days of strong wind are the reflections of the periodical change in climate. Why the strong sand stormy weather took place is that the anti El Nin~o case is at top, and the land cover deteriorates in the whole area but with part area improving. The dust weather mainly originated from mid west Inner Mongolia and northwest of Hebei Province. The dust is mainly consisted of soil dust from the origination area and the trace. The ground bare soil and sandy dust from construction site in urban extension areas also supplied materials for the local dust. In order to alleviate and control the dust damage, some suggestions were proposed as following: Firstly, the natural vegetation must be restored through planting tree or grasses in cultivated land instead of planting crops. Especially effective ecological protective shield must be established for Beijing and Tianjin city. The bare land of urban marginal areas must be treated in order to control local dust. Secondly, eco environmental construction must be paid more attention to during implementing Western Development Plan. Ecological benefits must be combined with economic and social benefits. Finally, the system for monitoring and predicting the sand stormy weather must be established and improved. Study on controlling and alleviating the dust disaster also need to be done.
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    Analysis on Asynchronism-synchronism of Regional Precipitation in Planned South-to-North Water Transfer Areas
    ZHENG Hong xing, LIU Chang ming
    2000, 55 (5):  523-532.  doi: 10.11821/xb200005002
    Abstract ( )   PDF (486KB) ( )   Save
    This paper proposes a method on analyzing the asynchronism synchronism of precipitation in different hydrological regions related to South to North water transfer areas of eastern China. In general, the analysis process includes three steps. Firstly, we created a Thiessen map based on data obtained from representative stations of the regions concerned and calculated the relative area controlled by each station. Then the areas were regarded as the weight of the corresponding stations in calculating average rainfall of the regions concerned. Secondly, we made an assessment on the dryness or wetness of the rainfall time series. According to the result we found that the Gamma distribution function is the best to describe the stochastic characters of rainfall series. Thus Gamma distribution function was called for the rainfall series and the probability values of 62 5% and 37 5% were determined as indexes to classify the wetness, evenness and dryness of each sample of the time series. As we had divided the rainfall conditions of each region into three types, there were nine kinds of encountering of every two different regions. Finally, by comparing the dryness wetness assessments of different regions, we calculated the frequency of the dryness wetness encountering of the regions. According to the above discussed method, we carried out two case studies, one was between North China and mid lower reaches of the Changjiang River, the other was between North China and upper reaches of Han River. In the case studies, the monthly rainfall material used ranged from 1957 to 1998 and rainfall time series of the regions were calculated by three kinds of time scales, i.e. annual, seasonal and monthly. The investigation has shown that synchronous frequency of rainfall encountering of in the two cases in annual scale was about 45% for both, which included the frequency of wetness wetness (NW SW), evenness evenness (NE SE) and dryness dryness (ND SD). The results of the other two time scales were quite the same to that of the annual scale. On the consideration of the asynchronism between North China and Changjiang River, the total frequency of SW ND, SW NE and SE ND is a little higher than that of NW SD, NW SE and NE SD. However, in this case, the condition of North China to upper reaches of Han River is just in a different way. As to the total frequency of the conditions (i.e. SW ND, SW NE, SE ND and SE NE) that are fit for water transfer, in annual scale it is 40% and 24% respectively of the two cases. While in spring, it is 28% and 35% respectively. According to the results of the investigation, in order to enhance the effectiveness and reliability of South to North Water Transfer Project in China, we strongly suggested that the ability of water resources adjustment and the capacity of the project must be given careful consideration.
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    Analysis on Spatial Distribution Characteristics of Soil Organic Carbon Reservoir in China
    WANG Shao qiang, ZHOU Cheng hu, LI Ke rang, ZHU Song li, HUANG Fang hong
    2000, 55 (5):  533-544.  doi: 10.11821/xb200005003
    Abstract ( )   PDF (324KB) ( )   Save
    The paper respectively adopted physiochemical properties of every soil stratum from2 473 soil profiles of the second soil survey. The corresponding carbon content of soil is estimated by utilizing conversion coefficient 0 58. First, we calculated the carbon content of every stratum of different soil stirp in the same soil subtype. Then, we took soil stratum depth as weight coefficient to acquire the average physiochemical properties of various kinds of soil stirp. Finally, we got the average depth, organic content, duck density and carbon density of different soil subtypes through area averaging. The total carbon quantity of different kinds of soil can be calculated by the following expression: C j=0 58S jH jO W j where j is the soil type, C j is the carbon storage of j soil type, S j is the distribution area of j soil type, H j is the average depth of j soil type, O j is the average organic content of j soil type, and W j is the average bulk density of j soil type. In the second soil survey, the total amount of soil organic carbon is about 924 18?10 8 t and carbon density is about 10 53 kgC/m2 in China according to the statistic country area 877 63?106hm2. The spatial distribution characteristics of soil organic carbon in China are that the carbon storage increases with the increase of latitude in eastern China and the carbon storage decreases with the decrease of longitude in northern China. There is a transition zone where carbon storage varies greatly in China. Moreover, there is an increasing tendency of carbon density with the decrease of latitude in western China. Soil circle has implications on global change, but the difference in soil spatial distribution is substantial in China. Because the structure of soil is inhomogeneous, mistakes will be resulted in estimating soil carbon reservoir. It is thus necessary to farther resolve soil respiration, organic matter conversion and others related problems, and build uniform and normal methods of measurment and sampling.
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    Strategies for Sustainable Use and Characteristics of Soil Quality Changes in Urban-Rural Marginal Area: A Case Study of Kaifeng
    QIN Ming zhou, ZHAO Jie
    2000, 55 (5):  545-554.  doi: 10.11821/xb200005004
    Abstract ( )   PDF (465KB) ( )   Save
    Impacts of land use on soil quality, especially soil quality change became clear when 1998 soil samples in the Kaifeng rural urban marginal area are compared with 1982’s. Selected soil attributes included pH level, organic matter (OM), total nitrogen (TN), available nitrogen (AN), phosphorus (AP) and potassium (AK), cation exchange capacity (CEC) etc. Then through standardization of these parameters according to PRC national standards, the modified Nemoro Formula was used to make a trail assessment on soil quality. Soil quality changes caused by changes in typical land use patterns were analyzed. The dominant quality of soil types is still at middle level. The 1998 soil quality is 1 16, lower than 1 44 of the 1982’s. pH, OM and TN of soil attributes increased by 2 27%, 23 9% and 11 35% respectively; AP, AK and CEC declined by 67 14%, 29 8% and 4 48%. So phosphorus and nitrogen are seriously deficient to crop growth; OM and AK are in middle range. In all investigated soil samples, increased soil attributes including pH, OM, TN, AP, AK and CEC accounting for 84 6%, 71 4%, 71 4%, 28 6%, 8 3% and 35 7% respectively. In other words, alkalization developed increasingly and pH level was raised, OM and TN in soil rose more or less, AP, AK and CEC declined. Regarding land use change in patterns, there were two different trends in soil quality changes. One was improvement, which included the shift of upland to orchard or irrigated field, and the field of single rice plantation to irrigated rice wheat rotation. The other is degradation including adjustment of vegetable plots to orchard and various other shifts from extensive management. General trends of those high and middle quality grades declined lightly through land use and low quality soils were improved significantly. Specifically, 13 3% of soil types were improved qualiatively and 53 3% of soil type clearly degraded in quality. In terms of soil areas, sandy soil accounting for 60% of urban rural marginal area declined slightly within the same quality range although soil quality in Kaifeng is still in middle range. The best soil in Kaifeng is soil devoting to vegetable plots with high material input, paddy soil and irrigated field ranking at the second level (middle grade). Upland soil is the worst. Comparisons of soil quality in 1998 with that in 1982, clearly indicated that soil fertility and productivity has dropped.
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    The Spatio-temporal Variability of Soil Nutrients in Zunhua Plain of Hebei Province: Semivariogram and Kriging Analysis
    GUO Xu dong, FU Bo jie, CHEN Li ding, MA Ke ming, LI Jun ran
    2000, 55 (5):  555-566.  doi: 10.11821/xb200005005
    Abstract ( )   PDF (609KB) ( )   Save
    Geostatistics has proven to be useful for characterizing and mapping spatial variability of soil properties, however, most of the previous geostatistical studies were focused on data at small scale. With the development of GIS and GPS, geostatistcs and GIS are becoming indispensable in characterizing and summarizing spatial information in large regions to provide quantitative support to decision and policy making for soil, agricultural and natural resources management. In this paper, we applied geostatistics combined with GIS to analyze the spatio temporal variabilities of the available potassium, available phosphorus and organic matter in soil surface (0~20 cm) in Zunhua county (municipality) of Hebei province over 20 years from 1980 to 1999. Results showed the average content of the available potassium, available phosphorus and organic matter in soil surface in 1980 was 77 78 mg/kg, 19 46 mg/kg, 1 14%, respectively; and that in 1999 was 90 12 mg/kg, 25 7 mg/kg, 1 54%, respectively. Paired samples t test of soil nutrients and the results estimated by block kriging indicated the content of the available potassium, available phosphorus and organic matter in 1999 was significantly higher than that in 1980. The ratio of nugget to sill of the three soil nutrients varied from 32 3% to 60%, indicating the spatial correlation of the three tested soil nutrients at this large scale was moderately dependent. The ratio of nugget to sill of available potassium was 56 5% in 1980 and 41 1% in 1999, that of the available phosphorus was 45 5% in 1980 and increased to 54 1% in 1999. The ratio of organic matter changed considerably from 60% in 1980 to 32 3% in 1999. The range of the available potassium and available phosphorus between the year of 1980 and 1999 was almost the same, however, the range of organic matter was from 4 8 km in 1980 to 2 7 km in 1999. The improvement of the available potassium and available phosphorus in soil was mainly contributed to the more application of fertilizers, and the content of organic matter was affected by the changes of tillage and land use, which also altered the spatial distribution of soil nutrients to different degrees over the past 20 years.
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    Surplus Summer Rainfall Along the Middle to Lower Reaches of Changjiang River in the 1990s
    GONG Dao yi, WANG Shao wu, ZHU Jin hong
    2000, 55 (5):  567-575.  doi: 10.11821/xb200005006
    Abstract ( )   PDF (452KB) ( )   Save
    There were a series of severe floods along the middle to lower valley of Changjiang River of China during the 1990s. The extensive summer (June, July and August) precipitation is mostly responsible for the flooding. The summer rainfall in the 1980s and the 1990s is much higher than that in the prior 3 decades. The means for 1990~1999 is +87 62 mm above normal, marked the 1990s the wettest decade since 1951. Six stations are selected to establish century long rainfall series. These two series correlate at 0 92 for the period 1951~1999. It is found that the 1990s is also the wettest decade during the last 120 years. Four of the all 12 abnormal wet years, in which more than +40% of the rainfall over the normal, has occurred in the 1990s. Composite analysis for the six samples since 1951 presented that the atmospheric circulation anomalies associated with the surplus rainfall along the middle to lower valley of Changjiang River show positive southern wind velocity and moisture flux anomalies in southern China and negative values in northern China in the lower troposphere. There are strong northern winds during the 1990s over the northern China, which may be responsible for the simultaneous summer rainfall along the Changjiang River valley. Although there are some uncertainties in the climate models, it is strongly suggested that the summer rainfall would increase under the global warming as the IPCC modeling results demonstrated. Frequencies of summer rainfall types during the Medieval Warm Period and the Little Ice Age were compared to the normal (AD950 to 1999). Contrast of greater frequency of types 2 and 1b in MWP and of types 4 and 3 in LIA is exciting. It proves again the statement that warm/dry (or cold/wet) climate was predominated in north and warm/wet (or cold/dry) climate in southeast China.
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    The Ecological Impacts of Hydropower Cascade Development in Lancang-Mekong River
    CHEN Li hui, HE Da ming
    2000, 55 (5):  577-586.  doi: 10.11821/xb200005007
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    The Lancang-Mekong is one of the most famous international rivers in the world. It flows through six countries. There are plentiful hydraulic resources in the basin. Since the mid 1980s, as a main development subject, attention has been drawn to large scale damming and its effect on the environment. Hydropower investigation had been begun since the late 1950s in Lancang Jiang (the upper Mekong River). Hydropower cascade of 14 dams was planned in the early 1980s. At the end of the 1980s, planned as the key development area and hydropower development base in China, the alternative cascade of 8 dams has been chosen, in which, one had been finished, one is under construction and nearly finished, and two will be built soon. All these actions have caused the widely attention and been criticized in the world, but some of these criticisms are not true, especial for the Xiaowang dam, the highest dam in the world, which will be constructed by 2002. Based on 40 years’ hydrological data series, comparing the hydropower cascade indexes and simulated adjusting of the runoff, This paper draw up the conclusion as following: (1) The hydrawic resource concentrates in the mainstream in Lancang river and the tributaries in lower Mekong; (2) The sediment of the lower Mekong mainly comes from northern part of the Laos, not from Lancang river; (3) No essential harms to the sediment of the Lower Mekong and its delta area, if hydropower cascade developed on the Lancang mainstream; (4) The comprehensive countermeasures should be put into practice, so as to reduce the ecological impacts of hydropower cascade development: hydropower cascade focusing on the mainstream of Lancang and tributaries of the Lower Mekong, and canceling all cascades in lower Mekong, mainstream facilitating the integrated power network construction; It will meet the energy demand, reduce the cost, the inundated land, the residents and the harm to water environment. So it will benefit the irrigation development, flooding control, salt control, navigation, and ecological conservation the lower Mekong river basin.
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    The Shift of Tropical Zone During The Last Glacial Maximum in China
    HUANG Zhen guo, ZHANG Wei qiang
    2000, 55 (5):  587-595.  doi: 10.11821/xb200005008
    Abstract ( )   PDF (369KB) ( )   Save
    According to paleoenvironment analyses of about sixty samples including sporopollen, oxygen isotope, sedimentary, diatom, nannofossil, mammal, stalagmite, coral reef, glacial remain, etc., location changes of tropical and southern subtropical zones during the last glacial maximum in China are discussed in this paper. Comparing with present limits, the north limit of central tropics moved southward by five latitudes with an amplitude of drop in temperature of about 3℃. The north limit of northern tropics moved southward by two latitudes and the drop in temperature was less than 2℃. The north limit of southern subtropics experienced the largest moving distance and amplitude in temperature drop, being the six latitudes and four degress respectively. The longitude zonal differentiation is also obvious. Five regions can be divided from east to west. The largest amplitude of temperature drop is 5~9℃ in Taiwan Island, and the second is 3~4℃, occurring in Guangdong, Guangxi and Hainan provinces. The drop amplitude in water temperature of 2 4~5 7℃ in winter was considerably large in the South China Sea but its summer temperature was similar to that at present. In soutern Fujian and eastern Guangdong the amplitude was relative small, being 2~3℃, and the minimum of 1~2℃ was only found in southern Yunnan. The paleoenvironmental conditions at that time tended to be more cold and wet generally except southern Fujian and South China Sea where the trend was cold and dry. The drop in temperature in China’s tropical zones is a result of the uplift of the Tibetan Plateau with an accelerated rate of 11 8~15 7 mm/a during the late Pleistocene which aggravated the worsening of climate during glacial period.
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    Sustainable Valuation for Resources and Environment and Development in Rural China ——the SEEA and Its Application Research
    YANG You xiao, CAI Yun long
    2000, 55 (5):  596-606.  doi: 10.11821/xb200005009
    Abstract ( )   PDF (310KB) ( )   Save
    Environmental and Economic Accounting (SEEA) is applied to analyze quantitative characteristics of genuine saving ( S g ), net resource product (NRP) and net environmental product (NEP) in the rural China from 1990 to 1996. The aim of the study is to measure resource and environmental costs for economic development. The conclusions are drawn as follows: (1) The annual damage of NRP in rural China amounts to 2 1~142 6 billion yuan, which is 0 11%~3 22% of GNP in rural China. The main contributors of damage of NRP decrease sharply in cultivated and severe soil eroded area. Shortage of water and decrease of grassland harm NRP lightly. Increase of forest area and standing stock volume in some years plays a key role in making up damage of NRP. (2) The annual damage of NEP in rural China amounts to 60 4~105 9 billion yuan, which is 1 20%~4 60% of GNP in rural China. The percentage of NEP occupying GNP in rural China is 1 35 times as high as that of NRP occupying GNP in rural China. That is exposited huge damage of pollutant emissions to Chinese rural sustainable development. Emissions of SO 2 and soot damage to NEP occupy first place; liquid waste, particulates, solid waste and NO X damages to NEP follow behind it respectively. (3) The annual genuine saving in rural China amounts to -128 3~80 0 billion yuan. The genuine savings in 1990 and 1991 are positive. They are 48 1 and 80 0 billion yuan, which are 2 89% and 4 21% of GNP in rural China. The genuine savings from 1992 to 1996 became negative. The lowest damage is 87 2 billion yuan, the highest damage, 128 3 billion yuan, which is 0 98%~3 43% of GNP in rural China. Continuous negative genuine savings mean that development in Rural China is non sustainable during our research period. The 0 59% indicator of annual average damage proportion of S g in GNP in rural China and the descending trend of proportion of S g damage in GNP in rural China imply that it is possible to realize sustainable development in rural China, as long as we do our best to use natural resources without waste and to protect the environment against pollution in the process of rural development.
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    The Calculation and Analysis of Ecological Footprints of Gansu Province
    XU Zhong min, ZHANG Zhi qiang, CHENG Guo dong
    2000, 55 (5):  607-616.  doi: 10.11821/xb200005010
    Abstract ( )   PDF (238KB) ( )   Save
    Because humans consume the products and services of nature, every one of us has an impact on the earth. Does the human load stay within global carrying capacity? The ecological footprint concept has been designed to answer this question and estimate man’s impact on nature. The ecological footprint of any defined population (from a single individual to a whole city or country) is the total area of ecologically productive land and water occupied exclusively to produce all the resources consumed and to assimilate all the wastes generated by population. The ecological footprint method presents a simple framework for national natural capital accounting. The concept of ecological footprint and its calculation method is introduced in this paper. The paper also analyses the advantages and disadvantages of the ecological footprint model, and addresses the types of ecologically productive lands. The article calculates and analyses the ecological footprint of Gansu province in 1998. The ecological footprint ledger is composed of three main section. The first ledger is basic biotic resources consumption including its byproducts, the second is energy consumption, the third is trade balance. Trade balance through more detailed trade flow analyses can mitigate the influence of import and export product on consumption variations. Based on the ecological footprint concept and analysis framework, human consumption can be compared with regional level natural capital production using existing data. In the case of Gansu province, the ecological deficit of Gansu is 0 564 2 hm2 per capita. Simplification of calculation methodology to certain extent results in over optimistic estimates. Finally, the ecological footprint model’s advantages and disadvantages are identified. Ecological footprint index is an excellent aggregate index that connects many issues of sustainability, development and equity. The model can reveal the extent to which local carrying capacity has been exceeded and allows a cumulative approach to impact analysis. The use of ecological productive area as a numeraire, rather than money or energy, makes the footprints easy to be understood, and also permits provocative calculations. The limitations of the model is that it doesn’t include several important issues, which are even directly related to land use: land areas lost to biological productivity loss of land because of contamination, erosion and urban “hardening” and dissertation (especially in north western China). Methodologically, the assessments could be more complete by including the ecological spaces used for freshwater use, a particular important issue in arid area of north western China.
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    Soil Nitrogen Loss by Erosion as Affected by Vegetation Cover and Comprehensive Managements in Zhifanggou Catchment of Hilly Loess Plateau
    ZHANG Xing chang, SHAO Ming an
    2000, 55 (5):  617-626.  doi: 10.11821/xb200005011
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    It is agreed in most researches that vegetation coverage can effectively reduce soil erosion and this reduction is attributed to interception of rainfall by stems and leaves, fixation of soil by roots and resistance of vegetation covers to runoff movement. But because total soil nitrogen mainly exists in organic forms and combines with soil particles, vegetation covers, preventing soil particles from being eroded, reduce soil total nitrogen loss. The effect of vegetation covers on soil erosion will be intensified as the vegetation coverage is increased. Unlike total soil nitrogen, mineral soil nitrogen (NH + 4 N and NO - 3 N) mainly exists inside soil liquid and attaches itself to the surface of soil particles. The catchment model of 1∶400 scale under artificial rainfall conditions with rainfall intensity of 2 mm/min and rainfall amount of 60mm in 1998 and the original catchment with an area of 8 05 km2 under natural rainfall conditions in 1992 and 1998 were utilized to study the relationship of the nitrogen loss of catchment flood with vegetation coverage and comprehensive control to erosion. This research is aimed at finding out the relationship between comprehensive catchment control and nitrogen loss by erosion that will serve as the scientific base for reseeding cropland to forest and grass. As the vegetation coverage was increased from 0 to 60%, the flow of runoff was reduced only by 19 9%; the time span from the end of rainfall to the occurrence of soil erosion was lengthened from 1 5 minutes to 10 2 minutes and multiplied 5 8 times as much; the concentration of mineral nitrogen in runoff was increased from 1 4 mg/kg to 4 5 mg/kg and multiplied 2 2 times as much. The results showed that as the vegetation coverage was increased in the catchment, the loss of mineral soil nitrogen was intensified. But further researches should be conducted as to the intensity and mechanism of the interaction between runoff and mineral topsoil nitrogen. On June 23 and August 21, 1998, the concentrations of NH + 4 N in rainwater were 1 4 mg/kg and 1 9 mg/kg and the concentrations of NO - 3 N in rainwater were 1 2 mg/kg and 1 7 mg/ kg respectively. On the corresponding period, those of NH + 4 N in river water were 1 2 mg/kg and 0 9 mg/kg respectively and those of NO - 3 N in river water 1 3 mg/kg and 0 6 mg/ kg respectively. Thus it can be seen that the catchment could function as a filter of the available rainfall nitrogen that retained more NO - 3 N than NH + 4 N. In the catchment, the loss of NH + 4 N was 6 525 kg/km2 and 2 725 kg/km2 respectively, the loss of NO - 3 N was 5 101 kg/km2 and 1 258 kg/km2 respectively, the loss of total nitrogen was 346 8 kg/km2 and 42 7 kg/km2 respectively, and the loss of organic matter was 5 201 4 kg/km2 and 552 3 kg/km2 respectively. The enrichment of micro aggregates with a diameter of less than 20 mm led to the enrichments of organic matter and total nitrogen in river sediment. Because of water and soil loss, the annual loss of total soil nitrogen content was 26 1%, 52 6%, 21 5%, 8 8% and 7 7% from 1992 to 1998 respectively in the lands for wood, economic forest, shrub, grass and crops. In the catchment, the erosion induced soil loss was 1 086 t/km2 and 1 119 t/km2 respectively. The erosion induced loss soil nitrogen was 8 758 5 kg and 7 562 2 kg respectively, which meant a decrease of 15 8% from1992 to 1998. The erosion induced loss soil nitrogen was decreased from 1 207 2 kg to 579 9 kg in the farm land in the catchment and thus the decreasing rate was 52 0% from 1992 to 1998. The erosion induced loss soil nitrogen in the farming land accounted for 13 8% and 7 7% of the total soil nitrogen loss in the catchment in 1992 and 1998.
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    Analysis of Impacts of Sedimentation on Flooding Disasters in China
    SHI Chang xing, ZHANG Dian
    2000, 55 (5):  627-636.  doi: 10.11821/xb200005012
    Abstract ( )   PDF (442KB) ( )   Save
    The flooding and inundated area in China has been increased since the end of the 1960’s. The area of farmland subjected to flooding or waterlogging increased at a rate of 4 69 million hm2 annually in 1965~1978, and rose to 16 74 million hm2 in 1991~1997. Climate condition is the number one important factor influencing the flooding disasters. However, changes in precipitation in China in the past decades did not support the considerable increase in flooded areas. Also, there was a decrease in the area damaged by flooding calculated in light of the disaster prevention level of the waterlogging prone farmland since lots of flood control and waterlogging mitigation works have been constructed in the same period. On the other hand, about 1 2 billion tons of sediment were deposited annually in river channels and lakes in the plains, reservoirs, irrigation areas, and flood detention areas in China. Supposing the annual increase in the volume of water flooding the farmland is equal to the annual volume of the 1 2 billion tons of deposits, there would be a good agreement between the calculated changes in flooded area and the actual increase in the flooded or damaged area after the year 1965. It suggests that the sediment accumulation in the rivers, lakes, reservoirs, etc. in China may be considered as one of the main causes for aggravating flooding disasters. The patterns of impacts of sedimentation on flooding disaster are generalized as deposition in lakes and reservoirs, deposition in river channels, deposition in river mouth, and high sediment concentration in flows. Sediment accumulation results in the decrease in the flood regulating capacity of the lakes and reservoirs. It lowers the drainage capacity of the channels and canals and also raises the riverbed to form a perching river channel. The extension of river mouth due to sediment accumulation will lower the slope of channel, enhancing sediment deposition upstreams and deteriorating the flood prone situation of delta areas. The large amount of sediment carried by water flows can obviously enlarge the discharge of peak flood, and the hyper concentrated flows show an unstable behavior, leading to abrupt rise and fall of peak flood, an unfavorable situation to flood control. To alleviate the impacts of sedimentation on flooding disasters, soil erosion control, enlargement of sediment transport capacity of the river channels, decrease of the height difference between the river channels and surrounding plains, and reduce of deposition induced extension of river mouth should be the measures. However, the feasibility of these measures in the senses of economy and technology is not so clear now and they may have negative effects on economic development, environment and society. Thus, the feasibility as well as the positive and negative effects of these measures should be studied and compared comprehensively in order to achieve the best results in reducing the impacts of sedimentation on flooding disasters in the future.
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