CHANGES IN STREAM CHANNEL CHARACTERISTICS AT TRIBUTARY JUNCTIONS

Several existing data sets are analyzed empirically to determine the nature of changes in channel characteristics at stream confluences. Classifications of the data indicate that such changes are diverse and do not conform well with predictions based on the hydraulic geometry of synchronous channel-forming flows on major and minor tributaries. Scatterplots reveal that at best only weak relation ships exist between rates of change for the variables and junction symmetry ratio. Most of the plots display abrupt discontinuities separating positive and negative changes at low symmetry values from changes at high symmetry values that are positive or negative only. These thresholds of discontinuity generally occur at a symmetry ratio of approximately 0.7. Such thresholds may be indicative of abrupt changes in the factors controlling junction morphology. More research is required both at site-specific and basin-wide scales to resolve this issue.     

ENVIRONMENTAL CHANGES IN NORTHWEST ARID CHINA DURINGRECENT 40 YEARS

ThearidlandsinNorthwestChina@WChina)includethefollowinggeograPhicalunits;theTarilllBasin,JunggarBasin,QaidamBasin,HexiCorridor,andAJasanPlateauwhicharealll0catedintheinlandofEurasiacontinent,farftomtheseawithadistancevaryingbetweenl500-l799kn.Theseareasbelongtocontinenta1climatewithverydrysummerandverycoldwinter.Environmentalconditions0ftheseareasarefragil,affectedbydrought,salinizationandshiftingsandhazards.Duringrecent40years,theeconomicdevelopmentofaridlandsinNWChinawasfast,theraPid…     

汉江丹江口水库下游河床演变

丹江口水库建库20多年来,下游河道由堆积性河道变成了冲刷性的河道,河床物质沿程粗化,比降得以调平。水深增加幅度大于河宽,流速并不随着水深增加而加快。深槽、浅滩分布明显化,宽深比不断在减少。整个库下游变化可分为三段:1)近库段,游荡河道向单一限制性曲流转化;2)大支流影响段,河道仍保持着游荡特性;3)下游段,游荡段游荡特性减弱,弯曲段则深蚀作用加强。     

DECADAL CHANGES OF INFLOW TO THE SACRAMENTO-SAN JOAQUIN DELTA,CALIFORNIA

The Sacramento-San Joaquin Delta is both an important environmental resource and a critical link in the water supply system for California. Concern for the adequacy of Delta water supplies increases with growing population and environmental maintenance needs and with the hydroclimatic uncertainty of global warming. Reconstructed or unimpaired discharges for Delta tributary areas are analyzed for trend and for changes in the seasonal regime of Delta inflows. Nonparametric tests indicate the absence of trend for annual inflows, but the low inflow months of September and October display increasing trends that are statistically significant. Additional changes in the Delta inflow regime are evident when inflow volumes are expressed relative to annual inflow. Decreasing trends in the spring fraction of annual total inflows and in the monthly fractions for April and May are statistically significant. March displays a significant increasing trend in the monthly fraction of inflow. The emerging decadal changes in monthly inflows have practical ramifications for water managers in the Delta. [Key words: Sacramento-San Joaquin Delta, hydroclimatology, climate change.]     

河型及其转化的判别

本文分析了影响河型的内外营力,建立了河型与河流能量、河岸不稳定系数、流域形状系数和地壳相对升降强度间的相关关系式。以汉江和黄河下游为例,用水力参数和河床形态指标,判别河型的转化趋势。     

LATE QUATERNARY CHANGES IN PINYON AND JUNIPER DISTRIBUTIONS IN THE WHITE MOUNTAIN REGION OF CALIFORNIA AND NEVADA

Data from packrat middens are used to reconstruct the migration of single-leaf pinyon (Pinus monophylla) and Utah juniper (Juniperus osteosperma) in the White Mountain region of California and Nevada for the last 20,000 years. Today this region is characterized by arid conditions with dry summers. Pinyon woodlands, which are favored by dry summer conditions, are dominant at middle elevations of the White Mountains. The midden record indicates that during the late Pleistocene and early Holocene, juniper was dominant at elevations now occupied by desert shrubs. A late Pleistocene-early Holocene record of desert shrubs was found only in the double rainshadow of the Sierra Nevada and the White Mountains. Pinyon entered the area during the Holocene (ca. 9000 yr B.P.) and subsequently became more important than juniper. During the late Holocene, desert shrubs became established at the lower elevation sites that they now occupy. The late Pleistocene-early Holocene record indicates that there may have been a more zonal upper-level wind flow and associated penetration of Pacific moisture, as indicated by the occurrence of desert shrubs downwind of the north-south-trending mountain ranges. Zonal flow probably became less important during the early Holocene based on the increasing amount of pinyon. The increase in pinyon also may indicate a change from an even seasonal distribution of precipitation to the dry summer conditions presently found in the region. [Key words: biogeography, packrat middens, Great Basin paleoenvironments, Pinus monophylla, Juniperus osteosperma, White Mountains.]     

SPECIALTY CROPLAND IN CALIFORNIA*

ABSTRACT. Census data do not support the widespread popular perception that urban encroachment on cropland in California is serious enough to justify programs of farmland preservation. Between 1949 and 1997 the acreage of harvested cropland declined near Los Angeles, in the San Francisco Bay area, and near Sacramento, but the high‐value specialty agricultural production displaced from these areas has been relocated to more distant areas, where it has replaced lower‐value field crops, and specialized agricultural production has increased steadily in the state. Vegetable production in the Salinas Valley and dairying near Los Angeles illustrate the twin processes of relocation and replacement. Urban encroachment actually has been a boon to California agriculture, because it has transferred massive amounts of urban capital to cash‐strapped farmers and enabled them to develop efficient modern operations. Much of the concern about loss of farmland really is concern about loss of open space and amenities, and urban demand for water probably will be a greater constraint on California agriculture than will urban demand for land.     

黄河口清水沟流路的河床演变及其影响

黄河口清水沟流路先后经历了游荡型,顺直型和弯曲型和河床演变阶段。与此同时,河流的冲淤变化经历了摆动淤积,造床冲刷和延伸淤积三个阶段,这些变化对河床纵剖面,水位和险情带来了某些较深刻的影响。     

SCIENCES IN COLD AND ARID REGIONS

正Aims and Scope The international journal Sciences in Cold and Regions.is dcvoted to publishing the latest rescarch achievements in proccssces and the patterns of the Earth surface system in cold and arid regions.     

SCIENCES IN COLD AND ARID REGIONS

正Aims and Scope The international journal Sciences in Cold and Arid Regions,is devoted to publishing the latest research achievements in processes and the patterns of the Earth surface system in cold and arid regions.Research in cold regions 1) emphasize particularly on the cold-region-characterized physical,chemical and biological processes and their interactions,and on the response of cryosphere to global change and     

SCIENCES IN COLD AND ARID REGIONS

Aims and Scope Sciences in Cold and Arid Regions, an international Engiish-language journal, is devoted to publishing the latest research achievements on the process and the pattern of Earth surface system in cold and arid regions. Researches in cold regions 1) emphasize particularly on the cold-region-characterized physical, chemical and biological processes and their interactions, and on the response of Cryosphere to Global change and Human activities as well as its effect to environment and the acclimatizable     

干旱区农业生态园策划、规划、设计理念初探——以新疆尉犁县生态园为例

将策划理论与旅游规划相结合，以尉犁生态园为例，集高科技农业示范、干旱区生态恢复研究、农业观光旅游于一体，并结合现代化农业培训和农业科技推广等的协调发展，尤其注重突出干旱区特色。从策划的角度对规划的背景、原则、设计及构思与进行讨论，以求对干旱区生态观光农业规划的发展有所启益。     

GEOMORPHIC RESPONSE TO WILDFIRE IN AN ARID WATERSHED,CROW CANYON,NEVADA

On August 28, 1981, the Crow Canyon drainage basin in central Nevada was burned by a lightning-generated wildfire that destroyed the vegetation cover consisting primarily of juniper trees, sagebrush, and desert grasses. The geomorphic impact of the wildfire was assessed on the basis of aerial photography, measurements of sediment movement on hillslopes using charred tree trunks as erosion indicators, and surveys of the valley floor, axial channel, and alluvial fan. Aerial photographs indicate the valley floor was untrenched prior to the fire. The combination of foliage destruction and heavy runoff in the spring following the wildfire initiated channel downcutting that has now reached as much as 3.9 m in depth. Entrenchment of the valley-fill in the lower 2.2 km of the drainage network produced as much as 48, 142 m³ of sediment. Much of the channel incision occurred during 1982 and 1983, years characterized by above-normal precipitation. Approximately 17,608 m³ of sediment were deposited on a preexisting alluvial fan at the mouth of the basin. Following initial channel entrenchment and deposition on the fan, a spatially out-of-phase episode of channel cutting was initiated on the fan apex, a process that is redistributing sediment down-fan. Thus, one geomorphic disturbance has produced two discrete depositional events on the fan. Moreover, the geomorphic instability was still evident over a decade after the wildfire. [Key words: wildfire, degradation, channel entrenchment, soil erosion, complex-response.]     

LAND DEGRADATION AND ITS MANAGEMENT IN THE ARID AND SEMI ARID AREAS OF CHINA

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干旱区绿洲棉田土壤水盐运动数值模拟

本文中利用预报校正法求解土壤水分运动的ｈ型Ｒｉｃｈａｒｄｓ方程。同时利用具有二阶高精度的差分法求解土壤盐分运移的对流扩散方程,此格式可避免利用一般差分格式时出现的数值弥散问题。最后,将上述数学模型对干旱区绿洲棉田,在蒸发条件下土壤水分运动和土壤盐分向上运动积累过程进行数值模拟,计算结果与测量值吻合较好。可为预报土壤盐碱化提供科学依据。     

HYDROCLIMATIC ANALYSIS OF SEVERE DROUGHT IN THE SACRAMENTO RIVER BASIN,CALIFORNIA

Climatic water budget principles provide an analytical basis for expressing drought severity as the cumulative moisture deficiency in biologic and hydrologic systems. This conceptual framework emphasizes the disparity between real-time and expected precipitation, soil moisture, and runoff in quantifying the cumulative moisture deficiency, and it facilitates incorporation of spatial characteristics in assessing drought severity. Comparison of the temporal and spatial properties of the severe 1924 and 1977 droughts in the Sacramento River Basin illustrate the procedure. The 1924 drought has a larger average water deficiency, or magnitude, but the 1977 drought has a longer duration and its cumulative moisture deficiency, or severity, is 1.4 times greater than the 1924 drought. Expressed volumetrically, the greater cumulative moisture deficiency for the 1977 drought is 13.3 times larger than the moisture deficit difference indicated by calculations of natural runoff for water years 1924 and 1977.     

TREE INVASION IN MEADOWS OF LASSEN VOLCANIC NATIONAL PARK,CALIFORNIA*

The causes of tree invasion into meadows in Lassen Volcanic National Park were investigated by dating invasive trees and associating age structure patterns with regional variations in fire history, grazing, and climate change. Massive invasion was associated primarily with cessation of livestock grazing and burning between 1905 and 1933, but climatic change may also have contributed. Maintenance of meadows may require continued use of unnatural processes due to uncertainties about pre-European fire regimes and other resource considerations.     

美国干旱区农业与干旱区农业发展

本文通过对美国干旱区农业发达的原因进行分析,揭示了干旱区农业发展的特色与优势,说明干旱区农业发展前景广阔。