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排序方式: 共有1073条查询结果,搜索用时 31 毫秒
71.
72.
双重排水条件下控制措施对银南灌区水稻田水盐关系的影响分析 总被引:4,自引:0,他引:4
针对宁夏银南灌区目前存在的排水过量问题,研究者提出了利用控制排水措施,在适当时段减少排水量的建议。然而,灌区在农沟以及更深层的支、干沟及黄河(低地)的影响下,受着深层排水和浅层排水的双重作用。其中深层排水一般不会受到田间措施的影响,它的存在“淡化或中和”了由于浅层排水的减少引起的田间含盐量的增加。本文在对双重排水情况下的水盐运动进行概化的基础上,建立了基于特定地区排水特性的“浅层排水与深层排水比-浅层排水控制率-排水含盐量增加率”的关系。最后结合根据地下水洗淋的临界情况,确定了达到这一目标控制指标以及浅层地下水控制率。结果显示,对于研究的典型情况,当地下水的盐分含量达到临界的1.62 g/l时,可以将1m深的农沟控制至30cm深;这时的浅层地下水仅为现状的14%,即从731mm减少到了102mm。 相似文献
73.
Quantifying the nitrogen retention capacity of natural wetlands in the large-scale drainage basin of the Baltic Sea 总被引:3,自引:0,他引:3
We estimate the nitrogen retention capacity of natural wetlands in the 1.7 million km2 Baltic Sea drainage basin, using a wetland GIS data base. There are approximately 138,000 km2 of wetlands (bogs and fens) in the Baltic Sea drainage basin, corresponding to 8% of the area. The input of nitrogen to natural wetlands from atmospheric deposition was estimated to 55,000–161,000 ton y1. A map of the deposition of both wet and dry nitrogen is presented. The input from the human population was estimated to 255,000 ton y1 in terms of excretory release in processed sewage water. There may also be leakage from forests and agricultural land into the wetlands. Due to lack of data on hydrology and topography, such potential nitrogen sources are not accounted for here. The capacity of the wetlands to retain the atmospheric deposition of nitrogen was estimated to 34,000–99,000 ton y1. The potential retention by wetlands was estimated to 57,000–145,000 ton y1 when the nitrogen input from the human population was added. If drained wetlands were to be restored and their area added to the present wetland area, the nitrogen retention capacity was estimated to increase to 196,000–261,000 ton y1. Our results indicate that existing natural wetlands in the Baltic Sea drainage basin annually can retain an amount of nitrogen which corresponds to about 5–13% of annual total (natural and anthropogenic) nitrogen emissions entering the Baltic Sea. The ecosystem retention service performed by wetlands accounts for a substantial nitrogen removal, thereby reducing the eutrophication of the Baltic Sea. 相似文献
74.
A model of arctic tundra vegetation derived from topographic gradients 总被引:10,自引:0,他引:10
We present a topographically-derived vegetation model (TVM) that predicts the landscape patterns of arctic vegetation types in the foothills of the Brooks Range in northern Alaska. In the Arctic there is a strong relationship between water and plant structure and function and TVM is based on the relationships between vegetation types and slope (tan ) and discharge (), two independent variables that can be easily derived from digital terrain data. Both slope and discharge relate to hydrological similarity within a landscape: slope determines the gravitational hydrological gradient and hence influences flow velocity, whereas discharge patterns are computed based on upslope area and quantify lateral flow amount. TVM was developed and parameterized based on vegetation data from a small 2.2 km2 watershed and its application was tested in a larger 22km2 region. For the watershed, TVM performed quite well, having a high spatial resolution and a goodness-of-fit ranging from 71–78%, depending on the functions used. For the larger region, the strength of the vegetation types predictions drops somewhat to between 56–59%. We discuss the various sources of error and limitations of the model for purposes of extrapolation. 相似文献
75.
76.
【目的】通过揭示不同土壤含水量对蔬菜产量及蔬菜地土壤水分渗漏量的影响,为蔬菜生产中合理灌溉提供指导。【方法】通过3年田间试验,利用TDR连续监测土壤含水量,运用田间定位通量法计算土体水分渗漏量,分析不同水分处理下蔬菜产量和土体水分渗漏量之间的差异。【结果】传统水分处理下蔬菜地水分累积渗漏量为982mm,其中蔬菜生长期内累积渗漏量为748mm,占蔬菜生长期内供水量的36%;优化水分处理和充分水分处理下蔬菜地水分累积渗漏量230mm和468mm,其中蔬菜生长期内水分累积渗漏量分别144mm和293mm,占各处理下供水量的9%和17%。除2002年优化和充分水分处理下花椰菜产量高于传统水分处理下花椰菜产量并达到显著水平外,其余年份蔬菜产量并无显著差异。【结论】保持土壤含水量在蔬菜生长有效土壤含水量50%~80%的优化水分处理在蔬菜生产中有很大的推广价值。 相似文献
77.
F. Ribeiro R. L. Orjuela M. F. Magalhães M. J. Collares-Pereira 《Ecology of Freshwater Fish》2007,16(4):559-569
Abstract – Ecological attributes behind the success of the South American cichlid Australoheros facetus in mediterranean-type rivers remain unclear. This study addresses this issue, by analysing its abundance and feeding patterns in the lower Guadiana drainage (Portugal), during the dry season of 2003. Despite slight spatial variations in abundance and individual size, A. facetus seemed well established in the Guadiana, Vascão and Ardila rivers. No seasonal or size-related changes in feeding intensity were found, but the diet changed across rivers, over time and throughout ontogeny. Feeding seemed highly generalist, with fish apparently reacting to both spatial and temporal changes in food supply and tending to use the most abundant and easily accessible food items. Overall prey breadth was broad relative to that of other native and non-native species. It is suggested that a generalist feeding strategy may play a significant role in the establishment of non-native fish in mediterranean-type rivers. 相似文献
78.
C. Kechavarzi Q. Dawson P. B. Leeds-Harrison J. Szatyowicz & T. Gnatowski 《Soil Use and Management》2007,23(4):359-367
The rate of oxidation of peat soils is highly seasonal and varies with temperature and soil moisture content. Large variations in soil moisture content result in wet–dry cycles that can enhance peat degradation. Water‐table management plays a crucial role in controlling and damping the effect of these environmental factors. However, maintaining high ditch water levels in fields bounded by ditches does not guarantee a high field groundwater level. The effect of installing subsurface irrigation at different spacings on water table elevation was studied in a low‐lying peat grassland. The water table elevation data were compared against values predicted with a water balance model. In addition, greenhouse experiments were carried out on undisturbed soil core samples collected from the peat grassland as well as a low‐lying peatland under intensive arable faming to measure CO2 evolution under different water regimes. The field data from the peat grassland suggest that sub‐irrigation spacing as low as 10 m is necessary during summer periods to maintain groundwater levels similar to those in the ditches. Over the same period of observation, the difference in water level between the ditches and the non‐irrigated fields is as high as 0.7 m. Modelled outputs are in good correlation with the field observations, and demonstrate that simple water balance models can provide an effective tool to study the effect of water management practices and potential changes in subsurface conditions, climate and land use on water‐table levels. The measurement of CO2 emission from undisturbed peat soil columns shows that the rate of oxidation of soil organic matter from peat soils is highly seasonal and that drainage exacerbates the rate of peat mineralization. 相似文献
79.
结合塑料排水管材的自身特点及目前这一管材在市政排水领域的应用现状,通过多年的工程实践,从国家相关的法律法规及经济技术比较等角度出发,笔者认为大口径埋地塑料排水管替代传统的钢筋混凝土排水管将成为市政工程管网建设中的新趋势。 相似文献
80.