全文获取类型
收费全文 | 386篇 |
免费 | 54篇 |
国内免费 | 87篇 |
专业分类
测绘学 | 83篇 |
大气科学 | 145篇 |
地球物理 | 114篇 |
地质学 | 84篇 |
海洋学 | 28篇 |
天文学 | 3篇 |
综合类 | 29篇 |
自然地理 | 41篇 |
出版年
2023年 | 3篇 |
2022年 | 4篇 |
2021年 | 16篇 |
2020年 | 12篇 |
2019年 | 10篇 |
2018年 | 9篇 |
2017年 | 11篇 |
2016年 | 25篇 |
2015年 | 20篇 |
2014年 | 30篇 |
2013年 | 26篇 |
2012年 | 27篇 |
2011年 | 18篇 |
2010年 | 26篇 |
2009年 | 23篇 |
2008年 | 29篇 |
2007年 | 28篇 |
2006年 | 43篇 |
2005年 | 19篇 |
2004年 | 17篇 |
2003年 | 9篇 |
2002年 | 16篇 |
2001年 | 14篇 |
2000年 | 14篇 |
1999年 | 11篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 8篇 |
1995年 | 4篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有527条查询结果,搜索用时 437 毫秒
101.
设计矩阵可以是秩亏阵,观测值的协方差阵可以是奇异阵的广义 Gauss-Markov模型(简称广义G-M模型),它是一种形式简单的统一模型。本文从最小二乘估计V~TQ~-V=min 出发,研究广义G-M 模型的参数估计理论和方法。说明了V~TQV~-=min与 Rao及Bjeharmmar等的平差原则一致。并对广义 G-M模型之解及其性质进行了系统讨论。 相似文献
102.
针对磁力计易受干扰和陀螺仪易漂移的问题,使用扩展卡尔曼滤波EKF(Extended Kalman Filter)融合多源传感器进行定向。利用基于四元数的EKF,在当地磁场和重力加速度的基础上融合加速度计、陀螺仪和磁力计观测信息,并通过自适应算法构建过程噪声向量和观测噪声向量的协方差矩阵以解决载体运动和周围磁场干扰对重力和地磁场观测值的影响。实验结果表明,该算法可以有效地减弱陀螺仪漂移和磁场对定向的干扰,获得更加精确和稳定的航向角信息。 相似文献
103.
Carbon dioxide flux techniques performed during GasEx-98 总被引:2,自引:0,他引:2
Wade R. McGillis James B. Edson Jonathan D. Ware John W. H. Dacey Jeffrey E. Hare Christopher W. Fairall Rik Wanninkhof 《Marine Chemistry》2001,75(4):851
A comprehensive study of air–sea interactions focused on improving the quantification of CO2 fluxes and gas transfer velocities was performed within a large open ocean CO2 sink region in the North Atlantic. This study, GasEx-98, included shipboard measurements of direct covariance CO2 fluxes, atmospheric CO2 profiles, atmospheric DMS profiles, water column mass balances of CO2, and measurements of deliberate SF6–3He tracers, along with air–sea momentum, heat, and water vapor fluxes. The large air–sea differences in partial pressure of CO2 caused by a springtime algal bloom provided high signals for accurate CO2 flux measurements. Measurements were performed over a wind speed range of 1–16 m s−1 during the three-week process study. This first comparison between the novel air-side and more conventional water column measurements of air–sea gas transfer show a general agreement between independent air–sea gas flux techniques. These new advances in open ocean air–sea gas flux measurements demonstrate the progress in the ability to quantify air–sea CO2 fluxes on short time scales. This capability will help improve the understanding of processes controlling the air–sea fluxes, which in turn will improve our ability to make regional and global CO2 flux estimates. 相似文献
104.
《水文科学杂志》2013,58(6)
Abstract Abstract Characterization of heterogeneity at the field scale generally requires detailed aquifer properties such as transmissivity and hydraulic head. An accurate delineation of these properties is expensive and time consuming, and for many if not most groundwater systems, is not practical. As an alternative approach, stochastic representation of random fields is used and presented in this paper. Specifically, an iterative stochastic conditional simulation approach was applied to a hypothetical and highly heterogeneous pre-designed aquifer system. The approach is similar to the classical co-kriging technique; it uses a linear estimator that depends on the covariance functions of transmissivity (T), and hydraulic head (h), as well as their cross-covariances. A linearized flow equation along with a conditional random field generator constitutes the iterative process of the conditional simulation. One hundred equally likely realizations of transmissivity fields with pre-specified geostatistical parameters were generated, and conditioned to both limited transmissivity and head data. The successful implementation of the approach resulted in conditioned flow paths and travel-time distribution under different degrees of aquifer heterogeneity. This approach worked well for fields exhibiting small variances. However, for random fields exhibiting large variances (greater than 1.0), an iterative procedure was used. The results show that, as the variance of the ln[T] increases, the flow paths tend to diverge, resulting in a wide spectrum of flow conditions, with no direct discernable relationship between the degree of heterogeneity and travel time. The applied approach indicates that high errors may result when estimation of particle travel times in a heterogeneous medium is approximated by an equivalent homogeneous medium. 相似文献
105.
Seasonal Variation and Meteorological Control of CO2 Flux in a Hilly Plantation in the Mountain Areas of North China
下载免费PDF全文
![点击此处可从《Acta Meteorologica Sinica》网站下载免费的PDF全文](/ch/ext_images/free.gif)
HUANG Hui ZHANG Jinsong MENG Ping FU Yuling ZHENG Ning GAO Jun 《Acta Meteorologica Sinica》2011,25(2):238-248
The carbon cycle of terrestrial ecosystems is an important scientific issue in global climate change re-
search. Plantation forest plays an important role in terrestrial carbon budget in China. In this study,
eddy covariance flux data measured at Xiaolangdi forest ecosystem research station (XLD) in 2007 and
2008 are used to analyze the seasonal variation and meteorological control of CO2 flux in a 30-yr-old mixed
plantation. The plantation forest mainly consists of Quercus variabilis, Platycladus orientalis, and Robinia
pseudoacacia. The results show that the seasonal variations of net ecosystem exchange of CO2 (NEE),
gross primary production (GPP), and ecosystem respiration (Re) display single-peak curves. The maximum
of carbon sequestration appears during May and June each year. The relative contribution of carbon re-
lease from ecosystem respiration to GPP varied slightly between 2007 and 2008. The relationship between
NEE and photosynthetic active radiation (Qp) accords with the rectangular hyperbola model on diurnal
scale, and shows a good linear correlation on monthly scale. The ecosystem photosynthetic parameters:
the maximum photosynthetic rate (Pmax), the ecosystem photosynthetic photonyield (α), and the daytime
ecosystem respiration (Rd) exhibit seasonal variations. Pmax reaches the maximum in August each year,
with small interannual di?erence. The interannual differences of α and Rd are obvious, which is attributed
to the changes of meteorological factors, such as solar radiation, vapor pressure deficit (D), precipitation,
etc. Parameters Re, GPP, and NEP (net ecosystem production) have obvious exponential relations with
temperature on monthly scale. There is a hysteresis in the response of GPP and NEP to temperature, i.e.,
the carbon sequestration is not the maximum when the temperature reaches the peak value. The Q10 values
were 1.37 and 1.45 in 2007 and 2008, respectively. On monthly scale, Re, GPP, and NEE increase as D
increases, but rise slowly and even decrease when D is higher than 1.5 kPa. 相似文献
106.
Fluxes of latent heat, sensible heat, and water vapor, including turbulent deposition of fog droplets, were measured for two months in autumn 2005 within a subtropical montane cypress forest in Taiwan. The goal of the study was to determine whether significant evapotranspiration can occur during foggy conditions. Water vapor fluxes, QW, as determined with the Bowen Ratio method, were compared to those simultaneously measured with the eddy covariance method. The median Bowen Ratio was 1.06, and the median QW flux was 5 · 2 × 10?5 kg m?2 s?1. The vertical gradients of temperature and specific humidity over the forest, ΔT and Δq, peaked around noon during days without fog, and were reduced during foggy conditions. For 66% of the data points, ΔT and Δq were negative, corresponding to positive (upward) fluxes of sensible heat QH and latent heat QE. A Monte Carlo simulation proved that statistically significant evapotranspiration rates, i.e., upward water vapor fluxes, occurred during fog. At the same time, deposition fluxes of fog droplets occurred. Our results show that even during fog events, significant evapotranspiration may occur. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
107.
Evaporation of intercepted rain by a canopy is an important component of evapotranspiration, particularly in the humid boreal forest, which is subject to frequent precipitation and where conifers have a large surface water storage capacity. Unfortunately, our knowledge of interception processes for this type of environment is limited by the many challenges associated with experimental monitoring of the canopy water balance. The objective of this study is to observe and estimate canopy storage capacity and wet canopy evaporation at the sub-daily and seasonal time scales in a humid boreal forest. This study relies on field-based estimates of rainfall interception and evapotranspiration partitioning at the Montmorency Forest, Québec, Canada (mean annual precipitation: 1600 mm, mean annual evapotranspiration: 550 mm), in two balsam fir-white birch forest stands. Evapotranspiration was monitored using eddy covariance sensors and sap flow systems, whereas rainfall interception was measured using 12 sets of throughfall and six stemflow collectors randomly placed inside six 400-m2 plots. Changes in the amount of water stored on the canopy were also directly monitored using the stem compression method. The amount of water intercepted by the forest canopy was 11 ± 5% of the total rainfall during the snow-free (5 July–18 October) measurement periods of 2017 and 2018. The maximum canopy storage estimated from rainfall interception measurements was on average 1.6 ± 0.7 mm, though a higher value was found using the stem compression method (2.2 ± 1.6 mm). Taking the average of the two forest stands studied, evaporation of intercepted water represented 21 ± 8% of evapotranspiration, while the contribution of transpiration and understory evapotranspiration was 36 ± 9% and 18 ± 8%. The observations of each of the evapotranspiration terms underestimated the total evapotranspiration observed, so that 26 ± 12% of it was not attributed. These results highlight the importance to account for the evaporation of rain intercepted by humid boreal forests in hydrological models. 相似文献
108.
Judith Fournier Antoine Thiboult Daniel F. Nadeau Nikki Vercauteren François Anctil Annie-Claude Parent Ian B. Strachan Alain Tremblay 《水文研究》2021,35(8):e14335
Hydrological models used for reservoir management typically lack an accurate representation of open-water evaporation and must be run in a scarce data context. This study aims to identify an accurate means to estimate reservoir evaporation with simple meteorological inputs during the open-water season, using long-term eddy covariance observations from two boreal hydropower reservoirs with contrasting morphometry as reference. Unlike the temperate water bodies on which the majority of other studies have focused, northern reservoirs are governed by three distinct periods: ice cover in the cold season, warming in the summer and energy release in the fall. The reservoirs of interest are Eastmain-1 (52°N, mean depth of 11 m) and Romaine-2 (51°N, mean depth of 42 m), both located in eastern Canada. Four approaches are analysed herein: a combination approach, a radiation-based approach, a mass-transfer approach, and empirical methods. Of all the approaches, the bulk transfer equation with a constant Dalton number of 1.2 x 10−3 gave the most accurate estimation of evaporation at hourly time steps, compared with the eddy covariance observations (RMSE of 0.06 mm h−1 at Eastmain-1 and RMSE of 0.04 mm h−1 at Romaine-2). The daily series also showed good accuracy (RMSE of 1.38 mm day−1 at Eastmain-1 and RMSE of 0.62 mm day−1 at Romaine-2) both in the warming and energy release phases of the open-water season. The bulk transfer equation, on the other hand, was incapable of reproducing condensation episodes that occurred soon after ice breakup. Basic and variance-based sensitivity analyses were conducted, in particular to measure the variation in performance when the bulk transfer equation was applied with meteorological observations collected at a certain distance (~10–30 km) from the reservoir. This exercise illustrated that accurate estimates of open water evaporation require representative measurements of wind speed and water surface temperature. 相似文献
109.
Comparison of methods for quantifying surface sublimation over seasonally snow‐covered terrain
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Snow sublimation can be an important component of the snow‐cover mass balance, and there is considerable interest in quantifying the role of this process within the water and energy balance of snow‐covered regions. In recent years, robust eddy covariance (EC) instrumentation has been used to quantify snow sublimation over snow‐covered surfaces in complex mountainous terrain. However, EC can be challenging for monitoring turbulent fluxes in snow‐covered environments because of intensive data, power, and fetch requirements, and alternative methods of estimating snow sublimation are often relied upon. To evaluate the relative merits of methods for quantifying surface sublimation, fluxes calculated by the EC, Bowen ratio–energy balance (BR), bulk aerodynamic flux (BF), and aerodynamic profile (AP) methods and their associated uncertainty were compared at two forested openings in the Colorado Rocky Mountains. Biases between methods are evaluated over a range of environmental conditions, and limitations of each method are discussed. Mean surface sublimation rates from both sites ranged from 0.33 to 0.36 mm day?1, 0.14 to 0.37 mm day?1, 0.10 to 0.17 mm day?1, and 0.03 to 0.10 mm day?1 for the EC, BR, BF and AP methods, respectively. The EC and/or BF methods are concluded to be superior for estimating surface sublimation in snow‐covered forested openings. The surface sublimation rates quantified in this study are generally smaller in magnitude compared with previously published studies in this region and help to refine sublimation estimates for forested openings in the Colorado Rocky Mountains. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
110.
????С?????????????任??????1????????????????????????????????????????Э????????????????????????????????????????????Tikhonov?????????????????????????????鷽?????????????????????Э?????????????? 相似文献