全文获取类型
收费全文 | 94篇 |
免费 | 19篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 9篇 |
地球物理 | 42篇 |
地质学 | 14篇 |
海洋学 | 1篇 |
天文学 | 1篇 |
综合类 | 2篇 |
自然地理 | 48篇 |
出版年
2024年 | 1篇 |
2023年 | 1篇 |
2021年 | 5篇 |
2020年 | 2篇 |
2018年 | 4篇 |
2017年 | 1篇 |
2016年 | 1篇 |
2015年 | 3篇 |
2014年 | 8篇 |
2013年 | 17篇 |
2012年 | 3篇 |
2011年 | 4篇 |
2010年 | 7篇 |
2009年 | 10篇 |
2008年 | 7篇 |
2007年 | 1篇 |
2006年 | 6篇 |
2005年 | 8篇 |
2004年 | 7篇 |
2003年 | 3篇 |
2002年 | 2篇 |
2001年 | 6篇 |
2000年 | 1篇 |
1999年 | 3篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1985年 | 1篇 |
排序方式: 共有120条查询结果,搜索用时 656 毫秒
21.
22.
The study developed an integrated reflectance model combining radiative transfer and geometric optical properties in order
to inverse leaf area index (LAI) of semiarid natural grasslands. In order to better link remote sensing information with land
plants, and facilitate regional and global climate change studies, the model introduced a simple but important geometrical
similarity parameter related to plant crown shapes. The model revealed the influences of different plant crown shapes (such
as spherical, cylindrical/cuboidal and conic crowns) on leaf/branch angle distribution frequencies, shadow ground coverage,
shadowed or sunlit background fractions, canopy reflectance, and scene reflectance. The modeled reflectance data agreed with
the measured ones in the three Leymus chinensis steppes with different degradation degrees, which validated the reflectance model. The lower the degradation degree was,
the better the modeled data agreed with the measured data. After this reflectance model was coupled with the optimization
inversion method, LAI over the entire study region was estimated once every eight days using the eight-day products of surface
reflectance obtained by multi-spectral Moderate-Resolution Imaging Spectroradiometer (MODIS) during the growing seasons in
2002. The temporal and spatial patterns of inversed LAI for the steppes with different cover degrees, swamps, flood plains,
and croplands agreed with the general laws and measurements very well. But for unused land cover types (sands, saline, and
barren lands) and forestlands, totally accounting for about 10% of the study region, the reasonable LAI values were not derived
by inversing, requiring further revising of the model or the development of a new model for them.
Supported by the National Natural Science Foundation of China (Grant No. 30500076), the National Basic Research Program of
China (Grant No. 2007CB714407), and the President Foundation of Graduate University of Chinese Academy of Sciences (Grant
No. YZJJ200205) 相似文献
23.
The present study aims the evaluation of bio-physical characteristics towards soil-water-vegetation stress and a rule is envisaged to assess the degree of temporal changes. The
digital rule for assessment is initialized through the index of land Instability (ILI) where the variance indicates the temporal instability of the pixel I.e., smallest land unit. It is assumed that the biophysical characteristic of land is in command of land-dynamics where there is no change in Land Use/Land Cover (LU&LC). The intensity map on tendency of albedo (IALB) assesses the intensity of soil erosion and water stress whereas intensity map on tendency of NDVI (INDVI) appraises the stress on vegetation. The carry-out study covers a part of semiarid Western India. Primarily remote sensing technique, which carries the digital information of land temporally and spatially, is adopted in this paper. A part of the study area is represented using two sets of IRS 1A/lB LISS-I data of March with a decadal time domain (1989-1998) as a test area. It is assumed that the soil-water-vegetation stress is maximum during summer(March-April-May) in any tropical belt and decadal data will stretch the possibility of climate as well as man-made activity over the land. 相似文献
24.
中国干旱半干旱区农业生态地质环境系统工程研究 总被引:1,自引:0,他引:1
干旱半干旱区是当前我国农业经济发展最有潜力的地区。可是,该区生态环境脆弱,并受到一定程度的破坏。利用系统工程学的原理,从农业生态地质环境容量的角度出发,对该区的农业生态地质环境的演化过程和演化模式进行了研究,进而提出了农业生态地质环境系统工程研究模式。在调控社会人口和经济发展的基础上,合理开发利用水、土、生物资源,保护治理生态环境,提高自然资源环境的承载力,抑制对资源环境的破坏力,使该区农业生态环境得以恢复和重建,建立起稳定持续发展的农业基地。 相似文献
25.
26.
半干旱区植物篱侵蚀及养分控制过程的试验研究 总被引:8,自引:2,他引:8
针对目前国内外对于温带及半干旱地区植物篱的侵蚀及养分控制过程研究较少的状况,利用冀西北黄土丘陵区较为完备的试验小区的长年观测资料,并结合野外全坡面的大型人工模拟降雨试验,定量地分析黄土丘陵区坡面侵蚀分异规律,得到了在暴雨条件下细沟产生并导致侵蚀剧烈增加的临界坡长为10~15m,从而以此作为植物篱在该区域布设的理论依据。本研究模拟和分析了植物篱对养分流失拦蓄的形态、植物篱改变坡面侵蚀过程、控制细沟产生以及拦蓄泥沙而达到控制侵蚀及养分的目的,对于植物篱———农作复合农林技术在温带及半干旱地区的实践具有科学价值。 相似文献
27.
《水文科学杂志》2013,58(1)
Abstract Abstract The exploitation of an alluvial aquifer (2 × 106 m3) has been developed since 1998 in the Valley of Forquilha (Quixeramobim, State of Ceará). For this purpose, 165 wells were drilled along the 23 km of the valley that supplies 500 families and their farms. Monthly monitoring of piezometric and electrical conductivity (2000–2003) show seasonal variations in the water volume (35%) and in the mean value of the conductivity (800–1200 µS cm-1). A conceptual hydrogeological model was developed. Analysis of the data and simulations highlight that the recharge of the aquifer is mainly due to infiltration from the river in the rainy season, which is of the order of 1% of the rain over the catchment area (195 km2). The abstractions increase the recharge between 30 and 60%. The model makes it possible to propose scenarios of sustainable exploitation of the water resource in the catchment. For the period between 1970 and 1988, other simulations show that it would have been possible to maintain irrigated cultures on 75 ha for 80% of the time. During the remaining 20%, the water level is too low, and farmers would have to reduce the irrigated area. 相似文献
28.
Abstract Proper agricultural land management strategies improve soil structural properties, thereby reducing soil loss by water erosion. This study was conducted to estimate soil losses from plots of different agricultural land management using the Water Erosion Prediction Project (WEPP) (95.7) model. The study took place in a semiarid region in Kenya. The mean annual rainfall was 694 mm. The WEPP (95.7) model was initially used to estimate total sediment loading from the catchment into a reservoir. The estimate was about 2871 t corresponding to an average sedimentation rate of 1063 t km?2 year?1, which was about 76% of the measured total sediment inflow into the reservoir. Soil losses were estimated within 10 plots on the catchment of different sizes and slopes with the following treatments: conventional tillage (hand hoeing) with maize and soybean intercropping (HOCOBE); conservation tillage (disc plough) with maize and soybean intercropping (COBEAN); conservation tillage with only maize cultivation (CNTCORN); and conservation tillage with only soybean cultivation (CNTBEAN). The soil loss reduction of COBEAN, CNTCORN and CNTBEAN relative to HOCOBE ranged between 27–47%, 16–29% and 12–25%, respectively, depending on the size and slope of the plot. In general, conservation tillage reduced soil loss relative to conventional tillage. However, with conservation tillage, the single cropping system resulted in greater soil loss than the intercropping system. In the case of single cropping with conservation tillage, the soil loss reduction for maize ranged between 4 and 9%, relative to soybean. Overall, the study showed that there would be a significant reduction of soil losses from plots if conservation tillage with an intercropping system (maize and soybean) were to be adopted on agricultural lands in semiarid regions. 相似文献
29.
Esteban G. Jobbágy Santiago Lorenzo Nicolás Buono Ricardo Páez Yésica Diaz Victoria Marchesini Marcelo D. Nosetto 《水文研究》2021,35(5):e14188
Our understanding of how groundwater mediates evapotranspiration/streamflow partitioning is still fragmented and catchment studies under changing vegetation conditions can provide a useful frame for integration. We explored this partition in a flat sedimentary dry catchment in central Argentina in which the replacement of native vegetation with rainfed crops was accompanied by the abrupt formation of groundwater-fed streams by subsurface erosion (i.e., sapping) episodes. Historical records indicated widespread water table rises (~0.3 m y−1 on average). Groundwater level and stream baseflow fluctuated seasonally with minima in the warm rainy season, indicating that evaporative discharge rather than rainfall shapes saturated flows. Diurnal groundwater level fluctuations showed that plant uptake was widespread where water tables are shallow (<3 m) but restricted to deep-rooted Prosopis forests where they are deep (7–10 m). MODIS and LANDSAT NDVI revealed a long-term greening for native vegetation, new wetlands included, but not for croplands, suggesting more limited evapotranspiration-groundwater level regulation under agriculture. Close to the deepest (20 m) and most active incisions, groundwater level and greenness declined and stream baseflow showed no seasonal fluctuations, hinting decoupling from evapotranspiration. Intense ecological and geomorphological transformations in this catchment exposed the interplay of five mechanisms governing evapotranspiration/streamflow partition including (a) unsaturated uptake and both (b) riparian and (c) distributed uptake from the saturated zone by plants, as well as (d) deepening incisions and (e) sediment deposits over riparian zones by streams. Acknowledging the complex interplay of these mechanisms with groundwater is crucial to predict and manage future hydrological changes in the dry plains of South America. 相似文献
30.
Because of high spatial heterogeneity and the degree of uncertainty about hydrological processes in large‐scale catchments of semiarid mountain areas, satisfactory forecasting of daily discharge is seldom available using a single model in many practical cases. In this paper the Takagi–Sugeno fuzzy system (TS) and the simple average method (SAM) are applied to combine forecasts of three individual models, namely, the simple linear model (SLM), the seasonally based linear perturbation model (LPM) and the nearest neighbour linear perturbation model (NNLPM) for modelling daily discharge, and the performance of modelling results is compared in five catchments of semiarid areas. It is found that the TS combination model gives good predictions. The results confirm that better prediction accuracy can be obtained by combining the forecasts of different models with the Takagi–Sugeno fuzzy system in semi‐arid mountain areas. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献