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991.
Sabine Douxchamps François-Lionel Humbert Rein van der Hoek Martin Mena Stefano M. Bernasconi Axel Schmidt Idupulapati Rao Emmanuel Frossard Astrid Oberson 《Nutrient Cycling in Agroecosystems》2010,88(3):447-462
Canavalia brasiliensis (canavalia), a drought tolerant legume, was introduced into the smallholder traditional crop-livestock production system of the Nicaraguan hillsides as green manure to improve soil fertility or as forage during the dry season for improving milk production. Since nitrogen (N) is considered the most limiting nutrient for agricultural production in the target area, the objective of this study was to quantify the soil surface N budgets at plot level in farmers fields over two cropping years for the traditional maize/bean rotation and the alternative maize/canavalia rotation. Mineral fertilizer N, seed N and symbiotically fixed N were summed up as N input to the system. Symbiotic N2 fixation was assessed using the 15N natural abundance method. Nitrogen output was quantified as N export via harvested products. Canavalia derived in average 69% of its N from the atmosphere. The amount of N fixed per hectare varied highly according to the biomass production, which ranged from 0 to 5,700 kg ha?1. When used as green manure, canavalia increased the N balance of the maize/canavalia rotation but had no effect on the N uptake of the following maize crop. When used as forage, it bears the risk of a soil N depletion up to 41 kg N ha?1 unless N would be recycled to the plot by animal manure. Without N mineral fertilizer application, the N budget remains negative even if canavalia was used as green manure. Therefore, the replenishment of soil N stocks by using canavalia may need a few years, during which the application of mineral N fertilizer needs to be maintained to sustain agricultural production. 相似文献
992.
A three-dimensional zinc arsenate with an interrupted zeolitic framework (?IIO), [C4N3H16]2[Zn5(AsO4)4(HAsO4)2], I has been synthesized solvothermally. The structure is built up from ZnO4, AsO4 and HAsO4 tetrahedral units connected alternatively through their vertices forming the 3-D structure possessing one-dimensional channels bound by 10 T-atoms (T = Zn, As). The framework density of the structure is 10.4 T-atoms which indicates considerable openness in its structure. 相似文献
993.
Wardah Fatimah Mohammad Yusoff Elias Salleh Nor Mariah Adam Abdul Razak Sapian Mohamad Yusof Sulaiman 《Building and Environment》2010
In the hot and humid climate, stack ventilation is inefficient due to small temperature difference between the inside and outside of naturally ventilated buildings. Hence, solar induced ventilation is a feasible alternative in enhancing the stack ventilation. This paper aims to investigate the effectiveness of a proposed solar induced ventilation strategy, which combines a roof solar collector and a vertical stack, in enhancing the stack ventilation performance in the hot and humid climate. The methodology selected for the investigation is physical experimental modelling which was carried out in the actual environment. The results are presented and discussed in terms of two performance variables: air temperature and air velocity. The findings indicate that the proposed strategy is able to enhance the stack ventilation, both in semi-clear sky and overcast sky conditions. The highest air temperature difference between the air inside the stack and the ambient air (Ti−To) is achieved in the semi-clear sky condition, which is about 9.9 °C (45.8 °C–35.9 °C). Meanwhile, in the overcast sky condition, the highest air temperature difference (Ti−To) is 6.2 °C (39.3 °C–33.1 °C). The experimental results also indicate good agreement with the theoretical results for the glass temperature, the air temperature in the roof solar collector’s channel and the absorber temperature. The findings also show that wind has significant effect to the induced air velocity by the proposed strategy. 相似文献
994.
This paper presents the simulation of pressurized underfill encapsulation process for high I/O flip chip package. 3D model of flip chip packages is built using GAMBIT and simulated using FLUENT software. Injection methods such as central point, one line, L-type and U-type are studied. Cross-viscosity model and volume of fluid (VOF) technique are applied for melt front tracking of the encapsulant. The melt front profiles and pressure field for all injection types are analyzed and presented. The pressure distribution within the flip-chip, fill volume versus filling time and viscosity versus shear rate are also plotted. The U-type injection is found to be faster in filling. The numerical results are compared with the previous experimental results and found in good conformity. The strength of CFD software in handling underfill encapsulation problems is proved to be excellent. 相似文献
995.
Multiuser MIMO OFDM Based TDD/TDMA for Next Generation Wireless Communication Systems 总被引:1,自引:1,他引:0
Multiple-input multiple-output (MIMO) wireless technology in combination with orthogonal frequency division multiplexing (MIMO
OFDM) is an attractive air-interface solution for next-generation wireless local area networks (WLANs), wireless metropolitan
area networks (WMANs), and fourth-generation mobile cellular wireless systems. In this paper, one multiuser MIMO OFDM systems
with TDD/TDMA was proposed for next-generation wireless mobile communications, i.e., TDD/TDMA 4G, which can avoid or alleviate
the specific limitations of existing techniques designed for multiuser MIMO OFDM systems in broadband wireless mobile channel
scenarios, i.e., bad performance and extreme complexity of multiuser detectors for rank-deficient multiuser MIMO OFDM systems
with CDMA as access modes, extreme challenges of spatial MIMO channel estimators in rank-deficient MIMO OFDM systems, and
exponential growth complexity of optimal sub-carrier allocations for OFDMA-based MIMO OFDM systems. Furthermore, inspired
from the Steiner channel estimation method in multi-user CDMA uplink wireless channels, we proposed a new design scheme of
training sequence in time domain to conduct channel estimation. Training sequences of different transmit antennas can be simply
obtained by truncating the circular extension of one basic training sequence, and the pilot matrix assembled by these training
sequences is one circular matrix with good reversibility. A novel eigenmode transmission was also given in this paper, and
data symbols encoded by space–time codes can be steered to these eigenmodes similar to MIMO wireless communication systems
with single-carrier transmission. At the same time,, an improved water-filling scheme was also described for determining the
optimal transmit powers for orthogonal eigenmodes. The classical water-filling strategy is firstly adopted to determine the
optimal power allocation and correspondent bit numbers for every eigenmode, followed by a residual power reallocation to further
determine the additional bit numbers carried by every eigenmode. Compared with classical water-filling schemes, it can also
obtain larger throughputs via residual power allocation. At last, three typical implementation schemes of multiuser MIMO OFDM
with TDMA, CDMA and OFDMA, i.e., TDD/TDMA 4G, VSF-OFCDM and FuTURE B3G TDD, were tested by numerical simulations. Results
indicated that the proposed multiuser MIMO OFDM system schemes with TDD/TDMA, i.e., TDD/TDMA 4G, can achieve comparable system
performance and throughputs with low complexity and radio resource overhead to that of DoCoMo MIMO VSF-OFCDM and FuTURE B3G
TDD. 相似文献
996.
3G时代,电信运营商经营的网络建设范围向移动+固定网络,2G+3G网络发展,网络结构愈加复杂。电信运营商投资控制难度不断加大,投资风险也逐渐升高。电信运营商需要加强投资风险管理意识.建立一套从规划层面到项目管理、贯穿投资前中后各个环节的投资风险管理体系,从而降低整体投资风险,提升投资效率。 相似文献
997.
998.
通过建立月降雨径流模型,采用月降雨量推求月径流量,并考虑了与产生径流量有关的气象要素和流域相关参数,对帕卡特水电站的设计径流量进行了计算。 相似文献
999.
西枝江水利枢纽工程建设水资源论证有关内容分析 总被引:1,自引:0,他引:1
对西枝江水利枢纽工程工程区域水资源时空分布特点及开发利用现状进行了分析,并对工程的取水地点、取水量、取水方式及用水、退水方式进行了研究,分析了该工程的用水和退水对周边环境和其他用水户的影响,论证了工程开发取水的合理性,并提出水资源保护措施和建议。 相似文献
1000.
Muhammad Masood Ahmad Abdul Razzaq Ghumman Sajjad Ahmad Hashim Nisar Hashmi 《Water Resources Management》2010,24(12):2971-2989
Water resources planning and management requires hydrologic models to estimate runoff from a catchment. For catchments with
limited data, the choice of model and identification of its parameters is very important for development of a direct runoff
hydrograph. A method is presented to determine a unique pair of hydrologic parameters of the Nash Model, number of linear
cascade (n) and storage coefficient (k), using optimization based on Downhill Simplex technique. In this study physical parameters
of the catchment are derived from (SPOT) satellite imageries of the basin using ERDAS software. Four different objective functions
of varying complexity are tested to find the best solution. Weighted root mean square error (RMSE) and Model Efficiency (Nash-Sutcliffe
coefficient) are used to evaluate the model performance. Using the NASH model, a direct surface runoff hydrograph (DSRH) is
developed. Kaha catchment is part of Indus river system and is located in the semi-arid region of Pakistan. This catchment
is dominated by hill torrent flows and is used in this work to demonstrate the applicability of the proposed method. Ten randomly
selected rainfall-runoff events are used for calibration and five events are used for validation. Model results during validation
are very promising with model efficiency exceeding 93% and error in peak discharge under 8%. The sensitivity of the Nash model
output in response to variation in hydrologic parameters n and k is also investigated. When evaluating the hydrologic response
of large catchments, model output is more sensitive to n as compared to k indicating that the runoff diffusion phenomenon
is dominant compared to translation flow effects. 相似文献