The frost-free period (FFP), first frost date (FFD) and last frost date (LFD) have been regard as the important climate variables for agricultural production. Understanding the spatio-temporal variations of the FFP, FFD and LFD is beneficial to reduce the harmful impacts of climate change on agricultural production and enhance the agricultural adaptation. This study examined daily minimum temperatures for 823 national-level meteorological stations, calculated the values of FFD, LFD and FFP for station-specific and region-specific from 1951 to 2012, estimated the gradients of linear regression for station-specific moving averages of FFD, LFD and FFP, and assessed station-specific time series of FFP and detected the abrupt change. The results as follows: at both the station level and the regional level, the FFP across China decreases with the increase of latitude from south to north, and with the increase of altitude from east to west generally. At the station level, the inter-annual fluctuations of FFD, LFD and FFP in south and west agricultural regions are greater than those in north and east. At the regional level, excluding the QT region, temporal changes of FFP are relatively small in both the low-latitude and the high-latitude regions, but for the mid-latitude regions. According to the linear trend gradients of the moving average values of station-specific FFD, LFD and FFP, FFD was delayed, LFD advanced, and FFP extended gradually over the 80% of China. Furthermore, the change magnitudes for FFD, LFD and FFP in the north and east agricultural regions are higher than that in the southern and western. Among the 659 station-specific time series of FFP examined by the Mann-Kendall test, 341 stations, located mainly in the north region, have one identifiable and significant abrupt change. And at the 341 stations with identified abrupt changes, most (57%) abrupt changes occurred during 1991-2012, followed by the periods of 1981-1990 (28%), 1971-1980 (12%), and 1951-1970 (3%). The spatio-temporal variations of FFD, LFD and FFP would provide 24 Journal of Geographical Sciences important guidance to agricultural practices.
The Huang-Huai-Hai Plain is the major crop-producing region in China. Based on the climate and socio-economic data from 1995 to 2018, we analyzed the spatial–temporal characteristics in grain production and its influencing factors by using exploratory spatial data analysis, a gravity center model, a spatial panel data model, and a geographically weighted regression model. The results indicated the following: (1) The grain production of eastern and southern areas was higher, while that of western and northern areas was lower; (2) The grain production center in the Huang-Huai-Hai Plain shifted from the southeast to northwest in Tai’an, and was distributed stably at the border between Jining and Tai’an; (3) The global spatial autocorrelation experienced a changing process of “decline–growth–decline”, and the area of hot and cold spots was gradually reduced and stabilized, which indicated that the polarization of grain production in local areas gradually weakened and the spatial difference gradually decreased in the Huang-Huai-Hai Plain; (4) The impact of socio-economic factors has been continuously enhanced while the role of climate factors in grain production has been gradually weakened. The ratio of the effective irrigated area, the amount of fertilizer applied per unit sown area, and the average per capita annual income of rural residents were conducive to the increase in grain production in the Huang-Huai-Hai Plain; however, the effect of the annual precipitation on grain production has become weaker. More importantly, the association between the three factors and grain production was found to be spatially heterogeneous at the local geographic level.
Based on the daily observation data of 824 meteorological stations during 1951− 2010 released by the National Meteorological Information Center, this paper evaluated the changes in the heat and moisture conditions of crop growth. An average value of ten years was used to analyze the spatio-temporal variation in the agricultural hydrothermal conditions within a 1 km 2 grid. Next, the inter-annual changing trend was simulated by regression analysis of the agricultural hydrothermal conditions. The results showed that the contour lines for temperature and accumulated temperatures (the daily mean temperature ≥0°C) increased significantly in most parts of China, and that the temperature contour lines had all moved northwards over the past 60 years. At the same time, the annual precipitation showed a decreasing trend, though more than half of the meteorological stations did not pass the significance test. However, the mean temperatures in the hottest month and the coldest month exhibited a decreasing trend from 1951 to 2010. In addition, the 0°C contour line gradually moved from the Qinling Mountains and Huaihe River Basin to the Yellow River Basin. All these changes would have a significant impact on the distribution of crops and farming systems. Although the mechanisms influencing the interactive temperature and precipitation changes on crops were complex and hard to distinguish, the fact remained that these changes would directly cause corresponding changes in crop characteristics.
This paper investigates model-checking Needham-Schroeder Public-Keyprotocol using Propositio nal Projection Temporal Logic (PPTL). To this end, the ProMeLa model of the protocol is firstly constructed then the properties to verify is specified by PPTL formulas, which is translated into automata and further to Never Claim. The transformation is by the method we present and is implemented by an automatic tool we developed. After that, the verification is done base on SPIN, and the results shows PPTL model checking approach is sound and can be used to verify more generalized communication protocols.
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