In this paper, we present a noise removal technique by combining the P-M model with the LLT model. The combined technique takes full use of the advantage of both filters which is able to preserve edges and simultaneou...In this paper, we present a noise removal technique by combining the P-M model with the LLT model. The combined technique takes full use of the advantage of both filters which is able to preserve edges and simultaneously overcomes the staircase effect. We use a weighting function in our model, and compare this model with the P-M model as well as other fourth-order functional both in theory and numerical experiment.展开更多
The accurate estimation of evapotranspiration (ET) in arid regions is important for improving the water use efficiency of vegetation. Based on successive observations from May to October of 2014, we estimated the ET o...The accurate estimation of evapotranspiration (ET) in arid regions is important for improving the water use efficiency of vegetation. Based on successive observations from May to October of 2014, we estimated the ET of a Populus euphratica Oliv. forest during the growing season in an extremely arid region using the PM (Penman-Monteith), SW (Shuttleworth-Wallace) and SSW (an improved canopy transpiration model) models. Estimated ET values were compared with those of the eddy covariance measurements. Results indicated that the actual ET of the P. euphratica forest was always overestimated by the PM model. The accuracy of the SW model was higher than that of the PM model. However, some data were not easily obtained because of the complicated structure of the SW model. The newly proposed SSW model gave the most accurate ET values, and its accuracy was higher at hourly than at daily time scale. In conclusion, the SSW model is more suitable for sparse vegetation system at large scales in extremely arid regions.展开更多
Remote sensing tools are becoming increasingly important for providing spatial information on water use by different ecosystems. Despite significant advances in remote sensing based evapotranspiration(ET) models in re...Remote sensing tools are becoming increasingly important for providing spatial information on water use by different ecosystems. Despite significant advances in remote sensing based evapotranspiration(ET) models in recent years, important information gaps still exist on the accuracy of the models particularly in arid and semi-arid environments. In this study, we evaluated the Penman-Monteith based MOD16 and the modified Priestley-Taylor(PT-JPL) models at the daily time step against three measured ET datasets. We used data from two summer and one winter rainfall sites in South Africa. One site was dominated by native broad leaf and the other by fine leafed deciduous savanna tree species and C4 grasses. The third site was in the winter rainfall Cape region and had shrubby fynbos vegetation. Actual ET was measured using open-path eddy covariance systems at the summer rainfall sites while a surface energy balance system utilizing the large aperture boundary layer scintillometer was used in the Cape. Model performance varied between sites and between years with the worst estimates(R2<0.50 and RMSE>0.80 mm/d) observed during years with prolonged mid-summer dry spells in the summer rainfall areas. Sensitivity tests on MOD16 showed that the leaf area index, surface conductance and radiation budget parameters had the largest effect on simulated ET. MOD16 ET predictions were improved by:(1) reformulating the emissivity expressions in the net radiation equation;(2) incorporating representative surface conductance values;and(3) including a soil moisture stress function in the transpiration sub-model. Implementing these changes increased the accuracy of MOD16 daily ET predictions at all sites. However, similar adjustments to the PT-JPL model yielded minimal improvements. We conclude that the MOD16 ET model has the potential to accurately predict water use in arid environments provided soil water stress and accurate biome-specific parameters are incorporated.展开更多
Based on ASTER （Advanced Spaceborne Thermal Emission and Reflection Radiometer） remote sensing data, bare soil evaporation was estimated with the Penman-Monteith model, the Priestley-Taylor model, and the aerodynami...Based on ASTER （Advanced Spaceborne Thermal Emission and Reflection Radiometer） remote sensing data, bare soil evaporation was estimated with the Penman-Monteith model, the Priestley-Taylor model, and the aerodynamics model. Evaporation estimated by each of the three models was compared with actual evaporation, and error sources of the three models were analyzed. The mean absolute relative error was 9% for the Penman-Monteith model, 14% for the Priestley-Taylor model, and 32% for the aerodynamics model; the Penman-Monteith model was the best of these three models for estimating bare soil evaporation. The error source of the Penman-Monteith model is the neglect of the advection estimation. The error source of the Priestley-Taylor model is the simplification of the component of aerodynamics as 0.72 times the net radiation. The error source of the aerodynamics model is the difference of vapor pressure and neglect of the radiometric component. The spatial distribution of bare soil evaporation is evident, and its main factors are soil water content and elevation.展开更多
This paper, using a revised Penman-Monteith model, computed the terrestrial surface humidity index of the Loess Plateau （China） based on climatic factors of monthly mean temperature, maximum temperature, minimum tem...This paper, using a revised Penman-Monteith model, computed the terrestrial surface humidity index of the Loess Plateau （China） based on climatic factors of monthly mean temperature, maximum temperature, minimum temperature, relative humidity, precipitation, wind speed and sunshine duration observed on the plateau from 1961 to 2008. The temporal-spatial distribution, anomaly distribution and sub-regional temporal variations of the terrestrial surface dry and wet conditions were analyzed as well. The results showed a decreasing trend in the annual average surface humidity from the southeast to the northwest in the research anna. Over the period of 1961-2008, an aridification tendency appeared sharply in the central interior region of the Loess Plateau, and less sharply in the middle part of the region. The border region showed the weakest tendency ol; aridification. It is clear that aridification diffused in all directions from the interior region. The spatial anomaly distribution of the terrestrial surface dry and wet conditions on the Loess Plateau can be divided into three key areas： the southern, western and eastern regions. The terrestrial annual humidity index displayed a significantly descending trend and showed remarkable abrupt changes from wet to dry in the years 1967, 1977 and 1979. In the above mentioned three key areas for dry and wet conditions, the terrestrial annual humidity index exhibited a fluctuation period of 3-4 years, while in the southern region, a fluctuation period of 7-8 years existed at the same time.展开更多
Based on the data of monthly precipitation and other monthly meteorological elements of 661 meteorological stations over China from 1961 to 2013, the temporal evolution characteristics of aridity in Hetao area of Nort...Based on the data of monthly precipitation and other monthly meteorological elements of 661 meteorological stations over China from 1961 to 2013, the temporal evolution characteristics of aridity in Hetao area of North China which is drying significantly were studied by using REOF, and the effects of summer monsoon and meteorological factors on the aridity index were discussed. The results showed that climatic aridity in Hetac area of North China tended to increase with time during 1961 -2013. The annual variation and overall trend of climatic aridity in Hetao area of North China was mainly influenced by /SASM1 before the 1990s, and the degree of the influence weakened with global warming. There were certain differ- ences between annual and decadal variations in the effects of the meteorological elements on climatic aridity. The impact of the thermal factors on aridity index was more significant than the dynamic factor after the 1990s, revealing that climate warming aggravated climatic aridity in Hetao area of North China.展开更多
Nonlinear elastidty of the earth medium produces a numerical difference between the dynamic and the static modulus of rock. The dynamic modulus is calculated with the ultrasonic velocity measurement, the small-cycle m...Nonlinear elastidty of the earth medium produces a numerical difference between the dynamic and the static modulus of rock. The dynamic modulus is calculated with the ultrasonic velocity measurement, the small-cycle modulus is calculated with small cycles in the load-unload experiment, the static modulus is calculated from the slope of the stress-strain curve in the large cycle, the Young＇ s modulus is obtained from the ratio of stress to strain in the measured point. The relationship between the modulus and the strain amplitudes is studied by changing the strain amplitude in the small cycles. The moduli obtained from different measuring methods are thus compared. The result shows that the dynamic modulus is the largest, the small-cycle modulus takes the second place, the static modulus of bigger-cycle is the third, and finally the Young＇s modulus is the smallest. Nonlinear modulus of rock is a function of the strain level and strain amplitude. The modulus decreases exponentially with the ascending of the strain amplitude, while increases with the ascending of the strain level. Finally, the basic concept of the P-M model is briefly introduced and the relationship between the modulus and strain amplitude is explained by the rock having different distribution densities and the different openand-close stresses of the micro-cracks.展开更多
Based on the meteorological data of 20 stations in the Hengduan Mountains region during 1961–2009, the annual and seasonal variation of potential evapotranspiration was analyzed in combination with the Penman-Monteit...Based on the meteorological data of 20 stations in the Hengduan Mountains region during 1961–2009, the annual and seasonal variation of potential evapotranspiration was analyzed in combination with the Penman-Monteith model. With the method of Spline interpolation under ArcGIS, the spatial distribution of potential evapotranspiration was presented to research the regional difference, and the correlation analysis was used to discuss the dominant factor affecting the potential evapotranspiration. The results indicated that the an-nual potential evapotranspiration showed a decreasing tendency since the 1960s, especially from the 1980s to 1990s, while it showed an increasing tendency since 2000. Regional potential evapotranspiration showed a rate of –0.17 mm a？1. Potential evapotranspiration in north, middle and south of the Hengduan Mountains exhibited decreasing trends over the studied period, and its regional trend was on the decline from southwest to northeast.展开更多
Evaporation acts as an important component and a key control factor in land hydrological processes.In order to analyze the trend of change on potential evapotranspiration from 1961 to 2013 and to discuss the existence...Evaporation acts as an important component and a key control factor in land hydrological processes.In order to analyze the trend of change on potential evapotranspiration from 1961 to 2013 and to discuss the existence of the evaporation paradox in Jiangxi province,China,monthly meteorological data spanning the years 1961–2013 were analyzed in this study,where the data were collected from 15 national meteorological stations in Jiangxi Province.The Penman–Monteith equation was employed to compute the potential evapotranspiration(ET0).Spatial interpolation and data mining technology were used to analyze the spatial and temporal changes of ET0 and air temperature,with the effort to explain the evaporation paradox.By solving the total differential and the partial derivatives coefficients of the independent variables in Penman–Monteith equation,the cause of the paradox was quantitatively evaluated.The results showed that the annual ET0 had been decreasing significantly in Jiangxi Province since 1979,whereas the air temperature had been rising significantly,presenting the evaporation paradox.The decreases in sunshine duration and wind speed reduced ET0 by 0.207 mm and 0.060 mm,respectively,accounting for 92.3%and 26.7%of the total ET0,respectively.It is concluded that sunshine duration and wind speed are the main causes to the decrease in potential evapotranspiration in Jiangxi Province.展开更多
文摘In this paper, we present a noise removal technique by combining the P-M model with the LLT model. The combined technique takes full use of the advantage of both filters which is able to preserve edges and simultaneously overcomes the staircase effect. We use a weighting function in our model, and compare this model with the P-M model as well as other fourth-order functional both in theory and numerical experiment.
基金supported by the National Natural Science Foundation of China(41271037)the Youth Foundation of National Natural Science of China(41401033)
文摘The accurate estimation of evapotranspiration (ET) in arid regions is important for improving the water use efficiency of vegetation. Based on successive observations from May to October of 2014, we estimated the ET of a Populus euphratica Oliv. forest during the growing season in an extremely arid region using the PM (Penman-Monteith), SW (Shuttleworth-Wallace) and SSW (an improved canopy transpiration model) models. Estimated ET values were compared with those of the eddy covariance measurements. Results indicated that the actual ET of the P. euphratica forest was always overestimated by the PM model. The accuracy of the SW model was higher than that of the PM model. However, some data were not easily obtained because of the complicated structure of the SW model. The newly proposed SSW model gave the most accurate ET values, and its accuracy was higher at hourly than at daily time scale. In conclusion, the SSW model is more suitable for sparse vegetation system at large scales in extremely arid regions.
基金supported by the South African Parliamentary Grant to the Council for Scientific and Industrial Research Project (ECHS014, EEEO024, ECHS058 and ECHS052)
文摘Remote sensing tools are becoming increasingly important for providing spatial information on water use by different ecosystems. Despite significant advances in remote sensing based evapotranspiration(ET) models in recent years, important information gaps still exist on the accuracy of the models particularly in arid and semi-arid environments. In this study, we evaluated the Penman-Monteith based MOD16 and the modified Priestley-Taylor(PT-JPL) models at the daily time step against three measured ET datasets. We used data from two summer and one winter rainfall sites in South Africa. One site was dominated by native broad leaf and the other by fine leafed deciduous savanna tree species and C4 grasses. The third site was in the winter rainfall Cape region and had shrubby fynbos vegetation. Actual ET was measured using open-path eddy covariance systems at the summer rainfall sites while a surface energy balance system utilizing the large aperture boundary layer scintillometer was used in the Cape. Model performance varied between sites and between years with the worst estimates(R2<0.50 and RMSE>0.80 mm/d) observed during years with prolonged mid-summer dry spells in the summer rainfall areas. Sensitivity tests on MOD16 showed that the leaf area index, surface conductance and radiation budget parameters had the largest effect on simulated ET. MOD16 ET predictions were improved by:(1) reformulating the emissivity expressions in the net radiation equation;(2) incorporating representative surface conductance values;and(3) including a soil moisture stress function in the transpiration sub-model. Implementing these changes increased the accuracy of MOD16 daily ET predictions at all sites. However, similar adjustments to the PT-JPL model yielded minimal improvements. We conclude that the MOD16 ET model has the potential to accurately predict water use in arid environments provided soil water stress and accurate biome-specific parameters are incorporated.
基金supported by the Ministry of Water Resources (Grants No. 200701039 and 200801001)the National Technology Supporting Program (Grants No. 2006BAC05B02 and 2007BAC03A060301)
文摘Based on ASTER （Advanced Spaceborne Thermal Emission and Reflection Radiometer） remote sensing data, bare soil evaporation was estimated with the Penman-Monteith model, the Priestley-Taylor model, and the aerodynamics model. Evaporation estimated by each of the three models was compared with actual evaporation, and error sources of the three models were analyzed. The mean absolute relative error was 9% for the Penman-Monteith model, 14% for the Priestley-Taylor model, and 32% for the aerodynamics model; the Penman-Monteith model was the best of these three models for estimating bare soil evaporation. The error source of the Penman-Monteith model is the neglect of the advection estimation. The error source of the Priestley-Taylor model is the simplification of the component of aerodynamics as 0.72 times the net radiation. The error source of the aerodynamics model is the difference of vapor pressure and neglect of the radiometric component. The spatial distribution of bare soil evaporation is evident, and its main factors are soil water content and elevation.
基金supported by the National Basic Research Program of China (2012CB955903,2012CB955304)the Special Fund for Public Welfare Industry(GYHY201106029,GYHY200806021)+2 种基金the National Natural Science Foundation of China (40830957)the China Meteorological Administration Special Program for Climatic Change(280200S011C00)the Drought Meteorology Science Research Program (IAM201111)
文摘This paper, using a revised Penman-Monteith model, computed the terrestrial surface humidity index of the Loess Plateau （China） based on climatic factors of monthly mean temperature, maximum temperature, minimum temperature, relative humidity, precipitation, wind speed and sunshine duration observed on the plateau from 1961 to 2008. The temporal-spatial distribution, anomaly distribution and sub-regional temporal variations of the terrestrial surface dry and wet conditions were analyzed as well. The results showed a decreasing trend in the annual average surface humidity from the southeast to the northwest in the research anna. Over the period of 1961-2008, an aridification tendency appeared sharply in the central interior region of the Loess Plateau, and less sharply in the middle part of the region. The border region showed the weakest tendency ol; aridification. It is clear that aridification diffused in all directions from the interior region. The spatial anomaly distribution of the terrestrial surface dry and wet conditions on the Loess Plateau can be divided into three key areas： the southern, western and eastern regions. The terrestrial annual humidity index displayed a significantly descending trend and showed remarkable abrupt changes from wet to dry in the years 1967, 1977 and 1979. In the above mentioned three key areas for dry and wet conditions, the terrestrial annual humidity index exhibited a fluctuation period of 3-4 years, while in the southern region, a fluctuation period of 7-8 years existed at the same time.
基金Supported by the State Key Development Program for Basic Research of China(2013CB430200)
文摘Based on the data of monthly precipitation and other monthly meteorological elements of 661 meteorological stations over China from 1961 to 2013, the temporal evolution characteristics of aridity in Hetao area of North China which is drying significantly were studied by using REOF, and the effects of summer monsoon and meteorological factors on the aridity index were discussed. The results showed that climatic aridity in Hetac area of North China tended to increase with time during 1961 -2013. The annual variation and overall trend of climatic aridity in Hetao area of North China was mainly influenced by /SASM1 before the 1990s, and the degree of the influence weakened with global warming. There were certain differ- ences between annual and decadal variations in the effects of the meteorological elements on climatic aridity. The impact of the thermal factors on aridity index was more significant than the dynamic factor after the 1990s, revealing that climate warming aggravated climatic aridity in Hetao area of North China.
基金The project was funded by the National Natural ScienceFoundation,China (Item No.40274057) .
文摘Nonlinear elastidty of the earth medium produces a numerical difference between the dynamic and the static modulus of rock. The dynamic modulus is calculated with the ultrasonic velocity measurement, the small-cycle modulus is calculated with small cycles in the load-unload experiment, the static modulus is calculated from the slope of the stress-strain curve in the large cycle, the Young＇ s modulus is obtained from the ratio of stress to strain in the measured point. The relationship between the modulus and the strain amplitudes is studied by changing the strain amplitude in the small cycles. The moduli obtained from different measuring methods are thus compared. The result shows that the dynamic modulus is the largest, the small-cycle modulus takes the second place, the static modulus of bigger-cycle is the third, and finally the Young＇s modulus is the smallest. Nonlinear modulus of rock is a function of the strain level and strain amplitude. The modulus decreases exponentially with the ascending of the strain amplitude, while increases with the ascending of the strain level. Finally, the basic concept of the P-M model is briefly introduced and the relationship between the modulus and strain amplitude is explained by the rock having different distribution densities and the different openand-close stresses of the micro-cracks.
基金National Basic Research Program of China,No.2010CB951404National Basic Research Program of China,No.2007CB411501+5 种基金 National Natural Science Foundation of China （NSFC）,No.40971019 No.90511007 No.40801028 NSFC,No.J0630966 Major Directionality Program of the Chinese Academy of Sciences,No.KZCXZ-YW-317 West Light Foundation of the Chinese Academy of Sciences,No.O828A11001
文摘Based on the meteorological data of 20 stations in the Hengduan Mountains region during 1961–2009, the annual and seasonal variation of potential evapotranspiration was analyzed in combination with the Penman-Monteith model. With the method of Spline interpolation under ArcGIS, the spatial distribution of potential evapotranspiration was presented to research the regional difference, and the correlation analysis was used to discuss the dominant factor affecting the potential evapotranspiration. The results indicated that the an-nual potential evapotranspiration showed a decreasing tendency since the 1960s, especially from the 1980s to 1990s, while it showed an increasing tendency since 2000. Regional potential evapotranspiration showed a rate of –0.17 mm a？1. Potential evapotranspiration in north, middle and south of the Hengduan Mountains exhibited decreasing trends over the studied period, and its regional trend was on the decline from southwest to northeast.
基金This research was supported by the 2015 Water Science and Technology Planning Project of Jiangxi Province,China(KT201540)the Foundation of Key Laboratory of Soil Erosion and Control of Jiangxi Province,China(JXSB201202)The meteorological data of this paper was supported by the China Meteorological Science Data Sharing Service Network.
文摘Evaporation acts as an important component and a key control factor in land hydrological processes.In order to analyze the trend of change on potential evapotranspiration from 1961 to 2013 and to discuss the existence of the evaporation paradox in Jiangxi province,China,monthly meteorological data spanning the years 1961–2013 were analyzed in this study,where the data were collected from 15 national meteorological stations in Jiangxi Province.The Penman–Monteith equation was employed to compute the potential evapotranspiration(ET0).Spatial interpolation and data mining technology were used to analyze the spatial and temporal changes of ET0 and air temperature,with the effort to explain the evaporation paradox.By solving the total differential and the partial derivatives coefficients of the independent variables in Penman–Monteith equation,the cause of the paradox was quantitatively evaluated.The results showed that the annual ET0 had been decreasing significantly in Jiangxi Province since 1979,whereas the air temperature had been rising significantly,presenting the evaporation paradox.The decreases in sunshine duration and wind speed reduced ET0 by 0.207 mm and 0.060 mm,respectively,accounting for 92.3%and 26.7%of the total ET0,respectively.It is concluded that sunshine duration and wind speed are the main causes to the decrease in potential evapotranspiration in Jiangxi Province.
