共查询到20条相似文献，搜索用时 46 毫秒
1.
Evapotranspiration is an important flux term in the water cycle that integrates atmospheric demand and surface conditions. Using the FAO Penman–Monteith method, we calculated monthly reference evapotranspiration (ET0) for 119 stations during 1961–2004 over Yunnan Province (YP), southwest China. Linear trend analysis shows that area-averaged annual and seasonal ET0 rates declined, with most remarkable decreases during pre-monsoon (?1.5 mm decade ?1, Mar–May) and monsoon (?0.6 mm decade ?1, Jun–Aug) seasons. Most of the stations with negative trends were concentrated in the eastern and northern parts of YP. Over the 44–year period, wind speed (WS), relative sunshine duration (SD) and relative humidity (RH) all showed decreasing trends, whereas maximum temperature (TMX) increased slightly. Multivariate regression analysis indicated that the variability of ET0 rates is most sensitive to the variations of SD, followed by RH, TMX and WS. The temporal evolution of these contributing factors was not stable during the study period, with an increasing contribution of SD and a decreasing contribution of TMX after the 1970s. Temporally changing contributions of climatic variables to ET0 should be taken into account when evapotranspiration rates are calculated with equations that rely on parameterization of climatic variables. Linking the changing contributions of climatic variables to ET0 rates to circulation features may help to better understand how ET0 responds to regional climatic change. 相似文献
2.
基于机器学习方法和多源数据构建高精度蒸散发（Evapotranspiration,ET）产品对研究气候变化背景下干旱、半干旱地区陆地水循环变化具有重要意义。本文利用西北地区12个草地通量站点与卫星遥感产品,基于随机森林、极端梯度提升、支持向量回归和人工神经网络4种机器学习方法构建ET估算模型,制作5 km分辨率ET产品,并分析ET的长期变化趋势。交叉验证结果表明,4种模型的均方根误差都低于0.57 mm·d -1, R2高达0.73～0.88。SHAP （SHapley Additive exPlanation）可解释性分析表明,4种模型均将净辐射、植被和土壤湿度作为ET估算的重要因子,也能刻画出土壤偏干时土壤水分对ET的限制作用,有较好的物理解释性。多模型集合的ET结果相比单一机器学习模型以及现有遥感产品误差分别降低7％～20％和45％～70％。趋势分析结果显示,西北地区非裸地下垫面在2001—2018年间整体呈现ET增加趋势,平均速率为19 mm/（10 a）。在河套平原和内蒙古中部和东北部地区,ET的增长速率超过降水,这可能会进一步加剧这些地区的干旱化。 相似文献
3.
利用1961~2011年江淮地区5~9月无缺测的71站逐日降水资料,做基于POT(Peaks-Over-Threshold)的广义Pareto分布(GPD),研究江淮地区极端降水的分布特征及其变化趋势。结果表明:(1)皖赣交界处阈值最大,西北和东南部较小,且江淮大部分地区阈值的线性趋势系数为正,其中湖北东部和江西北部的站点,趋势达0.8 mm(10 a)-1以上,并通过了显著性水平0.01的MK(Mann-Kendall)检验。(2)江淮地区中东部多存在连续性极端降水,因此文中采用基于极值指数的自动分串技术获得近似独立的极值样本。(3)尺度参数大值区位于江淮南部,西北、东南以及淮河以北较小,且线性趋势系数在大部分地区均表现为正值,表明出现降水极大值的概率增加。(4)皖赣鄂交界处是极端降水发生概率大值区,而西北、东南及安徽中部地区较小,且极端降水在大部分地区有增加的趋势,特别是在大别山附近及河南东部,2年和20年重现水平的趋势分别达6 mm(10 a)-1和20 mm(10 a)-1以上。 相似文献
4.
Sunshine duration data are desirable for calculating daily solar radiation ( R s) and subsequent reference evapotranspiration (ET 0) using the Penman–Monteith (PM) method. In the absence of measured R s data, the Ångström equation has been recommended by the Food and Agriculture Organization (FAO) of the United Nations. This equation requires actual sunshine duration that is not commonly observed at many weather stations. This paper examines the potential for the use of artificial neural networks (ANNs) to estimate sunshine duration based on air temperature and humidity data under arid environment. This is important because these data are commonly available parameters. The impact of the estimated sunshine duration on estimation of R s and ET 0 was also conducted. The four weather stations selected for this study are located in Sistan and Baluchestan Province (southeast of Iran). The study demonstrated that modelling of sunshine duration through the use of ANN technique made acceptable estimates. Models were compared using the determination coefficient ( R 2), the root mean square error (RMSE) and the mean bias error (MBE). Average R 2, RMSE and MBE for the comparison between measured and estimated sunshine duration were calculated resulting 0.81, 6.3 % and 0.1 %, respectively. Our analyses also demonstrate that the difference between the measured and estimated sunshine duration has less effect on the estimated R s and ET 0 by using Ångström and FAO-PM equations, respectively. 相似文献
5.
Daily precipitation records of 267 European rain gauges are considered to obtain dry spell length (DSL) series along the second half of the twentieth century. A dry spell consists of consecutive days with daily rain amount below a given threshold, R 0. Four DSL series are obtained for R 0 values equal to 0.1, 1.0, 5.0, and 10.0 mm/day, and their empirical distributions are properly fitted to different statistical models: Pearson type III (PE3), Weibull (WEI), generalised Pareto, (GPA) and lognormal distributions. The parameters of every model are estimated by L-moments, and the goodness of fit is assessed by quantifying discrepancies between empirical and theoretical distributions in the L-skewness–kurtosis diagrams. The most common best-fitting model is PE3, especially for 0.1 and 1.0 mm/day. Nevertheless, a few stations in southern Europe are better modelled by the WEI distribution. For 5.0 and 10.0 mm/day, the spatial distribution of the best-fitting model is more heterogeneous than for the lowest thresholds. While PE3 is still the preferred model for Western Europe, some DSL series are better fitted to WEI or GPA models. Maps of DSL average and standard deviation and expected lengths for return periods of 2, 5, 10, 25, and 50 years show some common features. Whereas for thresholds of 0.1 and 1.0 mm/day, a N–S gradient is detected, especially in Mediterranean areas; for 5.0 and 10.0 mm/day, a NW–SE gradient is observed in the Iberian Peninsula and a SW–NE gradient in the Scandinavian Peninsula. Then, the vicinity to Atlantic and Arctic Oceans and the Mediterranean Sea, as well as orographic features, are more determining factors than the latitude in patterns associated with the highest R 0 thresholds. Finally, a regional frequency analysis based on a clustering algorithm is attempted for the four thresholds R 0, with the PE3 model as the parent distribution for the largest clusters. 相似文献
6.
During the April-June raining season,warm-sector heavy rainfall(WR) and frontal heavy rainfall(FR) often occur in the south of China,causing natural disasters.In this study,the microphysical characteristics of WR and FR events from 2016 to 2022 are analyzed by using 2-dimensional video disdrometer(2DVD) data in the south of China.The microphysical characteristics of WR and FR events are quite different.Compared with FR events,WR events have higher concentration of D<5.3 mm(especially D <1 ... 相似文献
7.
The extreme daily precipitation in Serbia was examined at 16 stations during the period 1961–2014. Two synoptic situations in May and September of 2014 were analysed, when extreme precipitation was recorded in western and eastern Serbia, respectively. The synoptic situation from 14 to 16 May 2014 remained nearly stationary over the western and central Serbia for the entire period. On 15 May 2014, the daily rainfall broke previous historical records in Belgrade (109.8 mm), Valjevo (108.2 mm) and Loznica (110 mm). Precipitation exceeded 200 mm in 72 h, producing the most catastrophic floods in the recent history of Serbia. In Negotin (eastern Serbia), daily precipitation of 161.3 mm was registered on 16 September 2014, which was the maximum value recorded during the period 1961–2014. The daily maximum in 2014 was registered at 6 out of 16 stations. The total annual precipitation for 2014 was the highest for the period 1961–2014 at almost all stations in Serbia. A non-significant positive trend was found for all precipitation indices: annual daily maximum precipitation, the total precipitation in consecutive 3 and 5 days, the total annual precipitation, and number of days with at least 10 and 20 mm of precipitation. The generalised extreme value distribution was fitted to the annual daily maximum precipitation. The estimated 100-year return levels were 123.4 and 147.4 mm for the annual daily maximum precipitation in Belgrade and Negotin, respectively. 相似文献
8.
This paper deals with the most recent trends in meteorological and hydrological variables, which include air temperature and precipitation ( P), potential and actual (ET) evapotranspiration, surface runoff (RO), water recharge into the soil ( R) and water loss from the soil ( L). Most hydrological variables were calculated via Palmer's algorithm. For this purpose, two rank-based statistical tests (the Mann?CKendall (MK) and a change-point analysis (CPA) approach) and the basic linear regression-based model were applied on the weekly precipitation and temperature from 17 stations all over Greece, during 1961?C2006. Only in winter, all variables except for R, which showed no clear signal, presented downward trends. The declining trends of P and L in spring and summer were counterbalanced by reductions in RO (and R in the case of summer) as opposed to increases in ET. In autumn, the declining tendencies of P and L were offset by RO reductions and R increases. Annually, the trends in water cycle components were analogous to that of spring, summer and autumn. The number of stations with statistically significant (at 95%) trends greatly varied with season and meteorological/hydrological variable. 相似文献
9.
Water loss by evapotranspiration (ET) is a principal component of the hydrologic cycle in wetlands. Using micrometeorological techniques, we measured ET from a Sphagnum-dominated open fen in northcentral Minnesota (U.S.A.) from May to October in 1991 and 1992. The daily ET rate ranged from 0.2–4.8 mm d -1 with a growing season average of 3.0 mm d -1. The evapotranspiration rate of the fen was near the potential rate of open water evaporation when the vascular plants were actively growing and the water table level was within or above the rooting zone. Using a dual-source modification of the Penman-Monteith equation (Massman, 1992), we partitioned the measured ET into evaporation from the non-vascular Sphagnum surfaces and transpiration from vascular plants. The analysis indicated that about two thirds of the water vapour flux to the atmosphere was from evaporation when the Sphagnum surface was wet. Such an evaporative flux was expected because of vertical distribution of vascular plant leaves which had a small leaf area index (0.4–0.7) and intercepted only about 30% of net radiation ( R
n
) during the day. The remainder of R
n
was thus available for evaporation from Sphagnum. Evaporation significantly decreased as the Sphagnum surface dried out. When the water table was within the rooting zone (0–0.4 m), the vascular plants absorbed Sphagnum-generated sensible heat, which amounted up to one third of their transpiration energy flux. Under these conditions, the total water vapour flux remained near its potential rate owing to the enhanced transpiration from vascular plants. A drop in water table of 0.15–0.2 m below the hollow bottom during vascular plant senescence resulted in ET rates lower than the potential rates by 5–65%. 相似文献
10.
The measurements of the photosynthetic photon flux density ( Qp) and other solar components have been in Beijing for 2-year period. The Qp, broadband solar radiation ( Rs) and the PAR fraction ( Qp / Rs) showed similar seasonal features that peaked in value during the Summer and reached their lowest value during the Winter. The PAR fraction ranged from 1.68 E M J − 1 (Winter) to 1.98 E M J − 1 (Summer) with an annual mean value of 1.83 E M J − 1. The analysis of the hourly values also revealed a diurnal pattern, with higher values of Qp and Rs being observed around noon. The PAR fraction increased from 1.78 to 1.89 μE J − 1 (hourly values), as the sky conditions changed from clear to cloudy. The monthly mean hourly PAR fraction also revealed a diurnal variation, however, with lower values being observed around noon during most months. In November, the diurnal variations showed an opposite feature in comparison with other months. This is mainly attributed to the diurnal variations in the water vapor concentration.Two models were developed to estimate Qp from Rs. The models consisted of atmospheric parameters that were found to cause substantial changes to the PAR fraction, such as sky clearness, brightness and path length. The estimated Qp obtained via different equations was much closer to the observed values, with relative errors below 20% in Beijing. The Qp and Rs data collected at three stations with featuring different climate types from within Beijing were used for verifying the transferability of the models. The correlation coefficients between the measured and estimated Qp values decreased at these stations, and the relative error increased. This indicates that the estimation models need to be modified accordingly for the local climatic conditions. 相似文献
11.
Monthly, seasonal and annual sums of precipitation in Serbia were analysed in this paper for the period 1961–2010. Latitude, longitude and altitude of 421 precipitation stations and terrain features in their close environment (slope and aspect of terrain within a radius of 10 km around the station) were used to develop a regression model on which spatial distribution of precipitation was calculated. The spatial distribution of annual, June (maximum values for almost all of the stations) and February (minimum values for almost all of the stations) precipitation is presented. Annual precipitation amounts ranged from 500 to 600 mm to over 1100 mm. June precipitation ranged from 60 to 140 mm and February precipitation from 30 to 100 mm. The validation results expressed as root mean square error (RMSE) for monthly sums ranged from 3.9 mm in October (7.5% of the average precipitation for this month) to 6.2 mm in April (10.4%). For seasonal sums, RMSE ranged from 10.4 mm during autumn (6.1% of the average precipitation for this season) to 20.5 mm during winter (13.4%). On the annual scale, RMSE was 68 mm (9.5% of the average amount of precipitation). We further analysed precipitation trends using Sen’s estimation, while the Mann-Kendall test was used for testing the statistical significance of the trends. For most parts of Serbia, the mean annual precipitation trends fell between −5 and +5 and +5 and +15 mm/decade. June precipitation trends were mainly between −8 and +8 mm/decade. February precipitation trends generally ranged from −3 to +3 mm/decade. 相似文献
12.
本文利用2014年全年北京市12个空气质量监测站的逐小时PM_(2.5)地面观测资料,以及Terra卫星和Aqua卫星的MODIS 3 km气溶胶光学厚度(AOD)产品,分析了地面PM_(2.5)和两颗卫星AOD的时空分布特征,并在时空匹配的基础上,建立了AOD与PM_(2.5)浓度之间的回归模型。结果表明:PM_(2.5)浓度在城区高、郊区低,最低值位于定陵站,城区站和郊区站的逐时PM_(2.5)浓度的日变化分别呈"双峰型"和"单峰型";两颗卫星AOD数值也均是城区高、郊区低,沿山区的边界有明显的AOD梯度,且城区上午Terra卫星的AOD高于下午Aqua卫星的AOD,而郊区上、下午的AOD基本相同;Aqua卫星AOD与PM_(2.5)的确定系数(R2)较Terra卫星AOD与PM_(2.5)的确定系数平均高0.11,且城区站点两颗卫星AOD与PM_(2.5)相关性均较郊区站点AOD与PM_(2.5)相关性偏高;综合来看,Aqua卫星的AOD与城区的PM_(2.5)相关系数最高,即Aqua卫星的AOD更适于监测和反演城区地面的PM_(2.5)。 相似文献
13.
Below-cloud aerosol scavenging is generally estimated from field measurements using advanced instruments that measure changes in aerosol distributions with respect to rainfall. In this study, we discuss various scavenging mechanisms and scavenging coefficients from past laboratory and field measurements. Scavenging coefficients derived from field measurements (representing natural aerosols scavenging) are two orders higher than that of theoretical ones for smaller particles (D p < 2 μm). Measured size-resolved scavenging coefficients can be served as a better option to the default scavenging coefficient (e.g. a constant of 10 ?4 s ?1 for all size of aerosols, as used in the CALPUFF model) for representing below-cloud aerosol scavenging. We propose scavenging correction parameter (C R) as an exponential function of size-resolved scavenging coefficients, winds and width in the downwind of the source–receptor system. For a wind speed of 3 m s ?1, C R decrease with the width in the downwind for particles of diameters D p < 0.1 μm but C R does not vary much for particles in the accumulation mode (0.1 < D p < 2 μm). For a typical urban aerosol distribution, assuming 3 m s ?1 air-flow in the source–receptor system, 10 km downwind width, 2.84 mm h ?1 of rainfall and using aerosol size dependent scavenging coefficients in the C R, scavenging of aerosols is found to be 16% in number and 24% in volume of total aerosols. Using the default scavenging coefficient (10 ?4 s ?1) in the CALPUFF model, it is found to be 64% in both number and volume of total aerosols. 相似文献
14.
2014年7月—2014年8月借助风速仪、微梯度集沙仪,通过野外监测系统获取的试验数据,对塔中地区2014年7月—2014年8月沙尘天气过程中贴地层输沙率进行分析,得出:0～85 mm高度内,随着风速的增大,35～85 mm无论是绝对的输沙量还是相对的输沙量都减少。0～85 mm高度内,各层输沙率最大值均出现在风速为8 m·s-1左右,波动较为显著;最小值出现在6.5 m·s-1左右,波动不明显;沙尘天气中,输沙率最大值出现在5～15 mm高度,最小值出现在35～85 mm高度。扬沙天气中,风速9.2 m·s-1时,输沙率最大值在0～5 mm处。沙尘暴天气,拐点风速为7.5 m·s-1,7.5 m·s-1时,输沙率增加不显著,7.5 m·s-1时,输沙率增加显著。通过微梯度集沙仪获得的上述试验数据是风沙工程设计的一个极重要工程参数,具有重要的实践意义。 相似文献
15.
Extreme rainfall events have serious implications for economic sectors with a close link to climate such as agriculture and food security. This holds true in the Central Rift Valley (CRV) of Ethiopia where communities rely on highly climate-sensitive rainfed subsistence farming for livelihoods. This study investigates changes in ten extreme rainfall indices over a period of 40 years (1970–2009) using 14 meteorological stations located in the CRV. The CRV consists of three landscape units: the valley floor, the escarpments, and the highlands all of which are considered in our data analysis. The Belg (March–May) and Kiremt (June–September) seasons are also considered in the analysis. The Mann-Kendall test was used to detect trends of the rainfall indices. The results indicated that at the annual time scale, more than half (57 %) of the stations showed significant trends in total wet-day precipitation (PRCPTOT) and heavy precipitation days (R10mm). Only 7–35 % of stations showed significant trends, for the other rainfall indices. Spatially, the valley floor received increasing annual rainfall while the escarpments and the highlands received decreasing annual rainfall over the last 40 years. During Belg, 50 % of the stations showed significant increases in the maximum number of consecutive dry days (CDD) in all parts of the CRV. However, most other rainfall indices during Belg showed no significant changes. During Kiremt, considering both significant and non-significant trends, almost all rainfall indices showed an increasing trend in the valley floor and a decreasing trend in the escarpment and highlands. During Belg and Kiremt, the CDD generally showed increasing tendency in the CRV. 相似文献
16.
Downscaling precipitation is required in local scale climate impact studies. In this paper, a statistical downscaling scheme was presented with a combination of geographically weighted regression (GWR) model and a recently developed method, high accuracy surface modeling method (HASM). This proposed method was compared with another downscaling method using the Coupled Model Intercomparison Project Phase 5 (CMIP5) database and ground-based data from 732 stations across China for the period 1976–2005. The residual which was produced by GWR was modified by comparing different interpolators including HASM, Kriging, inverse distance weighted method (IDW), and Spline. The spatial downscaling from 1° to 1-km grids for period 1976–2005 and future scenarios was achieved by using the proposed downscaling method. The prediction accuracy was assessed at two separate validation sites throughout China and Jiangxi Province on both annual and seasonal scales, with the root mean square error (RMSE), mean relative error (MRE), and mean absolute error (MAE). The results indicate that the developed model in this study outperforms the method that builds transfer function using the gauge values. There is a large improvement in the results when using a residual correction with meteorological station observations. In comparison with other three classical interpolators, HASM shows better performance in modifying the residual produced by local regression method. The success of the developed technique lies in the effective use of the datasets and the modification process of the residual by using HASM. The results from the future climate scenarios show that precipitation exhibits overall increasing trend from T1 (2011–2040) to T2 (2041–2070) and T2 to T3 (2071–2100) in RCP2.6, RCP4.5, and RCP8.5 emission scenarios. The most significant increase occurs in RCP8.5 from T2 to T3, while the lowest increase is found in RCP2.6 from T2 to T3, increased by 47.11 and 2.12 mm, respectively. 相似文献
17.
The changes in rainfall erosivity have been investigated using the rainfall erosivity factor ( R) proposed for USLE by Wischmeier and Smith ( R W-S ) and some simplified indexes (the Fournier index modified by Arnoldus, F, a regional index spatial independent, R Fr , and a regional index spatial dependent, R Fs ) estimated by indirect approaches. The analysis has been carried out over 48 rainfall stations located in Calabria (Southern Italy) using data collected in the period 1936–2012 and divided in three sub-periods. The series of the erosivity indexes and of some precipitation variables have been analyzed for evidence of trends using standard methods. The simplified indexes suggested a general underestimation of the rainfall erosivity with respect to R W-S . The mean underestimation ranged between 23 and 54 % for R Fr and from 10 to 15 % for R Fs . Both the sign and the magnitude of the trends were different for the different stations depending on the variable and sub-period considered. In general, the erosivity increased during the period 1936–1955 (1 st sub-period) and during the more recent sub-period (1992–2012, 3 rd sub-period), whereas it decreased during 1958–1977 (2 nd sub-period). The evidence of trends was generally higher for R W-S than for R Fr and R Fs . Focusing on the most recent sub-period (3 rd sub-period), all the variables analyzed showed mainly increasing trends but with different magnitude. More particularly, R W-S showed a mean increment of 29 %; F, R Fr and R Fs increased by 11, 15 and 18 %, respectively; the maximum intensity of 0.5-h precipitation increased by 5 %; and the annual precipitation increased by 22 %. Consequently, it remains difficult to define which precipitation variable plays the dominant role in the temporal variation of rainfall erosivity in the region. However, the overall results suggest that the indexes estimated by indirect procedures ( F, R Fr , and R Fs ) should be used with caution for climate change analysis, despite they are used for practical purposes considering they are based on easily available information. 相似文献
18.
Spatial variation of long term annual precipitation volume weighted concentrations of major chemical constituents (SO 4 ?2, NO 3 ?, Cl ?, NH 4 +, Ca +2, Mg +2, Na + and K + ) at all the ten Global Atmospheric Watch (GAW) stations in India for the period from 1981 to 2012 is studied in this paper. Ionic abundance and balance is studied as well. The range of long term annual mean pH at ten stations was 5.25?±?0.82 to 6.91?±?0.76, lowest at Mohanbari and highest at Jodhpur. The long term annual mean pH for the period 1981–2012 showed decreasing trend at all the stations (significant at 5 % level). Decadal mean pH among ten stations for 1981–1990, 1991–2000 and 2001–2012 ranged between 7.31 to 5.76, 7.45 to 4.92 and 6.16 to 4.77 respectively and showed decreasing trend at all the stations during 1981–1990 to 2001–12. The percentage occurrence of acidic pH (<5.65) at ten stations ranged from 3 to 72 %, lowest at Jodhpur and highest at Mohanbari and it increased from 1981–1990 to 2001–2012 almost at all the stations. Temporal variation of annual mean values of nssSO 4 ?2, NO 3 ?, Ca +2 and pH for the study period were attempted. Variation of nss K (non sea salt Potassium) at all the stations was studied to assess the biomass burning contribution in different regions. Non-marine (terrestrial) contribution dominated for majority of ionic constituents at most of the stations. However marine contribution was found to be dominant for Mg at Port Blair and Minicoy. Also sea salt fraction of SO 4 was higher than terrestrial at Minicoy. Sources of measured ionic constituents in rain water are assessed through correlation analysis. The concentrations of all the ionic species were lowest at Kodaikanal, a high altitude hill top station and the total ionic mass was 136.0 μeq/l. Jodhpur, an arid station not only had highest concentrations of Ca +2, SO 4 ?2 and K + but also had highest total ionic content (1051.8 μeq/l) among all the stations. At Srinagar, Jodhpur, Allahabad, Nagpur and Pune stations Ca +2 was the dominant cation while dominant anion was NO 3 ? for Srinagar, Allahabad, and Nagpur and Cl ? for Jodhpur and Pune; at Mohanbari NO 3 ? and Ca +2; at Visakhapatnam, Port Blair and Minicoy Na + and Cl ? were abundant. Temporal variation had shown an increasing trend for nssSO 4 ?2 and NO 3 ? and obviously decreasing trend for pH at all the stations. However, Ca +2 showed a decreasing trend at all the stations except at Port Blair. With the exception of Pune and Jodhpur stations, nssK showed a decreasing trend at all the stations revealing decreasing influence of soil/biomass burning over Indian GAW stations. Negative correlation of pH with SO 4 ?2 was found to be weak compared to NO 3 –. 相似文献
19.
The present study evaluates firstly the ability of the FAO-56 methodology, based on the two-step approach “reference evapotranspiration (ET 0)—crop coefficient ( K c),” to accurately determine the actual evapotranspiration (ET) of irrigated crops and proposes, secondly, the alternative approaches for improving this determination. The FAO-56 methodology is supported by two hypotheses: (1) ET 0 represents all effects of weather and (2) K c varies predominately with specific crop characteristics and only marginally with climate, which enables the transfer of K c standard values among locations and climates. On the base of the theoretical analysis and experimental observations, a critical examination of the previous hypotheses demonstrates that they are not verified by reality. The first hypothesis is not verified for two reasons: (a) The formulation adapted by the Penman–Monteith equation and proposed in FAO-56 methodology for calculating ET 0 uses climatic variables determined at a 24-h average scale. However, in principle it is only valid in permanent regime, in other words at least on an hourly scale. (b) The FAO-56-proposed formulation attributes a constant value to the canopy resistance of the reference surface; but in reality, this resistance is variable in relation to the climatic variables. The second hypothesis, concerning the two-step approach, is also not verified because the values of K c largely vary in relation to climatic variables (radiation, air vapour pressure deficit and wind speed). This fact does not support the possibility of the transferability of K c values into locations where the local conditions deviate from the conditions where the adjusted values of K c were determined. The weakness of the ET estimation, observed on several crops cultivated in the Mediterranean region, through the application of the FAO-56 methodology, is the result of errors accumulation, associated with that affects the determination of either ET 0 or K c. The present study underlines the advantage of using a one-step approach in the calculation of ET, since it is based on fewer computation steps and, consequently, on fewer error sources than the two-step model. Two models adopting this approach are proposed and validated, one of which can be considered as operational, i.e. it only needs standard meteorological data as input. The use of these models enables an improvement of the ET estimation. This objective is a key component of any strategy to improve agricultural water management in Mediterranean region. 相似文献
20.
Long-term data from diffuse and global irradiances were used to calculate direct beam irradiance which was used to determine three atmospheric turbidity coefficients (Linke T L , Ångström β and Unsworth–Monteith δ a ) at seven sites in Egypt in the period from 1981 to 2000. Seven study sites (Barrani, Matruh, Arish, Cairo, Asyut, Aswan and Kharga) have been divided into three categories: Mediterranean climate (MC), desert Nile climate (DNC) and urban climate (UC, Cairo). The indirect method (i.e., global irradiance minus diffuse irradiance) used here allows to estimate the turbidity coefficients with an RMSE% ≤20 % (for β, δ a and T L ) and ~30 % (for β) if compared with those estimated by direct beam irradiance and sunphotometeric data, respectively. Monthly averages of T L , β and δ a show seasonal variations with mainly maxima in spring at all stations, due to Khamsin depressions coming from Sahara. Secondary maxima is observed in summer and autumn at DNC and MC (Barrani and Arish) stations in summer due to dust haze which prevails during that season and at UC (Cairo) in autumn, due to the northern extension of the Sudan monsoon trough, which is accompanied by small-scale depressions with dust particles. The mean annual values of β, δ a , and T L (0.216, 0.314, and 4.6, respectively) are larger in Cairo than at MC stations (0.146, 0.216, and 3.8, respectively) and DNC stations (0.153, 0.227, and 3.8, respectively). Both El-Chichon and Mt. Pinatubo eruptions were examined for all records data at MC, UC and DNC stations. The overburden caused by Mt. Pinatubo’s eruption was larger than El-Chichon’s eruption and overburden for β, and T L at DNC stations (0.06, and 0.58 units, respectively) was more pronounced than that at MC (0.02, and 0.26, respectively) and UC (0.05 and 0.52 units, respectively) stations. The annual variations in wind speed and turbidity parameters show high values for both low and high wind speed at all stations. The wind directions have a clear effect on atmospheric turbidity, and consequently, largest turbidities occur when the wind carries aerosols from the main particle sources, such as industrial particle sources around Cairo or to some extent from the Sahara surrounding all study stations. 相似文献
|
