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1.
Accurate estimation of reference evapotranspiration (ET0) becomes imperative for better managing the more and more limited agricultural water resources. This study examined the feasibility of developing generalized artificial neural network (GANN) models for ET0 estimation using weather data from four locations representing different climatic patterns. Four GANN models with different combinations of meteorological variables as inputs were examined. The developed models were directly tested with climatic data from other four distinct stations. The results showed that the GANN model with five inputs, maximum temperature, minimum temperature, relative humidity, solar radiation, and wind speed, performed the best, while that considering only maximum temperature and minimum temperature resulted in the lowest accuracy. All the GANN models exhibited high accuracy under both arid and humid conditions. The GANN models were also compared with multivariate linear regression (MLR) models and three conventional methods: Hargreaves, Priestley–Taylor, and Penman equations. All the GANN models showed better performance than the corresponding MLR models. Although Hargreaves and Priestley–Taylor equations performed slightly better than the GANN models considering the same inputs at arid and semiarid stations, they showed worse performance at humid and subhumid stations, and GANN models performed better on average. The results of this study demonstrated the great generalization potential of artificial neural techniques in ET0 modeling.  相似文献   

2.
This paper examines the potential for the use of artificial neural networks (ANNs) to estimate the reference crop evapotranspiration (ET0) based on air temperature data under humid subtropical conditions on the southern coast of the Caspian Sea situated in the north of Iran. The input variables for the networks were the maximum and minimum air temperature and extraterrestrial radiation. The temperature data were obtained from eight meteorological stations with a range of latitude, longitude, and elevation throughout the study area. A comparison of the estimates provided by the ANNs and by Hargreaves equation was also conducted. The FAO-56 Penman–Monteith model was used as a reference model for assessing the performance of the two approaches. The results of this study showed that ANNs using air temperature data successfully estimated the daily ET0 and that the ANNs with an R 2 of 0.95 and a root mean square error (RMSE) of 0.41 mm day?1 simulated ET0 better than the Hargreaves equation, which had an R 2 of 0.91 and a RMSE of 0.51 mm day?1.  相似文献   

3.
This paper focuses on the primary causes of changes in potential evapotranspiration (ETo) in order to comprehensively understand climate change and its impact on hydrological cycle. Based on modified Penman-Monteith model, ETo is simulated, and its changes are attributed by analyzing the sensitivity of ETo to influence meteorological variables together with their changes for 595 meteorological stations across China during the period 1961–2008. Results show the decreasing trends of ETo in the whole country and in most climate regions except the cold temperate humid region in Northeast China. For China as a whole, the decreasing trend of ETo is primarily attributed to wind speed due to its significant decreasing trend and high sensitivity. Relative humidity is the highest sensitive variable; however, it has negligible effect on ETo for its insignificant trend. The positive contribution of temperature rising to ETo is offset by the effect of wind speed and sunshine duration. In addition, primary causes to ETo changes are varied for differing climate regions. ETo changes are attributed to decreased wind speed in most climate regions mainly distributed in West China and North China, to declined sunshine duration in subtropical and tropical humid regions in South China, and to increased maximum temperature in the cold temperate humid region.  相似文献   

4.

Evapotranspiration estimation is of crucial importance in arid and hyper-arid regions, which suffer from water shortage, increasing dryness and heat. A modeling study is reported here to cross-station assessment between hyper-arid and humid conditions. The derived equations estimate ET0 values based on temperature-, radiation-, and mass transfer-based configurations. Using data from two meteorological stations in a hyper-arid region of Iran and two meteorological stations in a humid region of Spain, different local and cross-station approaches are applied for developing and validating the derived equations. The comparison of the gene expression programming (GEP)-based-derived equations with corresponding empirical-semi empirical ET0 estimation equations reveals the superiority of new formulas in comparison with the corresponding empirical equations. Therefore, the derived models can be successfully applied in these hyper-arid and humid regions as well as similar climatic contexts especially in data-lack situations. The results also show that when relying on proper input configurations, cross-station might be a promising alternative for locally trained models for the stations with data scarcity.

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5.
The monthly rainfall data from 1901 to 2011 and maximum and minimum temperature data from 1901 to 2005 are used along with the reference evapotranspiration (ET0) to analyze the climate trend of 45 stations of Madhya Pradesh. ET0 is calculated by the Hargreaves method from 1901 to 2005 and the computed data is then used for trend analysis. The temporal variation and the spatial distribution of trend are studied for seasonal and annual series with the Mann-Kendall (MK) test and Sen’s estimator of slope. The percentage of change is used to find the rate of change in 111 years (rainfall) and 105 years (temperatures and ET0). Interrelationships among these variables are analyzed to see the dependency of one variable on the other. The results indicate a decreasing rainfall and increasing temperatures and ET0 trend. A similar pattern is noticeable in all seasons except for monsoon season in temperature and ET0 trend analysis. The highest increase of temperature is noticed during post-monsoon and winter. Rainfall shows a notable decrease in the monsoon season. The entire state of Madhya Pradesh is considered as a single unit, and the calculation of overall net change in the amount of the rainfall, temperatures (maximum and minimum) and ET0 is done to estimate the total loss or gain in monthly, seasonal and annual series. The results show net loss or deficit in the amount of rainfall and the net gain or excess in the temperature and ET0 amount.  相似文献   

6.
Meteorological stations, which measure all the required meteorological parameters to estimate reference evapotranspiration (ETo) using the Food and Agriculture Organization Penman?CMonteith (FAO56-PM) method, are limited in Korea. In this study, alternative methods were applied to estimate these parameters, and the applicability of these methods for ETo estimation was evaluated by comparison with a complete meteorological dataset collected in 2008 in Korea. Despite differences between the estimation and observation of radiation and wind speed, the comparison of ETo showed small differences [i.e., mean bias error (MBE) varying ?0.22 to 0.25?mm?day?1 and root-mean-square-error (RMSE) varying 0.06?C0.50?mm?day?1]. The estimated vapor pressure differed considerably from the observed, resulting in a larger discrepancy in ETo (i.e., MBE of ?0.50?mm?day?1 and RMSE of 0.60?C0.73?mm?day?1). Estimated ETo showed different sensitivity to variations of the meteorological parameters??in order of vapor pressure?>?wind speed?>?radiation. It is clear that the FAO56-PM method is applicable for reasonable ETo estimation at a daily time scale especially in data-limited regions in Korea.  相似文献   

7.
Estimation of reference evapotranspiration (ET0) is needed to support irrigation design and scheduling, and watershed hydrology studies. There are many available methods to estimate evapotranspiration from a water surface, comprising both direct and indirect methods. In the first part of this study, the generalized regression neural networks model (GRNN) and radial basis function neural network (RBFNN) are developed and compared in order to estimate the reference ET0 for the first time in Algeria. Various daily climatic data, that is, daily mean relative humidity, sunshine duration, maximum, minimum and mean air temperature, and wind speed from Dar El Beida, Algiers, Algeria, are used as inputs to the GRNN and RBFNN models to estimate the ET0 obtained using the FAO-56 Penman-Monteith equation (PM56). The performances of the models are evaluated using root mean square errors (RMSE), mean absolute error (MAE), Willmott index of agreement (d) and correlation coefficient (CC) statistics. In the second part of the study, the empirical Hargreaves-Samani (HG) and Priestley-Taylor (PT) equations are also considered for the comparison. Based on the comparisons, the GRNN was found to perform better than the RBFNN, Priestley-Taylor and Hargreaves-Samani models. The RBFNN model is ranked as the second best model.  相似文献   

8.
Direct calculation of actual evapotranspiration ETc based on Penman-Monteith type models gives more accurate values than indirect models, which need the determination of reference evapotranspiration and crop coefficient. However, the direct models need the measurement of weather variables above the crop, which is limiting and not easily feasible in practice. An operational version of a known ETc direct model is described and tested. This new version is based on the determination of the weather variables collected in a standard agro-meteorological station. The original and the operational versions of the ETc model were validated on two crops with contrasting height: soybean (0.8 m) and sweet sorghum (3 m). For soybean, ETc calculated with the two versions gave results very similar at both hourly and daily scales. For sweet sorghum, ETc calculated with the operational version is good at daily scale and not as good, although acceptable, at the hourly scale.  相似文献   

9.
Global warming has caused unevenly distributed changes in precipitation and evapotranspiration, which has and will certainly impact on the wet-dry variations. Based on daily meteorological data collected at 91 weather stations in Northeast China (NEC), the spatiotemporal characteristics of dry and wet climatic variables (precipitation, crop reference evapotranspiration (ET0), and humid index (HI)) are analyzed, and the probable reasons causing the changes in these variables are discussed during the period of 1961–2014. Precipitation showed non-significant trend over the period of 1961–2014, while ET0 showed a significant decreasing trend, which led to climate wetting in NEC. The period of 2001–2012 exhibited smaller semiarid area and larger humid area compared to the period of 1961–1980, indicating NEC has experienced wetting process at decadal scale. ET0 was most sensitive to relative humidity, and wind speed was the second most sensitive variable. Sunshine hours and temperature were found to be less influential to ET0 in the study area. The changes in wind speed in the recent 54 years have caused the greatest influence on ET0, followed by temperature. For each month, wind speed was the most significant variable causing ET0 reduction in all months except July. Temperature, as a dominant factor, made a positive contribution to ET0 in February and March, as well as sunshine hours in June and July, and relative humidity in August and September. In summary, NEC has experienced noticeable climate wetting due to the significantly decreasing ET0, and the decrease in wind speed was the biggest contributor for the ET0 reduction. Although agricultural drought crisis is expected to be partly alleviated, regional water resources management and planning in Northeast China should consider the potential water shortage and water conflict in the future because of spatiotemporal dry-wet variations in NEC.  相似文献   

10.
Identifying changes in reference evapotranspiration (ETo) can help in future planning of crop water requirements and water resources for high water-use efficiency. This study analyzes the ETo trends on a seasonal and annual timescale by applying various statistical tools to data from 41 Iranian weather stations during the period between 1966 and 2005. The Mann–Kendall test after removal of significant serial correlation was used to determine the statistical significance of the trends, and the change point in the ETo time series was determined using the cumulative sum technique. The results showed that (1) the significant increasing trends of annual ETo were observed at seven stations which are located in different parts of Iran, (2) the stations located at the southeast, northeast, and northwest corners of Iran experienced the highest positive change of annual ETo, and (3) the changes in seasonal ETo were most pronounced in the winter season, both in terms of trend magnitude and the number of stations with significant trends.  相似文献   

11.
The present study evaluates firstly the ability of the FAO-56 methodology, based on the two-step approach “reference evapotranspiration (ET0)—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) ET0 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 ET0 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 ET0 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   

12.
近30年安徽省地表干湿时空变化及对农业影响   总被引：5，自引：2，他引：5       下载免费PDF全文
采用FAO Penman-Monteith模型, 并利用安徽省辐射观测资料对其净辐射项进行修正, 计算近30年安徽省的参考作物蒸散量。用此计算值和相应时段的降水量计算干燥度 (Ia), 并进行了基于干燥度指标不同时间尺度的区域地表干湿状况变化分析。分析表明:1971—2000年安徽省年干燥度平均值Ia=1的等值线为湿润区和半湿润区的分界线, 该分界线与1000 mm的年雨量线有很好的一致性, 同时也具有清晰的农业意义。20世纪70—90年代Ia=1的等值线南北波动, 其波动区域正是安徽省江淮分水岭易旱区。在此基础上分析了半湿润区、波动区域和湿润区降水量、参考作物蒸散量和干燥度年代际、年际和半年际的变化趋势及变异率以及逐月干旱频率及其对农业的影响。  相似文献   

13.
In this study, weighing lysimeters were used to investigate the daily crop coefficient and evapotranspiration of wheat and maize in the Fars province, Iran. The locally calibrated Food and Agriculture Organization (FAO) Penman–Monteith equation was used to calculate the reference crop evapotranspiration (ETo). Micro-lysimetry was used to measure soil evaporation (E). Transpiration (T) was estimated by the difference between crop evapotranspiration (ETc) and E. The single crop coefficient (K c) was calculated by the ratio of ETc to ETo. Furthermore, the dual crop coefficient is composed of the soil evaporation coefficient (K e) and the basal crop coefficients (K cb) calculated from the ratio of E and T to ETo, respectively. The maximum measured evapotranspiration rate for wheat was 9.9 mm?day?1 and for maize was 10 mm?day?1. The total evaporation from the soil surface was about 30 % of the total wheat ETc and 29.8 % of total maize ETc. The single crop coefficient (K c) values for the initial, mid-, and end-season growth stages of maize were 0.48, 1.40, and 0.31 and those of wheat were 0.77, 1.35, and 0.26, respectively. The measured K c values for the initial and mid-season stages were different from the FAO recommended values. Therefore, the FAO standard equation for K c-mid was calibrated locally for wheat and maize. The K cb values for the initial, mid-, and end-season growth stages were 0.23, 1.14, and 0.13 for wheat and 0.10, 1.07, and 0.06 for maize, respectively. Furthermore, the FAO procedure for single crop coefficient showed better predictions on a daily basis, although the dual crop coefficient method was more accurate on seasonal scale.  相似文献   

14.
Summary In regions such as west Texas where water is scarce, changes in the water balance may have a significant impact on agricultural production and management of water resources. We used the mechanistic soil-plant-atmosphere simulation model ENWATBAL to evaluate changes in soil water evaporation (E) and transpiration (T) in cotton and grain sorghum that may occur due to climate change and elevated CO2 in west Texas. Climatic and plant factors were varied individually, and in combination, to determine their impact onE andT. Of the climatic factors,E was most sensitive to changes in vapor pressure, andT to changes in irradiance. Simulations suggest that if warming is accompanied by higher humidity, the impact of climate change may be minimal. However, if the climate becomes warmer and less humid,ET may increase substantially. Simulations also suggest that enhanced growth due to elevated CO2 may have a greater impact onET than climatic change.With 9 Figures  相似文献   

15.
Reference crop evapotranspiration (ET0) is one of the most important climatic parameters which plays a key role in estimating crop water demand and scheduling irrigation. Under global warming and climate change conditions, it is needed to survey the trend of ET0 in Iran. In this study, ET0 values were determined based on FAO-56 Penman-Monteith equation over 32 synoptic meteorological stations during 1960–2005; and analyzed spatially and temporally in monthly, seasonal and annual time scales. After removing the significant lag-1 serial correlation effect by pre-whitening, non-parametric statistical Mann–Kendall (MK) test was used to detect the trends. The slope of the changes was determined by Sen’s slope estimator. In order to facilitate in trend analysis, the 10 moving average low pass filter were also applied on the normalized annual ET0 time series. Annual ET0 time series and filtered ones were then classified by hierarchical clustering in three clusters and then mapped in order to show the patterns of different clusters. Results showed that the significant decreasing trends were more considerable than increasing ones. Among surveyed stations, and on an annual time scale, the highest and lowest annual values of Sen’s slope estimator were observed in Tabas with (+) 72.14 mm per decade and Shahrud with (?) 62.22 mm per decade, respectively. Results also indicated that the clustered map based on normalized and filtered annual ET0 time series is in accordance with another map which showed spatial distribution of increasing, decreasing and non-significant trends of ET0 on annually time scale. Exploratory and visual analysis of smoothed time series showed increasing trend in recent years especially after 1980 and 1995. In brief, the upward trend of ET0 in recent years is a crucial issue with regard to the high cost of dam construction for agricultural aims in arid and semi-arid regions e.g. Iran.  相似文献   

16.
The FAO Penman–Monteith (F-PM) method is a frequently applied approach for calculating the daily reference evapotranspiration (ET0). This method requires long records of meteorological data, which makes it quite hard to employ in locations with no or limited available data. Evaporation pans are widely used to estimate the reference ET0, but this method requires reliable estimates of the pan coefficient (K p). The objectives of this study were to determine the proper values of monthly and annual K p, as well as the best method among those available for the estimation of K p values in the study area. Measured weather data from 1992 to 2006 were obtained from 18 stations in the North and Northwest of Iran. Daily ET0 calculated using methods by Bernardo et al. and Pereira et al. were compared with those calculated by the F-PM method. The employed methods at all stations, except those located in the north of the study area with high relative humidity, overestimated the ET0 compared to the F-PM method. The constant parameters of these methods were optimized by a trial and error scheme to minimize the root mean square error. The results indicated that modified K p coefficients from Bernardo et al.’s method ranged between 0.41 and 0.87 and the optimal coefficient of Pereira et al.’s method ranged between 0.49 and 0.95. Modified monthly K p from Bernardo et al.’s method ranged between 0.3 and 1.07 and those from Pereira et al.’s method ranged between 0.4 and 1.18. Modified K p of the methods by Bernardo et al. and Pereira et al. showed the higher estimation accuracy of daily ET0 values. In general, the performance of the modified K p of Bernardo et al.’s method was higher than Pereira et al.’s method for all stations. Thus, in the study region and under the same climatic conditions [in areas with only pan evaporation (E p) records], the use of climatic monthly modified K p to calculate ET0 based on class A E p is recommended.  相似文献   

17.
湖北省潜在蒸散估算模型对比   总被引：2，自引：0，他引：2  
利用湖北省74个气象站1961~2011年逐日气象资料,通过与FAO56-Penman-Monteith(FAO-PM)模型潜在蒸散估算结果对比,从月、季、年际变化及不同干湿状况条件比较了PriestleyTaylor、Hargreaves及Thornthwaite 3种简化的经验模型在湖北省的适用性。结果表明:3种简化模型与FAO-PM模型计算的逐月潜在蒸散均存在一定偏差,Thornthwaite模型偏差最大且冬夏季偏差反位相,Priestley-Taylor模型偏差最小,Hargreaves模型各月间的偏差最为稳定。3种简化模型与FAOPM模型估算结果具有良好的线性关系,且在各区域间相对稳定,但不同季节和干湿状况下各有差异,其中Hargreaves模型各季节间和不同干湿状况下与FAO-PM模型的关系均最为稳定。在年际变化上,Priestley-Taylor和Hargreaves模型与FAO-PM模型计算结果年际波动基本一致,Thornthwaite模型与FAO-PM模型计算结果尽管在量值上较为接近,但年际波动偏小;Priestley-Taylor和FAOPM模型年潜在蒸散趋势变化基本一致,Hargreaves模型年潜在蒸散趋势变化微弱,而Thornthwaite模型年潜在蒸散趋势变化与FAO-PM模型相反。建议在湖北省气象资料匮乏或不便应用的情况下,作物模拟模型及气候变化等研究中采用Priestley-Taylor估算模型,日常干旱监测及水资源规划中采用Hargreaves模型,干湿气候区划等工作中可采用Thornthwaite模型。同时,使用中应基于FAO-PM模型对经验模型进行适当订正,模型订正应建立在季节或月尺度。  相似文献   

18.
The accuracy of nine solar radiation (R s ) estimation models and their effects on reference evapotranspiration (ET o ) were evaluated using data from eight meteorological stations in Canada. The R s estimation models were FAO recommended Angstrom-Prescott (A-P) coefficients, locally calibrated A-P coefficients, Hargreaves and Samani (H-S) (1982), Annandale et al., (2002), Allen (1995), Self-Calibrating (S-C, Allen, 1997), Samani (2000), Mahmood and Hubbard (M-H) (2002), and Bristow and Campbell (B-C) (1984). The estimated R s values were then compared to measured R s to check the appropriateness of these models at the study locations. Based on root mean square error (RMSE), mean bias error (MBE) and modelling efficiency (ME) ranking, calibrated A-P coefficients performed better than all other methods. The calibrated H-S method (using new K RS 0.15) estimated R s more accurately than FAO-56 recommended A-P in Elora, and Winnipeg. The RMSE of the calibrated H-S method ranged between 1-6% and the RMSE of the calibrated and FAO recommended Angstrom-Prescott (A-P) methods ranged between 1-9%. The models with the least accuracy at the eight locations are the Mahmood & Hubbard (2002) and Self-Calibrating models. The percent deviation in ET o calculated with estimated R s was reduced by about 50% as compared to deviation in measured versus estimated R s .  相似文献   

19.
The objective of this study was to test an artificial neural network (ANN) for estimating the evaporation from pan (E Pan) as a function of air temperature data in the Safiabad Agricultural Research Center (SARC) located in Khuzestan plain in the southwest of Iran. The ANNs (multilayer perceptron type) were trained to estimate E Pan as a function of the maximum and minimum air temperature and extraterrestrial radiation. The data used in the network training were obtained from a historical series (1996–2001) of daily climatic data collected in weather station of SARC. The empirical Hargreaves equation (HG) is also considered for the comparison. The HG equation calibrated for converting grass evapotranspiration to open water evaporation by applying the same data used for neural network training. Two historical series (2002–2003) were utilized to test the network and for comparison between the ANN and calibrated Hargreaves method. The results show that both empirical and neural network methods provided closer agreement with the measured values (R 2?>?0.88 and RMSE?<?1.2 mm day?1), but the ANN method gave better estimates than the calibrated Hargreaves method.  相似文献   

20.
Variations of global evapotranspiration (ET) and fresh water discharge from land to oceans (D) are important components of global climate change, but have not been well monitored. In this study, we present an estimate of twenty years (1989 to 2008) variations of global D and ET derived from satellite remote-sensed measurements and recent reanalysis products, ERA-Interim and CFSR, by using a novel application of the water balance equations separately over land and over oceans. Time series of annual mean global D and ET from both satellite observations and reanalyses show clear positive and negative trends, respectively, as a result of modest increase of oceanic evaporation (E o ). The inter-annual variations of D are similar to the in-situ-based observations, and the negative trend of ET supports the previous result that relative humidity has decreased while temperature has increased on land. The results suggest considerable sensitivity of the terrestrial hydrological cycles (e.g., D and ET) to small changes in precipitation and oceanic evaporation.  相似文献   

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