FAO Penman-Monteith Method Research Articles

Kalibrasi Persamaan Thornthwaite Dan Evaporasi Panci Untuk Memprediksi Evapotranspirasi Potensial Pada Daerah Dengan Data Cuaca Terbatas

Determination of irrigation channel dimensions is determined by the number of consumptive use values that are empirically determined by the potential evapotranspiration value (ETo). Due to the limited lysimeter measurement data, the ETo value is usually approached by an empirical method using weather data input. The recognized standard method for its reliability and can be used universally in determining ETo values is the FAO Penman-Monteith method. However, the use of this method is only possible if complete weather data is available. This study modified the simplest ETo calculation method, the Thornthwaite method and the evaporation pan method to obtain ETo values as accurate as FAO PM resulted. Callibrations are made by finding the correction number to get the best results shown by the high of correlation number and the efficiency index value. The results obtained are for the area with limited weather data condition, the Thornthwaite method both before and after calibration has better accuracy than the evaporation pan method. While the calibration carried out successfully increased the correlation number and efficiency index of both methods significantly. The results of the two calibrated methods, the Thornthwaite method provides better and satisfactory performance with a ratio between 0.86 and 0.89 and the efficiency index reaches 0.78.

Evaluating of eight evapotranspiration estimation methods in arid regions of Iran

Evapotranspiration (ET) plays an important role in efficient irrigation and water resources management. ET can be accurately estimated via the FAO–Penman–Monteith method (ETFPM), which is known as the standard reference method for the ET estimation. However, this model requires more detailed meteorological data than are generally available at most weather stations. Alternative methods for predicting daily ET should be found which are simpler to work with less input data without scarifying prediction accuracy. The main purpose of this study was to determine a reliable alternative ET models that requires fewer input data compared with the ETFPM as a reference method. Here, five radiation-based models including Makkink (ETMAK), Priestley and Taylor (ETPT), Abtew (ETABT), Jensen-Haise (ETJH), McGuinness and Bordne (ETMB), and three temperature-based models including Hargreaves and Samani (ETHS), Hamon (ETHAM), Linacre (ETLIN) were evaluated by comparing them with PETFPM on a daily and growing season scale, using long-term data from 13 meteorological stations in northeast Iran. The statistical analysis revealed that the ETHS, ETJH, ETHAM, and ETLIN were the best methods for predicting daily ET in 54%, 15%, 15% and 15% of areas, respectively. In general, the temperature-based methods outperformed the radiation-based-methods in the study area. The cumulative values of daily ET during the growth periods showed that the Jensen−Haise method performed the best for the warm growing season compared to other alternative methods, while the Hargreaves–Samani method was the best prediction method for the cool growing season. Nevertheless, divergence between estimations of the best alternative methods and the reference method showed that the best ET alternative methods might be unreliable in some regions, and could not be recommended for estimating crop water requirements. Accordingly, the spatiotemporal variability in predictability performance of ET models should be taken into account prior to use.

INTEGRATIVE USE OF PENMAN-MONTEITH EQUATION WITH REMOTE SENSING AND GEOGRAPHICAL INFORMATION SYSTEM TECHNIQUES TO ESTIMATE EVAPOTRANSPIRATION VRIANCES IN IRAQ

This research was aimed to construct an integrative system which is capable of accurate determining and analyzing evapotranspiration rates in Iraq for long period , Since high evapotranspiration rates and extreme shortage in precipitation are the main causes of aridity, which considered principal reason for land degradation and land desertification eventually. FAO Penman-Monteith method was adopted because it's the international standard method. In this work meteorological readings of nine stations with comprehensive covering for Iraq's area were taken for every ten years in a long-term range (31 years). The daily evapotranspiration values had been calculated, then after the annual summation value determined for the years (1987, 1997, 2007, and 2017). The use of spatial analysis schemes proved that generally eighties decade of the last century had climax (Etr) values, then ETr rates rapidly decreased in whole Iraq except some anomalies. There were two reasons for this decrement, firstly air temperature value decent which increase relative humidity. Secondly wind speed rates falling (which considered the principal cause for reference evapotranspiration rates descending in this case).

A comparison of empirical equations of Estimating Potential Evapotranspiration (PET) from climatological data in Erbil city

Potential evapotranspiration (PET) is an important index of hydrologic budgets at different spatial scales and it is a critical variable for understanding regional hydrological processes. The objective of this study is to find the most suitable method for estimating PET in Erbil city which was done by comparing seven commonly PET methods. Radiation based method: Pristley –Taylor(PT), Turc(TU), Makkink (MK), and temperature based method: Ivanov (IV),  Penman–Monteith method (1965) , FAO Penman–Monteith method, and Thornthwaite(TW). The data were collected and used in the models to find PET for the period (1992-2015). The performance indicators were applied by using statistical parameters such as: the Root Mean Square Error (RMSE), Mean Absolute Error (MAE), person correlation coefficient (R2), MBE Mean bias error. The monthly PET results from the models were compared with the actual evaporation. Current work shows that Makkink (MK) model is better than the other models for estimating the potential Evapotranspiration in Erbil depending on the values of statistical parameters.

Estimation of reference evapotranspiration (ETo) at Parbhani, Maharashtra

Evapotranspiration (ET) is an important parameter for climatological and hydrological studies, as well as for irrigation planning and management. The FAO Penman-Monteith method (P-M) is the method recommended by FAO-56 for estimation of ETo over a wide variety of climatic situations over the world and it requires many types of climatic data. A study has been carried out to estimate the mean daily ETo characteristics of Parbhani in Maharashtra. The results showed that, the variation of average daily ETo values over the period (1982-2017) ranged from 3.45 mm day-1 (26th December) to 10.46 mm day-1 (29th May). The weekly mean ETo varied between 3.59 mm day-1 (52nd SMW) to 10.17 mm day-1 (22nd SMW). The total ETo is highest in the month of May (292.46 mm) followed by June (244.13 mm) and lowest in December (114.47 mm) followed by January (126.14 mm). It is observed that, there is progressively decrease in the total seasonal ETo from summer season (February to May) to kharif (June to September) and rabi (October to January) seasons. The average annual ETo was 2155.85 mm.

Evaluation of uncertainty in evapotranspiration values by FAO56-Penman-Monteith and Hargreaves-Samani methods

Methods of calculating evapotranspiration are subject to uncertainty. It is important to evaluate their uncertainty. Using 54 years of data, this study evaluated the uncertainty values of monthly reference crop evapotranspiration calculated with the FAO-56 Penman-Monteith and Hargreaves-Samani methods. The objective of this study was to determine the range of variation in the results of each method. It was found that for both methods, the bandwidth uncertainty obtained with 95% confidence interval was more in warm months than in cold months, and the mean and variance by the Hargreaves-Samani method were always less than by the FAO-56 Penman-Monteith method. The uncertainty value of the FAO-56 Penman-Monteith method was more than of the Hargreaves-Samani method, because the number of parameters used in the FAO-56 method was more than in Hargreaves-Samani method which increased uncertainty resources.

EVAPOTRANSPIRAÇÃO E COEFICIENTE DE CULTURA DO CAFEEIRO IRRIGADO A PARTIR DE IMAGENS DE SENSORES ORBITAIS

EVAPOTRANSPIRAÇÃO E COEFICIENTE DE CULTURA DO CAFEEIRO IRRIGADO A PARTIR DE IMAGENS DE SENSORES ORBITAIS
 
 
 THALLES LOIOLA DIAS1; ALEX PORTO RODRIGUES2; MILLER RAIK ARCANJO BATISTA3; Marcelo Rossi Vicente4 e Ronaldo Medeiros dos Santos5
 
 1 Instituto Federal do Norte de Minas Gerais, Campus Salinas, Fazenda Varginha Km 02 Rod. Salinas/Taiobeiras - Salinas/MG -CEP:39560-000, Salinas, MG, Brasil. E-mail: thallesloiola1@gmail.com.
 2 Instituto Federal do Norte de Minas Gerais, Campus Salinas, Fazenda Varginha Km 02 Rod. Salinas/Taiobeiras - Salinas/MG -CEP:39560-000, Salinas, MG, Brasil. E-mail: alex-porto@hotmail.com.
 3 Instituto Federal do Norte de Minas Gerais, Campus Salinas, Fazenda Varginha Km 02 Rod. Salinas/Taiobeiras - Salinas/MG -CEP:39560-000, Salinas, MG, Brasil. E-mail: raik.miller@gmail.com.
 4 Instituto Federal do Norte de Minas Gerais, Campus Salinas, Fazenda Varginha Km 02 Rod. Salinas/Taiobeiras - Salinas/MG -CEP:39560-000, Salinas, MG, Brasil. E-mail: marcelo.vicente@ifnmg.edu.br.
 5 Instituto Federal do Norte de Minas Gerais, Campus Salinas, Fazenda Varginha Km 02 Rod. Salinas/Taiobeiras - Salinas/MG - CEP:39560-000, Salinas, MG, Brasil. E-mail:ronaldo.medeiros@ifnmg.edu.br. 
 
 
 1 RESUMO
 
 O uso de sensoriamento remoto na agricultura é uma realidade. Dentre os diversos usos, destaca-se a determinação da evapotranspiração dos cultivos para o auxílio do processo de gerenciamento da irrigação. O presente trabalho objetivou determinar a evapotranspiração e o coeficiente da cultura do cafeeiro através do algoritmo SEBAL (Surface Energy Balance Algorithm for Land) na região Oeste da Bahia. Para a realização do estudo foram utilizadas imagens do satélite LANDSAT 7. A evapotranspiração de referência foi estimada pelo método Penman-Monteith FAO e, posteriormente, calculou-se o coeficiente da cultura (Kc) com base na evapotranspiração obtida via SEBAL. Os índices estatísticos para avaliar a eficácia do modelo SEBAL foram: o desvio da raiz quadrada média (RMSE); o erro médio absoluto (MAE); o coeficiente de determinação (R2); e o erro relativo (RE). O modelo SEBAL mostrou-se eficiente na determinação da evapotranspiração da cultura do cafeeiro e no coeficiente de cultura.
 
 Palavras-chave: índice de vegetação; sebal; manejo de irrigação.
 
 
 DIAS, T.L.; RODRIGUES, A.P.; BATISTA, M.R.A.; VICENTE, M.R.; SANTOS, R. M.
 EVAPOTRANSPIRATION AND CROP COEFFICIENT OF COFFEE PLANTS FROM ORBITAL SENSORS IMAGES
 
 
 2 ABSTRACT
 
 The use of remote sensing in agriculture is a reality. Among the various uses, the determination of crop evapotranspiration to aid the irrigation management process is detached. The present work aimed to determine the evapotranspiration and the crop coefficient of coffee through the Surface Energy Balance Algorithm for Land (SEBAL) in western Bahia. LANDSAT 7 satellite images were used to perform the study. The reference evapotranspiration was estimated by the FAO Penman-Monteith method and subsequently the crop coefficient (Kc) was calculated based on the evapotranspiration obtained by SEBAL. The statistical indexes for evaluating the effectiveness of the SEBAL model were the root mean square error (RMSE), the mean absolute error (MAE), the coefficient of determination (R²) and the relative error (RE). The SEBAL model proved to be efficient in determining coffee crop evapotranspiration and crop coefficient.
 
 Keywords: vegetation index; sebal; water management.

Spatiotemporal trends in reference evapotranspiration over South Korea

The universal phenomena of global warming caused by climate change have direct linkage with different hydro-meteorological variables which in turn affect the evaporative demand of the atmosphere. Therefore, this study evaluated the seasonal and spatial changes in reference crop evapotranspiration (ETo) during last 4.7 decades and identified the forcing mechanism behind the seasonal changes in ETo using a stepwise regression equation. To remove the effect of serial correlation, the modified Mann–Kendall test was adopted together with Sen's slope and linear regression method to identify and compare the spatial and temporal trends between Thornthwaite, Hargreaves and FAO Penman–Monteith (FAO-PM) methods. Results indicate that the annual average ETo value is higher along the south and east coastal areas and lower along the northwest side of South Korea. The highest number of stations with significant increasing trends was detected in Thornthwaite and significant decreasing trend in FAO-PM method. Spatial and temporal correlation analysis showed the existence of strong correlation between Hargreaves and FAO-PM methods with the Pearson r value ranging from 0.84 to 0.98 and R2 of 0.8. Wind speed is found to be the most influencing climatic variable, especially in autumn, early winter and early summer, and maximum temperature during spring and late summer.

Can artificial neural networks estimate potential evapotranspiration in Peruvian highlands?

Evapotranspiration (ETo) is one of the most important variables of the water cycle when water requirements for irrigation, water resource planning or hydrological applications are analyzed. In this context, models based on artificial neural networks (ANN) of the retro-propagation type can be an alternative method to estimate ETo in highland regions using a number of input variables limited. The objective of this study is to develop ANN models to estimate ETo for the Peruvian highlands using input variables such as maximum air temperature (Tmax), minimum air temperature (Tmin), hours of sunshine (Sh), relative humidity (Rh) and wind speed (Wv), as an alternative method to FAO Penman–Monteith method (FAO-PM56) and Hargreaves–Samani (HS). Daily climatic datasets recorded at 12 meteorological stations between 1963 and 2015 were selected in this study. For evaluation reason, the ETo calculated using the FAO-PM56 was also considered. The main input variable to ANN modeling is Tmax, followed by Sh and Wv or combinations between them. Hargreaves–Samani (HS) showed a poor performance in the estimation of the ETo in the Peruvian highlands compared to the 13 ANN models. Additionally, it was determined that in stations with lower thermal amplitude (< 14.2 °C) the lowest performance levels are presented in the estimation of the ETo with HS equation, which does not occur markedly with the ANN models that they estimate adequately ETo. Therefore, ANN models represent a great option to replace the FAO-PM56 and HS method, when ETo data series are scarce.

Using a lysimetric station to determine the irrigation water requirements for chufa crop (Cyperus esculentus var. sativus)

Chufa, also known as tigernut, is a typical irrigated crop in Valencia, Spain, where it is traditionally cultivated for its tubers. Its cultivation uses large amounts of water, in the order of 10,000 m3 ha(‑1) year‑1. During the last years, the research team in which this study has been developed, has carried out several studies leading to improve the irrigation water use efficiency in chufa. Although these improvements are considerable, they are not definitive, as the water requirements of the crop are still unknown. To know these needs, it would be recommended to determine the crop evapotranspiration (ETc). The experiment was carried out on an experimental plot of the Universitat Politecnica de Valencia, included in the traditional cultivation area. This plot includes a lysimetric station (Smart Field Lysimeter) which allows determining the daily ETc. In addition, the plot is equipped with a meteorological station, which provides the necessary parameters for the determination of the daily reference crop evapotranspiration (ETo) through the FAO-56 Penman-Monteith method. Relating the values of ETc and ETo, the crop coefficient (Kc) values corresponding to the different growth stages, has been estimated from the data obtained during the crop cycle. Considering the crop development, the length of the four growth stages were considered as 21, 76, 43 and 29 days for initial, development, mid-season and late season stages, respectively. The single Kc values for the initial, mean and end of the late season stages were established as 0.25, 1.43 and 0.75, respectively. The use of these Kc values will allow adjusting irrigation water doses to water requirements, thus reducing the volume of water applied and improving the irrigation application efficiencies in chufa crop.

Climate-induced historical drift of reference evapotranspiration in Mymensingh region of Bangladesh

Reference crop evapotranspiration (ETo) is essential for planning and management of irrigation to ensure optimum utilization of a region’s available water resources. ETo being an indicator of atmospheric evaporative demand provides a measure of the integrated effect of climatic parameters like solar radiation, wind, temperature and humidity. Variation of these climatic parameters over long period of time alters ETo. The modified ETo is crucial for periodic adjustment of irrigation planning and management. This study evaluated variation of ETo and contribution of the climatic parameters to ETo-variation in Mymensingh region of Bangladesh by analyzing climatic data of 28 years (1990–2017). ETo was determined by FAO Penman-Monteith method and trends of ETo and its governing climatic parameters were evaluated by MAKESENS trend model. The ETo-governing climatic parameters revealed contrasting trends, which also varied in different months of the year. Net radiation and wind speed showed decreasing trend, while temperature and saturation vapor pressure deficit showed increasing trend. In spite of contrasting contributions of the climatic parameters, their combined effect reduced ETo with a resulting decreasing trend of the monthly average daily ETo over the months of the year except July. These results enhance our understanding of the effects of climate change on ETo and can help correct-planning of water resources for irrigated agriculture.&#x0D; J. Bangladesh Agril. Univ. 17(2): 258–264, June 2019

Change of winter wheat planting area and its impacts on groundwater depletion in the North China Plain

The North China Plain is one of the most water-stressed areas in China. Irrigation of winter wheat mainly utilizes groundwater resources, which has resulted in severe environmental problems. Accurate estimation of crop water consumption and net irrigation water consumption is crucial to guarantee the management of agricultural water resources. An actual crop evapotranspiration (ET) estimation model was proposed, by combining FAO Penman-Monteith method with remote sensing data. The planting area of winter wheat has a significant impact on water consumption; therefore, the planting area was also retrieved. The estimated ET showed good agreement with field-observed ET at four stations. The average relative bias and root mean square error (RMSE) for ET estimation were −2.2% and 25.5 mm, respectively. The results showed the planting area and water consumption of winter wheat had a decreasing trend in the Northern Hebei Plain (N-HBP) and Southern Hebei Plain (S-HBP). Moreover, in these two regions, there was a significant negative correlation between accumulated net irrigation water consumption and groundwater table. The total net irrigation water consumption in the N-HBP and S-HBP accounted for 12.9×109 m3 and 31.9×109 m3 during 2001–2016, respectively. Before and after 2001, the decline rate of groundwater table had a decreasing trend, as did the planting area of winter wheat in the N-HBP and S-HBP. The decrease of winter wheat planting area alleviated the decline of groundwater table in these two regions while the total net irrigation water consumption was both up to 28.5×109 m3 during 2001–2016 in the Northwestern Shandong Plain (NW-SDP) and Northern Henan Plain (N-HNP). In these two regions, there was no significant correlation between accumulated net irrigation water consumption and groundwater table. The Yellow River was able to supply irrigation and the groundwater table had no significant declining trend.

Estimation of Potential Evapotranspiration by Different Methods in Handan Eastern Plain, China

Potential evapotranspiration estimation is the foundation of water resources assessment. Based on the daily meteorological data during 2000-2005 of Linzhang Meteorological Station in Handan Eastern Plain, temperature-based Hargreaves method, radiation-based Priestley-Taylor method, and FAO Penman-Monteith method with comprehensive consideration of aerodynamics were used to estimate potential evapotranspiration. Correlations between monthly potential evapotranspiration and water surface evaporation were conducted. The results indicated that potential evapotranspiration calculated by Hargreaves method was the largest, while the potential evapotranspiration calculated by Priestley-Taylor method was the smallest. The seasonal potential evapotranspiration values for the three methods were summer > spring > autumn > winter. The correlation between potential evapotranspiration calculated by the FAO Penman-Monteith method and water surface evaporation during the same period was best (r=0.991). In contrast, Penman-Monteith method is more suitable for estimating the potential evapotranspiration in Handan Eastern Plain.

ESTIMATING THE POTENTIAL EVAPOTRANSPIRATION (PET) USING SATELLITE IMAGERY IN ARID LANDS

Evapotranspiration is one of the main sources of water loss in arid regions. Although FAO Penman-Monteith (FAO-PM) method is one of the accurate method to estimate potential evapotranspiration (PET). Nevertheless, it is a point-based method and requires several weather data such as radiation, wind speed, air humidity and temperature, which is not available in all regions. Therefore, this study is aimed to estimate PET equivalent to FAO-PM model using satellite data and Priestly-Taylor (PT) model in Yazd province, located in Central Iran. Daily net radiation and soil heat flux, which are used in PT method, were calculated using Landsat 5 data. Then, the net radiation data were integrated with ground data to estimate PET through PT equation. Although similarity between ground-based Priestly-Taylor (PT land) and satellite-based Priestly-Taylor (PT image) was observed, FAO-PM had higher PET values (RMSE=4.74 mmd-1) compared to other methods in all climatic zones of the study area. Therefore, results of PT image method can be replaced with PT land results to estimate potential evapotranspiration, in regions with limitation or lack of weather data. Finally, linear regression model were used for presenting relationship between ground-based FAO-PM and PT land. For this purpose, 70% and 30% of ground data were used for modeling and testing, respectively at different Selianinov climatic zones of the study area. Maximu RMSE for modeled and test data were 4.6 and 2.93, respectively. The model output was applied on PT image to get the FAO-PM image with maximum RMSE of 3.67. It can be concluded that the defined regression equations for each climate zones in this study, can be used to convert values of satellite- based Priestly Taylor method (PT image) as independent variable (X) to equivalent of FAO-PM PET.

Water requirement of crops using CROPWAT model-A case study of Buxar district in Bihar

India with a geographical area of nearly 3.3 million square kilometers is home to 16% of the population of the world whereas it has only 4% of the total freshwater resources of the world. Moreover, there is huge inequality in the distribution of water resources within the country. The scarce natural resource is fundamental to life, livelihood, food security and development. Sustainable management of this scarce resource has become a challenge nowadays owing to increased demands of increasing population, growing urbanization and rapid industrialization. It has become very important to define appropriate strategies for planning and management of irrigated farmland. One of the major practices adopted by the researchers for estimating water requirement of the crop is modeling. Understanding crop water requirement is essential for irrigation scheduling and selection of cropping pattern in any particular area. This study has been conducted to estimate irrigation requirement, crop evapotranspiration and yield responses in Buxar district of Bihar, using CROPWAT model. Historical climate data from weather stations in the region along with soil and crop data were used as input to FAO Penman-Monteith method to estimate reference evapotranspiration (ETo). Effective rainfall was calculated using USDA soil conservation method.

Rainfall and potential evapotranspiration patterns and their effects on climatic water balance in the Western Lithoral Hydrological Zone of Nigeria

This study examined the climatic water balance and aridity in the Western Lithoral hydrological zone of Nigeria. Monthly climatic data from 1976-2015 collected from six Nigeria Metrological Agency (NIMET) stations located in Ikeja, Ibadan, Akure, Abeokuta, Osogbo and Ilorin were used. The climatic variables are rainfall amount, temperature, sunshine hour, relative humidity and wind speed. The FAO Penman-Monteith method was adopted in estimating the potential evapotranspiration (PET). Water balance was determined from PET and rainfall amount. The spatial and temporal pattern of rainfall amount and PET were evaluated using universal kriging interpolation method, Mann-Kendall and regression trend analysis. The results of the Mann-Kendall and regression trend analysis revealed a statistically significant (Z= 2.74, P= 0.005) upward trend in rainfall amount between 1976 and 2015. Similarly, a statistically significant (Z= 4.40, p= 0.001) increasing trend was observed in PET during the same period. Six months of water surplus (May-October) and six months of water deficit (November-April) were identified. Overall, 27 years of water deficit was discovered over the 40 years period. Water balance pattern revealed the highest annual mean water surplus (324.51 mm) in the hinterland of Idanre, Akure, and Owo rather than the coastal city of Ikeja which can be attributed to orographic effect in the hinterland. The study area with a mean Aridity index (AI) of 0.94 can still be regarded as humid environment, although the spatial variability of AI indicates that the northern part is tending towards a dry sub-humid condition (AI= 0.76). This study recommends conservation of water surplus in the wet months for the augmentation of deficit in the dry months; this will not only boost agricultural production, but also alleviate water supply problem in the study area. Keywords. Aridity index, potential evapotranspiration, water balance.

Evaluation of reference evapotranspiration methods and sensitivity analysis of climatic parameters for sub-humid sub-tropical locations in western Himalayas (India)

ABSTRACT The estimation of crop evapotranspiration (ETc) uses reference evapotranspiration (ET0) as a key variable. Physically based FAO-56 Penman–Monteith method provides accurate ET0 estimates; however, various methods are still used in different climatic regions worldwide, either due to simplicity of the methods or due to insufficiency of data required for Penman–Monteith method. The present study is conducted at sub-humid sub-tropical locations of western Himalayan agro-climate in the Indian state of Himachal Pradesh. Performance of 12 ET0 methods, representing combination type, pan evaporation, radiation and temperature based, is compared with Penman–Monteith method. Different ET0 methods are ranked on the basis of quantitative comparison performed on mean daily ET0 obtained for a period of 6 years. The temperature-based methods performed better than radiation and pan evaporation-based methods. Linear regression equations are developed between Penman–Monteith ET0 (dependent variable) and ET0 estimates from alternate method (independent variable). Relative significance of meteorological factors essential for accurate ET0 estimation in the climate considered is investigated using sensitivity analysis. Solar radiation is identified as the most sensitive parameter for ET0 estimation in the climate of the study area, followed by maximum temperature and relative humidity.

Spatio-temporal changes of the climatic water balance in Romania as a response to precipitation and reference evapotranspiration trends during 1961–2013

Changes in precipitation (P), evapotranspiration (ET) and, implicitly, in the climatic water balance (CWB), are imminent effects of climate warming. However, changes in the CWB, as a response to changes in P and ET, have not yet been analysed thoroughly enough in many parts of the world, including Romania. The present study explores the spatio-temporal changes of the CWB (difference between P and reference evapotranspiration, ETo) in Romania, based on a wide range of climatic data (P, as well as temperature, relative air humidity, sunshine duration and wind speed, necessary for computing ETo with the FAO-56 Penman-Monteith method) recorded at 70 weather stations across the country in the 1961–2013 period. As a secondary objective, the study attempts to identify the possible connections between the index's trends and large-scale atmospheric circulation, assessed based on the dynamics of certain European-scale relevant teleconnection indices. Thus, the Mann–Kendall test and Sen's slope methods were used to analyse CWB trends (but also P and ETo trends, in order to explain CWB pattern changes) annually, seasonally and in the maize and wheat growing seasons. Also, the Spearman correlation procedure and a composite analysis between interannual series of teleconnection indices and CWB were used to assess the influence of atmospheric circulation on the index's variability for all analysed time scales. The results generally showed CWB decreases (for the most part of up to −2 mm/yr, yet with a relatively low statistical significance) and highlighted an overall amplification of drier conditions on all time scales, except for autumn (CWB increases, generally of up to 1 mm/yr, but with low statistical significance). Moreover, net changes of even under −200 mm/53 yrs annually and −175 mm/53 yrs in summer and for the maize and wheat growing seasons were found in the CWB. Spatially, it was found that the country's southwestern and southeastern regions are the main epicentres of drier trends, while the northwest appears to have become wetter. Overall, the negative CWB trends are due to partial P decreases (statistically insignificant) and general ETo increases (highly statistically significant, even 100% in summer). It seems that the amplification of the climatic water deficit across the country is especially linked to the positive phases of the Arctic Oscillation and North Atlantic Oscillation, but also, in part, to those of several other teleconnection indices that affect Europe. Our results signal the necessity to adapt anthropic and ecological systems to future dryness trends countrywide, which will most likely intensify against the background of climate change expected to occur by the end of the century.

Irrigation water management in arid regions of Middle East: Assessing spatio-temporal variation of actual evapotranspiration through remote sensing techniques and meteorological data

Spatial and temporal distribution of reference (ETo) and actual evapotranspiration (AET) over the central region of Saudi Arabia during 1950–2013 are estimated using remote sensing and GIS techniques. Firstly, the FAO Penman-Monteith method was used to model the spatial distribution of ETo on a grid-by-grid basis using data collected from meteorological stations and GIS techniques. Then, crop coefficients (Kc) were modeled as a function of 16-day time-series MODIS normalized difference vegetation index (NDVI). Next, using Kc maps and ETo as input, daily AET was simulated by the soil water balance (SWB) model and aggregated to monthly and annual AET. From empirical NDVI-Kc relationships developed and applicable at pixel level, Kc derived from the NDVI-Kc relationships agree well with Kc recommended by FAO over various crop growth stages in the field. The monthly AET maps for 1950–2013 show a gradual increase in AET during the crop-growing season in January to May but a subsequent decline as the season progresses from June to December. The AET estimated for January to June are arranged in descending order, which are May (3.67–44.7 mm/day), April (5.99–36.8 mm/day), March (2.96–32.7 mm/day), February (0.68–20 mm/day), June (2.42–17.7 mm/day) and January (1–11 mm/day), respectively. Statistical analysis shows that statistically significant change point in daily AET generally occurred in 1990, such that the long-term average daily AET of 1950–1990 at 3.6 mm/day increased to about 5.3 mm/day between 1990 and 2016 with a positive trend of 1.5 mm/decade. The annual AET estimated for irrigated cropland in northern and central regions of Riyadh, Al-Qassim province and Hail province range from 1200 to 2900 mm/year. In these regions, low AET values are found in shrubland, grassland, and other natural vegetation. The annual AET estimated by the SWB model are about 9–11% higher than modeled AET in the study area, where the long-term average daily AET estimated for 1950–2013 range from 2 mm/day to 30 mm/day. Representative AET maps derived from applying the NDVI-Kc relationships to the SWB model will be useful to achieve the planning and management of sustainable water use in arid regions of Middle East.

Evapotranspiration and water response function of squash cv. ‘Italiana’ under different cultivation conditions

ABSTRACT Aiming to support projects and managements of irrigation for vegetables in the Mid-North region of Mato Grosso state, Brazil, this paper determined crop evapotranspiration (ETc), crop coefficients (Kc) and water response function of summer squash (Curcubita pepo) cv. ‘Italiana’ under different cultivation conditions. The plantations were conducted in the dry season of the region (July to October 2016), using 2.5 kg m-2 of millet straw (grass) and crotalaria straw (legume) and bare soil. ETc was obtained by soil water balance through tensiometry, whereas Kc values were given by the ratio between ETc and the reference evapotranspiration (ETo) obtained by the FAO-56 Penman-Monteith method. For the water response function, the rates of 40, 60, 80, 100 and 120% ETc were applied, in which case the Class A pan method was used to obtain ETo. The different soil covers did not influence the ETc, Kc and accumulated thermal sum in the phenological stages of the summer squash. Kc values were 0.1, 0.25, 0.38 and 0.41, for the phenological stages I to IV, with average durations of 8, 46, 16, and 20 days, respectively. Increase in irrigation depths did not influence the rates of production of male and female flowers, but affected yield and decreased water use efficiency, regardless of soil cover.