ETo Values Research Articles

LANDSCAPE COEFFICIENT TO DETERMINE WATER REQUIREMENT FOR A COMPUTER MODEL

The two factors used to determine landscape water requirement (ETL), the landscape coefficient and reference evapotranspiration, are solely responsible for producing differences in water loss estimates. For plantings in the same location (i.e., where the same ETo values will be used), the differences will arise solely from the landscape coefficient. To produce useful estimates of water loss, therefore, it is important to carefully determine the value of KL. In agriculture, irrigation water requirements are well established for many crops. In urban landscapes, irrigation requirements have been determined for turf grasses, but not for most landscape species. This study adapts this method for application to landscape plantings. The method used for estimating water needs for landscape plantings is basically the same as that used for crops and turfgrasses. One key change, however, has been made: instead of using the crop coefficient (Kc), a landscape coefficient (KL) has been substituted. The main objective of this work is building computer program to determine water requirement for some multi-plant landscape and modern system for their irrigation. Audit in the accounts of landscape coefficient (KL) closest to the prevailing conditions by application Landscape Irrigation Scheduling program outputs, led to save water use by 60% for landscape plants in Giza (latitude 30o 05', longitude 31o12') with good appearance and growth for landscape plants and grass. The program could offer a simple tool for planning ornamental plants and turf grass water requirements for landscape projects.

EFFECT OF SPRINKLER IRRIGATION MANAGEMENT AND STRAW MULCH ON YIELD, WATER CONSUMPTION AND CROP COEFFICIENT OF PEANUT IN SANDY SOIL

Field experiments were conducted at El-Bustan Agricultural Research Station, in summer season of 2013 on peanut (Arachis hypogaea L.) variety Giza 5, to study the effect of sprinkler irrigation management and the application of rice straw mulch on yield, quality, water consumptive use and crop coefficient of peanut in sandy soil. Two irrigation treatments, (I3) and (I5) were imposed based on the soil water depletion to bring the soil water deficit to the field capacity at two irrigation intervals of 3 and 5 days. Mulching treatments were applied randomly during the 3-leaf growth stage for one half of the experimental area (M) at a rate of about 500 g/m2, while the other half remained bare (non-mulched, Mo). The experimental design was split- plot with three replications and consisted of irrigation treatments in the main plots, mulching and non-mulching treatments as subplots. The results revealed that, higher yield and better crop growth were observed in the mulched plots as compared to nonmulched plots under the same irrigation treatments. Also, the effect of I3 treatment was more pronounced than the effect of I5 treatments. The average values of maximum plant length, No. of pods/plant, pods weight/plant, total Bio-mass, 100 kernels weight ,total pods yield and total kernels yield, were 65.00 cm, 39.45, 53.20 g., 4012.05 kg/fed, 67.10 g, 1658.80 kg/fed and 1176.00 kg/fed, respectively observed with I3M treatment. An increase in the tested quality parameters such as shelling percent, protein and oil percentage, of 8.9, 4.40 and 2.13%, respectively were obtained with I3M treatment as compared to the control treatment (I3MO). The computed ETO value in course of the peanut growth season was 752.25 mm. I3MO treatment had the maximum ETC and applied irrigation water (AIW) values of 2415.0 and 2952.60 m3/fed, respectively. Applying rice straw mulch conserved irrigation water by about 14.37 and 21.62% with I3 and I5 treatments, respectively. WUE ranged from 0.60 with I3M and I5M to 0.36 kg/m3 with I5MO treatment, meanwhile, I5M and I3M treatments had the maximum IWUE values of 0.48 and 0.47 kg/m3, respectively. The maximum crop coefficient (KC) value of 0.88 occurred during July, which was less than the generic value listed in FAO-33and-56 by about 16.2 and 10.2%, respectively, however the average KC values of peanut at El-Bustan area of 0.73 was close to FAO-33 and-56 listed values. These results showed that precise management of the available soil moisture content in the course of the growing season, and accurate estimation of ETC as well as, developing site and cultivar specific KC value can be effective way for appropriate irrigation scheduling and water allocation, saving water, maximizing yield and consequently optimizing the economic return.

GIS Tool for Distribution Reference Evapotranspiration under Climate Change in Egypt

Current climatic data were collected to estimate ETo from 1998 to 2007 in the major Egyptian agro-climatic regions, i.e. Delta region, Middle Egypt and Upper Egypt, to generate future mean air temperature unde r climate change conditions using Magicc\Scengen model. FAO -56 Penman -Monteith equation was used to estimate ETo by using the collected and generated climatic data. The climate change data were generated under four scenarios (A1, A2, B1 and B2, according t o IPCC results revealed that ETo significantly increased in 2050s and 2100s compared to the current ETo values. Results indicated that the values of ETo in 2100s were higher than those for 2050s for all studied regions. The average percentages increase in the four tested scenarios of the 2100s for Delta, Middle Egypt and Upper Egypt were 14, 17 and 18%, respectively. The most affected region by climate change was Upper Egypt because it already has the highest ETo under the current conditions; this increase in ETo could affect agricultural expansion negatively, especially if the water resources become a limiting factor. Moreover, the spatial analysis using kriging interpolation showed that the ETo in Egypt will be increased by uneven values under future clima te change, the highest increasing values will appear under the A1 scenario, while the lowest increasing will be observed under B1 and B2 scenarios.

A Predictive Approach to Simulate Possible Impacts of Climate Change on the Agricultural Sector

The aim of this paper is to investigate the possible implications of climate change of maize and beans crop in the country of Belize. The approach utilized PRECIS and observed input data in combination with the Food and Agriculture Organization (FAO) CROPWAT program. Six scenarios were predicted looking at Maximum Temperature (Tmax), Minimum Temperature (Tmin) and Rainfall for the years 2050-2055 along with observed data of similar climatic variables for the year 1998. The approach was use for three sites namely Towerhill which is located to the north of the country, Central Farm which is located at the center of the country and Toledo Research and Development Project Site (TRDP) which is located at the southernmost part of the country. The results showed that there was an increase in both maximum and minimum temperature at two of the three sites with the exception of the Towerhill site and there was a reduction in rainfall in all predicted scenario with the exception of Towerhill site in 2051 which exhibited a 4.28 % increase over the baseline scenario. The reference evapotranspiration ETo value illustrated more precisely the possible impact of climate change as there was an increase in of all ETo values at the three observation sites. In regards to yield percent reduction the scenarios determine that climate change would have an impact on crop yield. Where yield percent reduction was as high as 80.1% at the Central Farm site. The results determined that climate change will affect these crops, by validating this, more suitable strategies and mitigating programs can developed and established to assist Belize in the prevention of a future food crisis.

Water requirements for major crops in different agro-climatic zones of Iraqi Kurdistan using by CROPWAT 8.0

The study was performed by using FAO Penman-Monteith equation. Required parameters were set for 9 synoptic stations in Iraqi Kurdistan for a period from 2004 to 2012. Sangasar and Karkuk were selected as two different agro-climatic zones. Data were processed by (CROPWAT 8.0) program, and the values of ETo and effective rainfall were calculated. Several crops were selected in the study including alfalfa, barley, beans, cabbage, grapes, maize, potatoes, sunflower, melons, tomatoes and wheat. Crop coefficients (Kc) were taken from tables of FAO under medium soil conditions. The results show that potential evapotranspiration in Sangasar zone was 0.9 -8 mm/day, whereas in Karkuk zone was 1.5 to12 mm/day. The highest water requirement was estimated for grape in the Sangasar zone (Soran) 987.6mm/day and 1635.1 mm/day for Karkuk zone (Makhmur). In the same way, the minimum water requirement was estimated for alfalfa (first mowing) in Sangasar zone (Penjwen) 224.1mm/day and 269.7mm/day for Karkuk zone (Chamchamal).

Evaluation of wheat and maize evapotranspiration determination by direct use of the Penman–Monteith equation in a semi-arid region

The Penman–Monteith (PM) equation was introduced as one of the most reliable equations to determine crop ETc, without using crop coefficient or ETo values. In this study, the PM equation was evaluated using lysimeters in a semi-arid region for wheat and maize. Different equations for aerodynamic resistance (r a) and canopy resistance (r c) were tested in the PM equation and they were ranked using statistical analysis. It was shown that the combined method of r a and r c in FAO-56 does not lead to a good prediction of ETc for wheat and maize in comparison with the lysimeter-measured data. The results indicated that a modified equation for r c was the most accurate method for both wheat and maize. Using this equation, the suggested model of FAO-56 and another investigation for r a led to the best results for wheat and maize, respectively. Furthermore, it was shown that the previously modified equation for r c was newly modified as a function of vapor pressure deficit (VPD) and the results were as accurate as before. Therefore, an equation as a function of VPD can be used when solar radiation (R n) is not available easily.

Regionalization of the Hargreaves Coefficient for the Assessment of Distributed Reference Evapotranspiration in Southern Italy

AbstractThe Hargreaves equation (HE) is the most reliable and diffused temperature-based method for the estimate of the reference crop evapotranspiration ETo in areas with limited meteorological data. Several studies proposed its further enhancement by adopting correction equations for recalibrating its parameters, specifically the empirical Hargreaves coefficient, C=0.0023. In this paper, a local recalibration for southern Italy is shown, taking as a reference the Food and Agriculture Organization of the United Nations (FAO) Penman-Monteith (PM) equation applied over 137 (34 coastal and 103 inland) meteorological stations, allowing the estimation of 357,920 daily ETo values in the period 1994–2011. Through a regression analysis, the adjusted Hargreaves coefficients (AHC), minimizing the errors for each individual station between the HE and the FAO Penman-Monteith equation, and providing better performances than the original HE applied at the monthly scale, were correlated to the available daily meteorolo...

Estimation of reference evapotranspiration using a user-friendly decision support system: DSS_ET

Due to wide application of evapotranspiration (ET) data, a number of indirect methods for estimation of reference ET (ETo) have been developed. Therefore, it becomes impractical for many users to select the best ETo estimation method for the available data and climatic condition. To overcome this problem, a decision support system (DSS_ET) was developed which supports 22 ETo estimation methods with varied options for calculation of various intermediate parameters, generalized data input format with copy–paste option from spreadsheet applications, visualize/check input data and results, features to estimate missing data, and user-friendly graphical user interface (GUI) that enhances its applicability as a handy research and teaching tool. Monthly meteorological data for 32 years (1971–2002) were procured from IMD, Pune, for 133 selected stations evenly distributed over five climatic ecosystems of India. Altogether 18 ETo estimation methods, including the globally accepted standard method FAO-56 Penman–Monteith (PM), were found applicable for the data availability conditions used in this study. In absence of lysimeter measured ETo data, the FAO-56 PM ETo values were used to estimate the normal monthly ETo for each station and climatic condition. Finally, ranks were assigned to each applicable ETo method based on weighted standard error of estimate (WSEE) with respect to the FAO-56 PM method for different climatic ecosystems. If, because of poor data availability condition, it becomes unavoidable to use methods other than FAO-56 PM, then these ranks can readily be used to select the best applicable method in the existing data and climatic conditions to get ETo values closer to FAO-56 PM estimates.

Estimativa da evapotranspiração de referência através de redes neurais artificiais

A estimativa da evapotranspiração por métodos indiretos propicia, de modo facilitado, a geração de dados para o planejamento de sistemas de irrigação e aplicação de modelos meteorológicos e hidrológicos, ambos, úteis na gestão de bacias hidrográficas. O objetivo deste trabalho foi elaborar uma Rede Neural Artificial (RNA) para estimar a evapotranspiração de referência (Eto) em função de dados diários de temperatura do ar. A RNA, do tipo FeedForward Multilayer Perceptron, foi treinada tomando-se por referência a Eto diária obtida pelo método de Penman-Monteith. Nas camadas intermediárias e de saída foram utilizadas funções de ativação do tipo tan-sigmóide e lineares, respectivamente. Os valores de Eto gerados pela RNA foram comparados com os obtidos pelos métodos de Blanney-Criddle e Hargreaves considerando meses referentes às quatro estações do ano. Em relação aos outros métodos analisados, os resultados obtidos a partir da RNA foram mais próximos ao método padrão Penman-Monteith. Assim, o desempenho da RNA desenvolvida foi satisfatório, podendo-se considerá-la como integrante do conjunto de métodos indiretos para estimativa da evapotranspiração, além de representar uma diminuição dos custos de aquisição de dados para estimativa desta variável.

Field Emission from One-Dimensional Submicron Diamond Rods

One-dimensional boron-doped submicron diamond rods (SDRs) were fabricated on diamond/Si substrates by oxygen plasma etching. The SDRs are ∼4.5 µm in height and ∼383 nm in diameter. Iron oxide coated on SDRs is essential in the formation of one-dimensional SDRs. However, the as-etched SDRs suffer with high turn-on field (ETO) and low field emission current density (JFE) due to the iron oxide. A huge improvement in the field emission characteristics can be achieved by removing iron oxide using a wet-etch process in a diluted HCl (37%). After the wet-etch, the SDRs exhibit a low ETO value of4.5 V/µm (at 10 µA/cm2) and a high JFE value of 30 mA/cm2 (at 8.5 V/µm). The FE emitter is only stable for a short period of time at high current stress owing to the rounding of the tips of SDRs.

Measurement of grassland evaporation using a surface-layer scintillometer

A dual-beam surface-layer scintillometer (SLS) was used to estimate sensible heat flux (H) every 2 min for a path length of either 50 or 101 m, for more than 30 months in a mesic grassland in eastern South Africa. The SLS method relies on Monin-Obukhov similarity theory, the correlation between the laser beam signal amplitude variances and the covariance of the logarithm of the beam signal amplitude measured using 2 laser detectors. Procedures for checking SLS data integrity in real-time are highlighted as are the post-data collection rejection procedures. From the H estimates, using SLS and measurements of soil heat flux and net irradiance, evaporation rates were calculated as a residual of the shortened energy balance equation and compared with grass reference evaporation rates (ETo). Inconsistent hourly ETo values occur in the late afternoon due to the incorrect assumption that the soil heat flux is 10% of net irradiance. The SLS estimates of H and the estimates of evaporation rate as a residual compared favourably with those obtained using the Bowen ratio and eddy covariance methods for cloudless days, cloudy days and days with variable cloud. There was no evidence for the eddy covariance measurements of H being underestimated in comparison to the Bowen ratio and SLS measurements. On many days, the diurnal variation in SLS H was asymmetrical, peaking before noon.Keywords: energy balance, Bowen ratio, eddy covariance, grass reference evaporation, rejection criteria

Avaliação de modelos de estimativa do saldo de radiação e do método de Priestley-Taylor para a região de Dourados, MS

Este trabalho foi realizado com o objetivo de avaliar modelos de estimativa da radiação líquida e o método de Priestley-Taylor para a região de Dourados. O ajuste dos parâmetros dos modelos foi realizado com base em uma série de 1.421 dados diários de radiação líquida, radiação solar global, radiação extraterrestre, temperaturas máxima e mínima. Uma segunda série de dados com 360 observações diárias foi utilizada para validar as equações geradas. A evapotranspiração de referência (ET0) foi estimada pela equação de Priestley-Taylor, como função da radiação solar global. Os valores de ET0 foram comparados com 218 medidas lisimétricas. As estimativas de radiação líquida geradas com base apenas nas temperaturas máximas e mínimas não foram satisfatórias, mas o modelo que utilizou radiação extraterrestre, além dessas duas variáveis, apresentou performance mediana. Os modelos que utilizaram a radiação solar global como variável independente tiveram desempenho classificados como ótimos. O método de Prietley-Taylor apresentou desempenho muito bom, possibilitando estimar a evapotranspiração de referência diária da região de Dourados com base na radiação solar global e na temperatura média do ar.

Temporal Trends in Estimates of Reference Evapotranspiration over India

Evapotranspiration (ET) is likely to be greatly affected by global warming because of the dependence of ET on surface temperature. The increasing atmospheric concentration of carbon dioxide (CO2) and other greenhouse gases is expected to increase precipitation and evaporation proportionally. However, a few studies have shown a decreasing trend for evaporation over the last 50years globally. In India, earlier works showed that there was a significant increasing temporal trend in surface temperature and a decreasing trend in grass reference ET (ETo). To study the temporal trend of ETo along with its regionwise spatial variation, 32years (1971–2002) monthly meteorological data were collected for 133 selected stations evenly distributed over different agro-ecological regions (AERs) of India. ETo was estimated by the globally accepted Food and Agriculture Organization (FAO) Penman Monteith (PM) method (FAO-56 PM). These ETo values were then analyzed by a nonparametric Mann–Kendall (MK) test (with modified effective sample size approach for serially correlated data) and Sen slope to determine the existence and magnitude of any statistically significant trend over the time period considered in this study. The same analysis was also performed on governing meteorological variables to identify the cause of existence of such trend in ETo. A significant decreasing trend was found in ETo all over India during the study period, which was mainly caused by a significant increase in the relative humidity and a consistent significant decrease in the wind speed throughout the country. However, a general increase in rainfall was not found in recent years.

Reference and Crop Evapotranspiration in South Central Nebraska. I: Comparison and Analysis of Grass and Alfalfa-Reference Evapotranspiration

In Nebraska, historically, there have been differences among the water regulatory agencies in terms of the methods used to compute reference evapotranspiration ( ETref ) to determine actual crop water requirements and hydrologic balances of watersheds. Because simplified and/or empirical temperature or radiation-based methods lack some of the major weather parameters that can significantly affect grass and alfalfa-reference ET ( ETo and ETr ) the performance of these methods needs to be investigated to help decision makers to determine the potential differences associated with using various ETref equations relative to the standardized ASCE Penman–Monteith (ASCE-PM) equations. The performance of 12 ETo and five ETr equations were analyzed on a daily basis for south central Nebraska from 1983 to 2004. The standardized ASCE-PM ETo and ETr values were used as the basis for comparisons. The maximum ASCE-PM ETo value was estimated as 12.6 mm d−1 , and the highest ETr value was estimated as 19 mm d−1 on June 21,...

Variability Analyses of Alfalfa-Reference to Grass-Reference Evapotranspiration Ratios in Growing and Dormant Seasons

Alfalfa-reference evapotranspiration (ETr) values sometimes need to be converted to grass-reference ET (ETo), or vice versa, to enable crop coefficients developed for one reference surface to be used with the other. However, guidelines to make these conversions are lacking. The objectives of this study were to: (1) develop ETr to ETo ratios (Kr values) for different climatic regions for the growing season and nongrowing (dormant) seasons; and (2) determine the seasonal behavior of Kr values between the locations and in the same location for different seasons. Monthly average Kr values from daily values were developed for Bushland, (Tex.), Clay Center, (Neb.), Davis, (Calif.), Gainesville, (Fla.), Phoenix (Ariz.), and Rockport, (Mo.) for the calendar year and for the growing season (May–September). ETr and ETo values that were used to determine Kr values were calculated by several methods. Methods included the standardized American Society of Civil Engineers Penman–Monteith (ASCE-PM), Food and Agriculture Organization Paper 56 (FAO56) equation (68), 1972 and 1982 Kimberly-Penman, 1963 Jensen-Haise, and the High Plains Regional Climate Center (HPRCC) Penman. The Kr values determined by the same and different methods exhibited substantial variations among locations. For example, the Kr values developed with the ASCE-PM method in July were 1.38, 1.27, 1.32, 1.11, 1.28, and 1.19, for Bushland, Clay Center, Davis, Gainesville, Phoenix, and Rockport, respectively. The variability in the Kr values among locations justifies the need for developing local Kr values because the values did not appear to be transferable among locations. In general, variations in Kr values were less for the growing season than for the calendar year. Average standard deviation between years was maximum 0.13 for the calendar year and maximum 0.10 for the growing season. The ASCE-PM Kr values had less variability among locations than those obtained with other methods. The FAO56 procedure Kr values had higher variability among locations, especially for areas with low relative humidity and high wind speed. The 1972 Kim-Pen method resulted in the closest Kr values compared with the ASCE-PM method at all locations. Some of the methods, including the ASCE-PM, produced potentially unrealistically high Kr values (e.g., 1.78, 1.80) during the nongrowing season, which could be due to instabilities and uncertainties that exist when estimating ETr and ETo in dormant season since the hypothetical reference conditions are usually not met during this period in most locations. Because simultaneous and direct measurements of the ETr and ETo values rarely exist, it appears that the approach of ETr to ETo ratios calculated with the ASCE-PM method is currently the best approach available to derive Kr values for locations where these measurements are not available. The Kr values developed in this study can be useful for making conversions from ETr to ETo, or vice versa, to enable using crop coefficients developed for one reference surface with the other to determine actual crop water use for locations, with similar climatic characteristics of this study, when locally measured Kr values are not available.

An integrated method for irrigation scheduling of potted plants

Hourly meteorological and soil water potential data collected during the summer season on a container crop of 2-year-old plants of Hypericum hidcote were used to develop a methodology that integrates climate-based (AET = ETo × K c) and soil-based methods (soil water potential measurements by tensiometers) for the automatic control of the irrigation of plants growing in pots. Hourly ETo values were calculated by the CIMIS equation, derived from the Penman formula. Tensiometric data, by a soil-specific tensiometric curve, were transformed into pot water weight and used to estimate actual hourly evapotranspiration (AETt). AETt values were compared with AET data provided by precision balances (AETb). A good relationship was found between AETt and AETb ( R 2 = 0.7***), so AETt values were used to calculate hourly crop coefficient ( K c) by the AETt/ETo ratio. As the crop coefficients obtained by using AETb ( K cb = AETb/ETo) and by AETt ( K ct = AETt/ETo) shows no significant statistical differences, hourly AET values were calculated as ETo × K ch ¯ , where K ch ¯ is a 7-day average. The relationships between AET, calculated by the integrated system ( ETo × K ch ¯ ), and AETb shows an R 2 = 0.685*** and slope = 0.77. The introduction of the actual K c (AETt/ETo) in K ch ¯ calculation determined an increase in the accuracy of AET estimation, with R 2 and slope increasing to 0.896*** and 0.907, respectively. According to the above results, the integrated system allows the calculation of an operational K c for ornamental crops that can be used to correctly manage the irrigation of plants growing in containers, also if tensiometer malfunctioning or lack of representativity occur.

Efficacy of preoxygenation using tidal volume and deep breathing techniques with and without prior maximal exhalation

We evaluated the influence of prior maximal exhalation on preoxygenation in 15 adult volunteers using tidal volume breathing (TVB) for five minutes and deep breathing (DB) for two minutes with and without prior maximal exhalation. Inspired and end-tidal oxygen, nitrogen and carbon dioxide were monitored continuously and recorded during room air breathing and at 30-sec intervals during 100% oxygen TVB or DB (rate of 8 breaths.min(-1)). Tidal volume breathing with prior maximal exhalation resulted in an end-tidal oxygen concentration (ETO(2)) slightly higher (P = 0.028) at 0.5 and 1.0 min as compared with TVB without prior maximal exhalation at the same time periods. Regardless of whether TVB was preceded by maximal exhalation or not, 2.5 min was required to reach a mean ETO(2) value of 90% or higher. With DB, there were no differences in ETO(2) values at any time period and 1.5 min was required to reach an ETO(2) of 90% or greater, with or without prior maximal exhalation. Maximal exhalation prior to TVB slightly steepens the initial rise in ETO(2) during the first minute, but confers no real benefit if maximal preoxygenation is the goal. Maximal exhalation prior to DB has no added value in enhancing preoxygenation.

Evaluation of Evapotranspiration Estimation Methods for Sweet Cherry Trees (Prunus avium) in Sub-humid Climate

This study was carried out in the summer of 2001 in a 3 year old and in the summer of 2002 in a 4 year old sweet cherry trees (Prunus avium, variety Z-900) on Mazzard rootstocks in Bayramic-Canakkale which is located in the west part of Turkey. Micro-sprinkler irrigation was selected as the irrigation method. The trees were subjected to four micro-sprinkler irrigation treatments (T-1, T-2, T-3 and T-4). The water applied in treatment T-3 was considered sufficient to satisfy fully needs of the crop (100% of ETc) and to allow good rooting and tree growth. The water balance relationship was used in estimating ETc. A total of 4 climatological methods were selected for estimating reference crop evapotranspiration on a daily basis. Some of these methods are based on combination theory and others are empirical methods based primarily on solar radiation, temperature ans relative humidity. An attempt was made in the current study to develop regional relationship between the evapotranspiration measured and that estimated by the climatological methods, such as FAO-Penman, Penman-Monteith, FAO-Radiation and FAO-Blaney-Criddle. Performance of the climatological methods in estimating the ETo values as compared to the measured values was evaluated on the basis of root mean square error (RMSE). In 2001, the Penman-Monteith equation gave the best results followed by FAO-Penman, FAO-Radiation and FAO-Blaney-Criddle. In 2002, the Penman-Monteith and FAO-Blaney-Criddle equations gave same results.

Estimation of the potential evapotranspiration by a simplified penman method

The numerous methods for calculating the potential or reference evapotranspiration (ETo or ETP) almost always do for a 24-hour period, including values of climatic parameters throughout the nocturnal period (daily averages). These results have a nil effect on transpiration, constituting the main evaporative demand process in cases of localized irrigation. The aim of the current manuscript was to come up with a model rather simplified for the calculation of diurnal daily ETo. It deals with an alternative approach based on the theoretical background of the Penman method without having to consider values of aerodynamic conductance of latent and sensible heat fluxes, as well as data of wind speed and relative humidity of the air. The comparison between the diurnal values of ETo measured in weighing lysimeters with elevated precision and estimated by either the Penman-Monteith method or the Simplified-Penman approach in study also points out a fairly consistent agreement among the potential demand calculation criteria. The Simplified-Penman approach was a feasible alternative to estimate ETo under the local meteorological conditions of two field trials. With the availability of the input data required, such a method could be employed in other climatic regions for scheduling irrigation.

Validation of the FAO methodology for computing ETo with limited data. Application to south Bulgaria

AbstractThe calculation of reference evapotranspiration ETo with the FAO Penman‐Monteith method (FAO‐PM) requires maximum and minimum air temperature (Tmax and Tmin), actual vapour pressure (ea), net radiation (Rn), namely computed from sunshine duration or short wave solar radiation (Rs), and wind speed at 2 m (u2). However, some of these weather variables, especially solar radiation, are often missing in irrigation practice. To overcome these problems, procedures to estimate ETo with missing climate data are proposed as part of the FAO‐PM methodology and it is important to assess the accuracy of these procedures for different climate conditions. This is the objective of this study relative to the Trace plain, south Bulgaria, where data relative to five meteorological stations in the Trace plain are analysed. All stations are located within a homogeneous physiographic region where the air masses governing the weather are the same. The procedures to estimate the missing radiation data refer to the use of the maximum and the minimum daily temperature differences and data from a neighbouring station. Procedures to estimate missing humidity data relate to the use of minimum temperature data and those for the wind speed data refer to regional average data. The accuracy of all different procedures analysed was assessed by regressing ETo values calculated with the FAO‐PM equation computed with limited data versus ETo values computed with complete data sets. Statistical parameters of the regression through the origin and the computed standard errors of estimates (SEE) were used. It could be concluded that the procedures proposed by FAO are accurate providing small SEE, including when only maximum and minimum temperature data are used, which yields lower SEE than when using the Hargreaves equation, which tends to overestimate ETo for the observed conditions. Copyright © 2006 John Wiley & Sons, Ltd.