ETCALC - Evapotranspiration online calculator
1. About ETCalc
ETCalc is an online evapotranspiration calculator developed through a collaborative research effort between Canadian Rivers Institute (CRI), University of New Brunswick (UNB), Agriculture and Agri-Food Canada (AAFC) and Environment and Climate Change Canada (ECCC). ETCalc has been developed as part of the research effort aimed at evaluating the effects of agricultural production practices on groundwater and surface water quality and on the quality of downgradient aquatic ecosystems. This calculator is part of a suite of tools that can be used for analyzing streamflow (i.e. baseflow / hydrograph separation), soil water balance, soil water deficit, crop water stress (deficit or excess), crop irrigation requirements, impact of irrigation on aquifer storage, impact of climate change on soil water balance, etc.). The tools are available at https://www.canadianriversinstitute.com/hydrotools or at https://www.hydrotools.tech.
ETCalc offers several customizable methods for calculating daily Potential Evapotranspiration (PET), Reference Evapotranspiration (ETR) and Actual Evapotranspiration (ET) based on user provided meteorological data and crop coefficients. The web-based tool provides various output and data visualization options through a user-friendly interface and a streamlined process.
For detailed instructions on how to use this tool please refer to the User Guide section.
For citing ETCalc, please use the following:
Danielescu S, MacQuarrie KTB, Lepir D, Popa A (2021) ETCALC version 4: An online calculator for evapotranspiration. Available at http://etcalc.hydrotools.tech.
For detailed instructions on how to use the evapotranspiration online calculator please refer to the User Guide section.
2. Background
Although several definitions exist, evapotranspiration can be simply conceptualised as the sum of evaporation of water from soil or surface of the ground and the transpiration of plants. Evapotranspiration is a key component of the hydrological cycle from local to global scales. Therefore, knowledge of the amount of evapotranspiration is important for example for understanding or estimating local or watershed scale water balances, plant / crop water stress and irrigation requirements, subsurface water storage, impacts of climate change on water balance components, etc.
There are currently many equations available for calculating the various forms of evapotranspiration. ETCalc integrates several of these methods into a streamlined interface that allows for the calculation of Potential Evapotranspiration (PET), Reference Evapotranspiration (ETR) and Actual Evapotranspiration (ET) (see table below). Of note, ETCalc will calculate the various evapotranspiration forms only for the methods for which data is included in the user input dataset (i.e. ETCalc will produce columns with zero values for the methods for which input data is not available).
ETCALC: available methods and data requirements*
MethodPPTmeanTmaxTminRHSRTOASRSFCReference
Blaney - Criddle (BC) x Brouwer & Heibloem (1986)
Turc (TU) x x xAmatya & Harrison (2016)
Priestley - Taylor (PT) x xPriestley & Taylor (1972)
Hargreaves (HA) xxx x Allen et al. (1998)
Jensen - Haise (JH) x x Tegos et al. (2013)
Abtew (AB) xAbtew (1996)
*PP - Total Precipitation; Tmean - Mean air Temperature; Tmax - Maximum air temperature; Tmin - Minimum air temperature; RH – Relative humidity; SRTOA - Solar radiation @ top-of-atmosphere; SRSFC - Solar radiation @ surface at the surface of the ground.
A diagram of ETCalc workflow is shown below. All methods integrated in ETCalc require daily weather data and depending of the method, the direct output consists in either Potential Evapotranspiration (PET) or Reference Evapotranspiration (ETR).
ETCalc Workflow
For detailed instructions on how to use the evapotranspiration online calculator please refer to the User Guide section.
3. Methodology
Currently, the ETCalc online evapotranspiration calculator includes 6 methods for calculating the various evapotranspiration forms. Brief descriptions of the concepts and methods used are included below.
3.1. Definitions
The definitions for the most relevant terms used by the ETCalc online evapotranspiration calculator are included below. It should be noted, that in literature, there are slight variations in the definition of some of these terms, and hence, the user is encouraged to thoroughly review the various concepts and definitions used in calculation of the various forms of evapotranspiration.
Crop coefficients: these are coefficients that allow for accounting for the difference between potential evapotranspiration (PET) or reference evapotranspiration (ETR) and crop or actual evapotranspiration (ET). The crop coefficient has been introduced for accounting for the differences between the evapotranspiration for the actual crop (or “land cover”) and the evapotranspiration for the reference surface or reference crop (i.e. the land cover or the crop used for calculating PET or ETR). Of note, the definition (and values) of crop coefficients vary in accordance with the specifics of each evapotranspiration calculation method (i.e. the crop coefficient is method specific). In ETCalc, the crop coefficients are denoted with the letter K and can be fixed (Kfix, one value for the entire year) or variable (Kvar, the value can change on a monthly basis).
PET (Potential evapotranspiration): this is considered to be equivalent to the maximum evapotranspiration that would occur if the water supply is not limiting. Note that specific evapotranspiration methods might use a slightly different definition for PET. For example, PET could be considered to be equivalent of the evaporation from an open water surface (e.g. lake). In ETCalc output, ET0 notation is used for PET, and the user has to refer to the description of the method to identify if for a particular method either PET or ETR is calculated.
ETR (Reference evapotranspiration): this is typically considered to occur for example when a short crop (i.e. grass) fully covers the ground and always has a good supply of water. Note that specific evapotranspiration methods might use a slightly different definition for ETR. ETR is always lower than PET. In ETCalc output, ET0 notation is used for PET, and the user has to refer to the description of the method to identify if for a particular method either PET or ETR is calculated.
ET (Crop evapotranspiration or Actual evapotranspiration): this form of evapotranspiration accounts for both weather (i.e. integrated in PET and ETR formulas) and land cover or crop (i.e. via crop coefficients). ET is the product of ET0 (PET or ETR, depending of the method) and K (crop coefficient). Note that specific evapotranspiration methods might use a slightly different definition for the various evapotranspiration forms or crop coefficients. ET is always lower than PET, however it can be higher or lower than ETR. In ETCalc output, depending on the method of calculation, ETF or ETV notations are used for actual (crop) evapotranspiration, depending on the type of crop coefficient used for its calculation. ETF is used for the cases when a fixed crop coefficient has been used (Kfix) and ETV is used for the cases when a variable crop coefficient has been used (Kvar) respectively.
Input data: this is a dataset consisting of daily weather data and validation data. The input data has to be uploaded using a file with a fixed number of columns (i.e. 11 columns, with 8 columns for weather data and 3 columns for validation data). The columns for which data is not available can be left blank. For example, if only daily mean air temperature data is available, then only the column Tmean should have values, with all other columns left blank. The methods for which required data is not present are disabled by ETCalc and the output for the respective methods will consist in zero values for all output parameters (i.e. ET0, ETF, ETV). In the example above, only Blaney-Criddle method will be available for calculating the evapotranspiration forms.
Validation data: this dataset is part of the Input data file (i.e. the last 3 columns in the input data file). The validation data provides means for directly comparing any of the data output from ETCalc with a dataset obtained using different methods or sources. The validation dataset can consist of evapotranspiration data, but could also consist of other data types such as soil moisture, soil temperature, groundwater table elevations, etc.
3.2. Crop Coeficients (K)
Depending on the method(s) selected, the direct output from each of the evapotranspiration methods included in the ETCalc online evapotranspiration calculator is either Potential Evapotranspiration (PET) or Reference Evapotranspiration (ETR). In order to obtain actual (crop) evapotranspiration (ET), the crop coefficients (K) need to be used. ET is simply the product of the method’s direct output (i.e. PET or ETR) and the crop coefficient (K). It should be noted the meaning of crop coefficients can be slightly different for each of the methods integrated in ETCalc. Hence, the best practice is to calibrate the crop coefficients separately for each method. The higher the crop coefficient is, the higher ET will be. ET is always lower than PET, however it can be either lower or higher than ETR.
ETCalc provides the option to use either fixed (Kfix) or monthly variable (Kvar) crop coefficients throughout the year, and these can be different for each of the methods used. In ETCalc, the resulting actual (crop) evapotranspiration (ET) is denoted either as ETF if a fixed crop coefficient has been used or as ETV, if a variable crop coefficient has been used. Generally, Kvar would be the better choice as it allows the crop coefficient to be adjusted in response to, for example, monthly weather and various growth stages of the plants. However, Kvar values might be more difficult to obtain and hence, Kfix option is provided if a simplified approach is deemed sufficient for the purpose of calculating evapotranspiration.
There are currently many sources of crop coefficients values readily available in literature. Typically, in literature either a single value (e.g. 0.3), a range of values (e.g. 0.2-0.5) or values for beginning, middle and end of the growing season (e.g. 0.2/0.65/0.2) are provided. In ETCalc initial values for both Kfix and Kvar are provided, however, the users are encouraged to adjust these values in accordance with their requirements and conditions of their study area (e.g. change Ks to reflect the crops for which evapotranspiration is calculated).
For Kfix a single value is provided for the entire year, while for Kvar, ETCalc uses an interpolation algorithm to calculate Kvar for each month based on values provided by the user for three periods of the year: January - February, July-August and December. Once an initial set of values for Ks(Kfix and/or Kvar) is selected, this set becomes a “reference” set and the Ks for each of the methods can be obtained by applying a user specified multiplier (i.e. factor) to the Ks used for the “reference” set. For example if for a selected method the user wishes to use the same K values as the one used in the reference set, a value of “1” has to be specified for the K multiplier.
3.3. Blaney-Criddle Method (BC)
Overview: This is a simplified method, ideal when only mean air temperature data is available. Due to its simplicity it is recommended that this method is used for monthly or greater intervals.
Reference: Brouwer C, Heibloem M (1986) Irrigation Water Management: Irrigation Water Needs. Part II – Detmination of Irrigation Water Needs. FAO Irrigation Water Management. Training manual No. 3, FAO, Land and Water Development Division, Rome, Italy.
Data requirements: Mean air temperature (Tmean); Air relative humidity (RH).
Data output: Reference Evapotranspiration (ETF and/or ETV), Actual evapotranspiration (ET) (using crop coefficients).
Formula:
Blaney-Criddle Method
Where:
ETR – daily reference evapotranspiration (mm)
p – mean daily percentage of annual daytime hours
Tmean – daily mean air temperature (oC)
Mean daily percentage (p) of annual daytime hours for various latitudes
(based on Brouwer & Heibloem [1986])
LatitudeNorthJanFebMarAprMayJunJulAugSepOctNovDec
SouthJulAugSepOctNovDecJanFebMarAprMayJun
60°0.150.200.260.320.380.410.400.340.280.220.170.13
55°0.170.210.260.320.360.390.380.330.280.230.180.16
50°0.190.230.270.310.340.360.350.320.280.240.200.18
45°0.200.230.270.300.340.350.340.320.280.240.210.20
40°0.220.240.270.300.320.340.330.310.280.250.220.21
35°0.230.250.270.290.310.320.320.300.280.250.230.22
30°0.240.250.270.290.310.320.310.300.280.260.240.23
25°0.240.260.270.290.300.310.310.290.280.260.250.24
20°0.250.260.270.280.290.300.300.290.280.260.250.25
15°0.260.260.270.280.290.290.290.280.280.270.260.25
10°0.260.270.270.280.280.290.290.280.280.270.260.26
5°0.270.270.270.280.280.280.280.280.280.270.270.27
0°0.270.270.270.270.270.270.270.270.270.270.270.27
3.4. Turc Method (TU)
Overview: This is an energy balance based method which uses mean air temperature, solar radiation, and air humidity. The method has been modified in ETCalc by introduction an additional condition (ET=0 if Tmean<0), to prevent the formula from resulting in negative ET values.
Reference: Amatya DM, Harriso CA (2016) Grass and Forest Potential Evapotranspiration Comparison Using Five Methods in the Atlantic Coastal Plain. J. Hydrol. Eng. 05016007.
Data requirements: Mean air temperature (Tmean); Solar Radiation at the surface of the ground (SRSFC), air relative humidity (RH).
Data output: Potential Evapotranspiration (PET), Actual evapotranspiration (ETF and/or ETV).
Formula:
If Tmean<0oC:
Turc Method
If Tmean>0oC and RH<50%:
Turc Method
If Tmean>0oC and RH>50%:
Turc Method
Where:
PET – daily Potential evapotranspiration (mm)
Tmean - mean daily air temperature (oC)
SRSFC – daily solar radiation incident on a horizontal surface at the surface of the earth (MJm−2 day−1)
RH – mean daily air relative humidity (%)
3.5. Priestley-Taylor Method (PT)
Overview: This is physically-based formulation, which is interpreted as a simplified version of the Penman – Monteith formula for calculating evapotranspiration. The Priestley-Taylor formula can be used for calculating daily evapotranspiration, but can also be used for smaller time-steps (i.e. hourly) provided the required data is available.
Reference: Priestley CHB, Taylor RJ (1972) On the assessment of the surface heat flux and evaporation using large-scale parameters. Mon. Weather Rev 100: 81-92.
Data requirements: Precipitation (PP), Mean air temperature (Tmean); Solar Radiation at the surface of the ground (SRSFC).
Data output: Potential Evapotranspiration (PET), Actual evapotranspiration (ETF and/or ETV).
Formula:
Priestley-Taylor Method
If Tmean<0oC
Priestley-Taylor Method
If Tmean>0oC
Priestley-Taylor Method
Where:
PET – daily potential evapotranspiration (m)
λv - latent heat of vaporization (2260 kJ kg-1)
ρw - density of water (1000 kg m-3)
Δ - slope of the saturation vapor density (kg oC-1)
γ - psychrometric constant1 (4.95x10-4 kg m-3 oC-1)
Tmean – mean daily air temperature (oC)
SRSFC – daily solar radiation incident on a horizontal surface at the surface of the earth (kJ m−2)
3.6. Hargreaves Method (HA)
Overview: Hargreaves formula provides a simple method for calculating evapotranspiration based on an empirical relationship with temperature and solar radiation.
Reference: Allen RG, Pereira LS, Raes D, Smith M (1998) Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56, Rome, 300.
Data requirements: Mean air temperature (Tmean); Maximum air temperature (Tmax), Minimum air temperature (Tmin), Solar Radiation at the top of atmosphere (SRTOA).
Data output: Reference Evapotranspiration (ETR), Actual evapotranspiration (ETF and/or ETV).
Formula:
Hargreaves Method (HA)
Where:
ETR – daily reference evapotranspiration (m)
Tmean – mean daily air temperature (oC)
Tmax – maximum daily air temperature (oC)
Tmin – minimum daily air temperature (oC)
SRTOA – Solar radiation incident on the top-of-atmosphere (MJ m-2 day-1)
3.7. Jensen-Haise Method (JH)
Overview: Jensen-Haise (McGuiness model) formula provides a simple method for calculating evapotranspiration based on daily temperature and solar radiation.
Reference: Tegos A, Efstratiadis A, Koutsoyiannis D. (2013) A Parametric Model for Potential Evapotranspiration Estimation Based on a Simplified Formulation of the Penman-Monteith Equation. In Evapotranspiration – an overview, Alexandris S (ed.), IntechOpen, eBook ISBN: 978-953-51-5369-6.
Data requirements: Solar Radiation at the top of atmosphere (SRTOA), Mean air temperature (Tmean).
Data output: Potential Evapotranspiration (PET), Actual evapotranspiration (ETF and/or ETV).
Formula:
Jensen-Haise Method (JH)
Where:
PET – daily potential evapotranspiration (m)
SRTOA – Solar radiation incident on the top-of-atmosphere (MJ m-2 day-1)
Tmean – mean daily air temperature (oC)
λ - latent heat of vaporization (2260 kJ kg-1)
ρ - density of water (1000 kg m-3)
3.8. Abtew Method (AB)
Overview: Abtew method for calculation of evapotranspiration is a simplified energy-based formulation that requires only solar radiation.
Reference: Abtew W (1996) Evapotranspiration measurements and modeling for three wetland systems in south Florida. J. Am. Water Resour. Assoc. 32: 465–473.
Data requirements: Solar radiation incident on a horizontal surface at the surface of the earth (SRSFC).
Data output: Potential Evapotranspiration (PET), Actual evapotranspiration (ETF and/or ETV).
Formula:
Abtew Method (AB)
Where:
PET – daily potential evapotranspiration (m)
K1 – dimensionless coefficient (0.53)
SRSFC - Solar radiation incident on a horizontal surface at the surface of the earth (MJ m-2 day-1)
λ - latent heat of vaporization (2.45 MJ kg-1)
For detailed instructions on how to use the evapotranspiration online calculator please refer to the User Guide section.
3.9. Input and Validation Data
Input data: this is a dataset consisting of daily weather data and validation data. The input data has to be uploaded using file with a fixed number of columns (i.e. 11 columns, with 8 columns for weather data and 3 columns for validation data). The columns for which data is not available can be left blank. For example, if only daily mean air temperature data is available, then only the column Tmean should have values, with all other columns left blank. The methods for which required data is not present are disabled by ETCalc and the output for the respective methods will consist in zero values for all output parameters (i.e. ET0, ETF, ETV). In the example above, only Blaney-Criddle method will be available for calculating the evapotranspiration forms.
The table below shows the weather data required for each of the methods currently included in the ETCalc online evapotranspiration calculator.
ETCALC: available methods and data requirements*
MethodPPTmeanTmaxTminRHSRTOASRSFCReference
Blaney - Criddle (BC) x Brouwer & Heibloem (1986)
Turc (TU) x x xAmatya & Harrison (2016)
Priestley - Taylor (PT) x xPriestley & Taylor (1972)
Hargreaves (HA) xxx x Allen et al. (1998)
Jensen - Haise (JH) x x Tegos et al. (2013)
Abtew (AB) xAbtew (1996)
*PP - Total Precipitation; Tmean - Mean air Temperature; Tmax - Maximum air temperature; Tmin - Minimum air temperature; RH – Relative humidity; SRTOA - Solar radiation @ top-of-atmosphere; SRSFC - Solar radiation @ surface at the surface of the ground.
Validation data: In addition to the weather data, the user can include validation data in the input dataset. The last three columns on the input file are reserved for the validation dataset. Validation data provides means for directly comparing any of the data output from ETCalc with a dataset obtained using different methods or sources. The validation data set can include evapotranspiration data, but could also include other data types such as soil moisture, soil temperature, groundwater table elevations, etc.
The following format and units are required for the input data file:
(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)
DatePPTmeanTmaxTminRHSRTOASRSFCVAL1VAL2VAL3
dd-mmm-yyyymm°C°C°C%MJ/m²/dMJ/m²/d???
Notes:
  • Columns have to be left blank for parameters for which data is not present
  • The first row in the input data file has to contain the column headings as specified in the table above
  • ETCalc will select the calculation methods based on the columns with available data (columns with zero values will be produced for methods for which input data is not available)
  • Data in columns Val1, Val2 and Val3 will be used for comparison of ETCalc results with the data in the respective columns (i.e. if Val columns are not left blank). Measurement units for validation data are not required.
For detailed instructions on how to use the evapotranspiration online calculator please refer to the User Guide section.
4. User Guide
ETCalc is a web application that allows for the calculation of various evapotranspiration forms (Potential evapotranspiration - PET, Reference evapotranspiration - ETR, Actual (or crop) evapotranspiration - ET) based on user provided weather data and crop coefficients (Kfix, Kvar). Currently, ETCalc includes 6 methods available in literature. The tool provides tabular and graphical representations of the input data (weather data and validation data), output data (PET, ETR, ET), comparison with validation data (included in the input data file), as well as representative statistics.
On the left, a contextual menu provides access to the various components of the tool. At the top of the page, a progress bar displays the current status of the analysis.
4.1. Quick Start
In order to run ETCalc the user has to complete the following steps:
  1. Load Input Data: provide required data [Source Data Menu];
  2. Perform Evapotranspiration Calculations: choose Crop Coefficients, Select ET methods and Run ETCalc [Analyze Menu];
  3. Investigate Results and Export Data: Review ETCalc output and export results [Output Data Menu].
The steps required for using this tool are described in more detail below.
4.2. Load Input Data
The first step in any analysis is to upload the Input Data file to be used by the online evapotranspiration calculator. The input data consists of a file containing 8 columns for daily weather data and 3 columns reserved for daily validation data. Not all the columns are required to be filled with data – ETCalc selects which methods for calculating evapotranspiration can be used based on the columns that contain data. The input data file can be uploaded using the Upload User Data menu entry under the SOURCE DATA menu section.
Alternatively, for a better understanding of ETCalc routines, the user can use the sample data set provided by using the Load Sample Data menu entry. The data set contains two years of daily weather and validation data.
The input dataset is a collection of daily weather data (i.e. total precipitation, mean air temperature, maximum air temperature, minimum air temperature, relative air humidity, solar radiation incident on the top-of-atmosphere, and solar radiation incident on a horizontal surface at the surface of the earth). The dataset has a fixed number of columns, with 8 columns dedicated to daily weather data and 3 columns reserved for daily validation data.
The tool accepts source data sets in Excel (xls, xslx) and Comma Separated File (csv) formats. The tool does not check for missing or erroneous data entries, hence it is recommended that the user verifies the integrity of the source data before uploading.
The table below shows the weather data required for each of the methods currently included in the ETCalc online evapotranspiration calculator. ETCalc selects which methods will be available for calculating the evapotranspiration forms based on the datasets that are uploaded (i.e. the user should leave the columns blank for the data that is not available). The methods for which required data is not present are disabled by ETCalc and the output for the respective methods will consist in zero values for all output parameters (i.e. ET0 – Potential Evapotranspiration or Reference Evapotranspiration; ETF – actual (crop) evapotranspiration using fixed crop coefficients, ETV – actual (crop) evapotranspiration using variable crop coefficients).
ETCALC: available methods and data requirements*
MethodPPTmeanTmaxTminRHSRTOASRSFCReference
Blaney - Criddle (BC) x Brouwer & Heibloem (1986)
Turc (TU) x x xAmatya & Harrison (2016)
Priestley - Taylor (PT) x xPriestley & Taylor (1972)
Hargreaves (HA) xxx x Allen et al. (1998)
Jensen - Haise (JH) x x Tegos et al. (2013)
Abtew (AB) xAbtew (1996)
*PP- Total Precipitation; Tmean - Mean air Temperature; Tmax - Maximum air temperature; Tmin - Minimum air temperature; RH – Relative humidity; SRTOA - Solar radiation @ top-of-atmosphere; SRSFC - Solar radiation @ surface at the surface of the ground.
In addition to the daily weather data, the input file can also contain daily validation data (the last three columns on the input file) Validation data provides means for directly comparing any of the data output from ETCalc with a dataset obtained using different methods or sources. The validation data set can include evapotranspiration data, but could also include other data types such as soil moisture, soil temperature, groundwater table elevations, etc.
The following format and units are required for the input data file:
(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)
DatePPTmeanTmaxTminRHSRTOASRSFCVAL1VAL2VAL3
dd-mmm-yyyymm°C°C°C%MJ/m²/dMJ/m²/d???
Notes:
  • Columns have to be left blank for parameters for which data is not present
  • The first row in the input data file has to contain the column headings as specified in the table above
  • ETCalc will select the calculation methods based on the columns with available data
  • Data in columns Val1, Val2 and Val3 will be used for comparison of ETCalc results with the data in the respective columns (i.e. if Val columns are not left blank). Measurement units for validation data are not required.
Once the input dataset is loaded to ETCalc, a series of options become available in the SOURCE DATA menu section. These allow for investigation of the input data (i.e. weather and validation data) using a tabular format (Table View) as well as for visualization of the various time series (Graphical View). The Graphical View allows for inspection of the full data set or of a reduced subset by changing the beginning and the ending values of the time interval that is displayed. In the Graphical View, the user can select which parameters to be displayed, on which Y axis (i.e. left/primary or right/secondary), and what colors to be used for each of the datasets (i.e. parameters). Once the selections are completed the user can use the Update Graphs button to display the data. Statistics for both the entire dataset and the selected subset are shown in Table View.
4.3. Perform Evapotranspiration Calculations
Evapotranspiration calculations can be performed once the input data is loaded and the Analyze menu entry becomes available. Once the user follows Analyze menu link, a page with two subsections becomes available: i) CROP COEFFCIENTS and II) METHOD SELECTION.
CROP COEFFICIENTS (K)
ETCalc provides the option to use either fixed (Kfix) or monthly variable (Kvar) crop coefficients throughout the year. The values of Kfix and Kvar can be different for each of the methods used, since these coeffcients might have a slightly different definition for each of the methods. For Kfix a single value is provided for the entire year, while for Kvar, ETCalc uses an interpolation algorithm to calculate Kvar for each month based on values provided by the user for three periods of the year: January - February, July-August and December. Once an initial set of values for Ks (Kfix and/or Kvar) is selected, this set becomes a “reference” set and the Ks for all methods can be obtained by applying a user specified multiplier (i.e. factor) to the Ks used for the “reference” set. For example if for a selected method the user wishes to use the same K set as the one used in the reference set, a value of “1” has to be specified for the K multiplier. For more details regarding the definition of the crop coefficients, refer to section 3.2. Crop Coefficients.
METHOD SELECTION
Once the selection of the crop coefficients is complete, the user can proceed to adjusting the parameters required for each evapotranspiration calculation method. The default values for the constants associated with each method are the same as the ones used in the references provided for each of the methods, and hence it Is recommended that caution should be used if these values are to be changed. For the Blaney- Criddle method (BC) the user is required to provide the hemisphere and the latitude of the location from where the weather data has been obtained.
ETCalc can run all available methods at once (Run All button at the bottom of the page) or run only the s selected by the user (Run Selected button at the bottom of the page). The methods for which required data is not present are disabled by ETCalc and the output for the respective methods will consist in zero values for all output parameters (i.e. ET0 – Potential Evapotranspiration or Reference Evapotranspiration, ETF – actual (crop) evapotranspiration using fixed crop coefficients, ETV – actual (crop) evapotranspiration using variable crop coefficients).
4.4. Investigate Results and Export Data
Following completion of the data analysis / evapotranspiration calculation, the tool redirects to the OUTPUT DATA (Table View) page, where the user can start to investigate the results using the method specific tabs located in the upper portion of this page. The user can browse the results for each method by clicking on each of the tabs. The Table View and Graphical View entries in the OUTPUT DATA menu section become visible only when either Run Selected or Run All has been completed at least once). In a similar fashion as the Graphical View menu entry in SOURCE DATA, The Graphical View in OUTPUT DATA allows the user to inspect the full data set or only a reduced subset by changing the beginning and the ending values of the time interval that is displayed. In the Graphical View, the user can select which parameters to be displayed, on which Y axis (i.e. left/primary or right/secondary), and what colors to be used for each of the datasets (i.e parameters). Once the selections are completed the user can use the Update Graphs button to display the data.
The data set resulted from the analysis (i.e. output dataset), as well as the graphical representations and statistics relevant for the analysis, can be downloaded by using Export Output Data / Export Metadata / Download Image buttons in the Table View and Graphical View pages of OUTPUT DATA menu entries. It should be noted that Export Output Data, results in exporting all columns used by ETCalc (i.e. includes both input and output data).Export Metadata results in exporting metadata associated with the dataset, including crop coefficient and parameters for all the methods used in on the Analyze page.
5. Limitations
a) Large data sets (i.e. over 5000 rows) take longer to load. Please be patient until loading is completed and you are redirected to the Source Data Graphical View page;
b) If the analysis cannot be conducted or takes too long even with only one method selected, it is recommended to split the input data set into smaller files. For example, running all methods simultaneously with a 2-year data set (730 rows x 8 columns = 5840 values) takes approximately 90 seconds. Running time for a single method with the above dataset ranges between 10 and 20 seconds;
6. Terms of Use
ETCalc can be used freely.
For citing ETCalc, please use the following:
Danielescu S, MacQuarrie KTB, Lepir D, Popa A (2021) ETCALC version 4: An online calculator for evapotranspiration. Available at http://etcalc.hydrotools.tech.
The authors do not assume any responsibility for the tool's operation, output, interpretation, or use of results.
For more information you may wish to contact the authors please see the Contact section.
7. References (TBD)
Abtew W (1996) Evapotranspiration measurements and modeling for three wetland systems in south Florida. J. Am. Water Resour. Assoc. 32: 465–473.
Allen RG, Pereira LS, Raes D, Smith M (1998) Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56, Rome, 300.
Amatya DM, Harrison CA (2016) Grass and Forest Potential Evapotranspiration Comparison Using Five Methods in the Atlantic Coastal Plain. J. Hydrol. Eng. 05016007.
Brouwer C, Heibloem M (1986) Irrigation Water Management: Irrigation Water Needs. Part II – Determination of Irrigation Water Needs. FAO Irrigation Water Management. Training manual No. 3, FAO, Land and Water Development Division, Rome, Italy.
Priestley CHB, Taylor RJ (1972) On the assessment of the surface heat flux and evaporation using large-scale parameters. Mon. Weather Rev 100: 81-92.
Tegos A, Efstratiadis A, Koutsoyiannis D. (2013) A Parametric Model for Potential Evapotranspiration Estimation Based on a Simplified Formulation of the Penman-Monteith Equation.In Evapotranspiration – an overview, Alexandris S (ed.), IntechOpen, eBook ISBN: 978-953-51-5369-6.
8. Contact
Serban Danielescu, Ph.D.
Research Scientist | Chercheur scientifique
Environment and Climate Change Canada | Environnement et Changements Climatiques Canada Agriculture and Agri-Food Canada | Agriculture et Agroalimentaire Canada
Fredericton Research and Development Centre | Centre de recherche et développement de Fredericton
850 Lincoln Rd., Fredericton, NB, E3B 4Z7
Telephone/Téléphone: 506-460-4468
Facsimile/Télécopieur: 506-460-4377
For detailed instructions on how to use evapotranspiration online calculator please refer to the User Guide section.