Thursday, 24 August 2016
The calculation of evapotranspiration (ETo) in DataSight can be undertaken by following the equation below. This formula is based on the Hargreaves Model. Whilst simplistic when compared to the more complex Penman-Monteith equation, correlation results with R2 >0.9 have been achieved (Wu, 1997). The Jensen Haise Model was also considered, however results were not as consistent and involved a number of extra inputs. Other models were also appraised including Romanenko and Lincare. None were deemed suitable for this application.
The Hargreave Model
ETo = 0.0135(Tm+17.78)Rs(238.8/(595.5-(0.55* Tm)))
Where:
ETo = Evapotranspiration in mmd-1
Tm = Mean daily temperature in oC
Rs = Solar radiation in MJ/m2/d
This model can only be applied to a complete previous 24 hour dataset.
To use this model, the following variables are required:
It is critical to correctly calculate MJ/m2/d from the common variable of W/m2.
In DataSight Tm and Rs will be replaced with one of the calculation variables such as “A” or “B” etc
For example the DataSight formula would be - 0.0135*(A+17.78)*B*(238.8/(595.5-(0.55* A)))
The correct multiplier is:
1 Watt = 1 Joule per second. The radiant exposure is a measure of the energy received per square metre (m2). Consequently, for example, 1000W (1kW) over an hour equates to 3,600,000J/m2/h or 3.6MJ/m2/h. (BoM, 2012). The Rs input into the equation requires the daily sum of MJ/m2/d.
The equation is:
ETo = 0.0135(Tm+17.78)Rs (238.8/(595.5-(0.55* Tm)))
DataSight can readily accommodate other evapotranspiration models.
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REFERENCES
Wu, I. P., (1997) A Simple Evapotranspiration Model for Hawaii: The Hargreaves Model. Engineer’s Notebook no. 106.
Bureau of Meteorology (2012) Solar Radiation Glossary. Available at: http://www.bom.gov.au/climate/austmaps/solar-radiation-glossary.shtml (Accessed: 30 September 2015). Last updated 13 June, 2012.