The calculation of evapotranspiration (ETo) in DataSight can be undertaken by following the equation 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, I. P., 1997, A Simple Evapotranspiration Model for Hawaii: The Hargreaves Model. Engineer’s Notebook no. 106.).

The Hargreaves Model

Where:

ETo = Evapotranspiration in mmd-1

Tm = Mean daily temperature in oC

RS = Solar radiation in MJ/m2/day

This model can only be applied to a complete previous 24 hour dataset.

To use this model, the following variables are required:

• Tmas mean daily temperature in oC;
• RSas daily solar radiation in MJ/m2/day; and
• EToas evapotranspiration in mmd-1

DataSight can readily accommodate other evapotranspiration models.

Calculate Evapotranspiration

1. Ensure that data containing Tmand RS values are visible in your datasheet.
2. Right-click in the datasheet and select Calculation.
3. In the Calculation window, select the appropriate Level for Master Variable A.
4. Select Mean daily temperature as Master Variable A.
5. Add a second Variable by clicking Add Variable.
6. Select Solar radiation as Variable B.
7. Enter the Formula required to complete the calculation.

0.0135*(A+17.78)*B*(238.8/(595.5-(0.55* A)))

where A = Mean daily temperature and B = Daily Solar radiation

8. Select the Output Variable Evapotranspiration at desired level.
9. Select the Calculation Chart tab, where you can review the calculation input variables and the result of the proposed calculation.
10. Similarly, select the Calculation Result tab to review the data in grid view, before completing the calculation.
11. Select the Database box and click Update.
12. Close the calculation tool. The Evapotranspiration (mmd-1) variable has been populated within the specified datasheet.