Halytech
hydroSpider2 User’s Manual v1.23
Page 33
5.5.3.3
Evapotranspiration
This type of super channel performs the calculation
𝐸𝑇
0
= 0.0135 × (𝑇
𝑚
+ 17.78) ×
𝑅
𝑠
1000
×
238.8
595.5 − (0.55 × 𝑇
𝑚
)
Where:
ET
0
= Evapotranspiration in mm/day
T
m
= Mean daily temperature in ⁰C
R
s
= Daily solar radiation in kJ/m
2
/day
When “Evapotranspiration” is selected as the source for a super channel the inputs to the
calculation can then be chosen via the advanced input options dialog (the ‘cog’):
Total Solar Radiation
is the total sunlight accumulated in a day, measured in kJ/m
2
/day.
To calculate total solar radiation, set up a channel which reads a solar radiation sensor, with the
logging period set to one day and a sample period short enough to capture sunlight variation
through the day, for example 15 minutes, and select the
Integrate
filter type.
If your radiation sensor outputs W/m
2
instead of kW/m
2
, you must also select the
Divide Samples by
1000
option, to convert W to kW. The integrated value will then be in kJ/m
2
/day, as required by the
evapotranspiration calculation.
Mean Temperature
is the average daily temperature. To calculate it, setup a channel which reads
an air temperature sensor calibrated in ⁰C, with the logging period set to one day and a sample
period short enough to capture temperature variation through the day, for example 10 minutes, and
select the
Average
filter type.
Note that the evapotranspiration, total solar radiation and mean temperature channels must all
have the same logging period which, for the calculation to be valid, should be set to 1 day.
