Dynamax Inc
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4.0 Lascano-VanBavel ETP Program
The value of ETP (Potential Evapotranspiration) is a reference measure of the evaporative demand, as
determined by weather conditions. Using Lascano-VanBavel ET algorithm we calculate ETP estimation
from a well-watered short grass. ETP is normally expressed in mm/hr or inch/hr and daily-accumulated
values are expressed in mm/day or inches/day. It can be related to hourly or daily data for the sap flow
rate, and used to normalize such data against day-to-day variations in the weather, or to identify
deviations in the sap flow rate from normal patterns. ETP is also used to create and index reference to
schedule irrigation and to calculate crop coefficients. Lascano-VanBavel ET algorithm is embedded in
Dynamax weather stations to calculate and output ETP. In addition custom weather station programs can
also be supplied with embedded ET calculations. The new Lascano-VanBavel RCM algorithm for ETP
does not assume a value for temperature and saturation humidity at the evaporation surface, but rather
derives both from closing the energy balance.
In InteliMet Advantage 6 weather station, weather measurements and ET computation are integrated
hence, there is no further processing of the collected data. Output of data loggers already contains average
weather measurements, computed ET, ready for observation and analysis.
4.1 Application of ETP Information
– with Sap Flow
In the interpretation of data on the sap flow rates in crops and trees it is essential to compare their hourly
pattern within that of the concurrent evaporation demand. A general comparison can also be made of
daily totals, as an indication of water stress, or other influences that cause the plant to use less water than
expected.
For example, the weather data may give a value of 10.3 mm/day for ETp and, from stem flow gauges, the
total water used by a tree on 10 m2 of land is found as 94 kg/day (converts to 9.4 mm/day). This converts
to a Kc of 0.91. On the next day, if the value for ETp is 10.6 mm/day, but the water use measured as 46
kg/day (4.6 mm), stomatal closure and reduced transpiration has occurred. The reduced transpiration can
be a result of reduced soil water availability, but possibly a result of other factors such as low root zone
temperature, vascular disease, or others.
In the case of irrigation management, a comparison of the ETp rates with the stem flow data serves the
dual purpose of diagnosing the need for supplying water and the basis for calculating how much water
should be applied. For example, if a four-day sequence of water use a transpiration showed, respectively,
94 kg/day (well watered), 96 kg/day, 50 kg/day (stressed), and 28 kg/day (stressed), while the ETp was
essentially constant, we would know that irrigation is needed and overdue, on the 4
th
day. In this case the
plant water stress caused a decrease in ETa by about 70%.
By adding up the sap flow, we also know that the total amount that has to be replaced equals 266 kg. To
this amount we must add the losses by soil evaporation, which must be estimated from previous data.
Assuming one tree occupies a 10 m2 area, and if the transpiration is 80% of ETp, then the suggested
