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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 Dynamet 5.CR1 and
Dynamet 5Tst.CR1 programs supplied with Dynamet 5 weather station. In addition custom
Dynamet 5 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.
Using Dynamet 5.Cr1 or any of the variation of these programs and a Dynamet 5 weather
station user can collect sensor data as well as the computed ET data (both hourly and daily)
from the station. Hence, there is not further processing of the collected data. Output of the data
logger already contains raw sensors data, computed ET data as well as daily-accumulated data.
4.1 Application of ETP Information
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). 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, 96 kg/day, 50 kg/day, and 28 kg/day, while the
ETP was essentially constant, we would know that irrigation is needed and overdue, on the 5
th
day. By adding up the sap flow, we also know that the total amount that has to be replaced
equals 266 kg.
Summary of Contents for Dynamet 5
Page 1: ...DYNAMET 5 Weather Station User Manual v 1 1 ...
Page 3: ...Dynamax Inc 2 ...
Page 5: ...Dynamax Inc 4 ...
Page 18: ...Dynamet 5 17 ...
Page 19: ...Dynamax Inc 18 Dynamet 5 Logger Assembly wiring internal battery and solar charge regulator ...
Page 72: ...Dynamet 5 71 Logger 4 Logger 3 Logger 2 Logger 1 ...
Page 74: ...Dynamet 5 73 ...
Page 75: ...Dynamax Inc 74 ...
Page 79: ...Dynamax Inc 78 Appendix C SENSOR SPECIFICATIONS ...