Sutron 8310 - Page 71

Sutron Corporation 8310 & 7310 Users Manual 8800-1125Rev. 2.7 (BETA) 4/16/2014 pg. 71
Evapotranspiration
Calculates daily potential evapotranspiration using the American Society of Civil Engineers
standardized reference evapotranspiration equation (January 2005) using 5 (normally averaged)
input values:
1. Temperature in degrees C or degrees F
2. Relative humidity expressed as percent
3. Barometric pressure in millibars or inches of mercury
4. Wind speed in kilometers per hour, miles per hour, meters per second, knots, or feet per
second
5. Solar energy in Langleys (cal/sq-cm/min), or Watts/sqare meter
The ET calculation can output any of three options:
ETo – Standardized Reference Evapotranspiration, Short (grass)
ETr – Standardized Reference Evapotranspiration, Tall (alfalfa)
Water surface – Same as ETo except uses albedo of 0.06, characteristic of open water
The calculation method used in the block is based on:
Me. Jensen, R. D. Burman, and R. G. Allen 1990.
Evapotranspiration and Irrigation water Requiremtnts
ASCE manuals and Reports on Engineering Practice NO.70
American Society of Civil Engineers NY 1-332
Walter, et. al. January 2005
The ASCE Standardized Reference Evapotranspiration Equation
ASCE-EWRI Task Committee Report
ASCE-EWRI Task Committee Report Appendixes A-C
When setting up a system to make an Evapotranspiration measurement, you’ll typically first
create Average measurements for each of the ET inputs. You’ll then create the
Evapotranspiration measurement, selecting the corresponding averaged output for each of the
Reference inputs.
This measurement has the following properties:
Time
Define when the measurement task runs. The evapotranspiration measurement type
always runs daily, i.e., every 24 hours. Time allows you to specify when the final result is
logged. For example, when Time is 06:00:00, the final result is logged at 6am.
TempUnits
Specify the units of the air temperature measurement input (“AT”). Choices are
Fahrenheit or Celsius. Be sure you configure the AT input to produce an air temp result
in the selected units, otherwise the ET result won’t be correct.
BPUnits
Specify the units of the barometric pressure measurement input (“BP”). Choices are in-
Hg or mb. Be sure you configure the BP input to produce a result in the selected units,
otherwise the ET result won’t be correct.
SolarUnits
Specify the units of the solar measurement input (“Solar”). Choices are Ly or W/m2. Be
sure you configure the Solar input to produce a result in the selected units, otherwise