Dynamax Dynamet 5, Руководство пользователя

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4.2 ET Program Basics
This section covers the basics of Evapotranspiration algorithms and the ET algorithm
implemented in Dynamet 5.Cr1.
The ET program contains two constants that are always set for the customers site, and two
constants that could be modified for various situations. Parameters lev and zot should be
modified only by persons thoroughly familiar with ETP modeling. Normally, the last two
constants are altered to account for the barometric pressure and the height of the wind
measurement. The present form of the program is adapted to metric units. Each user needs to
know the height of the weather station above ground level, and the altitude above sea level in
feet.
zot = 0.0005 surface roughness parameter in m, for ¾ in. high turf grass
zom = 2.0 wind speed height of measurement in meters, 2 m typical.
elf = 100 elevation of station in feet , 100 ft being only an example
has = 1004.0 Specific air heat capacity in J/KgC
In this section we discuss definition of these parameters and their units followed by procedure to
modify these parameters in Dynamet 5 program to meet the geography of the location of
weather station.
4.2.1 Elevation and Average Barometric Pressure Calculation
The user should calculate the Average Barometric Pressure as indicated in the following formula
and substitute the calculated average barometric pressure (abp) figure for line two of the select
elements. The Average Barometric Pressure is computed as follows:
Average barometric pressure in mb,
 
5 . 0 817 . 3
2 . 1013
    
E elf
e abp
As a simple example, at sea level, elf is zero, so the ABP is 1013 Mb.
If the altitude is 100 ft (elf=100), enter abp=1009 in line two of the select elements
calculation.