Derek,
 
The FAO-56 reference for the DSSAT crops was developed from the standard short-grass reference ET equation (Allen et al., 1998).  There is a parameter (called ETRATIO, I believe that is the name) at the very bottom of the "read-in" species file of the CROPGRO model V4.0, which will increases the potential EO as a function of LAI increase from 0 to 6 from 1.0 to some value (ETRATIO) higher than 1.0.  The default is 1.0, but we have tried out values of 1.1 and 1.2 for some commodities.  A value of ETRATIO of 1.2 would, for example, increase EO from 1.0 to 1.2 times reference, as LAI increases from 0.0 to 6.0.  This is not the same as the LAI effect on energy absorption (for potential T=transpiration) versus transmission to the soil (for potential E=soil evaporation) which is set by KEP (presently about 0.70 for CROPGRO).
 
Unfortunately, this ETRATIO does not apply for CERES-Maize model.  We anticipate the Maize model may be modified to allow this in the next release.
 
Ken Boote

________________________________

From: DSSAT - Crop Models and Applications on behalf of Heeren, Derek
Sent: Mon 7/30/2007 3:17 PM
To: [log in to unmask]
Subject: Tall reference for ET



Hello DSSAT users,

It appears that when "FAO-56" is selected for the method of calculating
ET, the constants for a short-grass equation are used.  Is there a way
to run a tall reference ET equation in DSSAT?

Thanks,
Derek


Derek M. Heeren, E.I.
Graduate Research Assistant
Agricultural and Biosystems Engineering Department
South Dakota State University