Ventana software support forum

Hello everyone.
i am having issues modelling. Would everyone help me, please?
I want to investigate the effect of Temp (temperature) and Prcp (precipitation) on NDVI as shown in the attached figure1. In the equation section for NDVI, i selected lookup for type: , and in the As graph section, i can only draw one graph. How can I enter two graphs together (such as the attached figure 2)?

What is NDVI?

I'm guessing here. I think what you need are two new parameters .
effect of temp on NDVI = effect of temp on NDVI table ( temperature )
effect of prcp on NDVI = effect of temp on prcp table ( precipitation )

"effect of temp on NDVI table" and "effect of prcp on NDVI" are the two graphs that you have hand drawn.

Then you have something like
NDVI adjusted for temp and prcp = NDVI * effect of temp on NDVI * effect of prcp on NDVI

NDVI : normal different vegetation index
NDVI is representive of vegetation.
I want to write a equation for ndvi based on temperature and precipitation. How can i do it?
Is this conceptual model correct?

neda65 wrote: ↑Fri Dec 30, 2022 5:12 pm NDVI : normal different vegetation index
NDVI is representive of vegetation.
I want to write a equation for ndvi based on temperature and precipitation. How can i do it?
Is this conceptual model correct?

Maybe someone else on the forum can help? I've got no knowledge about any of this.

You are right. But my question was how to draw two graphs together with lookup?

Hi
When two variables have both an effect on another variables and these effects are independant from one another, the total effect is equal to the multiplication of both effects. But if the effects are not independant from one another then you must build a two dimension lookup. There was in the past an explanation in the Vensim guides how to build a two dimensions or three dimension lookups. I have tried to find it, but I did not. Maybe an adminstrator knows where to find it.
JJ

Hi

It is possible too, to use the vector lookup to build a two dimensional lookup see the function example. it was maybe not possible to do that years ago and the solution at this time was more complex. It must be too possible to build a three dimensional lookup using the vector lookup.

JJ