I want to make a relationship to allocating water. e.g. :
if Q of River<5 cms then link outflow=2 cms

or something like this.

I'm not quite sure what you want to do. Allocations are based on demands
(and supply available)--are you saying that the demand is function of
streamflow, or that the amount available to allocate is a function of
streamflow? Obviously, you cannot allocate more water than is available. For
example, if Q < 2 CMS, then you cannot allocate 2 CMS.

WEAP will try to allocate such that all demands are satisfied, with
preference to the highest priority demands. If your intent it to make sure
that a certain minimum flow is maintained in the river, you could achieve
this goal with a minimum instream flow requirement, immediately below the
outflow point for the link. If the flow requirement had a higher priority
than the demand, then it would be satisfied before any water was withdrawn.
For example, if the flow requirement was 3 CMS, then a flow of 5 would allow
at most 2 CMS to be allocated. A flow of 4 would allow at most 1 CMS.

Jack

I didn't mean that. I wanted to know that is it possible that define a withdrawal water by an exprision or not. some thing like writing codes, as LINGO.

No, it is not possible to define an expression, as a function of streamflow, that will determine how much to withdraw from the river.

Jack