Dirac inputs initial value problem

[tfwoodroof]

Dear STANMOD team and users,

I am using CXTFIT to solve the inverse problem for a column experiment. If the boundary value problem is set as a 'Dirac delta input' there is only one parameter to set (the mass M). When setting the initial value problem, if 'Dirac delta initial distribution' is selected there are three parameters: mass, position and U. I have two questions:

- What is the difference between the two masses?
- From my understanding of the manual, U is associated with stepwise inputs. What is the reason for including it as a parameter for Dirac inputs?

I am new to CXTFIT and advection-dispersion modelling in general, so please let me know if any more information is needed to answer these questions. Thanks in advance.

Tom Woodroof