Change the units for the area column in the time area diagram to m2 (or have the ability for user to change the units between m2 and ha). When using time area diagram, as per the Green Roof Runoff Method (link below), it results in areas of 0.001 ha or less when inputting the catchment area at each time step. When less than 0.001 it puts a value of 0 so no area is defaulted therefore it doesn't represent the total catchment correctly - if it was m2 the values would be much more accurate
https://help.innovyze.com/space/infodrainage/16124094/Green+Roof+Runoff+Method