Inflows can be specified with a mathematical expression. Typical expressions include: constants (e.g., groundwater recharge that doesn't vary over time), a specified value for each month (this is usually how the Current Accounts inflow data is specified when you are using the Water Year Method to project future inflows--using the Monthly Time-Series Wizard can be helpful in establishing these data), or some other relationship (e.g., the headflow for an ungaged stream could be modeled as some fraction of the headflow in another river for which good data exists). You may also use the ReadFromFile function to read in data from a text file (not to be confused with the obsolete Read from File Method, which uses a different ASCII file format).
WEAP allows you to mix these methods, using the Read From File method for some sources (e.g., one or two rivers for which you have historical streamflow data), the Water Year Method for others, and expressions for the rest. For example, the natural recharge for an aquifer might be relatively constant over time, so you would enter a constant for this value. As another example, you could use the Read From File method for the headflow and surface inflow of a major river, then use an expression for the minor rivers' inflows--perhaps a fraction of the flow in the main river. In this way, you could replicate the actual historical variation of the main river on the smaller rivers.