USER'S MANUAL FOR XSRAIN--A FORTRAN IV PROGRAM FOR CALCULATION OF FLOOD HYDROGRAPHS FOR UNGAGED WATERSHEDS

XSRAIN is a computer program written to calculate flood hydrographs for ungaged watersheds by means of the Soil Conservation Service (SCS) runoff index, the curve number (CN). The curve number (CN) for a watershed is determined by the soil and land use types occuring within it. XSRAIN does not, however, use SCS equations for computing excess rainfall. Abstraction from variable intensity rainfall events is accomplished with physically based infiltration equations. The hydraulic soil parameters needed to make use of these equations have been calibrated against CN, and the computer program makes use of this equivalence. Routing of excess rainfall is performed with the SCS dimensionless unit hydrograph (mass curve) to produce a runoff hydrograph. Users may input variable intensity storms or they may simply specify a depth and duration. In the latter case, one of four possible time distributions is imposed at user's option on the rain. Options permit accounting for antecedent moisture conditions. Subroutines are described and variables defined. Examples of the four main options are provided, as well as a hand calculated example. This user's manual is totally self-contained and includes all tables and figures needed for the use of XSRAIN. (FHWA)