The precipitation and air
temperature files are directly entered or selected using
the buttons to the right. The files must be stored in the same
directory as the case file. If the user selects a file from a different
directory, there is an option to copy it into the case directory.
The precipitation scale factor (normally = 1) is multiplied by values in the precipitation file during simulation. This feature can be used to re-scale design storms or test sensitivity of results. For example, the sample input file 'TYPE2.PCP' defines a 1-inch, 24-hr storm with SCS TYPE II distribution. To run a 2-inch storm using this file, set PRECIP VOLUME FACTOR = 2.
To flush out initial conditions (particle accumulation in the watershed, device volumes and concentrations), the simulation can be repeated with multiple passes through the precipitation time series. Results are kept only on the last pass through the file. This is generally applicable to short time series or design storms.
An alternative way to flush out initial conditions is to run the model for a while before saving the output. The simulation begins on the 'Start Date' (if = blank, first date in input precipitation file). Results are kept on or after after 'Keep Date' ( if = blank, Start Date used). The simulation ends on the 'Stop Date' (if = blank, simulation runs to the end of the precipitation file). If the 'Stop Date' is past end of precip. file, missing days are assumed to have zero precipitation.
The simulation starts at the beginning of the first storm after the 'Start Date'. If the first storm occurs after the 'Start Date', the initial dry period is ignored.
Some output tables report the total load (lbs) and the loading rate (lbs/yr). The rates are computed based upon the actual duration of the simulation, not upon the difference between the Keep and Stop Dates. For that reason, the loading rate may be slightly higher than that computed based upon the total load and the difference between the Keep and Stop dates.
The total duration and date ranges actually used in the simulation are reported in various output tables and on the 'Case Info' screen.
While input precipitation values are specified on an hourly basis, P8 summarizes results on a storm-event basis. Each storm event begins with a wet period and ends with a dry period. The Minimum-Interevent Time (hours) specifies the minimum length of the dry period. If the dry period is greater than or equal to this value and precipitation occurs, a new storm event is begun. If the dry period is less than this value, the precipitation is part of the current event. The default value (10 hours) was assumed in the initial calibration of P8 and would normally not be changed by the user.
The Number of Time Steps per Hour determines the integration step size in solving the P8 differential equations. Specifying higher values will increase the time required for the simulation but yield more accurate results. Specifying lower values will decrease the simulation time but results may be less accurate, depending on the case. At least 4 time steps per hour are recommended. Users should test the sensitivity of results to the assumed step size. Use higher values to be on the "safe side" if you dont mind waiting a little longer while the simulation is run. Results may be sensitive to step size, particularly in devices where flow is directed in either of 2 directions, depending on elevation (e.g., splitter, detention pond with both a normal outlet and flood pool, or infiltration basins that overflow).
Impervious area runoff coefficients specified on the Edit Watersheds screen apply to storms with cumulative rainfall + snowmelt less than or equal to the Rainfall Breakpoint specified here. A runoff coefficient of 1.0 is applied when the cumulative rainfall + snowmelt exceeds the breakpoint. The default value of (0.8 inches) is consistent with SLAMM calibrations. This will impact runoff simulations in watersheds with specified impervious runoff coefficients < 1.0. A breakpoint value of 999 in (greater than the largest storm event) will provide results consistent with previous versions of P8.