This program simulates catchment dynamics using the diffusion wave model developed by Ponce (1986).
The program is a numerical model of overland flow using an open-book schematization.

A significant feature of the model is its grid independence,
allowing it to calculate the same runoff hydrograph, regardless of the grid specification [provided the problem is well defined]. The only grid input is the time interval Δt.
Upon execution, the program adjusts the value of the space interval Δx in planes and channel to assure grid independence and preserve modeling accuracy.

**Units:** The program can use either SI (Metric) units of U.S. Customary units, selectable on input. The applicable units are displayed to the left of the input box.

The **rainfall depth ***P* is specified in cm or in., depending on the units selected.
The input value is limited to a value greater than zero (0).

The **runoff curve number ***CN* is limited to a value greater than 20.

The **rainfall duration ***t*_{r} is specified in hours.
The input value is limited to a value greater than zero (0).

The **rainfall distribution **is specified in three parts: (1) the number of points *N*_{p} in the cumulative rainfall distribution,
(2) the dimensionless cumulative time *T*_{*}, and (3) the dimensionless cumulative depth *D*_{*}.
For instance, for a uniform distribution, the number of points *N*_{p} is 2; the dimensionless cumulative time *T*_{*} is entered as 0.,1.;
likewise, the the dimensionless cumulative depth *D*_{*} is entered as 0.,1.

The **total simulation time ***T*_{t} is specified in hours.
The input value is limited to a value greater than zero (0).

The **number of time intervals ***N*_{Δt} is limited to an integer greater than zero (0).

The **number of time intervals for printing ***N*_{pr} is an integer greater than zero (0). A normal value is
*N*_{pr} = 1, that is, output is reported every time interval. For *N*_{pr} = 2, output is reported every two time intervals.

The **fraction used to estimate reference discharge ***F*_{r} is specified in the range 0 ≤ *F*_{r} ≤ 1.
Normally, the value of
*F*_{r} = 0.5; leave the input box blank to specify the default value *F*_{r} = 0.5.

The **overland flow area ***A* is specified in hectares or acres, depending on the units selected.
The allowable range is 0 ≤ *A* ≤ 2500 in SI (metric) units; 0 ≤ *A* ≤ 2500 in U.S. Customary units.

The **fraction of area in the left plane ***F*_{r} is specified in the range 0 < *F*_{r} ≤ 1.
Enter *F*_{r} = 1 to specify only one (left) plane.

The **slope of the left plane ***S*_{lp}, limited in the range 0.000001 ≤ *S*_{lp} ≤ 0.1.

The **Manning coefficient for the left plane ***n*_{lp}, limited in the range 0.008 ≤ *n*_{lp} ≤ 1.

The **rating exponent for the right plane ***β*_{lp}, limited in the range 1.5 ≤ *n*_{lp} ≤ 3.
To specify default value of β = 2, enter the value -1 in the input box.

The **slope of the right plane ***S*_{lp}, limited in the range 0.000001 ≤ *S*_{rp} ≤ 0.1.

The **Manning coefficient for the right plane ***n*_{lp}, limited in the range 0.008 ≤ *n*_{rp} ≤ 1.

The **rating exponent for the right plane ***β*_{lp}, limited in the range 1.5 ≤ *n*_{rp} ≤ 3.
To specify default value of β = 2, enter the value -1 in the input box. Leave input box blank to specify right plane to exactly mirror
left plane for *F*_{lp} = 0.5 only.

The **length of the channel ***L*_{ch} is specified in m or ft, depending on the units selected. The input value is limited to a value greater than zero (0).

The **slope of the channel ***S*_{ch}, limited in the range 0.000001 ≤ *S*_{lp} ≤ 0.1.

The **Manning coefficient of the channel ***n*_{ch}, limited in the range 0.008 ≤ *n*_{ch} ≤ 0.250.

The **bottom width of the channel ***B*_{ch} is specified in m or ft, depending on the units selected. The input value is limited to a value greater than zero (0).

The **design depth of the channel ***y*_{ch} is specified in m or ft, depending on the units selected. The input value is limited to a value greater than zero (0).

The **side slope of the channel ***z* [z H : 1 V] is limited in the range 0 ≤ *z* ≤ 10.

Program ONLINEOVERLAND will trigger one of thirty-two (32) fatal error messages if any input value is out of range.
Additionally, the program will trigger a warning error message (Error 33) if the [calculated] maximum channel flow depth is greater than the design depth of the channel.

The current version [130818] can only handle one book. The capability to handle several books is under development at this time.