This linear IUH is used mainly for rural areas. With Nash, the peak discharge increases with N and decreases with Tp. Measurements in Ontario and in Switzerland indicate that an average of 3 number of linear reservoirs may be appropriate.
The time to peak, Tp, is obtained from the time of concentration, Tc:
Tp = (N-1)/N Tc where, N, is the number of linear reservoirs
Tp = 0.67 Tc
In general, the time of concentration, Tc, can be determined using one of three methods:
- Empirical formulas (only recommended if they are based on regional verifications).
- Velocity methods, Tc = 3 (Li/Vi). The overland velocities are determined with an SCS graph, and channel velocities can be determined from Manning’s equation.
- Kinematic wave method (which accounts for the rainfall intensity).
The NASHYD command is used for non-homogeneous areas, and in the case of SCS abstraction methods, uses a weighted average of CN. Comparisons with measurements show a better performance if the response from the pervious and impervious areas are simulated separately.
Fore very large urban areas (> 200 hectares), NASHYD requires calibration.
Furthermore, if the response time of an urban watershed is increased by significant channel storage, this effect must be simulated by channel routing (unless Tp is calibrated).