SSARR Routing Method

The SSARR routing method ("Figure: Reach Editor - SSARR Routing Method") is an implementation of the channel routing method used in the computer program Streamflow Synthesis & Reservoir Regulation (SSARR) developed the Corps' Northwestern Division (USACE, 1991) and NOOA's North West River Forecast Center.

Figure: Reach Editor - SSARR Routing Method

SSARR is conceptually similar to Modified Puls in that it represents the reach as a "chain of lakes" and the Number of Subreaches defines the number of "lakes in the chain. The routing of each lake in the chain is described by the equation:

$\begin{array}{l}\displaystyle O_{t}=O_{t-1}+\Delta t* \frac{I_{m}-O_{t-1}}{T_{s}+\frac{\Delta t}{2}}\end{array}$

Where:
O_t = Outflow for this timestep
O_t-1 = Outflow for the previous timestep
∆t = length of the timestep
T_s= time of storage
I_m = the mean inflow

Two options are available for defining the parameters for this routing method:

Outflow vs Time of Storage (Interpolation table)—In this form of the SSARR method, the outflow vs. reach storage is defined by Time of Storage values, T_s in units of hours.

Time of Storage equation—this equation defines T_s as a power function of outflow:

$\begin{array}{l}\displaystyle T_{s} = \frac{KTS}{Q^n}\end{array}$

Where:
T_s = Time of Storage (per increment, in hours)
Q = Outflow
KTS = Coefficient, determined empirically
n = The power coefficient on outflow; usually between -1.0 and 1.0.

As evident from the above equation, T_s is a nonlinear function of discharge except when n = 1. A negative value of n is used when time of storage increases as discharge increases. According to the SSARR User's Manual, a value of n = 0.2 is reasonable for most streams in the Columbia River Basin.