Calculation of Baseflow

From Eq. 30.30, if the water level in the linear reservoir is above the threshold water level

(30.48)   

where h is the depth of water in the baseflow reservoir, h_thresh is the depth of water required before baseflow occurs, and k_b is the time constant for base­flow. If the water level is below the threshold there is no baseflow.

Similar to Eq. (30.39), for each Baseflow Reservoir

(30.49)   

where q_IN is the amount of inflow to each base flow reservoir, q_B is the spe­cific baseflow out of the reservoir, and q_pump is the amount of water removed via extraction wells from each reservoir. Both q_v and q_pump are controlled by split fractions that distribute q_v and q_pump between the two parallel baseflow reservoirs.

Each Baseflow Reservoir can be treated as A Single Linear Reservoir with One Outlet (p. 614). Thus, as long as the water level is above the threshold water level for the reservoir (i.e. there is still baseflow out of the reservoir),

(30.50)   

where k_b is the time constant for the Baseflow Reservoir.

The formula for a single outlet is applicable because there is no time constant associated with the pumping. However, Q_pump, is also controlled by a thresh­old level, in this case, a minimum level below which the pump is turned off. Since this minimum level is independent of the threshold level for the reser­voir itself, a case could arise, whereby there was pumping, but no baseflow from the reservoir. In this case,

(30.51)   

If there is no pumping and no baseflow out, then the expression for the water level in the reservoir simply becomes

(30.52)   

In general, during a time step, the water level may cross one or more of the pumping or the baseflow thresholds. If this occurs, the program uses an itera­tive procedure to split the time step into sub-time steps and applies the appro­priate formulation to each sub-time step.