Runoff Model (reservoir) - Recharge

Recharge

The recharge, also called effective rainfall or rainfall excess, can be modeled by a pre-reservoir (figure 6) giving the recharge as overflow. The pre-reservoir knows the following elements:

• a maximum storage (Sm) with unit length
• an actual storage (Sa) with unit
• a relative storage: Sr = Sa/Sm
• a maxmimum escape rate (Em) with units length/time . It corresponds to the maximum rate of evaporation plus percolation and groundwater recharge, which will not take part in the runoff process (figure 5, 6)
• an actual escape rate: Ea = Sr.Em
• a storage deficiency: Sd = Sm + Ea − Sa

The recharge during a unit time step (T2−T1=1) can be found from R = Rain − Sd
The actual storage at the end of a unit time step is found as Sa2 = Sa1 + Rain − R − Ea, where Sa1 is the actual storage at the start of the time step.

The Curve Number method (CN method) gives another way to calculate the recharge. The initial abstraction herein compares with Sm − Si, where Si is the initial value of Sa.