create a tank model for runoff simulation

In this model, i am considering a soil depth of watershed into 3 layers one below the other as storage depth and process runoff simulation by calculating runoff each layer. u can refer the followingf pdf for a better understanding of the model.

please find the image below and let me know how they are adding storage depth of tank 2 (ST2) to the the remaining storage of tank1

the part where i have doubt with i marked it with a box in blue in picture.

u can find the total picture of the model in the 9th page of pdf.