Where At is the area of the ...
Published by Miguel Angel Pavon Gutierrez, Data and Mapping Solutions Lead at Texas Water Development Board
Where At is the area of the tank (18 m X 18 m)
Ao is the cross area of the open valve
g is gravity and
Cd is the coefficient for the tipe and size of valve, I believe is 3076, see https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjs0p2jq7DuAhUlnq0KHaK9D4cQFjAAegQIBRAC&url=https%3A%2F%2Fglobalsupplyline.com.au%2Fwp-content%2Fuploads%2F2014%2F10%2FGate-Globe-Check-Valves-Flow.pdf&usg=AOvVaw1sSUMuyRoM9SuVUF7yN93K