If tanks are hydraulically close, as in the case of several tanks adjacent to each other, it is better to model these tanks as one composite tank with the equivalent total surface area of the individual tanks.

This process can help to avoid fluctuation that may occur in cases where the tanks are modeled individually. This fluctuation is caused by small differences in flow rates to or from the adjacent tanks, which offset the water surface elevations enough over time to become a significant fluctuation. This results in inaccurate hydraulic grades.