Performance Analysis of Rainwater Tanks at California State University Long Beach

A daily water balance model is developed for California State University Long Beach campus to investigate rain water saving potential of a rain water tank system. When it rains, the model stores runoff from 28,225 m(2) roof top area in the tank and uses it for landscape irrigation. The model evaluates the performance of tanks ranging from 10,000 L to 100,000 L in size for the historical driest, average, and wettest years. Results show that in an average rainfall year, runoff volume stored in a 100,000 L tank can supply 50 days of irrigation demand (< 52,000 L/day) of cool grass turf on North campus. However, whenever the rainfall was more than 10% of average annual rainfall, a large fraction of the rain water was lost as overflows from the 100,000 L tank. Reliability of tank system increases with tank sizes but is highly dependent on the annual volume of rainfall.