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 m² 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.