How to Determine the Capacity of a Water Storage Tank? Should It Be Calculated Based on Dry-Season Water Demand?
How should the capacity of a water storage tank be determined? This is a common question in water supply and irrigation engineering. In general, the capacity should be determined based on water balance analysis, verified against dry-season water demand, and checked against relevant design standards. Firefighting and safety storage requirements should also be considered to achieve a balance between economy and reliability.
The effective storage volume of a water storage tank generally consists of three components: regulating storage, firefighting (safety) storage, and losses from evaporation and leakage. Among them, regulating storage is the key factor, mainly determined by the balance between water inflow and water demand.
1. Water balance analysis is the fundamental method.
Using hourly, daily, or monthly intervals, compare water inflow with water demand for each period. The water deficits during periods of insufficient supply are accumulated, and the maximum cumulative deficit represents the required regulating storage volume. In other words, the capacity is determined by the longest continuous period of water shortage under the most unfavorable conditions—the classic Rippl (mass curve) method.
2. Dry-season demand calculation is an important verification method.
The dry season is generally the period with the lowest water supply reliability throughout the year. The storage tank should be capable of maintaining continuous supply throughout the longest consecutive dry period. The required volume can be estimated using V = W × T, where W is the average daily water demand during the dry season, based on the maximum daily demand under the required design reliability, and T is the longest consecutive dry period in days. The result should then be divided by an effective volume coefficient, generally 0.85–0.90, to account for evaporation and leakage losses.
3. Standard-based ratios can be used as an additional check.
According to GB 50013-2018, Standard for Design of Outdoor Water Supply Engineering, when the distribution network has no regulating structures and sufficient data are unavailable, the effective volume of a clear-water tank may be determined as 10%–20% of the water treatment plant’s maximum daily design flow. Under the Technical Code for Rural Water Supply Engineering (SL 687-2014), the effective volume of clear-water tanks or elevated storage tanks for Type I–III projects is generally controlled at 15%–25% of the maximum daily water demand. In actual design, the volume can first be calculated based on the water balance and then compared with these standard-based ratios to avoid excessive or insufficient storage capacity.
Therefore, calculating storage requirements based on dry-season water demand is not optional, but an important verification step—it should not, however, be used as the sole basis for sizing. Dry-season calculations ensure continuous supply during the most critical period, water balance analysis addresses the dynamic relationship between inflow and demand, and standard-based ratios provide an additional reference. By combining all three approaches, engineers can achieve a reasonable balance between project cost and water supply reliability.
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