A large cistern can look reassuring while the proposed roof supply remains untested. For a Texas home, compare the water available through the year before choosing tank capacity. The Texas Water Development Board still publishes its 2005 third-edition manual; its calculation method is useful, while its historical weather examples, prices and incentive discussion should not be mistaken for current project facts.
Define the Job the Tank Must Perform
The manual ties storage decisions to intended uses and demand (printed pp. 32–33; PDF pp. 37–38). Write a short purpose statement: occasional garden watering, a defined irrigation zone, or a separately designed household supply. Those are different promises to ask an installer to evaluate.
Give each bidder the same assumptions. Identify which uses can stop during dry weather, which need another supply and whether landscaping will change. For household drinking water, obtain a separate water-quality and treatment design; this storage discussion does not establish that collected roof water is safe to drink.
Request a Running Balance, Including the First Month
The worked example carries stored water forward, adds collection and subtracts demand. It also specifies water already in the tank at startup (printed pp. 38–39; PDF pp. 43–44). Ask to see that opening assumption in your own worksheet.
As a simple illustration, 1,000 gallons already stored, plus 400 collected, minus 700 used leaves 700 gallons. Starting empty gives a different outcome. These invented inputs demonstrate bookkeeping only. Have the proposal identify whether startup water is collected before use begins, purchased separately or left as an unresolved assumption.
Challenge the Rainfall Assumption
The manual compares average and median monthly rainfall and illustrates how the result changes under different inputs (printed pp. 31–32 and 39; PDF pp. 36–37 and 44). Its old Dallas tables are teaching examples, not a forecast for your property.
Ask the designer to identify the location, period and source of the rainfall data used today. Request a dry-period scenario and a version with lower demand. A comparison is more useful when it shows the first month that needs supplemental water, rather than presenting only one annual collection total.
Compare a Tank Expansion With a Demand Change
TWDB’s current documents page offers a system-size calculator for estimation and newer rainfall/runoff maps. Those resources help start a discussion; they do not settle a particular installation’s performance.
Ask for separate alternatives: more storage, a changed collection area, reduced irrigation or a backup arrangement. For each, record the assumed starting level, unmet demand and excess water that cannot be retained. Require the selected alternative’s calculation with the contract. Also assign responsibility for overflow routing, cleaning access and any utility connection review, rather than leaving those items inside an undefined tank-installation allowance.
Questions Homeowners Ask
Can I use the manual’s Dallas tank size for my home?
Use the example to understand the worksheet. Replace its weather, demand, roof area and starting-water assumptions with values appropriate to the proposed installation.
Does increasing storage create more collected water?
A tank adds capacity to retain supply; the proposal still needs to explain where that supply comes from. Compare the collection and demand assumptions before enlarging the order.
Are the manual’s old cost and incentive figures current?
No current price or benefit is established here. Obtain dated quotes and verify any claimed incentive separately with its administering authority before including it in the budget.
Sources And Local Resources
Official sources checked October 17, 2026. Confirm current requirements with the authority responsible for your property.
- Texas Water Development Board: The Texas Manual on Rainwater Harvesting, third editionPrinted pp. 31–33 and 38–39 correspond to one-based PDF pp. 36–38 and 43–44. Supports monthly balance, startup storage, supply/demand distinction and average-versus-median example. Title page is PDF p. 4 and states 2005. Original arithmetic does not reuse the manual’s Dallas inputs.
- Texas Water Development Board: Rainwater Harvesting DocumentsCurrent official catalog identifies the manual as the 2005 third edition, describes the calculator as an estimate and links rainfall/runoff maps using 1991–2024 data.
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