Size storage for the dry season
Storage turns an uneven supply into a steady one. Size it for the longest dry spell you must survive, plus a locked fire reserve.
How
- Take the conserving daily demand from water-3 and the dry season length from water-5.
- Subtract any reliable inflow during the dry period (for example sustained well yield or a spring's dry season flow).
- Multiply the shortfall by the number of dry days and add a safety margin for longer droughts.
- Cross-check with the Texas manual's monthly water balance: start with a tank volume, add each month's harvest, subtract demand, and see if the tank ever runs dry. The manual also gives a simple rule of one quarter of annual demand, while noting it can oversize.
- Run the monthly water balance calculator on this page with your own 12 monthly rainfall figures. It shows the lowest tank level and any month that falls short.
- For solar-pumped wells without batteries, Colorado State, Virginia Tech and Wyoming extension guidance use at least three days of demand as a rule of thumb, to cover nights and cloudy days.
- Add the fire reserve from water-18 as a separate volume, then use the storage calculator on this page.
Sources
- The Texas Manual on Rainwater Harvesting, 3rd edition. Texas Water Development Board, accessed 2026-11
- Solar-Powered Groundwater Pumping Systems. Colorado State University Extension, accessed 2026-11
- BSE-346: Solar water pumping for livestock (design steps). Virginia Cooperative Extension, Virginia Tech, accessed 2026-11
- B-1289-1: Solar-powered water pumping systems. University of Wyoming Extension, accessed 2026-11