Skip to main content
Close contact information
Get in touch

Josef Hargrave

Global foresight manager

Phone icon +44 (0) 20 7636 1531
Close contact information

Topup: GIS tool to optimise rainwater harvesting tanks


GIS tool to optimise rainwater harvesting tanks

Focus Area

Digital, Water


Digital Environments, Sustainability, Water





Harvesting of rainwater is an important element of regional water supplies. Rainwater harvesting improves water resilience and saves money.

The largest cost item in medium- to large-scale rainwater harvesting systems is the cistern (storage tank), and so it is important to optimise the size of the cistern. Smaller cisterns are more effective in terms of volume of rainwater harvested per cistern unit volume. The optimal cistern size occurs when the additional benefit of increasing the size of the cistern, in terms of available harvestable volume of rainfall, declines steeply. Some practitioners simply adopt thumb-in-the wind approaches to pick the cistern size: this typically this fails to identify the optimum size, and usually results in inefficiently oversized systems.

Arup developed a GIS (geographic information system) tool called Topup that uses rainfall data records to generate location-specific rainwater tank size optimisation curves. This removes the guesswork from cistern sizing, and allows users to determine the optimum size for a rainwater harvesting system by analysing local climate and project characteristics.

Key Findings

Topup can be applied wherever suitable rain gauge data are available. In the US, Topup uses data from National Oceanic and Atmospheric Administration (NOAA) rain gauges. Topup scrubs the data to ensure that gauges with less than 20 years of records are omitted.

Sizing of rainwater harvesting cisterns has often been ill-informed: for example, setting out to meet 100% of non-potable demand using harvested rainwater can preempt optimal sizing of the installed cistern.

Rainfall can vary significantly over relatively short distances: at the University of Hawaii, Manoa, rainfall trends at the upper and lower ends of campus vary so greatly due to the local terrain that two separate optimisation curves were developed to size cisterns for rain harvesting at the campus.


The Topup tool has been successfully used on several projects to determine site-specific optimum cistern sizes for rain harvesting.

The user inputs basic project-specific information to Topup such as site location, roof size, indoor demand for rainwater, and irrigation demand. Topup determines the site-specific daily historic rainfall record based on a distance-weighted triangulation of rain gauges within a user-defined radius from the project site. The resulting tank size optimisation curve allows the designer to select the cistern size that will provide the greatest benefit for a reasonable cost. Topup also produces charts showing days without rain over the historical record, and the maximum number of consecutive days without precipitation, which can be used to plan for droughts.


Topup is a first-of-its-kind tool for cistern sizing that combines the power of GIS to access and process large data sets with a pragmatic approach to efficient design. Within minutes, Topup allows designers and clients to visualise the feasibility of rainwater harvesting systems at a specific site, based on their specific water demands and available harvest areas, coupled with detailed site-specific daily historical rainfall records.