6 Steps to Designing a Rainwater Harvesting System

The principles of designing a rainwater harvesting system with a water tank or rain barrel can be applied worldwide. Simple rainwater tank systems can be designed and installed by anyone with some DIY skills. More complex systems may require professional assistance. The article below is a good start for anyone thinking of designing and installing rain water tanks.

jojo rainwater tank

Simple rainwater collection: downpipe from a gutter to a JoJo water tank on a steel tank stand

Rainwater harvesting systems can be as simple as a rain barrel or drum for garden irrigation at the end of a downspout, or as complex as a domestic potable system or a multiple end-use system at a large corporate campus.

Basic Rule For Sizing

The basic rule for sizing any rainwater harvesting system is that the volume of water that can be captured and stored (the supply) must equal or exceed the volume of water used (the demand).

The variables of rainfall and water demand determine the relationship between required catchment area and storage capacity. In some cases, it may be necessary to increase catchment surface area by addition to capture enough rainwater to meet demand. Water tank capacity must be sufficient to store enough water to see the system and its users through the longest expected interval without rain.

  1. Determine What The Captured Water Be Used For

    Will you be using the rainwater for irrigation only? If so, a rough estimate of demand, supply, and storage capacity may be sufficient. On the other hand, if rainwater is intended to be the sole source of water for all indoor and outdoor end uses, a more precise calculation will ensure adequate supply.

  2. Determine How Much Water Can Be Captured

    In theory, approximately 0.62 gallons per square foot of collection surface per inch of rainfall can be collected. In practice, however, some rainwater is lost to first flush, evaporation, splash-out or overshoot from the gutters in hard rains, and possibly leaks. Rough collection surfaces are less efficient at conveying water, as water captured in pore spaces tends to be lost to evaporation. Also impacting achievable efficiency is the inability of the system to capture all water during intense rainfall events. For instance, if the flow-through capacity of a filter-type roof washer is exceeded, spillage may occur. Additionally, after water storage tanks are full, rainwater can be lost as overflow.

    For planning purposes, therefore, these inherent inefficiencies of the system need to be factored into the water supply calculation. Most installers assume an efficiency of 75 percent to 90 percent.

  3. Determine the Collection Surface

    The collection surface is the “footprint” of the roof or structure. In other words, regardless of the pitch of the roof, the effective collection surface is the area covered by collection surface (length times width of the roof from eave to eave and front to rear). Obviously if only one side of the structure is guttered, only the area drained by the gutters is used in the calculation.

  4. Calculate the Volume of Rainfall

    To ensure a year-round water supply, the catchment area and storage capacity must be sized to meet water demand through the longest expected interval without rain.

    If the rainwater harvesting system is intended to be the sole water source, the designer must size the system to accommodate the longest anticipated time without rain, or otherwise plan for another water source, such as a well backup or hauled water.

    Also, rainfall from high-intensity, short duration rainfall events may be lost to overflow from water tanks or splashout from the gutters. Although these intense rainfall events are considered part of the cumulative annual rainfall, the total available volume of such an event is rarely captured.

    Another consideration is that most rainfall occurs seasonally; annual rainfall is not evenly distributed throughout the 12 months of the year. The monthly distribution of rainfall is an important factor to consider for sizing a system.

  5. Estimate Indoor Water Demand

    A water-conserving household will use between 25 and 50 gallons per person per day. Households served previously by a water utility can read monthly demand from their meter or water bill to find monthly demand for purposes of building a new rainwater harvesting system. Divide the monthly total by the number of people in the house, and the days in the month to get a daily per capita demand number.

  6. Estimating Outdoor Water Demand

    Outdoor water demand peaks in hot, dry summer. In fact, as much as 60 percent of municipal water demand in the summer is attributable to irrigation. The water demands of a large turfgrass area almost always preclude the sole use of harvested rainwater for irrigation.

    For planning purposes, historical evapo-transpiration can be used to project potential water demands. Evapotranspiration is the term for water use by plants, the combination of evaporation from the soil and transpiration from the plant leaves.

    It is recommended to install landscapes of native and adapted plants, and also ascribe to the xeriscaping principles. A water-wise landscape can be quite attractive, while conserving water and demanding less care than a garden of non-native or non-adapted plants.

Source: About.com

If rainwater is to be used for drinking, it must be filtered first. Importantly, water tanks used for storing potable water should be certified for this purpose. We recommended JoJo water tanks as these are made from 100% food-grade polyethylene and rated for storing drinking water. Click on the links below to see the range of vertical tanks, horizontal tanks, septic tanks, silo tanks, tank stands and other high quality JoJo Tanks products.

JoJo Tanks South Africa offer a wide range of products such as plastic water tanks and chemical tanks to suit every household, business or agricultural need, including silo tanks and silo tank stands as well as other high quality JoJo Tanks products.  See JoJo Tanks’ VERTICAL TANKS, HORIZONTAL TANKS (transporter tanks), STEEL TANK STANDS and OTHER JOJO PRODUCTS and see JoJo Tanks’ NEW 6000 LITRE UNDERGROUND TANKSAlso see our FAQ and WATER TANK PRICES.

JoJo’s superior polyethylene plastic water tanks and steel water tank stands make JoJo Tanks the leaders in plastic water tank technology and the quality, affordability and 8-year guarantee on these water tanks make JoJo products one of the best choices in South Africa.  *All JoJo Tanks polyethylene tanks and products now carry a 8 year warranty (on tanks purchased on or after 1 July 2013)*  JoJo’s water tanks and chemical tanks come standard with a number of features that are often lacking on cheaper/inferior plastic tanks made by other companies in South Africa.

We are authorised JoJo Tanks dealers in South Africa and dispatch orders directly from JoJo Tank depots to save on transport costs. Full range of JoJo Tanks products and JoJo water tanks for sale.  CONTACT US for a quote on the right JoJo water tank or other JoJo product for you.  Special discounts are available on multiple orders of chemical tanks and water tanks and to our commercial and government customers. We can usually delivery tanks and tank stands free of charge to business/commercial addresses in South Africa.


(15000 litre and 20000 litre JoJo vertical water tanks & chemical tanks are available on order directly from JoJo Tanks’ main factory)

, , , , , , , , , , , , , , , , , , , , ,