Rainwater harvesting: How to save for a less rainy day
Given the ongoing challenges posed by extreme weather conditions – particularly the widespread drought affecting much of the UK’s growing season – now is an ideal time to explore the potential benefits of rainwater harvesting on your farm.
As average temperatures rise and weather patterns become less predictable, making the most of the UK’s naturally abundant rainfall is becoming more important than ever.
Water plays a vital role across UK farms, whether this is for livestock drinking, sprayer filling, or equipment and yard cleaning. With increasing pressure on utility mains supply driving up costs, using harvested rainwater can offer significant savings on metered bills.
Where to place your collection tank
Choose a location according to your gutter positions, but bear in mind it must also be easily accessible for use and allow for overflow management during heavy rainfall periods.
How to manage water quality
Livestock and crops do not typically require treated water. However, it is still good practice to periodically empty and clean your water storage tanks to prevent algae growth and bacterial contamination.
If your enterprise requires potable water, you can install additional filtration systems—such as sand and ultraviolet light treatments—for approximately £200 to £1,000, depending on tank size, flow rate, and system complexity.
Debris filtration
Simple mesh filters help prevent sediment build-up, or it’s possible to purchase specialised filters for enhanced protection.
Most tanks feature raised outlets that trap sediment, which you can periodically empty for cleaning. For larger setups, you may benefit from installing full filter kits and pumps to direct water where it’s needed most.
Low-cost solutions for small holdings
For smaller businesses or holdings with limited roof area, rainwater harvesting can still be a cost-effective solution. Affordable water butts and connection kits can easily be fitted to barns, or polytunnels to collect runoff.
Intermediate bulk containers (IBCs), which have a capacity of 1,000 litres, can be cleaned and repurposed. IBC tanks are widely available second-hand and can offer a cost-effective solution for increasing rainwater storage without significant investment.
Advanced rainwater collecting systems
If your farm includes large, roofed structures such as barns, or you’re considering a more advanced rainwater harvesting setup, then it may be worth consulting a specialist consultant or an equipment provider. Such companies can assess your site and calculate the potential volume of rainwater you could collect, advise on appropriate storage capacity, and the most suitable system for your specific needs.
Alternatively, free online calculators are available to help you estimate collection potential and system requirements yourself. For further technical guidance, see Environment Agency’s ‘Rainwater Harvesting – An on-farm guide‘, co-authored by ADAS.
Calculating the amount of rain you can collect
The amount of rainwater available for collection depends on:
- Rainfall statistics (monthly and annual)
- Roof area
- Run-off coefficient
- Filter efficiency
The collectable rainwater can be calculated by:
- (a) Roof area m² x (b) run-off coefficient x (c) filter efficiency x (d) annual rainfall (mm)= (litres/year)
- a) Roof area is the surface that will collect and channel rainwater to the storage tank. Multiply the width by the length of the roof, e.g. a roof that is 10m wide and 10m long would have an area of 100m².
- (b) Run-off coefficient determines how much water will flow from the roof surface, and how much is lost. A value of 0 would mean that no run-off occurs, and a value of 1 would mean it’s possible to collect all rain falling on the roof.
| Roof type | Coefficient |
|---|---|
| Pitched Roof | 0.85 |
| Flat Roof (smooth surface) | 0.55 |
| Flat roof (gravel layer or thin turf) | 0.45 |
- (c) Filter efficiencies are best obtained from your supplier; however, in the absence of specific data, use an efficiency factor of 0.8 (80% efficiency).
- (d) Annual Rainfall can be determined through the AHDB Slurry Wizard using a National Grid Reference for your farm.
A calculation example:
A farm building has a roof area of 600m² (a). It has a pitched roof run-off coefficient of 0.85 (b), and an assumed filter efficiency of 0.80(c). The farm is in an area with an annual rainfall of 500mm (d).
(a) x (b) x (c) x (d)
(600) x (0.85) x (0.80) x (500) = 204,000 litres
How much water will you need?
An example from an arable farm
To illustrate how much water you may need, here is a very simple example with a system collecting from a single roof for a single use – water for a crop sprayer.
On an arable farm with 150ha of land sprayed, at an average water volume of 150 litres/ha, there is an annual requirement for 22,500 litres on each pass. The farm may typically have up to seven passes over a cereal crop.
For spraying operations, peak rainfall is rarely at the time of peak demand, so it’s advisable to have enough capacity to carry some water over into drier periods.
If additional harvested rainwater is required for other uses, such as washing down machinery, this should be considered in the tank capacity.
The recommended minimum storage capacity is an average of one month’s rainfall off the proposed harvested roof area. For the above example, a 20m3 tank may be enough (a standard tank size) to supply the crop sprayer from harvested rainwater.
If there’s spare capacity, softer water can be used as a diluent for chemical spray. Reducing the amount required of some products like glyphosate saves money as well as saving on utility mains water.
Linked collection tanks
If you require a more detailed system collecting from multiple roofs, linked tanks may work best. With this system, water is directed off multiple roofs to a collection chamber to pump to a central storage reservoir, termed an ‘indirect system’.
A single central storage tank may be easier to manage water distribution compared to multiple tanks, but this may involve a private water supply pump to distribute, for example, to multiple animal water troughs.