Once water is pulled from its source, whether that source is surface water such as rivers, streams or natural lakes, it is usually stored in an artificial pond or reservoir on the farm, where access is convenient and immediate. Underground sources may be used this way, even when the well is onsite, since the volume of pull during irrigation may exceed the pumping capacity.
Water stored in an open-air facility is vulnerable to loss through evaporation, transpiration and seepage. Storage efficiency in this case is the ratio of the volume of water available from the storage reservoir available for irrigation use vs the volume originally drawn from the source. It’s expected that the ratio will be less than 1.0, but the drive will always be to bring it as close to 1.0 (100%) as possible.
Seepage, evaporation and transpiration are the major sources of water losses during storage in a reservoir. Storage in a closed tank will avoid most of these almost entirely, but the volume of water needed to irrigate even small farms makes this an impractical solution.
A surprising source of loss during storage can be transpiration due to vegetation growth around the perimeter. Roots reaching into the reservoir can draw moisture up into the plant itself as well as passing moisture into the surrounding soil, which can act like a sponge. A simple solution for this is preventing or minimizing growth in and around the reservoir. Some vegetation may be required to stabilize soil and minimize sediment incursion, but other options such as a protective berm constructed with riprap or rubble may be acceptable.
Seepage in a reservoir depends a lot on the construction of the reservoir. Building a “reservoir” by simply digging a hole in the ground isn’t going to work - if the area doesn’t already hold water on its own, the soil will quickly drain away any water that’s added. Even when a reservoir is built by damming up a river, some sort of liner is needed to make sure the water doesn’t simply drain back to the water table.
Storage reservoirs must be lined in some fashion with an impermeable material in order to keep the water in place until it’s needed. Clays are sometimes used, although they often require such a thick layer that a significant volume of the reservoir is lost to the liner itself. Cement is almost never cost-effective for large impoundments such as reservoirs. In most cases, impermeable geomembranes are the best option for lining an irrigation reservoir. EPDM and PE liners are popular options, but RPE (reinforced polyethylene) offers superior characteristics that make ordering, shipping, and installation much simpler. RPE is also much more durable than other options, and it doesn’t require underlayment, which adds to overall cost for the alternative materials.
Evaporative losses can also be a significant concern for reservoirs of any size, particularly in hot arid climates. One solution to consider if evaporative losses are exceptionally high, especially if the reservoir has already been constructed and put into service, is the addition of a cover. A cover of Reinforced Polyethylene (RPE) from BTL Liners is lightweight, easy to deploy, and resistant to direct UV exposure. Talk to one of our experts about your needs and we can help you envision the most practical solution and help you choose the best materials.
If you’re still in the planning or construction stage, another option would be to adjust the dimensions of your reservoir. Deep reservoirs with a smaller surface footprint will significantly reduce evaporative losses -- just remember to adjust for safety.
