Excess tailwater is still likely going to be present with the amount of water one uses to ensure proper soil saturation, even with these high-tech precautions (although runoff is exceptionally minimal in the case of drip irrigation). The use of a runoff pond or pump system that allows one to store and utilize these leftovers may lead to significant water savings. A retention pond equipped with a liner and filtering system will also protect the local watershed and groundwater from highly concentrated, agricultural nutrients and chemicals. In places where stormwater retention is legal and practical, adding another source of water income can be a major boon for drought-affected farmers. Due to the cyclical nature of drought, installing stormwater retention systems in time to take advantage of the next rainy period or snowmelt could allow farmers to store excess water for the long term. These savings can make a massive difference in the following dry shift.
Standing water and runoff are very often found in the case of flood or furrow irrigation. The high nutrient load of fertilizer and chemical-rich runoff is dangerous for local wildlife and the watershed and groundwater that humans use. When left to sit in the trenches or burrows surrounding the field and crops, excess moisture or standing water is desirable to pests and weeds alike. Excess water can also lead to increased erosion of your fields, pushing sediment out into the watershed and further downstream. This increases the turbidity of the local water, makes a mess of your parcels, and agricultural soil is often full of the same harmful chemicals as irrigation water; nitrogen, phosphorus, and other toxins. In some places, drainage management of this runoff is required by law due to the severe environmental and community-wide consequences.
While reuse of this water adds installation and maintenance considerations, the savings to water costs and crop and environmental health are major. Biological forces in tailwater reuse systems allow microorganisms to grow, which absorb and utilize many of the chemicals found in runoff. Hungry bacteria and microbes will gobble up excess nitrogen in the water before releasing it into the air. Allowing time for the water to rest in a pond or lagoon also gives sediment time to settle, meaning many of the chemicals caught inside the soil are allowed to settle as well. The designs for tailwater reuse systems vary depending on conditions and local ordinance but should primarily consider similar core aspects. Certain circumstances may require a more complex tailwater reuse system, in which case consulting your local Soil Conservation Service expert, or hiring an irrigation engineer, can be a major resource.
Tailwater Reuse System Design
Redirection Management
Where is the tailwater being redirected? Frequently, the field the tailwater has flowed from is already saturated enough to allow runoff, so reapplying tailwater to its origin is rarely helpful. Usually, redirecting tailwater downhill towards another field in a sequential system is a better idea. This water can be used to supplement another irrigation system or, if enough tailwater is collected, can be the primary source of irrigation for another field. However, this requires a return pump strong enough to produce the flow rate necessary for adequate irrigation.
- Some designs can assist in the flow rate of tailwater to avoid extra costs involved with a more substantial return pump system. A smaller retention pond will only hold the tailwater from an irrigation system or two at a time. This can be drained quickly with larger pipes. However, this will increase irrigation flow and may require installing or opening more valves. Smaller lines and the resulting slower drainage may require less equipment to handle such an increase in water flow.
- Other irrigation designs will use pumps to move tailwater back to the beginning of the system to be reused over the same fields. This solution may be ideal in places where the soil is incredibly sandy or where the conditions otherwise require frequent soaking.
The Storage Reservoir
Is the design utilizing a natural body of water to collect tailwater or digging a new retention pond? Where will the pond be placed? Ensuring the pond is in-ground and below the draining field will allow gravity to assist in the redirection.
How big will the pond be? A larger pond will offer more flexibility in design while taking up more space and requiring a larger budget. How often one intends to empty the pond will also affect size considerations. A smaller pond will need to be monitored more closely for overfilling. Pond storage capacity and how quickly it drains further into the tailwater system will determine how often units need to be inspected and maintained. In general, it's best to ensure there's enough storage room for an entire cycle of irrigation. This will be incredibly helpful in the case of electrical interruption or if your tailwater discharge needs to halt at sudden notice. Suppose runoff from precipitation or snowmelt is a likely factor. In that case, these rates will need to be considered in your total storage expectancy (this will limit natural seepage into local aquifers and waterways and may be restricted in some jurisdictions).
Calculating Capacity
To determine how much water your retention pond will need to hold, consider the following:
- How much runoff are you expecting? What is the flow rate?
- Do you have a minimum required depth for pumps to function properly?
- Does the flow rate change? If unable to regulate the rate in which water enters your pond, a larger container may be necessary.
- What’s the chemical makeup of the runoff? A higher concentration of nutrients or toxins will require a longer retention time for breakdown to occur. Larger ponds provide longer holding times. Certain chemicals will require more or less time to degrade.
- How much sediment is being carried in the runoff? Different soil types will take different amounts of time in order to settle.
- How reliable is your power source? Interruptions in energy sources could render you unable to open new pumps, valves, or emergency releases. In this case, a storage pond that can hold an entire irrigation set is ideal.
- Are you collecting stormwater? Sedimentation and chemical makeup of external water sources flowing into the pond should also be considered.
A pond built too shallow will allow sunlight to reach the pond's bottom, encouraging unwanted weed growth. A minimum depth of around 5 feet should discourage most bottom-dwelling weeds. A deeper pond minimizes the amount of field space sacrificed while still satisfying irrigation storage needs.
Generally, the steepest slope acceptable is a grade of 1:1. A steeper slope will minimize the space needed, sunlight permeation, and the resulting weed growth. Yet, a gentler slope makes the pond more accessible and safer for people and wildlife alike. Desired plants, allowed to grow on gentler slopes, will discourage erosion and provide a habitat for wildlife.
The width of the pond will depend on the equipment needed to access and clean the pond. Width considerations will also need to include conditions of mosquito larvae growth in the area (your local mosquito abatement district authority will be the expert here).
Other Retention Pond Considerations:
Some farmers utilize a two-stage pond. Sediment control occurs in an initial, smaller pond (called a trap) designed to be easily cleaned and maintained. Water then flows into a second larger pond that provides primary storage of the tailwater. A general recommendation for the initial sediment trap specifies that the trap's length is three times its width. Suggested trap volume allows for sufficient storage space to contain a season's worth of sediment while still providing enough room for sedimentation to settle. In order to enable larger soil particles to settle before being directed into the larger storage pond, it's recommended that flow velocity be kept beneath 1 foot per second.
- A pump placed too low in the storage pond will pick up unwanted sediment, meaning a certain water level will be necessary to maintain water flow. Automating drainage controls and pumps may add some cost but will help prevent overflowing or overtraining.
- Water discharging into the pond may cause erosion of the closest bank where it is anchored. This can disrupt sediment settling in your tailwater pond and cause significant damage to the surrounding soil stability. Several kinds of protected inlets can minimize this.
- It will sometimes be necessary to remove sediment, weeds, crop residue, trash, etc., from a tailwater pond. This will be made much easier if the pond is easily accessible by required equipment. This could mean leaving safe walkways near pond edges or enough room to drive smaller machinery to and around the water storage.
Lining a Tailwater Pond
Water caught in tailwater ponds for reuse can still carry dangerous chemicals down into the groundwater as it seeps into the soil. This effect is amplified if all the seepage occurs from the same small area, like a retention pond. Most traditional liners will prevent some water from soaking through the pond. However, heavy nutrients and toxins will still eventually make their way through even durable liners like concrete. Ensuring that the pond is emptied after each full irrigation set will prevent water from saturating deeper into the soil. This will decrease the amount of chemical leaching that long-sitting water would allow. However, this won't be a viable solution for farmers looking to invest in long-term water savings.
Additionally, fully emptying a pond and allowing the soil to dry will damage many pond liners (although RPE liners are exceptionally resilient). Using a liner in a retention pond prevents nutrients and toxins from soaking into the soil (or salt leaching into the water). This also ensures a watertight seal, preventing further water loss in your irrigation system. There are numerous kinds of geomembrane liners that are plant-safe while protecting precious water supplies.
Clay Liners
Clay liners, most often made of bentonite clay, are convenient for their relatively small environmental impact and low initial cost. However, puddled clay is easily spread too thin during the application, resulting in a less than watertight seal. The sensitivity of this step makes clay liners tricky to install. Clay also allows the permeation of chemicals and nutrients between water and the underlying soil. A soft clay liner is also easily disturbed by wildlife or erosion. Damage is tough to identify and locate, and frequently complete reinstallation of another lining solution is necessary.
PVC Liners (Polyvinyl Chloride)
These are very common in small backyard pond projects due to their affordability and ease to purchase and set up. Conventional PVC isn't as durable as other liners and usually has a lifespan of only ten years. PVC also lacks the UV protection found in newer liner technology, meaning they need to be buried to avoid brittleness and damage from sun and movement. Tears are common throughout installation and use, and frequent disturbance for maintenance only contributes to the problem.
EPDM Liners (Ethylene Propylene Diene Monomer)
This is a popular kind of synthetic rubber liner. This is more durable than PVC and is accordingly thicker and heavier. This more robust liner is still somewhat flexible, but its weight and general unwieldiness complicate shipping and installation. EPDM liners are sealed using adhesive tape, and any mistakes during installation will significantly impact the liner's permeability. EPDM is less sensitive to U.V. damage than PVC but still needs protection from long-term sunlight and decay. Leaks are easier to locate and repair in EPDM liners than the above. However, the rubber is susceptible to punctures from wildlife or damage during installation.
HDPE Liners (High-Density Polyethylene)
These geomembranes are increasingly popular options as they become cheaper and more available. These are stiffer than PVC or EPDM liners and are incredibly resistant to U.V. degradation and tearing. Despite their durability, HDPE liners are lighter than their EPDM cousins. However, the material's stiffness means that any folds or creases during installation increase the risk of rips and tears. Their seams are heat-welded, leaving their watertight seal virtually permanent. Holes and damage are patchable, saving you the cost and trouble of choosing and installing an entirely new solution. HDPE liners are also chemical-resistant, protecting the surrounding soil and local watershed. (In situations where RPE liners involve unavailable expense, HDPE is a worthy alternative).
RPE Liners (reinforced polyethylene)
RPE liners are on the frontlines of geotextile technology today. These are lightweight, durable, and UV-resistant. RPE liners are welded using the same heat technology as HDPE liners, reducing the frequency and severity of damage from installation or wear-and-tear. They are also built to be chemical-resistant, preventing seepage from chemical-rich irrigation water into groundwater aquifers beneath the soil. RPE is also built to avoid the leaching of internal chemicals, essential for water used on crops. BTL has manufactured RPE liner panels in immense sizes beyond 150,000 sq ft. The lightweight flexibility of RPE liners means that even the largest panels can be factory-welded to be installation ready upon delivery. RPE is also simpler to install, meaning that in all but the largest projects, manual installation is possible. For example, BTL and our team of experts installed an RPE liner in a 40-acre irrigation reservoir manually in only four days.
Concrete Liners
Frequently, retention ponds are built using precast or poured concrete. Concrete is porous, and capillary action will quickly pull water through the material towards the earth below. Sealants will protect the concrete from permeation for some time but aren't permanent. A geomembrane liner serving as an underlayment between the concrete and the soil will be a waterproof layer. This will keep any water from soaking into the ground when it makes its way through cracks or pores. Additionally, this will protect the concrete from underground movement (like traffic or minor tremors).
The Return Flow Pump
The tailwater return flow pump is responsible for moving water from the tailwater pond and moving it back into the irrigation system for reuse. The pressure required for a return flow pump will depend on the depth of the retention pond, the friction within the pipes, and any changes in elevation during transportation. Additionally, the water flow rate and the total water pressure may change due to rising or lowering water levels within the pond. These considerations, known as operating conditions, will help someone pin down the right return flow pump for a particular system's solution.
Some farmers choose to automate the return flow process, installing float or probe systems that alert the pump when the water rises or lowers beyond a certain point. This can help prevent overfilling, though some choose to manually operate the return flow pump to actively adjust specific valves and water flows.
A screen will help protect your return flow pump from any debris that has collected within the retention pond. Blockages in your return flow can cause major issues within the irrigation system. A sudden increase in pressure within the pipes or sudden overfilling due to a backup can lead to a chain reaction in damage throughout the water system. In addition, valves installed to maintain flow rate or separate distinct irrigation and return systems may be required if these pipes are connected. In this case, a check valve installed between the pump discharge point and the pipeline will prevent backflow. Vents and air-release valves can be installed in order to prevent surge; however, this is a much more complicated design process, and would require further investigation/consultation.
