Water security

Quality freshwater is essential for growing plants, directly impacting profits when producing nursery and greenhouse crops. The increasing frequency of sporadic weather patterns that increase the risk of short rainless periods (meteorological drought) and reduced water supply (hydrological drought) can cause crop damage or loss. Recent and recurring droughts have made us rethink water security now and into the future. The continual burden of assessing risk has many wondering, now aloud, “How much fresh water do I need and how long will my water last?” While reservoirs are commonplace where nursery and greenhouse plants are produced, little has been done to holistically understand and thoughtfully design irrigation reservoirs for specialty crop production.

Irrigation reservoirs are not a new technology, but their design and successful implementation on-farm can help achieve water security, ensuring a supply of quality fresh water to put your mind at ease when water availability from typical sources (e.g., municipal, well, river, etc.) becomes limited. This approach also ensures freshwater availability in highly competitive U.S. markets where demand outpaces supply.

The beauty of an irrigation reservoir (aka tailwater recovery pond or containment pond) is that it can capture, recycle and retain water on your site from both irrigation and rain. In this context, there are two types of water: operational and stormwater. Operational water is any water that has traveled through production areas or their waterways (i.e., drainage systems) and could contain chemical (e.g., pesticides, herbicides, fertilizers) or biological (e.g., weed seeds, plant diseases, nematodes) contaminants. Stormwater is water collected from non-production areas or roofs of farm structures and remains isolated from operational water and thus avoids contaminants. Stormwater is typically high quality water that can safely be used for even sensitive phases of crop production (propagation, for example) or can be safely released off-farm with negligible environmental impacts. Be aware of local and state laws, as some states don’t allow stormwater capture.

Reservoir design

When considering reservoir installation and design, you need to be able to answer a few important questions that directly impact specifications, including:

How much water do I need per day during full production on a hot summer day?

How many days of water do I want as a backup water source to address drought periods now, and those expected in the future?

How much land area can I dedicate to a reservoir and still balance crop production with water security?

If you can’t answer these questions because you don’t know how much water you apply, consider investing in a pump meter or in-line water meter to measure your total and annual water use. Knowledge is power, and data-based evidence for water use could help you justify water allocations in the future. There are also online calculators that can help you estimate these numbers, so you can plan for reservoir sizing. The grower tools section of cleanwater3.org has an Irrigation Volume Tool (www.cleanwater3.org/growertools.asp) to help estimate how much water you use.

Once you can answer those three questions — then we can get into the nitty-gritty details of reservoir design. Irrigation reservoirs tend to be at least 10 feet deep unless a shallow groundwater table limits their depth. Deeper reservoirs can hold more water, so if land is limiting and water security is a major concern, reservoir design would likely incorporate depths of 15 or more feet. Another factor to consider when designing reservoir capacity is weather. Some regions experience rain only during a single season, while others have rain almost year-round. In recent years, the intensity of storm events seems to have increased in that more rain falls over a shorter period. So, the sizing of your reservoir might depend on how much water can reliably be captured over a relevant amount of time.

When designing a reservoir, it is important to consider:

How much water will leave/enter the reservoir per day?

How long should the water stay in the reservoir before leaving (aka hydraulic retention time)?

How “dirty” is the operational water entering the reservoir?

How much water from storms do I want to retain to minimize non-point source pollution?

What is the depth of the ground water?

If you are designing a new reservoir from scratch, base the size of the reservoir on how much water you would need to irrigate all your crops over an extended period of drought. To find the answer to how much water enters your reservoir per day, you can estimate an average refill rate after an irrigation event using the Pond Refill/Runoff Volume Tool (cleanwater3.org/growertools.asp).

If you already have a reservoir on-farm and want to know how much water it holds — you can use the Reservoir Calculator Tool (cleanwater3.org/growertools.asp) to determine how much water your reservoir should hold. Reservoir age and management influences the actual volume of water that can be held. Reservoirs should maintain a consistent or permanent pool (water surface) that does not fluctuate, exposing the reservoir embankment/liner or allowing water depth to become less than 10 feet. Additionally, the reservoir should be plastic or clay lined with embankment slope of 3:1 or 4:1.

Other factors also influence reservoir sizing decisions, including reservoir location, the production surface area (square feet or acres) feeding the reservoir, rainfall intensity and regularity, site topography, soil type, the depth to groundwater (reservoir should be at least 3 feet above the water table), and accessible to a power supply. Typically, the size of the reservoir surface is up to 3% of the production area draining to the reservoir and depth is related to volume needing to be stored. To minimize non-point source runoff and maximize storage, we suggest sizing the flood storage (maximum surface height or spillway height) level of the reservoir to hold a 24-hour rainfall that occurs with 50-year frequency.

Water can flow into/from the reservoir via various inlets, outlets, pump intakes and spillways. Inlets into the reservoir can be pipes or ditches. Ditches are typically lined with gravel, plastic, or concrete, or vegetated to better minimize sediment reaching the reservoir. Bare ditches result in large volumes of sediment transport, erosion of banks and destabilization of production areas that eventually diminish the volume of water reservoirs can hold and can also carry contaminants into the reservoir.

Ideally, retention reservoirs should be designed with at least a 3 feet deep forebay(s) that are in total 15% to 20% of the reservoir size prior to any water entry point. The forebay facilitates settling of suspended solids (sediment) from water to protect the water storage capacity of a reservoir. Sediment forebays should be easily accessible and designed for ease of removing accumulated sediment using readily available equipment. These measures ensure you maintain the volume of water you can store, reduce pond maintenance over time, and even reduce the potential for pathogens and pesticides to enter the reservoir.

Water quantity and quality

Consider an irrigation reservoir not only an investment in freshwater security — but also an investment in water quality management. When properly designed and maintained, irrigation reservoirs can help contaminants settle out of the water column. We briefly mentioned hydraulic retention time (HRT) or the time returning water remains in your reservoir before being reapplied to your crop. To expand on HRT — the longer of a time it takes for water entering a reservoir to leave the reservoir, the more likely that any water contaminants carried into the reservoir have time to settle and be degraded by sunlight and the existing microbiology. The goal is to have water remain in the reservoir for a minimum of 24 hours with a more optimum time being 72 hours. So, when planning your reservoir, think about how to design water inlets so that water enters the reservoir on one end and leaves the reservoir on the other end, to maximize the time for natural processes to degrade or settle contaminants. Also, think about your pump intake as a reservoir outlet, not just the spillway or the pipe where water typically leaves the reservoir. Ideally, your pump intake should be situated well away from water entering the reservoir (inlets), as well as away from where pump backwash, full of debris, enters the reservoir.

Water quality is a critical factor influencing crop health — yet it is often overlooked as an input that affects crop quality. Depending upon your water source, varying concentrations of chemical or biological contaminants and sediment could be present; however, most operational water is actually clean enough to reuse for irrigation with minimal treatment. In some cases, operational water needs to be treated before it can be used on-site (i.e., if applied to sensitive/ high-value crops) or before release off-farm. One way to improve water quality is by increasing retention time. Designing a series of smaller, but interconnected irrigation reservoirs, rather than one large reservoir, is yet another method to enhance removal of pathogen and nutrient contaminants. Any best management practice installed on-farm to help reduce sediment movement (e.g., grass filter strips, vegetative buffers, sediment forebays, turbidity curtains) will help reduce sediment movement in irrigation return flow and help to mitigate contaminant movement on-farm.

Reservoir monitoring and management

To ensure your reservoir can supply the quality of water needed at the right time, it is important to develop and implement a monitoring and management plan. To understand how water quality varies over the year, collect at least one water sample near your pump intake once a season (e.g., spring, summer and fall) and send it to a lab for water quality analysis. Continue collecting water samples, using the same laboratory, until you know if and how your water quality varies over time and by season.

Regularly inspect the reservoir infrastructure. Is the spillway still structurally sound? Is an inlet clogged? Is there a crack in an outlet pipe? It is important to complete these inspections after storm events, when fast moving water could carry larger debris that could damage reservoir infrastructure. If algae become problematic (clogging pump intake or irrigation lines), consult with your local extension agent or specialist and develop a plan to manage the algae. Treatment technologies that can be implemented within a reservoir to manage a variety of contaminants include aeration (which helps keep the water column mixed and limits potential for some types of algal blooms) and floating treatment wetland installation (compete with algae for nutrients in the water column).

If water quality issues persist in the reservoir that could impact crop quality, then other treatments may be required prior to water use for irrigation. Typical chemical (e.g., chlorination, ozonation, UV) and physical (e.g., media or disc filters) treatments are effective and commonly used to manage water quality.

Nursery producers should consider irrigation reservoirs as critical infrastructure — serving as a secure water supply and a means to detain operational water on farm, protecting the environment and quality of surface waters adjacent to the operation. When designing a reservoir, carefully determine the minimum volume of water storage needed to supply your operation during an extended drought (e.g. 30 days, 60 days?). The volume needed depends on your operation, the land area you can devote to water storage, and local rules and regulations related to water storage. Develop a monitoring plan both for water quality and reservoir structural integrity.

About the authors: Dr. Sarah White (swhite4@clemson.edu) is a professor in the Department of Plant and Environmental Sciences and Nursery Extension Specialist at Clemson University. Dr. James Owen Jr. (jim.owen@usda.gov) is a research horticulturist at the USDA-ARS Application Technology Research Unit in Wooster, OH. See their recent publication ‘Specialty crop retention reservoir performance and design considerations to secure quality water and mitigate non-point source runoff’ (tinyurl.com/3nsjp5xw), ‘The Basics of Irrigation Reservoirs for Agriculture’ (tinyurl.com/5n7c46n6), or their short YouTube video entitled ‘Five Tips on Managing Irrigation Reservoirs’ (tinyurl.com/bdcsccfm) for more information.