If rainwater harvesting and storage is one of the key solutions to help agricultural producers work through California’s notable drought periods, the next question becomes, what types of rainwater harvest systems are available, and what are their best uses? Rainwater capture is not a new concept. In fact, it is a technique has been employed for centuries, albeit in different forms. In the 1970s and 1980s, rainwater harvesting received a lot of attention due to widespread droughts in Africa that posed serious threats to crops, livestock and humans.
Consequently, there are many ways to harvest rainwater—including some methods one might not expect. In essence, there are two overall types of collection. One type focuses on tillage, land contours, and related techniques to capture water. The other type relies on reservoirs that collect and preserve water for later use, usually via irrigation systems. Here, we’ll explore some of these methods and discuss their merits.
Tillage, Contouring, and Runoff Utilization
These types of rainwater harvesting and collection systems rely on one crucial component: the ability to collect rainwater within the soil, one way or another. Different techniques under this category are suited to different areas and climates. Below, you’ll learn more about some of these types of systems.
Deep Tillage
Deep tillage is one of the more common methods for maximizing rainwater use. It’s ideally suited to climates that experience fairly regular rainfall in order to periodically replenish moisture levels within the soil.
This method works by increasing soil porosity, which increases the soil’s water infiltration rates and water holding capacity, thereby reducing runoff. Deep tillage has the advantage of keeping moisture levels high, within the root zone, where it is most needed.
Contour Farming
Where slopes are 3% above grade and higher, contour farming can be an important way to capture rainwater. It works in conjunction with deep tilling. The idea is to plow along contours so that rainwater collects in ridges as it flows down the slope. This impedes the flow of rainwater enough that it can absorb into the soil where it is available to crops rather than being lost to evaporation or runoff.
Pitting
Pitting is a type of micro catchment system that is best used on a small scale due to the amount of labor involved. This practice involves creating shallow depressions. In one example from West Africa, these depressions are about 30 centimeters in diameter and 20 centimeters deep. Seeds are planted in each depression, and as the growing season progresses, rainwater and nutrients also gather in the depression to make them better available to the crops’ roots.
Strip Catchment Tilling
This is another common form of rainwater capture that works well on sloped land where it may be necessary to preserve water during dry periods. Strip catchment tilling involves creating strips of tilled land interspersed with untilled ground. Runoff flows over the untilled strips to collect in tilled strips.
Bunding
Bunds are small embankments made of earth or stone—and there are two different types of bunding that agriculturalists can use. The first is referred to as “contour bunding.” This practice involves creating bunds along the contours of farmland. The second type is “semi-circular bunding,” which requires bunds to be built in semicircular formations at the bottoms of slopes.
With both methods, rainwater flows down the slope behind the bunds. The bunds stand in the way to slow or stop the flow of water so that it has more time to absorb into the soil where crops can make use of it.
Rainwater Harvesting with Storage
In many places, storage capacity is crucial. While some of the above methods can help mitigate concerns for California producers specifically, California also features a climate that offers rainy seasons followed by extended dry periods. No matter the tillage or contouring method used, many crop farmers in this area will find that reservoir supplies are necessary to keep crops healthy during these dry periods.
Where water storage is concerned, there are two primary ways to go about it. Above-ground storage tanks are one method. In-ground basins are another. Each has advantages and disadvantages, which we’ll explore below.
Rainwater Harvesting Tanks
This type of system is very similar to the rain barrels that a homeowner might use to supplement water in their landscape—only much larger. Rainwater harvesting and storage tanks range in sizes anywhere between a few hundred gallons to tens of thousands of gallons of water. With these systems, producers can harvest water from rooftops, driveways, and other flat areas where water can be diverted into a plumbed catchment system that channels it into the tank.
Advantages include:
- The ability to filter water. Because these systems rely on plumbing to collect the water, it’s easy to install filtration systems for water entering the tank. This provides a clean source for crops and livestock.
- It is also easier to treat water if necessary. This can be done by adding treatment solutions directly to the water in the tank.
- Water storage tanks have a relatively small footprint compared to reservoirs. Often, these tanks are somewhat taller than they are wide—and that depth means they take up less space.
- Less water loss. Though you may not be able to harvest field runoff in this way, the water that you collect in this closed storage system is protected against evaporation. You also won’t have to worry about losing water through seepage in earthen-lined ponds.
- These systems give you quite a bit of flexibility. Water stored in tanks can be used by tractor-driven sprayers if you only need sporadic irrigation, or if you need water to dilute fertilizer mixes. Most systems also allow you to hook up hoses so that you can direct the water wherever you want—into watering troughs for animals, for example, or to individual irrigation lines in the fields that need it most.
There are also a few disadvantages that come with these systems. These include:
- Higher maintenance. Tanks are more likely to spring leaks than lined reservoirs—especially when you consider that if they’re not well protected then it’s easy to bang into them with equipment or hit them with stones thrown while mowing. Exposure to the elements can also take a toll on a metal tank’s seams, while UV rays will eventually make plastic tanks brittle.
- More components mean there is more that can go wrong. This ties in with maintenance. You’ll have to consider maintenance not only on the tank itself, but on plumbing, the collection system, and the drainage or irrigation system.
- Potential energy costs. Tank systems may require electricity for filtration, pumps, and other equipment designed to keep the water clean and channel it where it needs to go.
- Cost is a factor. Tank-style rainwater capture systems are far more expensive to build than in-ground reservoirs.
- Limited capacity is another issue. While rainwater storage tanks can be built to extremely large sizes, in-ground ponds can be much, much larger. Tanks are better when you have a limited need for supplemental water. To water acres upon acres of crops, in-ground reservoirs may be the way to go.
- Limited collection capacity. These systems are designed to collect water from rooftops and gutter systems, driveways, and other flat areas. It’s much more difficult to collect field runoff with them. Therefore, you’ll be limited to the number of rooftops and flat spaces you have—which may not be enough water if you don’t have all that much surface area to collect from.
Reservoirs for Rainwater Collection
Reservoirs for rainwater harvesting and collection consist of in-ground ponds or lakes designed to capture and contain rainwater runoff. There are a few different types of ponds that can be created, and there are also a variety of ways to build these reservoirs.
One type of rainwater collection pond would be the earthen dam style. This type captures rainwater runoff from slopes. To build a dam pond, you would choose a low-lying area at the base of a slope, then create the pond by constructing raised earthen walls to block outlets, so that water collects at the bottom of the slope. This is an especially effective technique in hilly areas where valleys between hills are relatively narrow and easy to wall off.
Another type of rainwater collection pond diverts excess rainwater from flooded streams. Often, these ponds rely on channels to direct that excess water into the excavated pond.
In flat areas, stormwater detention basins are useful to collect rainwater runoff in nearby areas. These are often designed as long, narrow channels that not only collect rainwater, but divert it to larger holding basins for later use.
When building any of these types of rainwater collection ponds, you’ll need to consider lining options. The three most common types of liners are clay, flexible geomembranes, and concrete liners.
Concrete liners are quite rare—and that’s because there are several issues with them. Chiefly, these liners are expensive. Not only do you have to pay for excavation, but also, the concrete itself is pricey. Additionally, they’re more prohibitive in terms of design. Only in the rarest of instances might someone build a concrete dam, for example, and those are typically used for flood control rather than irrigation.
Concrete also comes with permeability issues and maintenance concerns. It is by nature a porous material, which means some water will seep through it—and that is counter to the purpose of creating a pond specifically designed to preserve water. Concrete is also known to crack relatively easily compared to other liner types, especially when ground shifts beneath it or when the ground around a concrete basin freezes and thaws. Cracks can be repaired, but those repairs aren’t always easy or particularly cheap. Meanwhile, leaving a cracked concrete liner to leak will cost you thousands or tens of thousands of gallons of water per day.
Another option is clay lined ponds. These are quite common because clay is a relatively inexpensive material and rather easy to work with compared to concrete. It can be a good option for ponds designed to hold harvested rainwater, but even here, there are some issues to be aware of. Clay lined ponds are prone to seepage just as concrete ponds are, which means you’ll lose some water through the soil even in perfectly functioning ponds. They’re also a bit harder to maintain than a pond lined with a geomembrane, namely because when a clay liner does start to leak, it can be difficult if not impossible to find where the leak is so that you can fix it.
Where rainwater retention is concerned, geomembranes are going to be your best bet. They can be used to cover dam walls, to create basins, in catchment pools at the base of slopes, to create trenches, and more. Because they are flexible—with RPE liners typically being the most flexible of all available types of geomembranes—they form easily to natural and manmade contours, which allows you to build a pond in the space you have rather than making the space fit the pond. Liners like BTL Liners’ AquaArmor are fish and plant safe, which means rainwater basins can double as recreational ponds. Even if you don’t use them recreationally, these liners won’t leach contaminants into water intended for crops and livestock.
Flexible liners are also a low-maintenance, durable option. Stick with reinforced polyethylene (RPE) liners, and you’ll find them tear resistant, puncture resistant, and able to withstand chemicals, UV rays and other environmental concerns known to degrade other types of liners over time.
Aside from the particulars about pond types and the kinds of liners one might choose, there are several advantages and disadvantages to reservoirs as opposed to tanks. Advantages include:
- Volume. Rainwater preservation ponds can be built in sizes far larger than tanks, capable of holding a lot more water, which is what you’ll need if you plan to produce water-intensive crops or farm a high volume of land.
- Placement is another advantage. While you may need to do additional excavation to place a pond in an ideal location, it does not need to be located near outbuildings or structures.
- Rainwater harvesting ponds can double as recreational ponds or fulfill another use, whereas other types of water harvesting systems, such as tanks or pumping the water into an underground storage basin, are purely utilitarian.
- Expenses. These ponds typically do not require electricity, complex plumbing, or much in the way of engineering aside from the construction of dam walls or other features. That makes rainwater containment ponds among the most inexpensive ways to collect and preserve rainwater for future use.
- Maintenance needs are relatively low since you won’t have to deal with issues that can arise with complex plumbing systems, electrical systems, or cracks and leaks that can arise in tank systems.
The advantages to these types of ponds make them look like the best option for rainwater collection and storage. In many cases, retention ponds might just be your best bet, but there are some disadvantages that you should be aware of. These include:
- Terrain limitations. In extremely flat areas, it can be more difficult to build a pond-based rainwater collection system since it may mean building a network of channels to collect as much water as possible to divert it into a containment basin.
- Acreage limitations. Ponds do have a relatively large footprint. It may be that you want to preserve as much acreage as possible for growing crops—and the land that you use for the pond is land that can be farmed. If that land is needed for crops, it may prove more profitable to pump collected rainwater into underground storage or into tanks, despite the added expense that comes with these systems.
- Water loss through seepage. In a pond lined with a geomembrane, this water loss will be negligible unless damage occurs that causes a larger leak somewhere within the geomembrane. However, in concrete and clay-lined containment systems, seepage can be a much larger problem.
- Water loss through evaporation. Evaporation will always cause some water loss in an open irrigation system—and California’s warm, sunny, dry climate does tend to exacerbate the problem. You’ll need to do calculations on the evaporation rate you can expect in your specific region given the surface area and depth of your pond in order to determine whether the loss of water through evaporation makes closed systems like underground reservoirs or tanks worthwhile.
In Summary
There are many methods available to harvest and preserve rainwater for later use in irrigation. This includes micro techniques useful at the small scale, macro techniques suited for large scale harvesting and preservation methods that you can use in conjunction with each other to maximize how rainwater is utilized.
California farmers are in a unique position to utilize these methods and refine them since it is one of the largest crop producing regions in the world. What’s more, the unique climate consisting of rainy seasons followed by extended dry periods makes rainwater harvesting and preservation almost essential to keep crops going—particularly during droughts.
