Irrigation Ponds Need Water
It’s a reasonable question: If water is already scarce, how am I going to fill up an irrigation pond? There’s real validity to that question, and there’s a few ways to address it. Of course, in arid western states a severe, extended drought where groundwater levels are falling precipitously, rivers are drying up, and cities are severely restricting or even cutting off water allotments, and farmers have already made the hard decision to let their cultivated fields go for the season, a new irrigation pond won’t be a benefit right away.
Feast or Famine
All droughts end, however. These days, they often end in torrential storms fed by atmospheric rivers, when devastating amounts of water are dumped suddenly on land that’s incapable of absorbing it. In January of this year, an incredible series of 9 atmospheric rivers dumped more than 32 trillion gallons of water across one state in just 3 weeks, setting the state up for disastrous flooding and deaths.
The question is, why can’t we take advantage of this incredible influx of water, which in California, typically arrives during the winter? When major reservoirs are forced to release millions of gallons of water to prevent dam collapse, it’s a frustrating situation for farmers who can’t support crops in the long, hot summer. Unfortunately, major reservoirs are very expensive to build, and the amount of water they can store, compared to the regional needs, doesn’t appear to be cost-effective. However, for small to medium farm operations, an irrigation reservoir could potentially make all the difference. When precipitation doesn’t follow a steady, moderate pattern throughout the season, reservoirs can provide irrigation water to carry crops through the long, hot weeks of late summer. In locations where the option some years seems to be either drought or deluge, properly sized reservoirs can potentially collect enough stormwater runoff to keep a farm operating for as much as 2 years.
The key here is to prepare in advance. It may feel ridiculous to build a pond when the ground is absolutely parched and there’s no rain in sight, but once a major storm is forecast, it’s usually too late to get one in place.
Collecting the Windfall
Even when rain continues to be inadequate, farms can benefit by collecting irrigation tailwater and recycling it back to the crops. This extends the usefulness of scarce water supplies and allows farmers to reduce their dependence on outside sources. Depending on your recycling strategy, tailwater can be stored in an irrigation pond, as well, and supplemented by rainfall.
An important distinction to be made here when sourcing stormwater is the difference between sheet flow, direct rainfall, and surface water that travels in a natural channel. Even seasonal creeks, that dry up during the summer, may be considered jurisdictional water. The use of it is heavily regulated. In most cases, though, rainfall that travels overland in a relatively uniform, shallow flow, called sheet flow, is considered non-jurisdictional. Sheet flow is part of the pattern of drainage in a basin that ultimately will empty into rivers or lakes in the valley bottom, but until it reaches that point, it can be captured and used by the public. Other unencumbered sources may include runoff collected from the roofs of agricultural buildings and other structures on your property.
Properly designed ponds will include sediment basins, which slow incoming water, allowing sediment to settle out before it reaches the main pond. While the water is retained in the main pond, finer sediments have an opportunity to settle, improving the overall quality of the stored water and preventing clogging or damage of pumps and irrigation equipment.
Collecting Stormwater
In the best-case scenario, a drainage and conveyance system will be designed alongside evaluating the overall drainage patterns on the parcels, siting the storage pond(s), identifying cultivated areas, and planning the irrigation system. When collecting stormwater, the capacity of the conveyance structure should be considered, as well as the effects of collecting and concentrating potentially large volumes of fast-moving water.
Smooth, solid structures like drainage pipes and culverts are designed to move water quickly and efficiently from one point to another. They typically perform this function well, but the result is quite damaging when the water is inevitably dumped into the environment. Erosion, high levels of sediment, and extremely poor water quality are typical results. Since agricultural operations require fairly high-quality water, other strategies are recommended for a successful irrigation strategy.
Controlling Stormwater: Holding the Tiger’s Tail
One of the most important goals in collecting and diverting stormwater is reducing the water’s velocity. We’ve all seen recordings of flash floods, where fast-moving water has the force to pick up and carry automobiles downstream and even move houses off their foundations. Slow moving water is considerably less destructive. The two most successful strategies for reducing water velocity are introducing non-smooth surfaces for water to travel across and spreading out the water flow into wider (but still contained) channels. In nature, prairie grasses and perennial wildflowers are effective in slowing the flow as well as capturing detritus that may have been picked up in route. When native vegetation isn’t an option, waterways lined with riprap may be a viable alternative.
Conveying the large volumes of water that come in severe storm events is no less important than slowing it down. Road flooding, destructive erosion, and waterlogged crops that result from overwhelmed ditches are a major concern, as well as the ecological effects of potentially contaminated flow into local rivers and streams. Two-stage ditches offer an effective strategy to manage water movement during both low-flow and peak-flow times. The design of two-stage ditches incorporates benches within the ditch, which mimic the function of floodplains within the channel. High volumes of water can expand beyond the ditch’s main channel and into the planted “floodplains,” which vastly expands the ditch’s overall capacity while reducing its velocity and even allowing large sediment to settle.
A system of ditches and culverts should be planned to collect runoff along the downslope of cultivated fields and direct it towards a storage pond. For a low point storage pond, gravity should do the work for you, while a high or midpoint storage pond will require the use of pumps to send it back up the hill. Important note: if you’re employing a mid or high point storage strategy, you’ll need to plan to turn the pump on at the beginning of a storm event and keep it running throughout. Low point storage ponds won’t need pumps to collect runoff.
