Building a network of acoustic rain gauges

UNL and Nature Conservancy partner with Arable to establish a network of sensors for better access to local weather data.

Fifty miles is a long way to drive to check a rain bucket on a center pivot. That's the challenge Chase Johnson dealt with whenever he wanted to know exactly how much rain fell on one of his pivots.

Johnson started farming about two years ago in York County, Neb., and as a beginning farmer, time management is a high priority.

When Johnson met with Trenton Franz, University of Nebraska-Lincoln hydrogeophysicist, and Adam Wolf of Arable, they asked him about what was important to him as a beginning farmer. The first thing that came up in Johnson’s mind was being able to reliably monitor rainfall.

"I've got farms that are 40 to 50 miles apart," Johnson says. "I talked with Trenton Franz and said, 'I'd like to know reliably, how much did it rain and when, and how far do I have to drive to turn a pivot on or off?' I had met Adam, and he told me about these Marks."

For the last two years, Johnson has been one of several irrigators involved in a network testing Arable's Mark sensors in Nebraska. The project is funded by The Nature Conservancy with support from UNL, the Daugherty Water for Food Global Institute and the South Platte Natural Resources District.

The multiyear project involves a network of 20 Marks installed throughout Nebraska — including the two on Johnson's fields. An additional one is located at the Testing Ag Performance Solutions (TAPS) field at the West Central Research and Extension Center at North Platte, and 17 more are in growers’ fields as part of a the Western Nebraska Irrigation Project. The goal is to validate the technology and demonstrate the value of having access to local rainfall data.

The Mark, which has a flying saucer-shaped acoustic rain gauge on top, has the ability to measure rainfall and distinguish it from hail and other objects.

"If you've ever been inside a shed with a corrugated tin roof while it rained, you can hear the sound. Your intuition can already calibrate it — maybe not to the exact millimeter — but you can intuit that heavier rain is louder than lighter rain. If you've been in your house when a hailstorm happens, it's a high-pitched 'tink' rather than a low-pitched 'thunk,'" explains Adam Wolf, CEO at Arable. "We can distinguish those by the pitch of the sound. Rhythm is another factor. For example, raindrops are more staccato [with a short, detached duration] than a human voice."

Using the sensor's radiometer, the Mark can also measure radiation. With this information, along with its spectrometer to measure normalized difference vegetation index (NDVI), the sensor can calculate evapotranspiration (ET) using a modified Penman-Monteith method. Connected to the Arable web tool and smartphone app, the sensor provides hourly updates to growers for the location. Wolf says the ability to measure both ET and rainfall locally gives users a more complete picture of their water budget.

Rainfall data disparities
One of the most reliable sources of weather data is regional weather stations like those available through Nebraska Mesonet, the High Plains Regional Climate Center, or the Nebraska Rainfall Assessment and Information Network. However, there are gaps in the data generated by these stations, especially when certain fields are located farther away from the station. The goal of the Mark network, says Trenton Franz, is not only to demonstrate the validity of the sensor, but also to fill in some of these gaps.

"I think there's room for both. Weather stations are designed to be much more spread out. But if you're an individual producer and need to make a decision today, how representative of your farm is that weather station?" Franz says. "This sensor can provide another source of reliable real-time data to supplement existing infrastructure."

That's especially helpful for growers in western Nebraska, where most — about 70% of rainfall — comes at night due through convective currents moving over the Rocky Mountains. These currents usually take a few hours to reach western Nebraska, and may be fairly spread out by the time they hit.

"You get small cells a few miles wide. Where exactly is the cell going to hit? If you farm over a span of 20 miles, you're going to get widespread difference in events based on convective current," Franz says. "Then you're stuck waiting until morning and driving around and checking all your fields."

That's a challenge faced by growers in the Western Nebraska Irrigation Project. The project, which includes 55 pivots, 11 producers and 8,000 acres, started in 2014 as a way to establish a watershed laboratory. This includes cost share for technology like soil moisture probes, variable-rate irrigation and telemetry. It also includes partnering with local natural resources districts, private industry and weather stations to provide rainfall and ET data to growers.

Although cost-share funds were no longer available last year, growers kept those technologies on 47 of the 55 pivots at their own expense. And Jacob Fritton, performance irrigation coordinator with The Nature Conservancy who is coordinating the project, hopes the same will be true for the Mark sensors.

This year, 17 Mark sensors were installed as part of the project. Growers can log into the website and see how much rainfall they received and check ET levels locally.

"Right now, the farmer might look out the window and see that it's raining, but that doesn't necessarily mean it's raining on their pivot 15 to 20 miles down the road," Fritton says. "That uncertainty leads to them waiting until they go out later and verify they got rain before they shut off the pivots. We'd like them to have enough data to take a different approach. If it's raining, the sensor shows it got rain, so I'll shut the pivot off remotely."

It's about trust
It didn't take Johnson long to see a return on investment by doing just that. In August, Johnson received 1.75 inches of rainfall — just after he started his final irrigation pass for the season.

"The corn was at half starch line, and we had a soil moisture probe and the Mark," he says. "I knew I needed 75/100 of an inch in the profile to finish the season. I started the pivot, and then we got an inch and three-quarters of rain. I turned off the pivot from my phone, and that was it for the season."

By eliminating one irrigation pass, Johnson saved about $500 in energy costs, along with three hours of drive time and an inch of water per acre. This more than paid for the Mark system.

"Between fuel and everything, it's probably close to $1,000 in savings," Johnson says. "The biggest thing is my time. I think farmers have a tendency to not value their time. Driving around messing with wells takes up a lot of my time. That's why things like this and pivot telemetry are great. It's a huge timesaver."

However, reaching this level of trust in technology takes time.

One of the next steps, Wolf says, is connecting the center pivot with the Mark through an automated system that shuts the pivot off when a certain threshold is reached.

"For drip irrigators in California, if the pressure in the drip line goes from 30 psi to 32, that's enough to know that there's algae building up in the emitters, and you must stop and flush it. You could imagine the same thing can happen for pivots. If it rains at night and other conditions are met, then through this action, we can automatically turn it off," Wolf says. "The rules are not hard to imagine, but it's the trust piece. People have to develop a level of trust with the equipment itself that they're willing to let it do that."

"Trust has to be built up over time. It has to do with the individual user and what their level of comfort is. We see it as a multiple-step project. In western Nebraska, we're about to start year four where we're building trust with producers," Franz says. “It will take a few years to reach that point with automation."

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