The Lessons of a Single Rainfall

The Lessons of a single Rainfall from Central Valley Ag on Vimeo.

by Keith Byerly

by Keith Byerly

Last week we had a significant rainfall. For some, it wasn’t that big of a deal, but for many of us, it was the first rainfall we had over 0.25” since the end of May. I know that I don’t need to remind most of you how hectic irrigation has been this season, so this was a welcome reprieve from the grind that we found ourselves in. But, this rainfall was much more than a break in the storm. It provided some valuable insight into our fields, especially around some water infiltration understanding, and compaction layers.

I know that Mike discussed this in his Reachout article this week, but I thought that I should help quantitate this with some real world data. Last week’s rainfall gave us a chance to see how irrigation events and rainfall events differ in a soil. Here I have some probe data from a field that would be a sandy loam texture. This probe was installed in June after a 3” rainfall event, so I was comfortable with where the full point was on it. But as the season progressed, and we decided it was time to begin irrigating soybeans, our irrigations just didn’t help us catch up. With limited water, at best we could refill the top 8”, but continued to see declines at our deeper depths, and found ourselves falling behind.

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And then this rainfall event happened. But what I want you to focus on is what we saw at our 12”, 16”, and 20” sensors. As Mike mentioned on Monday, for Nebraska, a good rule of thumb is that we can support an infiltration rate of about 2” per hour. This field has a water holding capacity of about 1.30” of available crop moisture per foot. So easy math would say that in about 1-2 hours we should be able to absorb a 2” rainfall. But what we saw here was significantly different.

Our rainfall began at 2 in the morning, and immediately began to infiltrate at 4” and 8”. Then it was 1:45 after the rainfall before we began to see infiltration at 12”. 22 hours before we saw infiltration at 16”, and 38+ hours before we saw it begin at 20”.

And so your take home for the day is that there are many lessons to be learned when you see a rainfall like this occur in the middle of a season. Is this issue caused by soil sub layers, or is it a compaction issue? The good news is that in either case, it is a mystery we can investigate and take action against for the future. In the short term, however, these sort of things are issues we take management action with today. This field will require a different management strategy from here on out. One that focuses on irrigation rates that fill the top foot and don’t over apply is step one. Understanding how our water infiltrates and making sure we have room to hold rain when we have a good chance in the forecast is another. Finally, understanding how the depletion is happening at deeper depths and understanding our ability to manage it this season is limited will help us with realistic expectations.

At the end of the day, this is just another example of how the technology helps us understand the agronomics of what is happening in our field. With a month plus of irrigation left, now is the perfect time to refine our strategy for the remainder of the year.