Weekly Irrigation Newsletter — Irrigation Scheduling Update for Aug. 19, 2025

Steve Melvin Author

08/19/2025 Added

7 Plays

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Nebraska Extension Educator Steve Melvin demonstrates how to apply the irrigation scheduling method outlined in "Irrigation Scheduling Strategies When Using Soil Water Data" (Nebraska Extension NebGuide EC3036). (Video No. 5 of series)

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[00:00:00.000]Hi this is Steve Melvin,
[00:00:07.240]Extension Educator with the
[00:00:09.320]University of Nebraska
[00:00:10.360]Extension and today
[00:00:12.000]we have the next installment of
[00:00:13.580]the Irrigation Scheduling
[00:00:15.030]Update Series for the week of
[00:00:16.570]August
[00:00:16.800]18th. As you recall we've been
[00:00:20.210]using a cornfield for an
[00:00:22.680]example at South Central Ag Lab
[00:00:25.260]near Clay
[00:00:26.000]Center. It was planted on April
[00:00:28.270]22nd, 117 day variety. We limit
[00:00:31.560]ourselves to two inches per
[00:00:33.440]week.
[00:00:33.680]It's a Clay Center if you're
[00:00:35.210]not familiar with 28 inches per
[00:00:37.240]year zone. The corn is now
[00:00:39.710]moved along
[00:00:40.320]to about the R5 stage, maybe
[00:00:42.950]not quite there but really
[00:00:45.180]close. So we get to beginning
[00:00:47.810]dent and we're
[00:00:48.880]going to use a four foot root
[00:00:50.360]zone from now on. We can pull
[00:00:51.970]water from that depth. We don't
[00:00:53.840]want to
[00:00:54.080]do it all in one time like at
[00:00:55.880]the end of the season but we
[00:00:57.700]want to start now using up some
[00:00:59.840]of that stored soil water down
[00:01:01.270]at the lower depths. As we've
[00:01:05.110]continued to see the trend
[00:01:07.520]from the highest levels up
[00:01:08.890]around silking of 0.3 inches
[00:01:10.740]per day we're clear down to 0.2
[00:01:12.800]inches per
[00:01:13.200]day. Remember the days are
[00:01:14.820]shorter, plants are getting
[00:01:16.520]more mature and in general
[00:01:18.960]temperatures
[00:01:19.440]tend to be a little bit lower
[00:01:20.700]when we move into the second
[00:01:21.960]half of August and so the water
[00:01:24.080]the crop needs each week
[00:01:25.290]continues to go down so we'll
[00:01:26.750]use 0.2 inches per day.
[00:01:28.320]We went and got the readings
[00:01:31.180]and from this week and remember
[00:01:34.440]we had some big rains last a
[00:01:37.520]week ago
[00:01:38.080]and got quite a bit of water
[00:01:40.410]into the profile so we've got
[00:01:43.010]50 51 41 and 39 and we're going
[00:01:47.760]to use
[00:01:48.080]this fourth foot now so we put
[00:01:49.710]those over here on the chart
[00:01:51.300]and again this chart as you
[00:01:52.550]recall
[00:01:54.080]this is field capacity so
[00:01:55.500]anything above there in blue
[00:01:57.130]means we're above field
[00:01:58.480]capacity. We've
[00:01:59.360]got a rain storage zone in
[00:02:00.970]other words the soil is pretty
[00:02:02.930]wet and we'd like to kind of
[00:02:04.640]have at
[00:02:04.960]least one sensor down in the
[00:02:06.570]green so we've got some room to
[00:02:08.320]store rain if we get it and of
[00:02:10.000]course
[00:02:10.240]the big rain a week ago we
[00:02:11.570]pretty well refilled the
[00:02:13.040]profile up and probably deep
[00:02:14.590]percolated a
[00:02:15.360]little bit of water and then of
[00:02:16.740]course we get clear down here
[00:02:17.930]in the bottom this is
[00:02:18.880]when things are getting pretty
[00:02:20.580]dry it's where we want to aim
[00:02:21.850]to be when the crop matures.
[00:02:24.080]So what do we think you know we
[00:02:26.800]could probably just say well we're
[00:02:28.080]not going to irrigate this week
[00:02:29.360]but let's go on and see what we
[00:02:30.900]can learn from these other two
[00:02:32.530]columns of numbers.
[00:02:33.600]Here's a chart that we pulled
[00:02:35.800]together and you can see that
[00:02:38.080]the white bars are the
[00:02:40.160]irrigation and
[00:02:41.360]we've came along here putting
[00:02:42.700]on some irrigation we did get a
[00:02:44.000]little bit of rain here and
[00:02:45.170]skipped
[00:02:45.680]some several days of irrigation
[00:02:48.480]put a little bit on a week ago
[00:02:50.850]and now last week and then a
[00:02:53.310]week
[00:02:54.080]past weekend we ended up with
[00:02:56.020]with a really good bunch of
[00:02:57.800]rain over about three days and
[00:02:59.760]if you
[00:03:00.000]take a look this is the top
[00:03:01.300]foot you know it got wet right
[00:03:02.560]away second foot started
[00:03:03.780]getting wet as
[00:03:04.720]the water moved on down then
[00:03:07.100]you can see the round black
[00:03:09.380]dots are the third foot and it
[00:03:11.780]started
[00:03:12.080]responding quicker than the
[00:03:13.870]fourth but we certainly got
[00:03:15.520]water on down to the fourth
[00:03:17.280]foot as
[00:03:17.680]well and so we ended up with
[00:03:19.150]water moving all the way down
[00:03:20.900]in fact when we look at the
[00:03:22.170]amount the
[00:03:24.080]last week versus the amount of
[00:03:26.150]of water that was in the in the
[00:03:28.030]soil profile from that we
[00:03:29.840]measured a
[00:03:30.240]week ago we probably deep percolated
[00:03:32.420]at least a half inch of water
[00:03:34.050]out the bottom of our root
[00:03:35.600]zone from this rain and so you
[00:03:37.330]know in retrospect we'd kind of
[00:03:38.680]like to went back and taken off
[00:03:40.320]maybe
[00:03:40.560]these last couple of irrigations
[00:03:42.610]here but you make the best
[00:03:44.140]decision you can and then if
[00:03:45.920]rain comes
[00:03:46.720]along be thankful for it but
[00:03:48.160]know sometimes we deep percolate
[00:03:49.920]water when we get some really
[00:03:51.510]great
[00:03:51.840]rains.
[00:03:54.080]Okay so let's take a look at
[00:03:55.440]the next column remember again
[00:03:56.880]that's the percent of plant
[00:03:58.340]available water so at field
[00:03:59.760]capacity is 100 percent if we're
[00:04:01.980]up in this zone it's greater
[00:04:03.640]than 100 percent as we start
[00:04:05.520]using
[00:04:05.920]that water out of the profile
[00:04:07.740]or drainage below the the root
[00:04:09.800]zone which has certainly
[00:04:11.520]happened
[00:04:11.840]this past week you know the
[00:04:13.760]numbers get less than a hundred
[00:04:16.040]percent and so we can these
[00:04:17.920]numbers
[00:04:18.320]over here the readings from the
[00:04:19.890]Centibar readings from the
[00:04:21.300]watermark sensors we can't just
[00:04:22.910]average
[00:04:24.080]because you can see they're not
[00:04:25.460]very linear but as soon as we
[00:04:26.660]get into the percent of plant
[00:04:28.160]available water we can and so
[00:04:29.990]we come over here and plot
[00:04:31.480]these the top couple feet were
[00:04:33.360]both 80s
[00:04:34.160]and then the third and
[00:04:36.130]fourth foot were just a little
[00:04:38.560]bit wetter at 88 we add those
[00:04:41.120]together and
[00:04:41.680]divide by four we've got 84
[00:04:43.840]percent of plant available
[00:04:45.620]waters where we're at so again
[00:04:47.520]we could
[00:04:47.840]probably say yeah we're
[00:04:48.910]probably good for this week let's
[00:04:50.260]go ahead and take a look at
[00:04:51.320]this third
[00:04:51.840]column and see what we can
[00:04:52.970]learn from there.
[00:04:54.080]Okay so first thing we always
[00:04:57.600]want to figure out is could we
[00:04:58.960]irrigate if we wanted to
[00:05:00.480]and if we take a look we
[00:05:01.920]probably have room to put on
[00:05:03.620]some water you know this is
[00:05:04.910]field capacity
[00:05:05.760]so the difference down to where
[00:05:07.260]we're at at 84 percent which is
[00:05:08.920]what we calculated when we take
[00:05:11.440]a look at that we've got the 0.36
[00:05:14.310]that we could refill it to four
[00:05:16.230]feet so we could put on a
[00:05:17.920]little bit of water if we
[00:05:19.240]wanted to so we could irrigate
[00:05:20.890]it's not like we'd fill above
[00:05:22.460]field
[00:05:24.080]So now how much water do we
[00:05:26.190]have left?
[00:05:27.380]And now this time of year when
[00:05:28.930]we get into that area, we're
[00:05:30.720]looking at the 40% remaining
[00:05:33.620]water as the target that we're
[00:05:35.890]trying to use the stored soil
[00:05:38.160]water up to to get down to as
[00:05:40.220]dry as we can without lowering
[00:05:42.390]the yield.
[00:05:43.440]So we've got room to store off-season
[00:05:44.810]precipitation.
[00:05:45.580]So now we've got the water from
[00:05:47.580]this 84% or 0.36.
[00:05:51.340]This is a minus number, of
[00:05:52.810]course, because it'd take 0.36
[00:05:55.150]to refill the profile and clear
[00:05:56.900]down to this number.
[00:05:58.320]So we put it over here.
[00:05:59.400]Both of these are negative, but
[00:06:00.820]we're trying to get the amount
[00:06:02.140]of water between these two
[00:06:03.290]points.
[00:06:03.880]So when you subtract two
[00:06:05.020]negative numbers, you end up
[00:06:05.950]with a positive.
[00:06:06.740]So there's 0.99 or about an
[00:06:08.840]inch of water in each foot of
[00:06:10.460]soil that we could use.
[00:06:11.940]So we multiply times four.
[00:06:13.840]We come up just a little less
[00:06:15.220]than four inches of water that's
[00:06:16.830]stored in our profile.
[00:06:18.600]And so we can see that we'd be
[00:06:19.850]in pretty good shape.
[00:06:21.020]We could probably stop here,
[00:06:22.320]but let's keep calculating just
[00:06:23.720]a little bit farther.
[00:06:24.840]We know it's 0.2 inches per day
[00:06:27.010]is what we're planning this
[00:06:28.450]next week we'll use.
[00:06:29.860]And looking at the forecast, it's
[00:06:31.980]kind of a typical middle of the
[00:06:33.790]August forecast.
[00:06:35.140]So that's probably about right.
[00:06:36.640]So that'd come up with about 20
[00:06:38.210]days.
[00:06:38.620]But again, I never want to
[00:06:39.770]predict out near that far.
[00:06:41.020]But I'm going to say for this
[00:06:42.530]next seven days, you know, we've
[00:06:44.420]got plenty of water for that.
[00:06:46.220]So if we add that up.
[00:06:48.580]If we add these all together,
[00:06:50.730]inch 40 is what we predicted
[00:06:53.040]water for this next use for
[00:06:54.930]this next week.
[00:06:56.300]We're below field capacity, so
[00:06:58.230]it could put water on, but we've
[00:07:00.010]got plenty of water for seven
[00:07:01.150]days.
[00:07:01.540]So the prediction or the
[00:07:02.870]recommendation would be no
[00:07:04.420]water applied to this field in
[00:07:06.100]the next seven days.
[00:07:07.460]So let's go on and figure out
[00:07:09.300]the rest of the season because
[00:07:11.130]we're getting down to that time
[00:07:12.380]when we want to use up the
[00:07:13.470]stored soil water.
[00:07:14.480]So again, we come back and we
[00:07:15.920]look and this is how much water
[00:07:17.420]we've got left.
[00:07:18.560]We can use before the end of
[00:07:20.380]the season.
[00:07:21.400]So if you remember this chart
[00:07:24.880]from last week, we started
[00:07:27.220]using it as we get towards the
[00:07:28.430]end of the season.
[00:07:29.340]It's from Nebguide G1871,
[00:07:31.410]predicting the last irrigation
[00:07:33.260]of the season.
[00:07:34.360]And it talks about how many
[00:07:36.400]days it will take to mature the
[00:07:38.680]crop and how many inches of
[00:07:40.610]water the crop will still use.
[00:07:43.200]It's also got grain, sorghum,
[00:07:44.600]soybeans and dry beans.
[00:07:45.820]But if we take a look at the
[00:07:47.440]corn.
[00:07:48.540]We're beginning dent and so it'll
[00:07:50.740]take about 24 days and about
[00:07:52.690]five inches of water to finish
[00:07:54.200]the crop.
[00:07:54.880]And so you can see we're
[00:07:55.880]getting on towards the end of
[00:07:57.140]the irrigation season.
[00:07:58.220]If we add 24 days on to today's
[00:08:00.870]date, it'll be about September
[00:08:02.770]11th, you know, give or take a
[00:08:04.630]few days.
[00:08:05.380]This is a prediction, so it's
[00:08:06.930]not exactly going to be that
[00:08:08.380]date, but getting close and we've
[00:08:10.380]got about 3.96 inches of water
[00:08:12.690]left.
[00:08:13.240]So if we subtract the five
[00:08:14.530]inches off, we can see that we
[00:08:16.050]need about 1.04 inches of water.
[00:08:18.520]So we can see that we need
[00:08:19.300]about 1.04 inches of water to
[00:08:20.410]get added to the profile to
[00:08:21.210]mature the crop.
[00:08:21.900]And so here we are sitting
[00:08:23.580]towards the middle, towards the
[00:08:25.850]end of August, and we could
[00:08:27.690]just say, well, let's just put
[00:08:29.220]this on and we can, you know,
[00:08:30.760]put the pivot away for the year
[00:08:32.400]and be done with our irrigation
[00:08:34.150]season.
[00:08:34.840]But hold on just a minute, it
[00:08:36.570]probably is going to rain some
[00:08:38.290]and so let's take a look at
[00:08:39.970]rain predictions and see what
[00:08:41.890]we might expect for the rest of
[00:08:43.360]the year.
[00:08:44.000]This is a chart that shows the
[00:08:46.260]amount of rain expected.
[00:08:48.500]On average across Nebraska for
[00:08:50.530]each week in August, and we can
[00:08:52.590]see that through Clay Center,
[00:08:54.590]it's about 0.7 inches.
[00:08:56.580]And in September, it's a little
[00:08:58.060]less, of course, and we end up
[00:08:59.740]taking a look and we're about 0.65
[00:09:02.650]inches per week.
[00:09:04.060]And so how would that impact
[00:09:05.600]our decision if we got average
[00:09:07.300]rainfall?
[00:09:08.080]Well, let's take a look at it.
[00:09:10.100]We know we need about another
[00:09:11.820]inch of water to finish the
[00:09:13.390]crop prediction, of course, and
[00:09:15.750]if we got two weeks left in
[00:09:18.480]August, that'd be about 1.4
[00:09:20.870]inches and about 1.3 inches for
[00:09:23.110]the two weeks in September.
[00:09:25.000]And so if we do the math, we
[00:09:26.230]can see that right now we would
[00:09:27.700]be predicting if we received
[00:09:29.200]average rainfall that we would
[00:09:31.470]have 1.66 inches of water left
[00:09:35.120]when the crop matured.
[00:09:36.660]Now, we don't know if it's
[00:09:37.620]going to rain or not, but
[00:09:38.680]unless we at least think about
[00:09:40.470]it and make some provisions and
[00:09:42.710]delay irrigation a little bit,
[00:09:44.980]we wouldn't be able to take
[00:09:46.240]advantage of that rain if it
[00:09:47.650]does come.
[00:09:48.460]So we'll keep figuring this
[00:09:49.900]number each week and see how
[00:09:51.290]kind of the season plays out.
[00:09:53.000]If we get normal rainfall, we're
[00:09:54.570]probably done irrigating for
[00:09:55.970]the season.
[00:09:56.720]But if we don't, we can always
[00:09:58.380]put on a little bit more water
[00:10:00.230]later on.
[00:10:00.820]You know, sometimes I
[00:10:02.010]understand this time of year a
[00:10:03.580]guy just wants to be done irrigating.
[00:10:05.760]But, you know, the pivot
[00:10:06.910]certainly doesn't care whether
[00:10:08.250]it goes around the last time in
[00:10:09.510]August or whether it goes
[00:10:10.580]around in the 10th of September.
[00:10:12.440]So we just need to keep in mind
[00:10:14.230]that a few things doing
[00:10:15.700]procrastination on the farm is
[00:10:17.610]good.
[00:10:18.560]But the last few irrigations is
[00:10:19.930]probably one exception to that.
[00:10:21.900]So for now, this is Steve Malvin,
[00:10:23.640]Extension Educator with the
[00:10:25.090]University of Nebraska
[00:10:26.310]Extension, signing off.

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Weekly Irrigation Newsletter — Irrigation Scheduling Update for Aug. 19, 2025

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