Irrigation Termination in Arkansas

by Bill Robertson, Cotton Extension Agronomist and Mike Hamilton, Irrigation Instructor - August 26, 2021

Furrow irrigation in a cotton field

Most fields have reached cutout and are starting to run out of gas as you would expect. We are determining cutout and using that as the basis for insecticide and irrigation termination. Some fields did not reach nodes above white flower of 5 prior to the latest possible cutout date. In Arkansas, the latest possible cutout dates using a 50-percent probability of collecting 850 heat units (HU) on a 30-plus year data set are: August 9, Keiser; August 14, Marianna; and August 17, Rohwer. If we use the last five years’ weather data, these dates may be extended slightly (3 to 5 days). The date of cutout is the date at which plants reach nodes above white flower (NAWF) of 5 or the latest possible cutout date whichever occurs first.

Our insect termination is straightforward. Once we accumulate 250 heat units after cutout, stop spraying plant bugs; bollworms, 350 heat units after cutout; stink bugs, 450 heat units if fields are clean.

Irrigation termination is not as straightforward. It is best to use multiple guides to determine irrigation termination. Our old rule of thumb for terminating irrigation is having good soil moisture all through the soil profile at first open boll. If you need to water, then do so at first open boll and roll up the pipe. However, September irrigations seldom add to lint yield and often result in a delay in getting a picker in the field.

To refine this, we can factor in heat unit accumulation after cutout, depending on the depth of the root system. For example, if you have a deep root system and good moisture in the top 2 feet of soil, then research has shown a final irrigation at 350 to 400 heat units after cutout will work. However, if you have a hardpan and a shallow root system, then you might need to irrigate as late as 600 heat units after cutout if the weather is extremely hot and dry. This generally equates to an additional two irrigations beyond the early termination, depending on rainfall.

Most furrow irrigated producers using conventional tillage without cover crops will likely fall into the window of terminating irrigation 500 to 550 heat units after cutout. It is not uncommon for producers using no-till with cover crops to eliminate irrigations during the season as well as allowing for earlier termination of irrigation approximately 400 heat units beyond cutout.

Soil moisture sensors are excellent for scheduling irrigations and in most crops irrigation termination because we can see a significant drop off of water use when the crop is near harvest. This is more difficult in cotton because the plant thinks it’s going to live forever. Sensors will help in evaluating rooting depths and how much available water is in the soil profile to determine if an additional irrigation is needed when we near the cutoffs mentioned above.

Water use and crop coefficient function for cotton in Stoneville, Mississippi.

Source: https://www.cottoninc.com/cotton-production/ag-resources/irrigation-management/cotton-water-requirements/

Using the table below or our phone app, the Arkansas Soil Moisture Calculator, it is possible to determine the available moisture in the profile. Use the values for each foot of soil the crop is showing water use at to determine the total available water and remember the cotton crop is still using around 0.25 inches per day at first open boll and we want good moisture at this point.

Soil Tension (cb)	Sand (1.0"/ft)	Sandy Loam (1.4"/ft)	Silt Loam with Pan (1.58"/ft)	Silt Loam (2.37"/ft)	Clay (1.6"/ft)
0	1.77	1.51	1.01	1.83	1.38
5	1.72	1.51	1.01	1.83	1.36
10	0.74	1.00	1.01	1.65	1.09
15	0.35	0.74	1.01	1.53	0.91
20	0.14	0.58	1.01	1.41	0.78
25	0.02	0.46	0.88	1.29	0.68
30		0.37	0.79	1.19	0.60
35		0.29	0.81	1.14	0.53
40		0.23	0.72	1.00	0.47
45		0.18	0.64	0.89	0.42
50		0.14	0.57	0.80	0.37
55		0.10	0.49	0.71	0.33
60		0.06	0.45	0.63	0.30
70		0.01	0.35	0.50	0.23
80			0.25	0.39	0.18
90			0.21	0.29	0.13
100			0.13	0.22	0.09
120			0.03	0.09	0.02
130				0.03
140

Source: Lab and model data of irrigated soils sampled and grouped from Arkansas farms.

Download the Arkansas Soil Sensor Calculator