August 12 – August 19 Crop Water Use and Soil Moisture Status Report

pic of datalogger

Figure 1. Watermarks sensor reading from the 2 ft at Location 3.

Location 1: 2 miles west of Hooper, rainfed no-till corn after soybeans on Moody Silty Clay Loam soil (about 2.2″ of available water per foot)

August 12 – August 19

Weekly Rainfall & Crop Water Use for August 12 – August 19:
Rainfall = 1.25″
ET Reference Reading = 1.3
Corn Water Use (R5 dent stage,  1/4 milk line) = 1.25″

Soil Water Depleted at Depth (Sensor Reading) on August 19:
1 ft = 0.68″ (78)
2 ft = 0.77″ (88)
3 ft = 0.00″ (20)
Average sensor reading = 62
Total water depleted = 1.44″
Total available soil water for use before crop stress (50% of soil water depleted in top three feet) occurs: 1.85″ or about 11 days from now

Seasonal Summary (view previous weekly crop water use at NE Ag Water Mgnt Network):

June 10 – August 19 Rainfall: 12.3″
June 10 – August 19 Corn Water Use: 12.1″
Total June 10 – August 19 (Rainfall – Crop Water Use) Surplus: 0.2″

Check out the Nebraska Drought Monitor Map too, new map released on Thursdays each week. View crop water use from around Nebraska at the Nebraska Ag Water Mgnt Network.

Location 2: 5 miles east of Winslow, irrigated soybeans after corn on Belfore Silty Clay Loam soil (about 2.0″ of available water per foot)

August 12 – August 19

Weekly Rainfall & Crop Water Use for August 12 – August 19:
Rainfall = 0.5″
Irrigation = 0.25″
ET Reference Reading = 1.0
Soybean Water Use (R5 beginning seed stage) =1.1″

Soil Water Depleted at Depth (Sensor Reading) on August 19:
1 ft = 0.50″ (59)
2 ft = 0.45″ (53)
3 ft = 0.56″ (65)
Average sensor reading = 59
Total water depleted = 1.51″
Total available soil water for use before irrigation should start (trigger point to start pivot is 35% of soil water depleted in the top three feet): 0.6″ or 3 days from now, August 22.

Seasonal Summary:

June 23 – August 19 Rainfall: 6.0″
June 23 – August 19 Soybean Water Use: 10.2″
June 23 – August 19 Irrigation: 1.6″
Total June 23 – August 19 (Rainfall + Irrigation – Crop Water Use) Deficit: 2.6″

Location 3: 2 miles north of Arlington, irrigated no-till corn after soybeans on Belfore Silty Clay Loam Terrace soil (about 2.0″ of available water per foot)

August 12 – August 19

Weekly Rainfall & Crop Water Use for August 12 – August 19:
Rainfall = 0.25″
Irrigation =0″
ET Reference Reading = 1.0
Corn Water Use (R4.7 beginning dent) = 1.10″

Soil Water Depleted at Depth (Sensor Reading) on August 19:
1 ft = 0.15″ (29)
2 ft = 0.46″ (54)
3 ft = 0.84″ (119)
Average sensor readings = 67
Total water depleted = 1.44″
Total available soil water for use before irrigation should start (trigger point to start pivot is 35% of soil water depleted in the top three feet): 0.8″ or about 5 days from now, August 24.

Seasonal Summary:

July 1 – August 19 Rainfall: 5.0″
July 1 – August 19 Corn Water Use: 9.7″
Total July 1 – August 19 Irrigation: 2.8″
Total July 1 – August 19 (Rainfall + Irrigation – Crop Water Use) Deficit: 1.9″

Check out the Nebraska Drought Monitor Map too, new map released on Thursdays each week. View crop water use from around Nebraska at the Nebraska Ag Water Mgnt Network.

Frequently asked questions (FAQ):

Why are we letting the soil dry down or get depleted when we have irrigation?  The goal of irrigating corn and soybeans in this area is to ensure we don’t cause any water stress (50% of soil water depleted) yet provide some storage capacity for future rainfall events. Because it can take about 3 days to irrigate a field, we want to leave about 4 days worth of crop water use stored in the soil above the point of causing water stress on the crop. Therefore we can deplete about 2″ of soil moisture in the top 3 feet in a silty clay loam soil in this area before we need to start the pivot. If we apply 1″ with the pivot, that still leaves room in the soil to store another 1″ from a rainfall event in the near-term.

Why doesn’t rainfall + irrigation – crop water = balance not equal the amount of soil water depleted in the top 3 feet measured by the sensors?  Look at information from Location 3. The total deficit for the monitoring period (July 1 to August 19) using the above is  1.9 inches However, the soil moisture sensors only show a moisture deficit in the soil of 1.4″ in the top three feet.  The likely explanation for the 0.5″ difference is that crop roots have extracted and used water from the soil below or deeper than 3 feet, were we don’t have sensors. Corn roots can grow 6-7 feet deep and soybeans 4-5 feet deep in well-drained loess soils like we have at all 3 locations.

August 12 – August 19 Crop Water Use and Soil Moisture Status Report
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