The Weekly Irrigation Newsletter on CropWatch features results from a collaborative Nebraska Extension research study involving three Nebraska growers, who are utilizing soil moisture sensors to schedule irrigation throughout the growing season. Results are updated every Monday.

Introduction

In 2024, Nebraska Extension launched the Weekly Irrigation Newsletter to share timely insights on irrigation decisions, featuring three growers located in east-central Nebraska. Due to the positive feedback, we are continuing the program in 2025.

This year, three new growers are participating. In each of their corn fields, we installed a set of three Watermark (Irrometer) sensors at 1-, 2-, and 3-foot soil depths to track soil moisture throughout the growing season. A rain gauge was also installed to monitor both rainfall and irrigation events. These instruments are connected to an IC-10 datalogger (Irrometer), which records data at one-hour intervals. With a telemetry system in place, the data is transmitted remotely, allowing for timely irrigation decisions.

How to Use Soil Tension Data from Watermark Readings

Watermark sensors measure soil water tension, or how tightly water is held in the soil. Lower tension values mean water is more readily available to plants, while higher tension values indicate less available water.

Below are two charts (Figure 2a and 2b, adapted from EC3036) showing the relationship between Watermark sensor readings, plant-available water, and soil water storage relative to field capacity. These examples reflect the same soil types where our sensors were installed.

For example:

• A grower managing irrigation in a silty clay loam should wait until the tension reading enters the desired depletion zone, then irrigate to refill the soil profile to the lower end of the rain storage zone (around 60 centibars).
• In contrast, a grower irrigating a loamy fine sand, which has low water-holding capacity, may need to irrigate more frequently to return the soil to field capacity before significant 

Early-season Crop Evapotranspiration and Irrigation

• Corn has reached its peak of water demand and will remain at this level through late August.
• Weekly crop evapotranspiration (crop water use) will depend on weather conditions.
• Crop evapotranspiration over the past seven days was 1.32 in, with accumulated since emergence of 8 in.
• With rising temperatures and if rainfall is not present in the incoming week, irrigation will be necessary to meet crop water demand.

Grower’s Irrigation Decision (July 9-14)

Understanding the chart:

• solid line — sensor at 1 foot soil depth
• dashed line — sensor at 2 feet soil depth
• dotted line — sensor at 3 feet soil depth
• blue arrows — rainfall in inches
• green arrows — irrigation in inches

• Sensors at all depths are still near field capacity.
• No irrigation is required at this time since there is still plenty of water remaining before soil moisture levels reach the managed allowable depletion zone.

• The sensor at 1 foot is no longer at field capacity. The reading at this depth is expected to increase in the coming days as corn plants begin using water from this soil layer.
• Bottom two sensors are reporting readings near field capacity.

• All sensors are still reporting reading near field capacity.
• Similar to Grower 2, the sensor at 1 foot is expected to show increased readings in the coming days.