May 28, 2010

Extension Educators Darrel Siekman and Gary Zoubek read water mark sensors in the field.

Irrigation management is crucial to raising a high yielding crop, improving efficiency, and ultimately, saving money. Two tools promoted by the Nebraska Agricultural Management Demonstration Network (NAWMDN) to accomplish efficient irrigation are the Watermark sensors and ETgage. While it is recommended that these tools be used together, sometimes producers prefer to use just one. The benefits of these tools and their purpose are briefly described below.

Watermark Sensors

Watermark sensors can be used throughout the growing season to manage irrigations (soil moisture-based irrigation management), using suggested trigger points in the UNL Extension publication, Watermark Granular Matrix Sensor to Measure Soil Matric Potential for Irrigation Management (EC783).

This approach does not require knowledge of actual crop evapotranspiration. Irrigations can be initiated every time the trigger point indicated by the Watermark sensors is reached, thus, the available soil water in the soil profile will fluctuate between the trigger point and approximately 35-38% depletion of the available water.

The primary reason for not waiting until the traditional 50% depletion is because it may take a few days to make a full circle application with the center pivot system. Thus, if the irrigation is initiated at the 50% depletion level, by the time the pivot completes the circle, the available soil water in some parts of the field may significantly exceed 50%, causing plant stress. The 35%-38% depletion level is used as a buffer or safety strategy to minimize the risk of imposing crop water stress.

The amount of water applied in each irrigation will depend upon several factors, such as soil type, irrigation system and well capacity, field slope, and other factors, but usually an application of 1.0 inch per center pivot revolution per week is common.

ETgages

ETgages also can be used alone to manage irrigations (crop water use-based irrigation management); however, this use requires knowledge of when to initiate the first irrigation. If you don't have Watermark sensors (or another type of soil moisture instrument) installed, you can determine the first irrigation trigger point with a traditional checkbook method. This method determines the soil water depletion from the estimated available water in the beginning of the growing season (can be assumed at field capacity for most locations in Nebraska) and adjusts it according to rainfall and ETgage information.

It is important to note that if the ETgage is used alone, this assumes that the first irrigation is triggered at a proper soil water depletion level (35%-38%). For example, the suggested trigger point for corn grown on a silt-loam soil (e.g., Hastings silt loam), is taken from an average of the top two sensors before tassel and an average of the top three sensors after tassel and when the value is approximately 90-100 kPa. These values would trigger an irrigation. After determining the first irrigation, the ETgage can be used to calculate actual evapotranspiration on a weekly basis and can be used to decide irrigation for the remaining part of the growing season.

If the first irrigation is not triggered at a proper level of depletion (i.e., if the irrigation is triggered before approximately 35-38% depletion), the ETgage will overestimate crop water requirement for the rest of the season. Thus, determining the first irrigation timing is very important for the success of using an ETgage alone for irrigation management.

Using Both Tools Together

The Watermark sensors and ETgages can be used together quite well for irrigation management. The Watermark sensors can be used to determine when to initiate the first irrigation, based on the suggested trigger points in the Watermark EC for a given soil texture. After the first irrigation, an ETgage can be used to estimate actual crop water use (ETa) since the last irrigation, using the reference ET (ETref) and crop coefficient (Kc) approach (i.e., ETa = ETref x Kc), to replenish the crop water needs on a weekly basis.

Let's say the 35% depletion trigger point is reached by using the Watermark sensors and the irrigation system is turned on to apply 1 inch of water. You will then go to the ET gage and move the second red ring 1 inch below where the current water line is. When the water level depletes in the ETgage to where the second red line is, you know it's time to irrigate again (and you can double check that fact with your Watermark sensors). If you receive a rainfall event before the water level reaches the second red ring, simply move the ring down for the amount of effective rainfall. For example, if you received 0.5 inches, move the ring down another 0.5 inches so your new trigger is set on the ETgage.

If you receive a large rainfall (see below), you may need to recalibrate by waiting for the Watermark sensors to again reach the 35% depletion point before correlating to the ETgage.

The Watermark sensors also can be used to determine the timing of the last irrigation. Between the first and last irrigation, ETgages can be used alone to make irrigation management decisions, saving you from having to make a weekly trip to the field to read the Watermark sensors.

Making Adjustments

It is also important to note that crop water use estimated using an ETgage and the value calculated from Watermark sensors may not correlate well in some cases. In other words, the irrigation timing determined using an ETgage vs. Watermark sensors may not match. This is due to the dynamics of changing available soil water. For example, even though the total water-holding capacity of a typical silt loam soil is 2.2 inches per foot of soil, after a heavy rain (e.g., 1.5-2.0 inch) or an irrigation event, the soil water holding capacity can exceed 2.2 inches and remain above field capacity for a few days, depending on weather conditions and crop growth stage.

It will take a few days for that extra (excess) water to be used by the crop or evaporate from the soil surface before the soil water decreases to field capacity. Depending on the soil texture and other factors, excess water above field capacity can be significant, ranging from 0.20 inch per foot for sandy soils up to 1.2 inch per foot for silt loam soils.

After soil water decreases to field capacity, it will take a few days for the Watermark sensors to dry and reach the trigger point of 90-100 kPa. Since there is no feedback related to soil conditions with the ETgage, the next irrigation will be triggered by the ETgage earlier than the Watermark sensors.

Resources

For more information on how to use ETgages and Watermark sensors for irrigation management, see these resources.

• Using Modified Atmometers (ETgage) for Irrigation Management, UNL Extension NebGuide G1579
• Watermark Granular Matrix Sensor to Measure Soil Matric Potential for Irrigation Management, Extension Circular EC783
• Nebraska Agricultural Water Management Demonstration Network 

Suat Irmak, Extension Irrigation Specialist, Lincoln
Jennifer Rees, Extension Educator, Clay County
Brandy VanDeWalle, Extension Educator, Fillmore County
Gary Zoubek, Extension Educator, York County
Jim Schneider, Extension Educator, Hamilton County
Nebraska Agricultural Water Management Demonstration Network (NAWMDN) team - UNL Extension