Using Soil Moisture Sensors and ETgage as a Guide for Irrigation Management

By Anne Nelson and Vasudha Sharma


Vasudha Sharma (irrigation specialist) and Anne Nelson (Extension educator) install watermark sensors.

In Minnesota almost 600,000 acres of cropland is irrigated, only about 3% of total cropland. However, the soils in these areas are highly susceptible to nutrient leaching and water loss. Using irrigation management tools can improve yields, grain quality, conserve water and energy, and reduce nutrient leaching. One of the easiest and most effective ways to improve irrigation efficiency is to implement soil moisture sensors and other tools in irrigation scheduling.

Much research has been done using soil moisture sensors elsewhere in the United States, especially in states with large amounts of irrigation. In Minnesota however, we have yet to quantify their usefulness in our soils and climate. As a demonstration trial we worked with three farmers in Rice, MN, Hastings, MN and Brooten, MN, along with partners from the Minnesota Department of Ag and the Benton County SWCD to test one soil moisture sensor called the Watermark and ET gage.

Our goal is to compare the farmer's irrigation recommendation, usually based on the hand feel method vs. what the Irrigation management tools recommended. Each farmer let us manage the irrigation for one field directly next to a field they managed to reduce weather variability and large changes in soil type.

The watermark sensors are about the size of a salt shaker with a wire at one end, these are then inserted into the soil at differing depths, and connected to a datalogger to record readings. These sensors measure how tightly water is held by the soil, which is reported in centibars, using a table this can be correlated to how much water is depleted in the soil and in turn how much you would need to irrigate. ET gage is a device that simulates the crop water use from a plant canopy. It has a canvas covered ceramic evaporator plate at the top that allows water to evaporate the same way as crop does.  ETgage is very handy in cases where weather stations are not available to measure crop water use for irrigation scheduling. It is an alternative tool to determine crop water use that do not require any calculations or weather stations. We installed the watermark sensors and ET gage soon after the emergence and checked them between one and three times per week to get readings. Once harvest is over we will compare the two systems to see which is more efficient and be able to make better recommendations to irrigators.

We hope to continue these types of studies, especially since 2019 has been a very wet growing season.