Irrigation Optimization Using Soil Moisture Monitoring & Remotely Sensed ETc

Jeff Tuel,
February 11, 2020

What is Irrigation Optimization & How Can Soil Moisture Monitoring Help?

To keep it simple, let’s go with irrigation optimization using soil moisture monitoring is efficiently applying water in frequency and duration to precisely match soil properties with plant water consumption to ensure optimal crop yield.  It’s about knowing how much water your crop consumed and how much evaporated, also known as crop evapotranspiration (ETc), along with how much moisture is in the soil.  Once these parameters are known, a grower can irrigate just enough to replenish the water consumed and lost (ETc) at a rate that the soil can hold.

Why is Irrigation Optimization Important

This question appears to have an easy answer – water scarcity and enhanced sustainability, of course. There are very few geographical locations on the planet today that are not under pressure and, in many cases, regulations to reduce groundwater use for irrigation. California’s Sustainable Groundwater Management Act is a good example where critically overdrafted water basin users must formulate and implement plans to become groundwater neutral (outflows = inflows) to achieve a sustainable balance. You can also visit the department of water resources website for more information. While many growers have improved irrigation system efficiency using drip, micro sprays, jets, etc., very few have adopted technology that would allow them to run their irrigation systems more optimally. It is widely accepted that most over irrigate using time-based schedules instead of a demand or replenishment approach.

Remotely sensed crop evapotranspiration (ETc) is readily available

State-of-the-art satellites observe crop water consumption and field evaporation are now capable of globe-scale coverage with 10-meter resolution (40 data points per acre) and revisit frequencies of 2-5 days. Field analytics showing ETc in inches of water per acre can be provided to growers weekly through services such as Jain Logic. This technology has been around for decades and is now refined and readily available to growers. The reason satellite-based ETc is so important is that it is specific to your field and crop, unlike methods using calculated ETc from remote weather stations that can be miles away from the field and crop coefficients that were developed decades ago. It’s not to say that these methods are bad or unacceptable, they’re simply not nearly as accurate or specific to the field.

Soil Moisture Monitoring is a Key Component

Soil moisture monitoring probes have been around and used by growers for many years. The idea here is to find a location in the field for the soil moisture monitoring probe that is considered marginal soil – not the best, not the worst. Chose a probe length that will cover the target moisture zone for the roots of the crop being monitored. This will give the best representation of the moisture in your soil profile. This concept is relatively well known, so we’ll use a couple of graphics to illustrate it.

By combining field-specific soil moisture monitoring information with satellite-based ETc, the grower can very precisely irrigate to replenish the water consumed in amounts and application rates to ensure the water is in the target moisture zone.  Putting too much water on the field means it will run off or drop below the root zone where it is not available to the crop, not to mention it would be more than the plant’s consumption.

Water savings are achievable year around, but the most significant savings come in the shoulder months of the spring and fall when ETc is lower (summer is peak ETc time).  During these times, it is possible to take advantage of rain events and the soil’s water-holding capacity by significantly reducing or skipping irrigation altogether.  Results vary depending on the crop, region, and grower practices, but we have seen 15% to over 30% reductions in applied irrigation water with no drop in crop yields.  It is important to understand that these results are not from under irrigating the crop, but from reducing over-irrigation by more precisely irrigating to the plant’s needs and managing the soil moisture monitoring in the target zone.