A look at crop water use trends and what it means for irrigation planning

By: Vasudha Sharma, Extension irrigation specialist

As we move through this growing season, one question we often hear is: “How are we doing this year compared to normal?” While each season brings its own set of challenges, comparing this year’s crop evapotranspiration (ET) and atmospheric demand to long-term averages provides critical insight into irrigation timing, water availability, and crop stress risk.

Reference Evapotranspiration (ET_o) or Atmospheric Demand

Let’s begin with an overview of atmospheric demand, also known as reference evapotranspiration.

Reference evapotranspiration (ET_o) is a fundamental metric in agriculture used to estimate the atmospheric demand for water from a well-watered reference crop, usually grass or alfalfa. It reflects how much water a crop would require to grow optimally under ideal soil moisture conditions. ET_o incorporates the effects of weather factors such as temperature, solar radiation, humidity, and wind, combining both evaporation from the soil surface and transpiration from plant leaves. In essence, higher ET_o values mean the atmosphere is "thirstier," drawing more moisture from the crop-soil system. This makes ET_o a critical input for irrigation planning and water resource management.

In Becker, MN, data from 2016 through 2024 (see figure above) reveal clear seasonal patterns in ET_o, with consistent annual peaks during the summer months. Each year, ET_o rises rapidly in late spring, peaks during the height of the growing season in midsummer, and declines again as fall approaches. This bell-shaped curve closely tracks the crop water needs of major Minnesota crops like corn and soybean, which undergo their most water-intensive growth phases during these high-ET periods. For instance, by mid-July, when ET_o often exceeds 0.4 inches per day, crops are typically in their reproductive stages and require significant water to support kernel and pod development.

Although the seasonal ET pattern is consistent, notable year-to-year variability exists in both peak ET_o values and the duration of high-demand periods. Years like 2021 and 2022 saw elevated ET_o peaks, suggesting hotter, drier growing seasons that increased atmospheric water demand. In such years, crops face a greater risk of water stress if irrigation or rainfall is insufficient. Conversely, cooler or wetter seasons may result in lower ET_o, which can reduce crop water use but also potentially slow down development or increase disease risk due to excess moisture.

Crop Evapotranspiration (ET_c) or Crop Water Use

Shifting focus to crop evapotranspiration, or ET_c, we look at the actual water use of a crop under specific field conditions. ET_c is calculated by multiplying ET_o by a crop coefficient (K_c), which adjusts for differences in crop type and growth stage: ET_c = ET_o × K_c. Essentially, ET_c reflects the crop’s “thirst”, the actual water needed to support physiological functions. High ET values typically occur during rapid growth stages and require timely irrigation. In contrast, low ET_c values, often associated with early or late growth stages or cooler conditions, may reduce irrigation needs but can sometimes delay crop maturity.

If you examine the figures above, you will notice that in a dry year like 2021 (green line), ET_c began to exceed the long-term average about 30 days after planting, indicating higher atmospheric demand due to elevated temperatures and limited rainfall. This led to greater reliance on irrigation. In contrast, 2025 data (orange line) show ET_c tracking close to the long-term average during the first 50 days after planting, suggesting a relatively normal season. A few spikes between days 60 and 80 indicate short periods of heat stress requiring supplemental irrigation. However, due to timely and widespread rainfall across central Minnesota this season, irrigation for both corn and soybean has been minimal so far.

While minimal irrigation is often a welcome relief, it’s important to remain cautious about the consequences of over-irrigation, especially during wet years. Excess water can lead to nitrate leaching, where unused nitrogen is washed below the root zone and lost to groundwater. This not only reduces nitrogen use efficiency but also poses environmental risks to local water quality. To prevent such losses, it’s essential to monitor soil moisture closely and avoid unnecessary irrigation when soil profiles are already at or near field capacity. Incorporating soil moisture sensors alongside ET-based scheduling can help ensure that irrigation supports crop needs without contributing to nutrient losses.

Ultimately, monitoring reference ET and crop ET over time provides more than just historical insight. It equips growers with the tools to anticipate crop water demand, fine-tune irrigation strategies, and respond proactively to shifting weather conditions. As climate variability increases, using data-driven approaches like these will remain critical to resilient and sustainable crop production in Minnesota and beyond.

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