Beyond wanting to know the range of water use by particular crops, producers sometimes question why and how crops use water. They may inquire about what causes crop water use to fluctuate. Kansas State University Research and Extension agricultural engineer Danny Rogers fields many of these questions, and in the past he has pointed producers to different research-based crop production handbooks to explain his answers.
This is why Rogers and several of his Kansas State colleagues recently consolidated crop water use information for common Kansas crops and included this in a single publication that speaks on maximizing irrigation efficiency. The publication, titled “Agricultural Crop Water Use,” is available now online at www.ksre.ksu.edu/bookstore.
“We discuss fundamentally why crops use water,” Rogers said of the publication. “We discuss why crop water use fluctuates on different scales. For example, why does it fluctuate between day and night? The simple answer is photosynthesis occurs during the day and not at night. We show some examples of how that might work and the impact [on water use].”
Other daily water use fluctuations depend on the type of crop, its stage of growth and weather conditions, Rogers said. Daily fluctuations over time develop seasonal water use fluctuations. This recent agricultural crop water use publication, part of a series of publications covering irrigation management, includes these causes of water use fluctuations and more.
Daily and Seasonal Fluctuations
Crop water use is also referred to as evapotranspiration (ET) demand, water demand that considers the combination of evaporation and transpiration. Evaporation is the water that moves from the soil or leaf surface to the atmosphere, while transpiration is the water that passes through, which the plant uses for cooling and growth.
Simply put, crops that are smaller and in the early stages of growth require a lower ET rate, Rogers said, at least until they start to go into the dry-down phase at the end of the growing season and again use less water. Weather conditions, however, play a more complex role in water use.
“For example, on a daily basis for a full cover crop—meaning that the crop has its maximum leaf area and is intercepting all the light it can—we may need an average ET rate of about 0.35 inch per day under normal climatic conditions,” he said. “When you have hot, windy conditions or other factors that drive up crop water use, the ET rate can peak above one-half inch per day. Conversely, on a cloudy day that is cool, or even during rainfall conditions, the ET can go to almost nothing on a daily basis.”
Rogers said this is why irrigation data calculated in KanSched, an irrigation scheduling program for producers provided by Kansas State’s Mobile Irrigation Lab, are used to make recommendations on water adjustments so crop needs are met without using excess water.
Seasonal crop water use variations for a specific crop are often due to year-to-year weather conditions and the specific variety of a particular crop, especially its maturity length, which often factors heavily into a producer’s decision to grow a particular crop.
“Obviously, an 80-day (maturity) crop would use less water than a 120-day crop,” Rogers said. “Why wouldn’t you plant an 80-day? It uses less water, but that means it also has less yield potential. It’s always a tradeoff on more than just a water decision.”
Leaf Area
Leaf area index (LAI) is important, Rogers said, as plants that fully shade the ground are intercepting solar energy that drives transpiration and maximizes water use. LAI is the leaf area of the crop per unit of land area. Producers tend to understand the leaf area index by imagining if all the leaves were stripped off a plant and placed on the ground to determine the square footage of cover.
“It takes a leaf area index of about 2.7 to capture all the sunlight or shade the ground, because the leaves are suspended in the air,” Rogers said. “It takes more than 1 square foot of leaf cover to capture all the sunlight. When you capture all the sunlight, you’re at your maximum photosynthesis rate. But, for most crops the optimal leaf area index for maximum production on some crops might be approaching 4 or more.”
Yield and crop water use are linearly related for given growing conditions, so more water drives greater yield, he said. Although lowering plant production reduces water use, it also reduces yield. Finding a balance is crucial, and considering LAI can help.
“We want to make sure to utilize all the light energy, nutrients, water resources and the land area to optimize crop production,” Rogers said.
Log on to the Kansas State Research and Extension Bookstore for the agricultural crop water use publication and others in the irrigation management series.