The 2017 growing season for much of Iowa so far has been characterized by low rainfall and high temperatures. This has generated concerns about water stress and yield reductions. In particular, 2017 maximum temperatures are 2-6 percent above average and precipitation 4-75 percent below average (equivalent to 0.3 to 5 inches deficit) from June 1 to July 15 (Figure 1). Radiation is 5-15 percent above average across Iowa. Minimum temperatures and growing degree days (GDD) are below average in northern parts of Iowa and above average in the central and southern parts of Iowa. The 2017 weather pattern is not much different from the 2016 weather with the exception of rainfall during the 1st and 2nd week of July. High minimum temperatures are being experienced in July of 2016 whereas both minimum and maximum temperatures moderated in July of 2016. In terms of heat stress, 22 days out of the 45 days from the June 1 to July 15 period had maximum temperatures above 86oF in 2017. Heat stress becomes very important above 96oF as it negatively affects several crop morphological and physiological processes.
The big question is how are crops responding to the severe heat and/or water stresses in 2017? The ability of crops to respond to these stresses is largely dependent on soil moisture reserves and the ability of roots to extract water from deep soil layers. Iowa soils can store more than 10-12 inches of plant available water down to five feet, allowing plants to compensate for the lack of precipitation up to a certain point. The Forecasting and Assessment of Cropping Systems (FACTS) team monitors soil moisture, rooting depth, and groundwater tables across Iowa along with other crop, soil, and weather parameters and uses a process-based model to provide more insight into crop growth aspects and yield forecasts. Results for the 2nd week of July of 2017 are summarized in Figure 2.
Our 2017 measurements and simulations have shown that the plant available water in the top soil (0 to 1 feet) has been depleted. Soil moisture content increases with increased depth and below approximately 5 feet it becomes fully saturated due to groundwater. Corn roots are mostly concentrated in the 0-2.5 feet but the maximum root depth is near the groundwater table depth. Roots from the deeper soil layers take up water and meet the high evapotranspiration demands. As of July 15, soils have between 2.8 and 6.4 inches left as plant available water to four feet. The groundwater table decreases daily (Figure 3) and subsequently the subsoil moisture decreases.
At this point in the growing season, soil water holding capacity and groundwater tables have buffered the lack of precipitation. Areas of the state with extremely low precipitation, deep water tables, and/or compromised root systems may already be showing severe water stress symptoms.
Check out the latest FACTS forecast for yield estimates, water uptake, and nitrogen uptake at a location near you.
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