Iowa soils can hold 9-12 inches of water in the soil profile and most parts of Iowa have shallow water tables at about 4-7 feet depth. These two sources of water, together with the precipitation, provide water for crop production. Typically, a 250 bu/ac corn yield requires about 22 inches of water as evapotranspiration.
The amount of precipitation received in the last three years was below normal, and as a result, soil moisture and water table depth have declined. Through the FACTS modeling system, we track soil moisture and water table depth dynamics in Iowa and, and more generally, in the US Corn Belt. Information is updated weekly on the website from April to November, and users can see dry and wet regions across the landscape. Soil moisture estimates are based on a well-calibrated process-based model for this region, the APSIM model (Archontoulis et al., 2020).
On March 18, 2024, the top 3-feet soil moisture was record-low in Northcentral and Northeast regions and below average across the state (Fig. 1). Subsequent rain events prior to April 8 have improved the water content in the top three feet by 14% compared to March 18 but levels are still below average (Fig. 1). The improvement in the top-foot soil moisture was even higher (28%), however, soil moisture at the topsoil is much more variable compared to the whole profile moisture. As of April 8, 2024, the top foot soil moisture is close to average, and levels are near optimum for field operations and planting.
Figure 1 – Relative soil moisture across the 0-3 feet soil depth. Zero means soil moisture at wilting point and 1 means soil moisture at saturation. Values below 0.2 indicate drought conditions and values above 0.9 indicate excess moisture conditions. The red line refers to 2024, the blue line refers to 2023, the black line to the 40-yr average, and the grey lines to the historical years. Source: https://crops.extension.iastate.edu/facts/soil-moisture-100cm
Depth to the water table is an important feature of Iowa soils because it can partially buffer drought stress. On March 18, the depth to the water table was 3 feet greater than the 40-yr average across Iowa and at a record low for Northcentral and Northeast Iowa crop reporting districts. Rain events from March 18 to April 8 raised the water table by 0.5 feet (Fig. 2) with the Northcentral and Central regions realizing the most benefit (up to 1 foot increase). Still, the water table depth is below average conditions across the state.
Figure 2 – Depth to the water table for different crop reporting districts in Iowa. The red line refers to 2024, the blue line refers to the last year, the black line to the 40-yr average, and the grey lines to the historical years. Source: APSIM model simulations.
Model simulations for the historical years show that the water table reaches the deepest depth in August, a critical period for corn and soybean growth (Fig. 3). Taking Boone County as an example, model simulations show that in only 3 out of the 39 historical years, the depth to the water table in August was less than that in April. The three years were 1990, 1993, and 2010. In 2 out of the 39 historical years, the water table depth in August was similar to April (years 1989 and 2015). In 34 out of the 39 years, the water table in August was from 1 to 5 feet below that of April. So, the probability that the water table depth will further decline by August is high. Corn and soybean roots in Iowa reach a depth of about 5 feet. The closer the roots are to the water table depth the greater the yield benefit from the water table in a dry year.
Figure 3 – Simulated water table depth for different years in Boone County, Iowa using the APSIM model.
In conclusion, we start the 2024 season with near optimum soil moisture conditions in the surface foot of soil and below normal subsoil moisture. Compared to last year or the average year, this year we start the growing season with less profile soil water. Moving forward, the two major drivers of soil moisture dynamics (evapotranspiration and rain) will determine the actual soil moisture and water table depth levels and temporal patterns in the coming months. Typically, both evapotranspiration and rain increase from April to August (Fig. 4). If the amount of rain we receive is higher than the evapotranspiration, then we will see improvements in soil moisture conditions.
Figure 4 – Mean monthly evapotranspiration (ET) and rain for central Iowa crop reporting district. Bars indicate mean monthly values and whiskers the year-to-year variability (39-years). Source: https://crops.extension.iastate.edu/facts/weather-tool and APSIM model simulations
Additional resources:
- Monthly weather summaries per crop reporting district (monthly updates)
- Rain and temperature forecasts (daily updates)
- 0-1 feet soil moisture maps and graphs (weekly update, every Tuesday)
- 0-2 feet soil moisture maps and graph (weekly update, every Tuesday)
- 0-3 feet soil moisture maps and graph (weekly update, every Tuesday)
- APSIM model description and simulation accuracy
- Crop yield and evapotranspiration
Sotirios Archontoulis Professor of Integrated Cropping Systems
Dr. Sotirios Archontoulis is an assistant professor of integrated cropping systems at the Department of Agronomy. His main research interests involve understanding complex Genotype by Management by Environment interactions and modeling various components of the soil-plant-atmosphere continuum. Dr...