Soil moisture benchmark April 19, 2021

The concern for not having enough water during the 2021 growing season is increasing. Observations from Iowa fields (i.e., deep soil samples and tile drainage experiments), and analytical frameworks (i.e., Drought monitor) indicate below normal moisture in parts of Iowa. Here, we benchmark the 2021 spring soil moisture across fields and across profile depths to

The concern for not having enough water during the 2021 growing season is increasing. Observations from Iowa fields (i.e., deep soil samples and tile drainage experiments), and analytical frameworks (i.e., Drought monitor) indicate below normal moisture in parts of Iowa. Here, we benchmark the 2021 spring soil moisture across fields and across profile depths to enhance our understanding of current soil moisture conditions. We simulated 25,000 fields across the US Corn Belt using the Agricultural Production Systems sIMulator (APSIM) from 1984 to 2021. The model has been extensively calibrated and validated at the field and regional scale. In this report, we first compare Iowa soil moisture to other US states and then we benchmark Iowa soil moisture status per crop reporting districts.

Simulation results for April 19, 2021 showed a range of relative soil moistures across the US Corn Belt and across soil profile depths (Fig 1). The general observation is drier topsoil moisture and wetter subsoil moisture across the landscape (Fig 1). The topsoil dryness is noticeable for central and northern Iowa. Considering the 0 to 3.5 feet profile, some areas in southern Illinois and eastern Indiana have excess water. A state-level comparison of the April 19, 2021 relative soil moisture compared to 37-year average for the 0 to 1.5 feet profile depth indicated that Dakotas have the largest deviation from normal among the 12 US Corn Belt states (data not shown). The April 19 simulated moisture values are a record low in North Dakota while in South Dakota moisture values are below normal but not record lows.

Fig 1. Simulated relative soil moisture on April 19, 2021 across the US Corn Belt and two depths (0 to 0.5 feet and 0 to 3.5 feet). A value of 1 means saturation and a value of 0 means wilting point (no water).    

Another soil water indicator of great interest is the depth to the water table (the depth at which the soil layer is saturated with water). A state-level comparison indicated the depth to the water table is below normal in all US Corn Belt states except Illinois and Missouri which are above normal. The Dakotas and Iowa have the largest deviations from normal. Statewide Iowa’s water table depth is 1.5 feet deeper than 37-year average and 2.5 feet deeper compared to 2020 (data not shown).  

Within the Iowa crop reporting districts, the depth to the water table is lower than normal in five out of nine districts (Fig 2). In the northwest district the depth to the water table depth is a record low, with current levels 2 feet below normal and nearly 3 feet below 2020. Other districts with noticeable below normal depth to the water table are the northcentral and central region. This explains the low drainage flow from artificially tile drained fields. Recent deep soil core sampling across multiple locations in Iowa confirm the simulation results. 

In contrast to the watertable, the topsoil moisture is closer to normal values for most crop reporting districts (Fig. 3). The northcentral region, followed by the central region, have the lowest topsoil moisture compared to 37-year average and a decreasing trend over the last month (Fig. 3). However, it is important to note the topsoil moisture is variable across and within year. An inch of rain can change the current trends.

In conclusion, half of Iowa has moisture levels (topsoil and watertable) below normal. In Iowa, we commonly have excess soil water with tile lines running non-stop in the spring. The low tile drainage flow in spring of 2021 to date does not mean that we do not have enough water to grow crops. Typically, it takes 25 inches of evapotranspiration to achieve a corn yield of 250 bu/ac. Precipitation is expected to increase in the following months (typically we get 4–5 inches per month precipitation in May to July compared to 1–3 inches per month in January to April). Evapotranspiration is expected to increase in the following months as well. Yet, it is to be seen how this will balance out. We will provide updates in the following weeks. On a positive note, not having excess moisture in the spring is very beneficial for environmental N losses and crop planting. No excess water, no nitrogen loss and spring of 2021 to date has resulted in one the least environmental N losses, while planting is progressing smoothly.

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