The rain that has occurred over the state this fall has been welcomed and sorely needed. However, rainfall totals have been variable across the state and not enough to make up the deficit from the growing season. Northwest Iowa has received the most rainfall, with totals ranging from 3-6”. Parts of east central Iowa have received some similar totals. Northeast Iowa and southern Iowa have received less, with 1-3”. While these are all helpful, the northeast and south are below average for the last 30 days. Growing season departures show the significant shortfall in rains since May 1. While the northwest is in decent shape, much of the east has large deficits.
There are various ways we can assess soil moisture status, each with their own pros and cons.
- The best way is to measure the soil moisture in a field by taking soil cores and drying, which is a time and labor-intensive process. The ISU Extension field agronomists in northwest Iowa do this each fall and spring. Stay tuned for those results in late November or early December.
- Another method to assess soil moisture status is with automated stations outfitted with soil moisture sensors, collecting real-time data. These are useful but expensive to deploy and maintain. The Iowa Environmental Mesonet does maintain a small network across the state. Conversion from electronic measurements to actual inches of water in the soil is also a bit difficult.
- Nationally there are products that incorporate precipitation, estimated evapotranspiration and other parts of the water cycle to assess soil moisture conditions. As for general patterns these are usually reasonable, but not as accurate at assessing soil moisture levels at the field-level.
When at field capacity, most Iowa soils hold 10-12” of plant available water in in the top five feet. This varies by soil type. For instance, sandier soils hold less plant available water compared to a silty loam soil. At the end of this growing season, it is fair to assume that in many parts of the state, the crops used all or almost all the plant available water in the top 5 feet. Consequently, at a bare minimum we know that we need at least 10-12” of precipitation to get back to field capacity, assuming all the precipitation infiltrates into the soil.
While precipitation will occur this fall and winter, remember that frozen precipitation (snow) usually contributes little to soil moisture, especially deeper in the soil profile, and moisture will not soak into frozen soil.
Drought status for the state will be updated for the week on Nov. 2. See the current status at https://droughtmonitor.unl.edu/.
Corn and soybean harvest are both ahead of the 5-year average. Soybean at 93% (9% ahead of the 5-year average). And corn at 77% (15% ahead). Early maturity and dry soils have allowed quick early progress. And even with recent rains, harvest progress has moved ahead steadily. These maps are available each Tuesday from the National Drought Mitigation Center web site via USDA Office of the Chief Economist and USDA-NASS.
After the sharp cold this week, temperatures will rebound closer to average and may again slip above average into the 2nd week of November. Warmer than average temperatures may also be a more frequent issue throughout the winter as a very strong El Niño is occurring in the Pacific Ocean. Typically El Niños in the winter increase chances for warmer than average conditions. Cold will still occur, but overall temperatures are more likely going to be above average.
El Niños are less clear about precipitation for Iowa. There is likely going to be less snow according to a recent NOAA blog. However, overall precipitation still could be above average, especially later in the winter. Iowa is often caught between areas during El Niños, more likely drier to the east and wetter in the Plains. Only limited drought easing is expected as overall precipitation in the winter is limited comparatively.