Ag Weather Forum

Water Year Moisture Update

Bryce Anderson
By  Bryce Anderson , Ag Meteorologist Emeritus
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Chronic drought in the western U.S. and developing dryness in the northern and north-central U.S. are notable in water year precipitation dating to Oct. 1, 2020. (NOAA graphic)

From time to time, a check of water year precipitation is useful to bring what may be called "effective moisture" to the discussion. The term "water year" is a U.S. Geological Survey (USGS) term. According to the USGS: "The term "water year" (is used) in reports that deal with surface-water supply (in) the 12-month period Oct. 1, for any given year through Sept. 30, of the following year. The water year is designated by the calendar year in which it ends and which includes nine of the 12 months ..."

Using this definition, we are in the 2021 water year, which began back in October 2020 and will conclude on Sept. 30, 2021.

The water year concept is useful to bring perspective to ground water supplies. The Oct. 1 start date is a useful time for the water year calendar to begin, since this is the end of the growing season over much of the U.S. and thus, very little precipitation will get used to maintain plant health; more moisture is likely to move into the soil structure or into streambeds. So, you have the fall, winter and early spring seasons for soil moisture recharge, and then the late-spring and summer time frames for soil moisture usage during the active stages of crops and other vegetation.

A look at the water year precipitation percentage related to normal, going back to Oct. 1 of 2020, shows just how deficient the precipitation has been in the Northern Plains and much of the northern and western Midwest. In the Northern Plains and northern and western Midwest, water year precipitation has been mostly 50% below normal. And in the western U.S., water year precipitation has been even more sparse -- mainly 75% below normal, and all the way to completely dry (zero percent of normal) in the Desert Southwest. (Even long-lived and tough-as-nails Joshua trees in Death Valley are dying because of the dryness along with excessive heat.)

The boundaries of these dry regions are also striking and, at least in this view, quite straightforward. From the spine of the Rocky Mountains along the Continental Divide west, conditions are bone-dry. Farther east, Interstate 80 is a fairly accurate divider boundary between drier north and wetter south. The territory north of I-80 is where the water year precipitation has been mainly no more than half the normal amount.

Such a lack of water year precipitation puts a keen focus on rainfall prospects during the next 10 days to two weeks. At this time, forecast models are in good agreement on chances for rainfall exceeding 2 inches in the eastern Midwest (east of the Mississippi River) along with the central and eastern portions of Iowa and northern Missouri. In Iowa, even a part of the state north of Interstate 80 is indicated to be in line for this substantial rainfall total. Further north, the rain tails off to a range of 0.25 to 1 inch -- offering only marginal benefit overall.

Even if the rain forecasts verify, the water year trends are not likely to change very much through the end of this cycle at the end of September. Forecast precipitation for August looks to be below normal for all crop areas except the Delta and Deep South. That forecast keeps dry conditions in the cards through the balance of this 2021 crop season.

Bryce Anderson can be reached at

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7/9/2021 | 1:54 AM CDT
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