Year-end soil moisture update | Aspen Global Change Institute

Why look at winter soil moisture?

Last year’s drought led to record-breaking low water levels in the Colorado River Basin. The coincident effort to understand the variables involved put fall/winter soil moisture in the spotlight. Over winter, soils at high elevations are frozen, meaning that the amount of water in the soil at the end of fall is the same amount of water in the soil at the start of spring when soils begin to thaw and take up snow melt. Drier soils will take up more of the snowmelt than would wet soils, meaning that a dry late autumn one year may mean lower runoff the following year. In this way, soils create a memory of drought from year to year.

How does 2021 soil moisture compare to 2020?

In summary:

Soil moisture values at an 8in depth going into winter were very low at most sites in the iRON network in 2020, comparable to dry soil conditions across the state of Colorado at that time. These conditions contributed to the low streamflows seen in the following spring and summer of 2021.
Across the stations, there was a variation in how 2021 December soil moisture values compared to 2020 December values at an 8in depth. At some sites, soil moisture values were similar for both years, at others Dec 2021 soils were wetter than Dec 2020, and at still others Dec 2021 soils were even drier than last year’s.
Even at sites where the Dec 2021 soil moisture values are similar to years preceding 2020, it is important to remember that the entire last decade (the time frame in which all the stations were installed) has been overall a period of drought for the Colorado River Basin.
November soil moisture estimates from the Colorado River Basin Forecast Center (see very bottom of blog), show that although soil moisture conditions across the Western Slope are generally less severely dry than last fall, we are again entering winter with soils drier than the 1980-2014 average.
Extreme drought such as we experienced this past year is a consequence of multiple factors beyond just soil moisture, including: snowpack, timing and intensity of spring and summer rain events, and temperature.

The graphs below show soil moisture records for an 8in depth for each iRON station for the 2021 calendar year as compared to 2020 and the station’s average for the period of record previous to 2020 (the average of the data values for the years that the station has been in operation). It is important to note that as the iRON stations came online at different times, individual stations have different periods of record.

This graph of data from Sky Mtn shows both 2020 and 2021 as having drier 8in depth soils than the average for the period of record (2013-2019) throughout the summer. However, 2021 enters winter with slightly wetter than average 8in depth soils.

This graph of data from Smuggler Mountain shows 8in depth soils for 2020 and 2021 as being drier than the average for the period of record (2014-2019) throughout the summer and both years entering winter with drier than previous soil at an 8in depth.

This graph shows 8in depth soil moisture for Glassier Ranch. The Glassier Ranch station is located below irrigated fields and near an irrigation ditch. It is consequently typical for these soils to remain saturated or more than saturated throughout the growing season.

This graph for the Glenwood Springs station shows soils at an 8in depth having similar moisture patterns to the period of record average (2015-2019) in both 2020 and 2021. However, soils at Glenwood Springs are rocky and tend to drain quickly, so dry soils are common at this site. At an 8in depth, 2021 entered winter wetter than the average for the period of record. Because this site is low elevation, soil moisture is not static throughout the winter– soils may dry during warmer periods or wet during snowmelt even in mid-winter.

This graph for the Spring Valley station shows soil moisture at an 8in depth following similar patterns across 2021, 2020, and the period of record (2017-2019). The soils at this site are a highly compacted clay. Consequently, water tends to bind tightly to the soil.

This graph for the Brush Creek station shows soil moisture at an 8in depth following similar patterns across 2021, 2020, and the period of record (2015-2019). The soils at this site are a highly compacted clay. Consequently, water tends to bind tightly to the soil.

This graph of soil moisture at an 8in depth at the Northstar Aspen Grove station shows 2021 has having wetter soils throughout the summer and entering winter than 2020 or the period of record (2015-2019).

This graph shows soil moisture at an 8in depth for the Northstar Aspen Grove. Although the two sites are located very near to one another, the Transition Zone occurs in an open meadow and the Aspen Grove site is surrounded by trees with undergrowth. At this site the 8in depth soil moisture for summer of 2021 was much lower than either 2020 or the period of record (2015-2019), and the soil at this depth entered winter drier in 2021 than in 2020 or the period of record.

This graph shows data from the newly added Castle Creek station, with a record that beings in 2020. At this site, soils at an 8in depth entered winter at a similar moisture value in both 2020 and 2021.

This graph shows data for an 8in depth for Independence Pass. There was a data loss during summer of 2021, but entering into fall and winter, soil moisture values for 2021 were below 2020 and period of record (2017-2019) values.

Gridded Soil Moisture From the Colorado River Basin Forecast Center 2021/2020 Comparison

November soil moisture estimates from the Colorado River Basin Forcast Center show that although soil moisture conditions across the Western Slope are generally less severely dry than last fall, we are again entering winter with soils drier than the 1980-2014 average. “Gridded” data refers to data that draws from a variety of observation locations to generate an average for a larger geographic area. Soil moisture data of this type are often “proxy” data, meaning that an estimate was generate using other values, such as rain, air temperature, etc.

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