Soil Moisture

Why does soil moisture matter?

Soil moisture plays an important role both as a critical component of ecosystem health and as an indicator of changes in the climate. The term "soil moisture" describes how much water is being held in the earth. Soil moisture influences forest health by determining how much water is available to plants, can help maintain cooler ground temperatures, and relates to groundwater flow and watershed health.

The figure below, from the Ontario Ministry of Natural Resources, shows the role of soil in the water cycle. Water is gained through snowmelt or precipitation. It is travels through evaporation, evapotranspiration (water lost by plants through photosynthesis), runoff, or percolation to the underground water table.

The Water Cycle. Image Credit: Ontario Ministry of Natural Resources.

Soil moisture can be an important indicator of climate change. Decreases in yearly precipitation translates directly to lower soil moisture. However, warmer temperatures or shift from precipitation falling as rain rather than snow can also impact soil moisture. Warmer temperatures can lead to early season drying of the ground if snow melt occurs early. It can also lead to water loss as higher temperatures increase evaporation and evapotranspiration. So if temperatures rise, water availability may decrease even if precipitation remains the same.  

Precipitation over the last 100 years for Colorado have not shown a clear trend up or down, and neither has precipitation in the Roaring Fork Valley. Yet both Colorado and our own Valley have shown increase in average temperature over the last decade. Measuring soil moisture and temperature and studying their relationship will offer insight into how climate change may affect water availability in the Valley. This has implications for water management, agriculture, and wildland management or restoration.

Metadata, or the background information about a site, is important in understanding soil moisture dynamics. Vegetation, soil type and texture, and slope of the land all play a role in the rate at which water moves through the soil.

For more information visit: agci.org/iron

Exploring Soil Moisture Using iRON Data

Soil moisture vs. max and min daily temperatures:

Soil Moisture and Temperature for Sky Mountain, 2013. Image Credit: iRON
(Click on graph to enlarge)

What does this tell us?

• The graph above shows the changes undergone by soil moisture during the transition from winter to spring and summer to fall as the soil respectively thaws or freezes.
• At the 2 in depth, soil thaw, represented by a sudden spike in soil moisture, occurs in early March.
• At the 8 in depth, a thaw occurs in early March followed by a period of stability, which may represent refreezing, with a second spike occurring in late March, and the third, largest spike occurring in mid-April.
• At the 20 in depth, the thaw occurs at the end of March/beginning of April.
• The 8 and 20 in depths remain saturated as a result of snow melt through mid-June, while the 2 in depth peaks from snow melt in mid-April.
• Sharp declines in soil moisture in November and December may indicate soil freezing at those times.
• Maximum temperatures (which stretch from June to September) coincide with a drying of soil moisture at all depths, with the exception of the moisture increases in response to rain events.

How does this relate to the ecological "big picture"?

Soil moisture provides an important source of water between rain periods for plants during times of active photosynthesis. At elevations similar to the Sky Mountain plant community, aspens (the dominant species) common leaf-out and increased photosynthetic activity in May and leaf-off in October. In addition to the relationship between plant health and soil moisture available, degree of photosynthetic activity may also play a role in drops in soils moisture levels during the summer.

Precipitation and soil moisture, 2013:

(Beginning in March, shortly before ground thaw and ending in mid-December). This data set does not include data from January and February because our precipitation device is a tipping-bucket rain-gauge and not designed to measure precipitation in the form of snow-fall.

Soil Moisture and Precipitation for Sky Mountain, 2013. Image Credit: iRON
(Click to enlarge)

What does this tell us?

• The 2 inch depth is the most responsive to rain events followed by the response at 8 inches.  
• The 20 inch depth is dominated by snow melt, showing little response to summer rain events.  
• Soil moistures at 2 and 8 inches were impacted by the large (0.8 inch) rain event at the end of July.
•  Subsequent events mid-September elevated the 8 inch soil moisture for the remainder of the growing season.  
• The data indicates that the effect of one large rain event (0.8 inches) has a greater and lasting effect on the 8 inch depth soil moisture compared to a series of smaller events of 0.1 to 0.2 inches more typical during the summer monsoon.

How does this relate to the ecological "big picture"?

Climate models suggest that in the future frequency and intensity of rain events may change. Since moderate rain events have a different impact on soil moisture than heavy rain events, a change in precipitation patterns may have an impact on soil moisture levels and curves even if yearly precipitation does not alter alter significantly, and will certainly have an impact on soil moisture if yearly precipitation does alter.

Day-by-Day look at a heavy vs. a moderate rain event:

     Soil Moisture and Precipitation for Sky Mountain Heavy Rain Event, 2013. Image Credit: iRON.
                                                                                      (Click on graph  to enlarge)

 Soil Moisture and Precipitation for Sky Mountain Moderate Rain Event, 2013. Image Credit: iRON.
                                                                                      (Click on graph  to enlarge)

What does this tell us?

• At least in this mid-summer instance, a rain event greater than about 0.3 inches is needed to noticeably affect soil moisture at the 8 inch depth.

How does this relate to the ecological "big picture"?

Soil moisture responds differently to rain events based on size of the event. This means that the amount of water available to plants will vary depending not just on the amount of rain that falls in the course of a month or a week but also on whether that amount falls a little at a time over a long time or all at once.