|Soil Moisture Content|
The soil moisture content has an important effect on soil thermal properties, soil heat flow, and vegetation. It is thus considered to be a critical parameter in analyzing the magnitude and variability of summer thaw.
At many locations, and especially those with a thick organic or moss mat, the soil moisture content in the upper soil varies greatly over time (precipitation events, evaporation) and space. Thus, most measurements should be collected beneath the organic mat, near the upper part of the mineral soil horizon. If possible, all measurements should be made at a depth of 15-20 cm.
Gravimetric MethodsAt most sites, digital recording of soil moisture content is not possible. In this case, we are asking that investigators collect measured volumes of soil for gravimetric measurement of soil moisture. Specifically, about 6-10 sites should be sampled over the grid. Using standard techniques, a known volume of soil should be collected from beneath the organic mat. After weighing, the samples should be heated in a drying oven to 105 C for 24 hours, then reweighed. It is useful to sample throughout the summer. If this is not feasible, end-of-summer measurements are most useful, especially if made in conjunction with thaw depths. The location should be marked with a stake, and samples collected from near (0.5 m) the same site each year. The following information should be recorded and sent to the CALM web site:
Stationary MeasurementsThis applies to digital measurements collected and recorded at a single site, and refer especially to TDR and similar devices. If possible, the time series should be sent to the CALM web site for archiving. Metadata should include the site location (see CALM map form), type and model of instrument, soil type and texture, depth of reading(s), and temporal frequency of measurements. The time/date should be included with the time series of soil moisture content.
Spatial MeasurementsIn some cases, measurements of soil moisture are made across the study area using a portable electronic probe. We are asking that 6-10 measurements be made across the study area, with measurements taken at the surface and at depth near the end of the thaw season.
At each sampling location, the probe should be inserted into the surface materials and readings (typically voltage) recorded. Because the probe tines are often 10 cm in length, this represents an integrated value for the upper 10 cm of material.
At the same location, a plug of soil about 5-10 cm in diameter and 15 cm deep should be removed. The probe should be inserted in the hole and a measurement made that represents a bulk reading for a depth of 15-25 cm. The soil plug should then be returned to its original position. Subsequent measurements should be made near (0.5 m) the same location.
The following information should be recorded and sent to the CALM web site: