How to Perform Your Soil Moisture Investigation
Part III/2: The Soil Moisture Protocols

Skill Levels

Beginning, Intermediate and Advanced

Purpose

To monitor the gravimetric water content of the soil.

Overview

A) All students will collect near-surface soil samples using grade appropriate sampling procedures,
B) A weight-based or gravimetric technique is used to determine soil wetness,
C) This data will be reported to the GLOBE data archive,
D) Additional calculations and analysis can be done by older students.

A) Advanced students will install the gypsum block sensors,
B) The blocks must be calibrated for your specific soil conditions,
C) Daily meter readings are recommended,
D) Locations with wet soils may need to do a lab or bottle soil calibration.

Time

Once you have learned the procedure, the sampling portion of this protocol should take about 20 minutes.

Frequency

Monthly (required); Daily (optional gypsum blocks)

Key Concepts and Skills

Concepts

• Soil moisture is directly related to precipitation, but decreases with time.
• Soil moisture is highly variable in space and time
• Soil moisture interacts with the atmosphere and biosphere
• The amount of water in a soil depends on site specific characteristics such as: precipitation, soil properties, biology and solar heating.
• Water held in soils are a critical element of the hydrologic cycles and affect air temperature, plant growth and flooding.
• Near-surface soil moisture can be related to satellite microwave data.
• Deeper soil moisture can be related to plant growth.

  Skills

• Observing and sampling
• Data recording and analysis

Preparation

• Start the introductory soil moisture activities,
• Locate the soil moisture site and
• Select the sampling strategy to use.

Overview of the Measurements

Study Site for the Investigation

Beginning students will make measurements in a star-shaped pattern, with samples obtained at different locations on the star each time. The soil samples will include soil from the bare surface to a depth of 5 cm. Each month one sample will be collected and every third month (seasonally), two additional samples will be acquired for quality control purposes.

Intermediate students will make measurements from locations along a 50-100 m line, called a transect. Students will measure soil moisture every 5-10 m along this line. The soil samples will include soil from the bare surface to a depth of 5 cm. Each month 5-10 samples will be collected and every third month (seasonally), two additional samples will be acquired at one station for quality control purposes.

Advanced students will make measurements in a star-shaped pattern, with samples obtained at different locations on the star each time. The soil samples will be collected at four different depths (10, 30, 60 ,90 cm). To begin, these depth profiles should be taken near your climate station in order to study the relationship between rainfall/snowfall and soil moisture.

OPTIONAL: Your most advanced students can make measurements of soil moisture and temperature using sensors they have buried in the ground. The sensors are easy to read using hand-held electronic meters, and simplify collecting lots of data. However, it takes a great deal of work to calibrate these sensors. Instructions for these procedures are in the Appendix.

Frequency

Make your observations at the same time every day, and avoid early morning when there is dew present. Soil moisture changes slowly, so the time of observation is not critical.

Improving Your Data: It is useful to take multiple samples occasionally to assess data quality and sample uniformity. You can do this by taking three samples within 10 cm of each other where you would normally take one. We recommend you collect triplicate samples at least seasonally. For consistancy, collect triplicates close to the first of the month in fall, winter, spring and fall. Making a collection schedule for the classroom would help remind you!

Pre-Protocol Preparation: Locating Your Site

• Unirrigated: Since we most want to investigate the soil's response to the sun and natural precipitation, it is important that your site not be irrigated.

• Uniform: Soil moisture can vary significantly across short distances - the challenge here is to find an area where the soil moisture you measure is representative of your site. Look for a place that has uniform soil properties, vegetation, water table and slope (a relatively flat site is best).

• Relatively undisturbed: Keep at least 3 m from buildings, roads, paths, playing fields or other sites where the soil may be compacted or disturbed by people.

• Safe for Digging: Check with local utility companies and site maintenance staff to be sure that you will not be digging into or disturbing a utility cable or sprinkler irrigation system.