ENVIRONMENTAL FIELD TECHNIQUES (GEOG 2043): 
LAB 9

MEASURING SOIL MOISTURE

OBJECTIVES:
A.  To measure soil moisture
B.  To determine if there are spatial differences in soil moisture with position on a hill slope
C.  To produce a lab write up using a scientific writing format

BACKGROUND:  Even in a very dry soil there is a thin film of water held onto soil particles by the force of adhesion.  This water is too tightly bound to the soil to be available for plant up-take.  Plants can use water that is held  in pore spaces (between soil particles) by capillary force.  When all pore spaces are filled, the soil is saturated, and the force of gravity acts on the water to create downward flow though the soil.  Infiltration is the process by which water moves into and through a soil.  The infiltration rate is governed by the texture and existing moisture content.  The maximum rate at which the soil can absorb water at a given initial soil moisture content is the infiltration capacity.  If rainfall intensity is less than the infiltration capacity, then the infiltration rate will be equal to the rainfall rate.  When the rainfall rate is greater than the infiltration capacity, the excess water becomes surface runoff.  Surface runoff  is an important mechanism in landscape development and evolution.  Antecedent soil moisture conditions control the type and amount of surface flow related to a given precipitation event.  If a soil has a high moisture content and receives lots of moisture from a high intensity storm, surface runoff and possible surface erosion is likely.  Surface runoff and surface erosion is less likely for the same storm on a soil with a lower initial moisture content.  Measurements of soil moisture and infiltration rates are important for agriculture, and for monitoring drought or flood conditions.  Soil moisture content is also an important factor in hazard mitigation, as soil moisture content will control the speed at which a contaminant will seep into the ground.

There are numerous ways to measure soil moisture, and the technique you choose will depend on factors such as the question you are asking and the resources available.  Some techniques, like Time-Domain Reflectometery (TDR), allow for in situ soil moisture measurements and are relatively fast, while others, like gravimetric techniques, are more time consuming and destructive, requiring the removal and destruction of soil samples to determine soil moisture content.  In this lab you will use a gravimetric technique to determine spatial variations in soil moisture content along a transect from the crest of a ridge to the bottom of the hill slope.  You may also get a chance to use TDR, depending on ground conditions and on how temperamental the instrument is being.   Since soil and vegetation type effect soil moisture, it is important that you note these differences in your field notes.  This will help you interpret the data for your lab write-up.
 

SOIL MOISTURE MEASUREMENT METHODS:
Time-Domain Reflectometer (TDR):
Description: Time-domain reflectometer (TDR) determinations involve measuring the propagation of electromagnetic (EM) waves or signals. Propagation constants for EM waves in soil, such as velocity and attenuation, depend on soil properties, especially water content and electrical conductivity.  The dielectric constant, measured by TDR, provides an accurate measurement of soil water content and is essentially independent of soil texture, temperature, and salinity.
Measured Parameter: Volumetric water content aided by propagation of electromagnetic wave measurements
Response Time: Approximately 30 seconds per measurement
Disadvantages: Costly; moody instrument
Advantages:  Independent of soil texture, temperature, and salinity;  possible to perform long term in situ measurements; can be automated

Gravimetric method:
Description: The oven-drying technique is probably the most widely used of all gravimetric methods for measuring soil moisture, and is the standard for the calibration of all other soil moisture determination techniques. This method involves removing a soil sample from the field, and determining the mass of the water contained in a soil relative to the mass of dry soil. Although the use of this technique ensures accurate measurements, it also has a number of disadvantages.
Measured Parameter: Mass water content (percentage of dry vs. wet soil weight)
Response Time: Approximately 24 hours
Disadvantages:  Destructive test; time consuming; inapplicable to automatic control; must know dry bulk density and transform data to volume moisture content
Advantages:  Ensures accurate measurements; not dependent on salinity and soil type; easy to calculate

EQUIPMENT:
You will need:

• tape measures
• soil auger instruments
• permanent marking pen (Sharpie)
• box of gallon sized Zip-lock plastic bags
• field note book
• access to a soils lab

• Try to find a hill slope with homogenous vegetation and soil types.  This may be hard here in the Front Range but try!
• Run a tape measure from the crest of the ridge to the bottom of the hill slope and note the total distance.
• Starting at the top, select at least five locations down the slope where you will take soil samples.  Try to space these locations at regular intervals.
• Once you have five down-slope locations, take another tape measure and run it across the hill slope, perpendicular to the first tape.  From this secondary transect, select five subsampleing locations.
• Use the soil auger to sample the soil, placing all five subsamples into one Zip-lock, SQUEEZE OUT ALL THE AIR, and label as "composite-1; top of ridge" or something equally as informative.  Keep samples cool until they are analyzed.  Include the date, general location, and perhaps sampler's initials.
• Remember:  Take really good notes on sample locations, note the sampling scheme you are using, and note vegetation and soil type changes you notice.

moisture content (%) = ((wet weight - dry weight)/(wet weight weight)) X 100

• Title page: Title, your name, class, date
• Introduction: One to two paragraphs.  Specify the purpose of you lab, what you did, and what method(s) you used.
• Methods: One paragraph on the method(s) you used.
• Results: Present and describe your results.  Include a graph of distance from ridge top vs soil moisture, and perhaps a table that shows the soil moisture, distance, and notes on vegetation and soil types.
• Discussion: Two to four paragraphs that discuss your results.  What do your results indicate?  Do you see any trends in moisture content with location on the slope?   To what can you attribute the trend or lack of trend?  Are any changes due to differences in soil or vegetation type?
• Literature cited:  List any references you used.  See a journal article for proper citation format.
• Note: Neatness, completeness and presentation count!