Evaluation of the GPR method for estimating soil moisture contents
in Puerto Rico
INTRODUCTION
Soil moisture content is a critically important parameter used in the analysis of various
scientific disciplines. Information about soil moisture is important for managing water
resources, especially in the area of agricultural water management, in which large quantities of
water are used for irrigation. Soil moisture status is essential for climate change studies, and for
conducting soil water balances. Soil moisture data is necessary for parameterizing numerical
models, which are used to estimate evapotranspiration from land cover, and deep percolation for
groundwater impact studies. Numerical models for predicting soil water movement are also
important in chemical transport studies.
Currently, investigators obtain soil moisture content by intrusive methods, which are limited
to point measurements. Interpolation of the point-measured moisture data can lead to errors
from interpolation within the nonlinear soil moisture regime. Ground-penetrating radar (GPR)
may provide a means of eliminating interpolation error because it can provide spatially
continuous estimates of soil moisture in multi-dimensions. The purpose of this study is to assess
the potential for using GPR for measuring soil moisture in seven Puerto Rico Soils. The selected
soils are representative of soil in the tropics, and therefore, may provide information generally
applicable to the highly weathered clayey soils common among tropical islands.
OBJECTIVES
GPR measurements will be taken at tests plots located at seven University of Puerto Rico
Experiment Stations. Specific objectives include:
1. Conduct literature study related to the theory and use of GPR for measuring soil moisture
content in fine textured soils.
2. Establish the test areas at each location.
3. Analyze soil for chemical and physical properties.
4. Obtain GPR and TDR measurements and gravimetric soil samples along the GPR
transects for comparison purposes.
5. Elucidate the best approach for conditioning the signal to obtain accurate values of
moisture content with the GPR.
6. Publish results.
MATERIALS AND METHODS
Objective 1: A literature search will be conducted to determine the most appropriate
frequencies to use for soil moisture measurement in fine textured soils. The literature review
will also consider potential problem related to moisture measurement in these soils.
Objective 2: Test areas will be selected at each of the seven UPR Experiment Stations. Figure 1
shows the locations of the tests sites and the NOAA Climate Division they are contained within.
Table 1 lists test site locations and the predominant types present.
Isabela
Rio Piedras
Corozal
Gurabo
Adjuntas
Lajas
Fortuna
Figure 1. Test site locations and Climatic Divisions in Puerto Rico as defined by NOAA.
Table 1. Test site locations, soil types and Climate Zones defined by NOAA.
Test Site
Soil Type
Climatic Zone*
1. Adjuntas
Toa Silty Clay Loam (Mollisol)
2. Corozal
Corozal Clay (Ultisol)
3. Fortuna
San Antón Clay Loam (Molisols)
4. Gurabo
Gurabo Clay (Inceptisol)
5. Isabela
Coto Clay (Oxisol)
6. Lajas
Fraternidad Clay
7. Mayaguez
Daguey Clay (Ultisol)
8. Rio Piedras
Rio Piedras Clay
* see Figure 1 for Climate Zones in Puerto Rico
Western Interior
Western Interior
South Coastal
Eastern Interior
North Coastal
South Coastal
Southern Slopes
North Coastal
The test areas need to have the following characteristics:
 Level topography
 Laterally homogeneous soil material
 Access to a irrigation water supply
Objective 3: Soils will be collected at each site for chemical and physical evaluation.
Undisturbed samples will be collected from 0 to 25 cm, 25 to 50 cm, 50 to 75 cm and 75 to 100
cm. Soil samples will be analyzed for pH, Point of Zero Net Charge, Anion Exchange Capacity,
Cation Exahange Capacity, exchangeable cations, organic C, free iron-alluminium oxides, and
electrical conductivity. The physical properties that will be determined are texture, aggregate
stability, bulk density, porosity, saturated hydraulic conductivity, specific surface of both the soil
and the clay fraction, and the soil characteristic curve (suction vs. moisture content).
Objective 4: Soil moisture content will be measured to a depth of 1 meter using a GPR at
locations where the vertical soil moisture distribution is known. This will be accomplished by
collecting spatially continuous GPR data along transects adjacent to TDR access tubes and
gravimetric soil sampling locations. The TDR (Time Domain Reflectometry) method is being
used because TDR is based on a similar principle of operation as the GPR method. Soil samples
taken adjacent to the TDR access tubes will provide the “true” moisture content for comparison
and calibration of the other methods. The experimental layout is shown in Figure 2.
Lines of Uniform
Sprinkler Application
Rate
GPR Transect
Sprinkler
TDR
Access Tube
Figure 2. Experimental Layout
The study area will consist of four groups of TDR access tubes aligned along two
perpendicular axes. The access tube spacing will increase exponentially from the inner tubes to
the outer-most tubes. At the center point, a sprinkler will be installed to apply water to the
surface as needed. Water from the sprinkler will be applied in a pattern similar to the
precipitation pattern shown in Figure 3. Note that the maximum depth of water is applied at the
sprinkler location and gradually decreases to zero at the outer edge. If the soil is relatively dry
before starting the test, the application distribution (Figure 3) produced by the sprinkler will
provide a range of moisture contents to be measured. The set-up shown in Figure 2 will provide
four groups of access tubes which will receive the same depth of water, and consequently, the
vertical moisture content distribution will be approximately identical as well. The area will be
carefully selected so that the soil is laterally homogeneous and will be free of vegetation.
Vertical soil variation may exist, and in fact, is desirable so that the GPR method can be tested
with several soil textures. During the study, water will only be applied during periods of low
wind speed (< 1 m/sec) so as to maintain a symmetric application of water over the circular area.
Figure 3. Distribution Pattern and Precipitation Profiles from a Typical Double Nozzle Sprinkler
under Fvorable Conditions (From Keller J. and R. Bliesner, 1990).
There will be four sampling events conducted over a one year period. During a sampling
event, the GPR will be moved along the transects indicated in Figure 2. TDR measurements will
be made at four depths within each access tube. Soil samples will be taken at four locations
along each transect as well at the four depths and analyzed for moisture content. The soil
samples will be weighed to determine the wet weight. The samples will then be dried in an oven
at 105 degrees C for at least 24 hours, after which the dry weight will be obtained. The moisture
content by mass will be obtained by taking the difference between the wet and dry weight
divided by the dry weight. The value multiplied by the soil bulk density and divided by the
density of water (assumed to be 1 g/cc) is the volumetric moisture content. The GPR and TDR
measurements and soil samples will be taken on the same day within several hours of each other.
The holes left by the soil sampling will be carefully backfilled.
Prior to starting the sampling events, the TDR will be calibrated for the soil conditions.
Based on our experience in using TDRs in Puerto Rico, it will be necessary to calibrate each
distinct soil horizon. The calibration will be made within the experimental areas, but away from
the locations of the access tubes and GPR transects. The calibration tests will be performed by
collecting soil samples next to five access tubes, at up to four depths corresponding to the
vertical TDR measurements. The calibration will be performed over a one to two month period.
This will allow a range of soil moisture contents to be achieved to be used for the calibration
curves.
As part of the TDR calibration, soil samples will be analyzed by the gravimetric soil moisture
content method. The moisture contents by weight (w) will be converted to the volumetric
moisture content by multiplying by the soil bulk density and dividing by the density of water.
Bulk densities will be obtained from the analysis of undisturbed soil cores collected at up to six
depths and at the five access tube locations.
Objective 5. HAMED YOU WILL NEED TO WRITE THIS SECTION
Objective 6. The results from this study will be presented at professional meetings and published
in peer reviewed journal.
HAMED – I WILL START TO WORK A BUDGET. THE ABOVE PROPOSAL IS FOR THE
ENTIRE EFFORT. I SUPPOSE I SHOULD WORK UP A TWO OR THREE YEAR BUDGET,
BUT THEN WE WOULD ONLY MAKE A REQUEST FOR THE 1ST YEAR, RIGHT?
THERE ARE TWO THINGS I WOULD LIKE TO DISCUSS WITH YOU.
1. MAYBE SEVEN LOCATIONS IS TOO MANY. WHAT I HAVE PROPOSED
WOULD BE NICE BUT IT REPRESENTS A LOT OF WORK. MAYBE TOO MUCH.
2. I HAVE ACCESS TO A NEUTRON PROBE. AS I MENTIONED TO YOU BEFORE,
WE HAVE HAD GREAT DIFFICULTY CALIBRATING THE TDR. SINCE THE
NEUTRON PROBE WORKS BY MEASUREING THE BACKSCATTER OF
NEUTRONS DUE TO THE PRESENCE OF HYDROGEN (PRIMARILY PRESENT
AT H2O), IT WILL PROBABLY PROVIDE A MUCH MORE ACCURATE
MEASURE OF MOISTURE CONTENT.