FE 537 Hillslope and Watershed Hydrology
Lab 1: The plot scale
Instructor: Chris Graham
Motivation
This lab is designed to provide some hands on experience with plot scale hydrological
processes. The experiment will first be demonstrated in the lab and then will be repeated
by each small group. This is a sprinkling experiment on a series of large (12 inch
diameter) soil cores with well characterized coarse sand from Schroth et al. (1996). Four
core types are provided, with different densities of artificial macropores embedded in the
matrix material. The experiments are designed to test the effects of preferential flow on
infiltration, flow and transport in 1D.
Learning Objectives
1) Observe core scale water and tracer fluxes.
2) Build a water content / unsaturated hydraulic conductivity curve
3) Determine macroporosity effects on tracer velocity and dispersion at core scale.
Activity (to be discussed in a hands-on lecture in the lab):
1) Measure dimensions of soil cores
2) Sketch experimental setup
3) Turn on Soil Moisture Probes
4) Install EC probe into collection cup
5) Turn on irrigation - Record time sprinkler on
6) Measure core discharge rate
7) When discharge is steady, add tracer cocktail. Record time tracer added
8) Record EC at outlet. Continue recording until EC is “near” background
9) Record final core discharge rate
10) Repeat steps 7 - 9 at other 3 cores
Suggested Reading
Janice McIntosh, Jeffrey J. McDonnell and Norman E. Peters. Tracer and hydrometric
study of preferential Flow in large undisturbed soil cores from the Georgia Piedmont,
USA. Hydrological Processes vol. 13, pp 139-155, 1999.
M. H. Schroth, S. J. Ahearn, J. S. Selker, and J. D. Istok. Characterization of MillerSimilar Silica Sands for Laboratory Hydrologic Studies. Soil Science Society of America
Journal, vol. 60, pp 1331-1339, 1996.
JH Horton and RH Hawkins. Flow path of rain from the soil surface to the water table
Soil Science, vol. 100(6), pp 377–383, 1965.
LW Gelhar, C Welty, and KR Rehfeldt. Critical Review of Data on Field-Scale
Dispersion in Aquifers. Water Resources Research, vol. 28(7) pp. 1955-1977, 1992
Questions/Assignment
1) Fill in Table 1 and email results to chris.graham@oregonstate.edu
2) Plot the tracer breakthrough for each core.
a) Calculate the dispersion coefficient for each core.
i) The dispersion coefficient can be estimated as the standard deviation of the
breakthrough curve if the breakthrough is symmetrical (Gaussian). Use the
portion of the breakthrough from initial rise until EC is “near” background
(Excel function STDEV). What is the skew (Excel function SKEW)? Is the
Gaussian assumption met?
b) How does the breakthrough change with increased macropore density?
i) How does macroporosity affect peak concentration?
ii) How does macroporosity affect tailing?
3) How do your unsaturated hydraulic conductivities compare with the unsaturated
hydraulic conductivity found by Schroth et al? Why is your result the same/different?
4) Using the values from classmates, build a soil moisture / hydraulic conductivity curve.
How does this curve look different than that of Schroth et al? How would this
difference affect soil response in the field?
Table 1 – Experiment Results
Core 1
Soil Type
Macropore Density
(MP/m2)
Precipitation Rate / Steady
State Discharge (L/min)
Time to Initial Tracer
Breakthrough (min)
Time to Peak Tracer
Breakthrough (min)
Mean Travel Velocity
(cm/min)
Dispersion Coefficient
Tracer Skew
Background Water
Content (m3/m3)
Steady State Water
Content (m3/m3)
Core 2
Core 3
Core 4