/div>
native prairie (1.4 ha) water sh eds.
The orientations of the cracks were categorized as par-
allel, perpendicular, or irregular (neither parallel nor
perpendicular) with respect to the slope of the land and
flow direction of runoff. A crack ( 10 mm) was consi-
dered parallel when it followed the direction of runoff
flow; and perpendicular when it was parallel to the con-
tour, both within ± 30 degrees of tolerance; and irregular
when it was neither parallel nor perpendicular. Finally,
the difference and similarities in cracks orientations
among and within the land use types were compared.
Figure 1. Topographic map of a native prairie with a 0.25 m
contour, located at the USDA-ARS Blackland, Soil and
Water Research Laboratory, Riesel, TX. Letters A, B and C
indicate the location where surveys started and lines indi-
cate transects of the survey.
Figure 2. Topographic map of the Grazed pasture with a
0.25 m contour, located at the USDA-ARS Blackland, Soil
and Water Research Laboratory, Riesel, TX. Letters A, B
and C indicate the locations where the survey started and
lines indicate transects of the survey.
3. Field Observation
The number of large cracks observed on the native prai-
rie and grazed pasture from the three transects were 53
and 58, respectively (Figure 3). Among the 53 cracks in
the native prairie, 61% were oriented parallel to the flow
direction, 28% were oriented perpendicular to the flow
direction and the remaining 11% were classified as irre-
gular to the flow direction. In the grazed pasture the 58
cracks had a distribution of 48% parallel, 45% perpen-
dicular and 7% irregular.
Copyright © 2012 SciRes. OJSS
A Technical Note: Orientation of Cracks and Hydrology in a Shrink-Swell Soil 93
Figure 3. Distribution of irregular, perpendicular and par-
allel crack orientation on a native prairie (NP) and grazed
pasture (GL) at the USDA-ARS Blackland, Soil and Water
Research Laboratory, Riesel, TX. The survey was con-
ducted on 11 August 2009.
The survey showed that most cracks, 61%, were ori-
ented parallel to the direction of flow in the native prairie.
However, number of parallel (48%) and perpendicular
(45%) cracks was not considerably different in the
grazed pasture. In both lands uses, the frequency of occu-
rrence of irregular cracks was less compared to parallel
and perpendicular cracks. In the native prairie, the per-
cent of irregular cracks was 11% and 7% in the grazed
prairie. The difference in orientations of cracks among
the land use types could be due to not only the differ-
rences in vegetation cover, but also to the inherent size
and shape of gilgai microhigh and microlow topographic
and subsurface features. In both land uses, cracks that
oriented parallel were mostly observed on the “lower
points” of the gilgai microtopography where runoff
would flow.
Two major types of gilgai were observed in the field:
circular and linear. Circular gilgai is a Vertisol feature
common in the Texas Gulf Coast Prairie, while linear
gilgais are formed on sloping land [15]. The gilgais in the
native prairie were very elongated (up to 10 m length and
0.50 to 1 m width) in the direction of a slope that formed
a natural channel for flow of runoff. The shape of gilgais
in the grazed pasture was circular with a diameter of up
to 3 m and a depth of up to 0.10 - 0.20 m. The existence
of parallel cracks in slopewise elongated microlows,
where runoff would flow, shows the possible influence of
cracks orientation on flow and distribution of water in a
vertic watershed. In contrast, a crack orientation in a cir-
cular microlow may not considerably affect the amount
of runoff generated from a vertic watershed. This is be-
cause the circular gilgai, regardless of the orientation of
cracks formed inside, would capture water. The influence
of crack orientation on runoff and water distribution de-
pends on the existence and type of microlows. Cracks
that oriented parallel to the runoff direction would likely
enhance more surface runoff as compared to cracks ori-
ented horizontally. However, since parallel cracks likely
trap greater volume of surface runoff, the volume of sub-
surface runoff and soil water distribution around the hori-
zontally oriented cracks could be high. Therefore, the
commonly observed spatial variability in occurrence of
gilgai [16] and orientations of cracks need to be ad-
dressed in a study of hydr ology of shrink-swell soils.
4. Challenges in Quantifying Orientation of
Cracks
Attempts have been made to study crack geometry using
photography in a laboratory [17] and in a field [18] set-
ting and a review on problems in studying the shrink-
swell and crack dynamics of Vertisol soils is given by
Dinka and Lascano [9]. The photographic technique has
the advantage that data can be easily and continuously
acquired and it is nondestructive. However, this tech-
nique does not provide other crack information such as
depth of cracks and it is difficult to get a quality data
when the land has vegetation. The direct measurement of
crack orientation in a field is another technique that is
available to measure crack geometry but its application is
challenging, especially on a wide area. First, categorizing
the direction of a crack is subjective, especially when it is
neither clearly parallel nor perpendicular. Second, apart
from the presence of gilgai, the existence of any other
microtopographic feature on the land has to be consid-
ered to classify the direction of the crack orientation and
deciding whether the influence of the microtopography is
important or not. If it is important, categorizing the di-
rection of cracks should not be based on the general
slope direction of th e entire land bu t rather the classifica-
tion should follow the slope of the microtopography be-
cause the aim of the categorization is to determine their
impact on runoff. This shows that the decision is site-
specific and also based on the flow direction of a runoff
on that particular site. In addition, quantifying the con-
tribution of minor cracks that might be part, i.e., branch
to a majo r crack also rep resents a ch allenge to cat egorize
the orientation of the cracks. Use of a survey quality
geographic positioning system (GPS) to measure crack
location would help reduce these problems.
5. Conclusion
The spatial variability of cracks orientations across two
land uses was investigated and the potential importance
of crack orientation and impact on the hydrology of Ver-
tisols was demonstrated. Results from this study showed
that crack orientation has an impact on the amount of
runoff; hence, hydrological models should incorporate
not only the size, depth and density of cracks but also
their orientation. Results also pointed out some of the
challenges associated with a study of crack orientations,
Copyright © 2012 SciRes. OJSS
A Technical Note: Orientation of Cracks and Hydrology in a Shrink-Swell Soil
Copyright © 2012 SciRes. OJSS
94
and suggested the importance of developing simple and
practical guidelines to determine crack orientation or
other techniques that could capture all the necessary
crack information such as the size, depth, density and
orientation of cracks needs to be used. Further studies are
necessary to understand whether there is a sp atial pattern
to crack orientations and whether there is a trend on the
distance be tween wid e cracks.
6. Acknowledgments
The Texas AgriLife Research, and a Cooperative Agree-
ment with the USDA NRCS Texas Soil Survey and Na-
tional Science Foundation Grant No. EAR 0911317 in
part supported this work.
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