Open Journal of Soil Science, 2012, 2, 91-94
http://dx.doi.org/10.4236/ojss.2012.22013 Published Online June 2012 (http://www.SciRP.org/journal/ojss) 91
A Technical Note: Orientation of Cracks and Hydrology
in a Shrink-Swell Soil
Takele M. Dinka1, Robert J. Lascano2*
1Department of Soil and Crop Sciences, Texas A&M University, College Station, TX, USA; 2USDA-ARS# Cropping Systems Re-
search Laboratory, Wind Erosion and Water Conservation Research Unit, Lubbock, TX, USA.
Received March 23rd, 2012; revised April 25th, 2012; accepted May 4th, 2012
ABSTRACT
Crack orientations are an important so il physical property that affects water flow, particularly in vertic soils. However,
the spatial and temporal variability of crack orientations across different land uses and gilgai features is not well-
documented and addressed in hydrology models. Thus; there is a need to quantify crack orientations for different land
uses and to incorporate their spatial and temporal dynamics into hydrological models. Our objectives were to document
the spatial variability of cracks orientations across two land uses and to demonstrate the potential importance of crack
orientation related to the hydrology of Vertisols. The exploratory field measurements of the spatial distribution of crack
orientations across two Vertisol catenae of two land uses and gilgai features are presented. The field survey showed the
complexity of crack geometry in a field, the potential impact of crack orientation on Vertisol hydrology and the chal-
lenges associated with measurement of crack orientations.
Keywords: Crack; Orientation; Hydrology
1. Introduction
Vertisols that shrink while drying and swell while wet-
ting can damag e building fou ndations, ro ads, utilities and
septic tanks. The management of Vertisols for agricul-
tural production such as fertilizer use, crop selection, so il
tillage, irrigation, and soil erosion is more problematic
compared to other soil group [1,2]. When shrink-swell
soils dry, cracks are formed, and these cracks facilitate
rapid transport of surface water into the sub-soil through
preferential flow. Furthermore, rapid transport of surface
water to the sub-soil reduces runoff and enhances flow of
chemicals to sub-soils and ground waters [3-5]. Hence,
the frequency, size and rate of crack development influ-
ence the transport of water, nutrients and gases in the so il
profile and plant growth processes in Vertisols [3,4].
Therefore, measurement of soil cracks is important to
monitor not only surface and sub-surface flow of water,
but also to monitor flow of gases in the soil-atmosphere
continuum, and to understand how roots grow and their
penetration pattern in the soil.
The importance of studying cracks and its field of ap-
plication is well documented by several researchers, e.g.,
[3-7]. Soil cracks affect several components of the water
balance, i.e., infiltration, drainage and runoff, therefore
impacting the hydrology of the soil. For example, soil
cracks enhance rapid flow of water into sub-soils [4] and
they increase the infiltration rate of water impacting sur-
face runoff [8]. The main soil physical property that is
associated with cracks is shrinkage and the associated
crack formation that impacts water flow is crack depth,
rate of development, crack area density and orientation.
Additional information on challenges and limitations in
understanding the shrink-swell and crack dynamics of
Vertisol soils is given by Dinka and Lascano [9].
Most studies of shrinkage and cracking of Vertisols
have focused on the size and areal density of cracks, e.g.,
[4,10-12] and not on the orientation of cracks. As a result,
hydrology models that account for the orientation of
cracks are not available because the genesis and forma-
tion of cracks orientations and their spatial variability
across different land uses are poorly understood. The ori-
entation of cracks can impact and affect the surface flow
and capture of water. For example, cracks that are paral-
lel to the direction of overland flow of water might cap-
ture less water than cracks that are perpendicular. How-
ever, current hydrological models like Soil Water As-
*Corresponding a uthor.
#The U.S. Department of Agriculture (USDA) prohibits discrimination
in all its programs and activities on the basis of race, color, national
origin, age, disability, and where applicable, sex, marital status, familial
status, parental status, religion, sexual orientation, genetic information,
p
olitical beliefs, reprisal, or because all or part of an individual’s in-
come is derive d from any public assis tance program.

A Technical Note: Orientation of Cracks and Hydrology in a Shrink-Swell Soil 92
sessment Tool (SWAT) [3] do not incorporate this crack
property, which may result in misrepresentation of water
flow in a Vertisol. The orientation of cracks may also
vary spatially with presence and shape of a Vertisol fea-
tures such as gilgai. In the Blackland Prairie of Texas,
the presence of linear and circular gilgai is reported [13].
Therefore, the spatial variability o f crack orientation on a
land with and without gilgai and with linear and without
circular gilgai can also have a different impact on the
flow water in the soil. However, no attempt has been
done to demonstrate the spatial variability of cracks ori-
entations in a field and their associated impact on the
hydrology of the soil. The objective of this technical note
was two fold. First, to investigate the variability of crack s
orientation across two land uses, and second, to demon-
strate the importance of crack orientation in studying the
hydrol ogy o f Vertisols.
2. Methodology of the Survey
The survey was conducted at the USDA-ARS Grassland,
Soil and Water Research Laboratory near Riesel, TX.
The climate is warm and sub-humid with a mean annual
rainfall of 910 mm. Two watersheds with different land
use systems were selected for the survey of crack orien-
tation. The land use types were native prairie and grazed
pasture. The dominant soil in the area is Houston Black
(Fine, smectitic, thermic Udic Haplusterts) that consists
of very deep, moderately well drained, very slowly per-
meable soils formed from weakly consolidated calcare-
ous clays and marls of Cretaceous age [14]. The domi-
nant vegetation in the native prairie is little Bluestem
(Schizachyrium scoparium) grass and in the grazed pas-
ture is Costal Bermuda (Cynodon dactylon) grass. The
average slope of the native prairie and grazed pasture is ~
5% and 2%, respective ly (Figure 1 and Figure 2).
A survey was conducted on 11 August 2009, when
there were many large cracks in the soil. Three slopewise
transects were selected for the survey in each watershed
(Figure 1 and Figure 2). The final destination of the
transect survey was at the outlet of the watershed. The
length of the transect lines ranged from 100 - 120 m in
the grazed pasture (1.5 ha) and from 110 - 125 m in the
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.

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,

A Technical Note: Orientation of Cracks and Hydrology in a Shrink-Swell Soil 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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