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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L24S24, doi:10.1029/2007GL030860, 2007
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Soil and topographic controls on runoff generation from stepped
landforms in the Edwards Plateau of Central Texas
Bradford P. Wilcox,1 Larry P. Wilding,2 and C. M. Woodruff Jr.3
Received 31 May 2007; revised 9 July 2007; accepted 31 July 2007; published 12 December 2007.
[1] The rugged Hill Country of Central Texas is part of the
extensive Edwards Plateau region. Significant portions of the
Texas Hill Country overlie the Glen Rose Formation, which is
characterized by a stair-step topography formed by the
weathering of interbedded carbonate materials having
different weathering susceptibilities. This process has
sculpted the strata into a series of ‘‘risers’’ and ‘‘treads’’
that mimic stairways. In this paper, we document the soil
hydrology within the riser –tread catena. Our results are
counterintuitive in that we find the highest infiltration and
deepest soils on the steep riser slopes. In addition, we find
that the riser subsoils are saturated or very wet for extended
periods. On the basis of these findings, we suggest that (1)
groundwater recharge on these hillslopes is minimal and
occurs only in highly fractured zones; (2) the water-holding
capacity of the subsoils is sufficient for supporting the woody
vegetation; and (3) runoff generation occurs as a combination
of surface and subsurface flow, with the risers serving as
sinks or recharge zones and the treads as source areas.
Citation: Wilcox, B. P., L. P. Wilding, and C. M. Woodruff Jr.
(2007), Soil and topographic controls on runoff generation from
stepped landforms in the Edwards Plateau of Central Texas,
Geophys. Res. Lett., 34, L24S24, doi:10.1029/2007GL030860.
1. Introduction
[2] It is often assumed that in rangelands of subhumid to
dry climates, runoff generation is exclusively Horton overland flow—that is, runoff occurs only when rainfall exceeds
the infiltration capacity of unsaturated soil. In many of these
rangelands, however, runoff generation can be more complex. For example, in the United States frozen soil runoff
[Seyfried and Wilcox, 1995], saturation overland flow
[Lopes and Ffolliott, 1993], and shallow subsurface flow
[Wilcox et al., 1997; Wilcox et al., 2005] have all been
documented. In addition, on rangelands where springs
occur, there is a groundwater flow component to runoff
[Huang et al., 2006].
[3] Runoff generation has been found to be especially
complex on rangelands underlain by limestone bedrock
[Wilcox et al., 2006]. Limestone and other carbonate rocks
in the Edwards Plateau proper often display solution features and/or fractures that facilitate the subsurface flow of
water. Also, soils on these landscapes often have relatively
1
Ecosystem Science and Management, Texas A&M University, College
Station, Texas, USA.
2
Soil and Crop Sciences Department, Texas A&M University, College
Station, Texas, USA.
3
Woodruff Geologic Consulting, Austin, Texas, USA.
Copyright 2007 by the American Geophysical Union.
0094-8276/07/2007GL030860$05.00
high permeability. Karst landscapes, areas in which dissolution of bedrock is one of the dominant geomorphic
processes, occupy 10 – 20 percent of the earth [Palmer,
1991]. Although extensive areas of carbonate and karst
terrains exist within subhumid and semiarid landscapes,
we know relatively little about how runoff is generated
from these landscapes.
[4] One of the more important karst rangelands in the
United States, and perhaps in the world, is the Edwards
Plateau region in Central Texas. In spite of the relatively dry
climate of this region, the margins of the plateau uplands are
drained by springs that feed perennial streams [Woodruff
and Abbott, 1979]. Even so, water is becoming a limited
resource as population pressures increase—which makes
the need to better understand runoff generation and its
relationship to vegetation, soil, and geologic properties even
more urgent.
[5] The Edwards Plateau region is bordered on the south
and east by the Balcones Escarpment and encompasses two
major physiographic features—the Texas Hill Country and
the Edwards Plateau proper (Figure 1). The Plateau tablelands are capped with a thick sequence of Cretaceous
limestones, consisting mostly of the Edwards Group
[Barnes, 1992]. Following the main displacement events
of the Balcones Fault Zone, the eastern and southern
margins of this limestone cap progressively eroded, forming
the Hill Country [Anaya, 2004; Woodruff and Abbott, 1979].
The principal bedrock unit underlying most of the Hill
Country is the Glen Rose Formation, a stacked, repetitious
sequence of limestone and dolomitic beds [Barker and
Ardis, 1996; Stricklin et al., 1972] with varying degrees
of weathering potential [Woodruff and Wilding, 2007]. It
thus exhibits a distinctive ‘‘stair-step’’ topography. The
more resistant layers (underlain by carbonate rocks having
generally homogeneous fabrics) form gently sloping
‘‘treads.’’ The intervening ‘‘risers’’ are underlain by limestone that (when fresh) is well indurated, with low porosity,
but they exhibit heterogeneous fabrics consisting of fossils
and myriad clay partings. These interbeds weather rapidly
when exposed to water (Figure 1). The soil sequence thus
produced is a counterintuitive one: the deepest soils are on
the risers and the more shallow soils are on the treads
[Wilding, 1993, 1997; Woodruff and Wilding, 2007]. Riser
soils comprise an upper depositional zone consisting of
colluvium and redeposited pedisediments and a lower
(inner) residual soil/regolith.
[6] Significant soil areas of this landscape have been
mapped in the Brackett series, which has been described as
thin, strongly calcareous, and exhibiting only slight pedogenic development [Werchan et al., 1974]. Wilding, however, has taken exception to this characterization [Wilding,
1993, 1997], arguing that these soils are often deeper, better
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Figure 1. (top) The major physiographic regions in central Texas including the Hill Country, Edwards Plateau, and the
Llano uplift. (middle) The stair-step topography that is typical for the Glen Rose limestone in the Texas Hill Country.
(bottom) Common features of soils along the riser-tread continuum.
developed, and richer in organic matter than previously
thought; and that the soil patterns change in predictable
ways depending on the topographic setting. He noted
[Wilding, 1997] that the map unit design needed to be
changed because the landscape was composed not as a
single soil (monotaxa consociation), but as a complex
assemblage of different soils that vary greatly in depth
and are found as disjunct bodies linked together in a
systematic pattern—from deep (on the upper risers) to
moderately deep (on the mid risers) to shallow (on the
lower risers) to very shallow (on the treads) (Figure 1).
[7] Soils on the risers are generally classified as Udic
Calciustolls, or occasionally as Petrocalcic Calciustolls, and
range in thickness from 1 to 3 m. They are developed in a
thin veneer of limestone/dolomite-rubble colluvium, which
serves as the parent material for organic-matter-rich, gravelly A horizons. These are uncomformably superposed over
partially weathered in-situ materials in which pedogenically
enriched carbonate horizons (calcic or petrocalcic) are
formed. In the common parlance this carbonate-rich layer
is called ‘‘caliche.’’
[8] On the gently sloping treads, soils are thinner (<0.5 m)
and classified as Lithic Haplustepts, Lithic Calciustepts,
Lithic Calciustolls, and Lithic Petrocalcic Calciustolls. Generally lighter in color, they contain less organic carbon than
the riser soils. But light soil colors belie the fact that even on
treads surface horizons commonly have more than 2.5%
organic carbon. The sola are formed in thin, loamy-gravelly,
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carbonatic pedisediments derived as erosional products
from upslope transport. These sediments are admixed with
weathered material or superposed directly over the hard
bedrock, in which many of the joint planes are partially
plugged with pedogenic calcite cement.
[9] Live oak– juniper woodlands have been present in the
Hill Country for millennia, but have probably become
more extensive and denser in the last century [Amos and
Gehlbach, 1988; Huxman et al., 2005; Wilcox and Thurow,
2006]. An ecological question of considerable importance—and one that continues to spark debate—is the
source of the water for these trees. A prevailing hypothesis
has been that the soils in the region are thin and lack the
water-holding capacity needed to support woody vegetation, and therefore the trees must be routinely accessing
water from deeper groundwater sources [Jackson et al.,
1999]. Alternatively, the ‘‘caliche’’ residuum is another
source of water that has been little appreciated [Duniway
et al., 2007; Hennessy et al., 1983].
[10] Relatively little is known of the dynamics of runoff
generation in this landscape. Small-plot infiltration studies
on the Edwards Plateau proper have repeatedly demonstrated that these soils have inherently high infiltration capacities, especially where vegetative ground cover is good
[Knight et al., 1984; Thurow et al., 1986]. For the Central
Texas Hill Country the hillslope hydrology of the tread/riser
slopes have yet to be comprehensively investigated. Largerscale rainfall simulation studies in the Edwards Plateau
region [Munster et al., 2006; Taucer, 2006], including some
at the small-catchment scale [Huang et al., 2006; Wilcox et
al., 2005], indicate that runoff can occur both as overland
flow and as subsurface flow. The purpose of this paper is
to evaluate and describe the hydrological properties—
including infiltration, runoff generation, and soil wetness—
associated with the soil continuum of the tread/riser topographic sequence.
2. Methods
[11] From 1991 to 2006, more than 100 trenches were
excavated by backhoe at three locations in the Central Texas
Hill Country of Travis County, Texas [Woodruff and Wilding,
2007]. The trenches, each 10 –15 m long, were excavated
down to indurated bedrock perpendicular to the slope,
incorporating a riser and tread element with horizontal
dimensions up to 30 m. At selected trench locations, soil
infiltration capacity, runoff and erosion, and soil matric
potential were determined.
2.1. Infiltration
[12] Soil infiltration capacity was estimated using a
portable, drip-type infiltrometer similar to the one described
by Blackburn et al. [1974]. The raindrop-producing module,
which measures 0.5 m2, is constructed from a pair of
plexiglass plates spaced about 1 cm apart, sealed with
caulking compound, and bolted to aluminum angle. The
module has more than 1000 hollow steel needles, evenly
spaced, for producing the raindrops.
[13] Runoff plots were constructed by inserting flexible
metal frames into the soil, each attached to a water collector
at the base of the plot. The tear-shaped plots thus formed
were around 0.3 m2. The area of each runoff plot was
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determined by placing a grid over the plot. Simulated
rainfall was applied at rates of 10– 35 cm/hr until steadystate runoff was reached (usually 30 – 60 minutes). The
difference between the application rate and the runoff rate
was considered to be the infiltration capacity.
[14] Soil infiltration rates were determined at 40 locations
adjacent to 11 of the trenches. The number of infiltration
tests per trench ranged from 2 to 7. For each trench,
infiltration was determined on at least one riser and one
tread, for a total of 21 risers (9 upper risers, 8 mid risers, and
4 lower risers) and 19 treads.
2.2. Runoff and Erosion
[15] At one trench location, seven of the runoff plots
established for the infiltration studies were subsequently
used for monitoring of naturally occurring runoff and
erosion. Between 1992 and 1995, rainfall (measured with
a manual rainfall gauge) and runoff were monitored after
each rainfall event, for 804 days. If runoff did occur, a 100ml sample was collected for determination of the sediment
concentration.
2.3. Soil Matric Potential
[16] Eight sites—six risers and two treads—were instrumented at various depths with ceramic cup tensiometers to
measure soil matric potentials from 0 to 900 cm of water
tension. The tensiometers were inserted horizontally 20–
25 cm into trench side walls, sealed with bentonite, and
brought to the surface with vertical PVC connector tubes of
the same diameter. Black plastic sheeting was then placed
between the side walls of the trench and the backfilled
materials to prevent extraneous hydrological effects from
the trench excavation. Field tensiometer values were corrected for the length of the installed tensiometer (total of
depth into soil and freeboard length) to obtain adjusted soil
moisture matric potentials. The deepest tensiometer was
placed near the interface of the soil and the underlying
indurated bedrock. Data were collected weekly (until soil
matric potentials exceeded > 900 cm water tension) and
after all rainfall events for about three years (1992 – 1995).
3. Results
[17] On average, the infiltration rates of the soils on the
upper risers were about double those of the tread soils, in
spite of the much steeper slopes of the risers (Figure 2).
Infiltration rates declined progressively from upper-riser
soils to lower-riser soils (those of the lower risers being
comparable with those of the treads). At the same time,
infiltration rates within each of the topographic positions
varied considerably, most likely because of individual
differences in soil thickness, vegetative cover, and surface
conditions.
[18] Soil depths (including soil/caliche), as highlighted in
Figure 2, declined progressively down the face of the riser.
The average soil depth on the treads was only about one
quarter or less that on the upper riser.
[19] The data obtained from the runoff monitoring are
consistent with patterns observed from the infiltration
experiments (Table 1). Runoff as a percentage of precipitation increased with distance downslope, from upper riser to
lower riser (runoff from the lower risers being comparable
with that from the treads). Marked differences occurred
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between the two tread plots: the one consisting mostly of
bare ground and rock produced higher amounts of runoff
while the one having good vegetative cover produced much
less. Erosion was quite low from the upper and mid risers
but was very high from the lower risers.
[20] Figure 3 presents corrected soil matric potential data
and associated precipitation data as a time series (weekly
averages) for three of the riser sites. These data are
consistent in that all three indicate saturated or very moist
conditions at the base of the riser soil for two extended
periods (6 months or more). The relatively high positive
pressures at sites 4 and 5 suggest that either water is
confined and under artesian pressure on a given step, or
that there is connectivity with water at a higher elevation.
The latter is less probable because positive pressures range
only from 0.5 –2m in the hillslope sequence of risers that
have elevation differences of 28m (Figure 3).
[21] Results from all eight tensiometer sites are summarized in Table 2. At four of the six riser sites, the deepest
tensiometer recorded saturated conditions for more than
25% of the time; and at the other two riser sites, soils were
near saturation for at least a comparable percentage of
the time. According to the 150-year meteorological record
of annual rainfall 1992 was one of the wettest years
(1170 mm), 1993 one of the driest (673 mm), and 1994
(1045 mm) and 1995 (863 mm) were at or above the longterm mean of 840 mm for Austin, TX.
4. Discussion and Conclusions
Figure 2. Box and whisker plots of infiltration rate, soil
depth, and percent slope at various topographic positions
along the riser – tread continuum. With sufficient data a box
and whisker diagram presents the median and mean (red
line); and the 5% (lower dot), 10% (lower whisker), 25%
(lower edge of box), 75% (upper edge of box), 90% (upper
whisker), and 95% (upper dot) distribution of the data.
[22] The predictable and systematic differences in soil
hydrology along the riser –tread have clear implications for
our understanding of both runoff generation and ecohydrological relationships. We find that the soils on the steepest
slopes have the highest infiltration rates; and that in general,
there is a progressive decline in infiltration rates from the
upper risers downward—the upper risers commonly exhibiting high or very high infiltration rates and the lower risers and
treads relatively lower values. Further, we find a similar
dramatic and progressive change in soil depth, and therefore
water-holding capacity, from riser to tread.
[23] Long-term monitoring of soil matric potential shows
that wet or saturated conditions persist for significant
periods of time at the base of the risers, which indicates
that (1) water is entering the riser; (2) water does move
through the Bk horizon (often referred to as caliche); and (3)
very little if any vertical flow penetrates the bedrock – soil
interface. Some of the tensiometers recorded relatively high
hydrostatic head, which indicates localized artesian condi-
Table 1. Soil and Hydrological Characteristics for a Sequence of 7 Runoff Plots Positioned Along a
Topographic Gradient of Risers and Treadsa
Topographic Position
Characteristic
Upper Riser
Mid Riser
Mid Riser
Lower Riser
Lower Riser
Tread
Tread
Runoff (%)
Erosion (g/m2)
Infiltration capacity (cm/hr)
Slope (%)
Soil Depth (cm)
2
2
14.8
26
66
11
8
11.0
38
97
28
58
7.7
36
91
32
364
2.9
16
41
24
306
4.7
26
66
33
98
0.0
4
10
12
56
7.7
6
15
a
Runoff and erosion were monitored for an 804-day period between 1992 and 1995. Rainfall during this period totaled
1625 mm.
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Figure 3. Soil matric potential at various depths at three riser sites (measurements taken from June 1992 to June 1995).
Data are presented only for observations when soil matric potentials < 900 cm of water tension. The hillslope sequence of
risers have decreasing elevations from 281 m (riser 4) to 261 m (riser 5), to 253 m (riser 6).
tions from water upslope (probably within the same riser
sequence.
[24] With respect to runoff generation, these hillslopes are
composed of an alternating sequence of sources and sinks
that facilitate both surface and subsurface flow. The data
from the runoff plots support the notion that most of the
surface runoff generated in this landscape comes from
the lower risers and the treads. Locally, it appears that the
downslope risers are serving as sinks (recharge) and the
upslope treads as hydrological sources (discharge). During
periods of high rainfall, overland flow is generated from the
lower risers and treads, which is then captured and stored
and ultimately used by vegetation or slowly released by
‘‘seeps’’ at the base of the riser. During the observation
period, around one-third of the rainfall ran off the lower
riser and tread plots. We believe it unlikely that the
ephemeral multiple water tables within the risers are
interconnected (see above comments), but more work is
required to confirm this.
[25] Our results have at least three major implications for
resource management. First, largely because of the high
infiltration rates and significant water-holding capacities,
Table 2. Topographic Position, Soil Depth, and Water State
Classes for Each of the Tensiometer Sitesa
Site No.
1
3
2
2
3
3
4
4
5
5
5
6
6
6
7
7
7
T1
T4
T1
T2
T1
T2
T1
T2
T1
T2
T3
T1
T2
T3
T1
T3
T4
Topographic
Position
Soil
Depth,
cm
Time
Saturated,
%
Time
Moderately
Moist and
Very Moist, %
Time
Slightly
Moist and
Dry, %
Tread
Tread
Riser
Riser
Riser
Riser
Riser
Riser
Riser
Riser
Riser
Riser
Riser
Riser
Riser
Riser
Riser
38
30
38
76
43
94
36
66
61
112
145
36
69
91
30
152
257
4
16
2
3
4
28
23
28
2
9
24
1
7
4
10
28
31
42
10
46
45
43
17
24
19
45
36
23
44
38
43
26
7
6
54
75
52
52
53
55
53
53
53
55
53
55
55
54
64
65
64
a
Data from Soil Survey Division Staff [1993]. All data based on corrected
tensiometer values reflect saturation (0 or + matric potential values),
moderately moist and very moist ( 1 to 899 cm water), and slightly moist
and dry (> 900 cm water) conditions.
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WILCOX ET AL.: RUNOFF GENERATION FROM STEPPED LANDFORMS
the risers serve as important buffers to storm water flow.
Water that would otherwise immediately run off is instead
stored on the landscape. This buffering mechanism can be
lost if the risers are covered or paved. Second, the ‘‘caliche’’
residuum (Bk and Bkm horizons) on the risers is likely an
important reservoir of water for deeper-rooted vegetation as
well as the source of water for the hillslope seeps often
noted at the base of risers. Finally, vertical recharge of
deeper groundwater is probably limited on these hillslopes,
and when it does occur is restricted to fractured bedrock or,
more likely, to valley bottoms.
[26] Acknowledgments. Gratitude is expressed to Bob Ayers, Barton
Creek Properties and Stratus Properties for supporting some of this
research. Appreciation is also expressed to USDA-Natural Resource Conservation Service and the Texas Commission on Environmental Quality for
additional support.
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B. P. Wilcox, Ecosystem Science and Management, Texas A&M
University, College Station, TX 77843, USA. ([email protected])
L. P. Wilding, Soil and Crop Sciences Department, Texas A&M
University, College Station, TX 77843, USA.
C. M. Woodruff Jr., Woodruff Geologic Consulting, 711 West 14th Street,
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