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The Inf-Iuence of Pinyon-Juniper on
Microtopography and Sediment Delivery of
an Arizona Watershed
Burchard H. Heede1
Abstract--Hummocks formed by litter fall of pinyon-juniper trees
resulted in soil formation. Overland flows were diverted and slope gradients decreased by about 57%. In turn, streampower decreased. It is
proposed that this was responsible for the decreased sediment delivery.
Where buffer strips exist below non-wooded areas, sediment delivery was
practically nullified .
.
present-day condition of the pinyon-juniper zone in the Southwest: overgrazed by cattle during the turn o( the century,
followed by controlled grazing and fuelwood harvest. During
our study, fuelwood cutting was not permitted on the watershed. The climate is semi-arid with 395 mm average annual
precipitation, of which 66% falls in summe·r. Summer storms
are of high intensity. From November to April, pre.cipitation
is mainly in the form of snow. The soils of the area are sandy
clay loams, with basalts forming the geologic base.
In the Southwest, woodlands cover an area of about 25
million hectares. Pinyon-juniper represents an important
vegetation type on this land, and occupies more than seven
times the area of chaparral in Arizona. The use of this large.
area is limited to cattle grazing, fuelwood cutting, and occasional re·c.reation. l\fore intense use of the resource, suc.h as
charcoal production, started in places but failed on e.cononuc
grounds. Vegetation type conversions to grass were unsuccessful because of increased erosion and insuffident water
yield increases (Gifford et a1. 1970, Gifford 1975, Roundy
1976). Unauthorized tree cutting is problematic in woodlands
near rural communities, largely because of insuffident manpower available to management.
With the exception of silvicultural and fire research,
pinyon-juniper has received little scientific attention. Thus
very little is known about the erosional processes operating in
this vegetation type, and the interactions between vegetation
and erosion processes.
Pinyon-juniper usually occurs in open stands with wide
spacings between individual trees. Some stands form large
clusters. The question therefore arises: can mosaic distributions of pinyon-juniper significantly influence overland flow
and sedinlent transport? The primary objectives of our ongoing study are therefore to quantify overland flow and sediment
delivery, and to determine their production is influenced by
pinyon-juniper.
J\.lethod
Thus far, only two years of data are available. Twelve
microwatersheds, or hillslope segments, were selected to
represent three different woodland cover types: (1) four
wooded, (2) si"{ non-wooded, and (3) two non-wooded with
buffer strips on the downslope borde.r. Vioode·d c.over formed
a mosaic pattern comprised of open areas and trees and tree
dusters. In this type, erosion pavements ave.raged 40% of the
area. The pavements were a matri"{ of different roc.k siz.es
cove.ring the ground surface. Ground c.over of the nonwooded or open areas consisted of erosion pa"ement in nearly
all c.ases.
Pinyon-juniper buffer strips consisted of 5- to 8-m-wide
strips of trees close to the topographic contour. Open areas
upslope from the strips had bare ground and erosion pavement. The trees in the strips were spaced so closely that erosion
pavements did not exist between them, and the ground was
fully covered by needle fall. Of the total area, erosion pavement averaged at 60%.
These small. mierowatersheds were neither subwatersheds
nor plots. Generally, subwatersheds are larger in size. In
contrast to plots, gentle topographic swales repre.sented the
Study Area
Located at an average elevation of 2,300 m in the Arizona
White Mountains, the study watershed represents the typical
1Hydrologist, Rocky Mountain Forest and Range Experiment Station, Forestry Sciences Laboratory, Arizona State University, Tempe, AZ
85287.
195
mi.crowatersheds where available. ~Then not available, the
miniature watersheds were hills lope segments bounded by 20~
em-wide sheet metal strips sunk about 10 em into the ground.
These strips were aJso place.d where the natural overland flow
divides were not sufficiently pronounced to prevent breaching
during strong runoff events.
At the downhill. drainage boundary, 4-m-Iong prefabricated metal troughs were installed. These conveyed the water··
sediment mixture into tanks, where overland flow volumes and
sediment concentrations were measured after each storm.
Where expected flow volumes were too large for the tanks,
home-made splitters (consisting of a steel blade installed
plumb into a 10-cm pipe) wasted 50% of the flow volume and
conveyed the remainder to the tank.
At least one continuously recording flow gaging station
was placed in ea.ch cover type drainage. Small supercriti.cal
flumes with waterstage recorder or bubble flow meter and
pumping sediment samples were:used. The bubble flow meter
and sediment sam pIer were synohronized so that flow hydrographs delivered by the meter could be correlated with sediment yields.
Sjnce microwatersheds occupied ,different aspects and
elevations on the mountain slopes, a precipitation gage network was installed so that individual estimates for each
drainage were obtained.
No unusual precipitation or flow events occurred during
the. study period. To determine whether precipitation during
the study years was normal, 14 years of data from a station, 5
mi from the study site, were compared. A linear re,gression
between the precipitation of the nearby-station and the study
area showed a coefficient of determination (r2) of 0.71. Based
on this regression, our precipitation data were calculated back
to 1972. The estimated mean precipitation (453 mm) did not
differ significantly from the actual mean 423 mm) (p = 0.5),
indicating that precipitation wa.s normal during the study
pe.riod.
Statistical analyses consisted of analysis of variance and
Student's t-test.
Results and Discussion
Sediment deliveries from the three vegetation cover types
varied greatly ('table 1). Wooded areas produced an annual
average of 1651~g ha- 1 yr- 1, non-wooded 556 kg ha- 1 yr- 1, and
non-wooded with buffer strip 31 kgha- 1 yr-1. However, when
overland flows were compared between wooded and non··
wooded conditions, no significant difference ( p== .05) could
be found. This contrasts with the sediment deliveries that were
significantly different (p< .01). Student's t-test was applied.
pavements. Earlier studie·s have shown that erc!sion pavements
are high sediment producers on a watershed 2, J (Heede 1984).
At times, annual herbaceous vegetation invaded the erosion pavement, but never represented more than 2-3% of the
cover. In contrast to the wooded area, erosion pavement
formed a continuous ground surface cover on the non-wooded
study sites. No apparent relationship was observed between
slope gradient and sediment delivery. For instance, on wooded
slopes, 175 kg ha- 1 yr- 1 were delivered from a 17(10 gradient
and 172 kg ha- 1 yr·· 1 from a 34~tJ gradient (table 1). This implies
that slope gradient was not the main influencing variable.
Inspection of the microtopography of the hillsides showed
that each tree and tree cluster had formed a mound protruding
up to 0.36 m above the surrounding ground surface. Average
mound height was 0.20 m with a standard deviation of 0.08 m.
The outline of this mound coincided with the dripline of the
tree. Viewed from some distance, the mound formations
transformed the hillsides into a landscape of miniature hummocks. Individual hummocks were formed by deposition of
dead needles. Hummock height tended to increase with age of
tree. In contrast with erosion pavements surrounding the
hummocks, soil had developed beneath the litter and duff
layers. Where trees had died or were removed, the hummocks
began to shrink in size and finally disappeared. As this
happened, the soils of the hummocks also disaI,peared, and
erosion pavements replaced the hummocks unde.rneath the
tree "skeletons."
Overall, hummocks appeared to be effective barriers for
overland flow, and only seldom was one overrun (i.ndicated by
rill formation). In nearly all cases, diversion of the overland
flow was diverted around the hummock, as evidenced from the
flow pattern after storm events. This diversion produced
c.onsiderable lengthening of the flow lines compa.red with
more or less straight downhill flows. If we consider the ideal
case of a hummock with a true circular circumference, the
increase in flow length necessary to reach the same elevation
as existing at the tre·e. (center of circle) is 57% (because the
length of the flow line r, the radius of the hummock, is
inc.reased to 1/2 r n, one fourth of the circle's perimeter).
From this follows a dec.rease of the flow gradient by 57%. Of
course, we are not dealing with a tilted tabletop and re.gular
geometric form; this decrease could therefore be somewhat
larger or smaller. But the point is that a substantial gradient
decrease ta.kes place which, in turn, leads to decrease of the
velocity of flow.
Slope and velocity are two important variable·s for streampower, an expression for sediment carrying capacity of the
flow. Bagnold (1973, 1977) described stream power ( ~) by the
equation
;~HeE~dE~, Burchard H. ThE~ influence of vegetation and its distribution
on sediment dE~livEHY from selectE~d Arizona forests and woodlands. In
preparation for Proceedings of Nineteenth Annual Confemnce of the
International Erosion Control Association. 1988.
3Heede, Burchard H. Overland flow and sediment delivery five years
after timber harvest in a mixed conifer forest, Arizona, U.S.A. Journal of
Hydrology (In press).
The apparent contradiction between similar flows but
dissimilar sediments was puz.zling. Both types had erosion
196
Table 1.--Averageannual overland flow and averages.nnusl sediment
delivery fl'om the three different vegetation cover types. Standard deviations given In parenthesls. 1
-------Mlcrowatershed
No.
Wooded
3
5
12
13
Avem.ge
Aver. slope
gl'adlent
Overland
flow
Sediment
delivery
m m- 1
mm yr- 1
~(gha-1 yr 1
0.17
.34
.34
.28
.28 (0.20)
3.8
1.6
3.6
.7
-2.4 (1.5)a
174.83
195.39
172.42
117.87
165.13 (28.71)a
.33
.35
.34
.41
.10
.12
.28 (.12)
4.0
3.1
2.7
4.5
2.0
7.8
4.0 (2.0)a
291.73
828.46
353.:38
623.85
405.71
835.69
556.47 (220.05)b
Non-wooded with bufferstrip
1
.29
15
.20
Average
.25 (.06)
.9
.7
.8 (0.2)b
46.94
15.45
Non-wooded
4
6
7
9
10
11
Average
31.20 (22.88)0
1Within a column, significant differences between classes are
indicated by different letters (flow, p:= .05, sediment, p < .01).
J!J == .y dSv
[1]
where q is the absolute density mass per volume, d is the mean
flow depth, S is the energy slope, usually substituted by bed
slope, and v the mean flow velocity. As can be seen from this
equation, the computation of absolute changes in stream
power induced by overland flow diversions would be theoretical, since mean flow depth, mean velocity, and their changes
would have to be estimated, while 'Y could be taken as constant. l\feasurements of the variables under field conditions
possibly would lead to closer estimates, but not to absolute
values, due to the difficulties of measurements in shallow flows
and rapidly changing depths and velocities. It can be reasonably assumed that slope gradient and velocity will substantially
decrease, exceeding any possible increase in flow depth due to
lack of channelization. In turn, this leads to wider flows,
increased wetted perimeter and roughness of flow, and decreased velocities.
Thus, stream power would decrease with overland flow
diversions induced by hummocks. Generally, diversions occur
several times as water flows downslope, re.sulting in flow
regimen changes from a turbulent to a more tranquil flow, and
further decreased sediment carrying capacity.
It is proposed, but has not been tested, that the signifkant
difference in sediment delivery between the pinyon-juniper
mosaic pattern and the open area is caused by the hummock
formations. The. apparent contradiction that hummocks led to
decreases in sediment delivery, but slope gradient-sediment
delivery relationships did not exist on the microwatersheds,
can be explained by the c.umulative effect of several hummock
diversions.
Unfortunately, only two sites of non-wooded areas with
buffer strip had acorn plete data set (table 1). If we assume that
average sediment production on the erosion pavement
upslope from the buffer strips was similar to that from the nonwooded sites, only an average of about 6% of sediment left the
buffer strip. In ponderosa pine (Heede 1984) and in chaparral
(Beede., in preparation2), only2~, and 0.4%, respectively, left
the buffer strips. The face value of the data is not as important,
as what they reveal about processes.
The data indicate that pinyon-juniper buffer strips were
more effective in reducing sediment delivery than the wooded
sites, because the upslope microtopography forced the overland flow to enter the extended hummock of the strips' tree
dusters. Therefore, due to i.ncreased infiltration, the litter-soil
ground cover underneath the trees reduced the flow and with
this the sediment load.
Conclusions
Sediment delivery from wooded areas was lower than from
non-wooded sites, but overland flow was not. Pinyon and
juniper trees changed the mic.rotopography by forming
mounds, or hummocks, whose edges corresponded to the
tree's dripline. Litter, duff, and soil maki.ng up these hummocks protrude.d up to 0.36 m above the. surrounding ground
surface. I observed that the elevational difference forc.ed
overland flow to circumvent the trees. This c.aused extension
of the flow lines and reducti.on of the slope gradient by about
57%. If the flow line extension is the predomi.nant va.ria.ble
responsible for the decrea.se of sediment delivery, resulting
decrease of sediment carrying capacity of the flow would
explain the reduced sediment delivery from wooded sites.
Sediment delivery from pinyon-juniper buffer strips was
practically nil. Similar results had been obtained by the author
below buffer strips in ponderosa pine and chaparral.
Figure 1.--Buffer stl'ip!~ consisted of a clustel' oftl'«~es tha.t had fOl'nled
on continuous mound undernl~ath theh' crowns. The ovel'land
flow and sediment collector tl'ough Is located aUhe bottom of the
figure (looking upslope).
197
Literature Cited
Gifford, Gerald F.1976. Impacts of pinyon-juniper manipulation on watershed values. In: The Pinyon-Juniper Ecosystems: A Symposium; May 1975; Logan, UT: Utah State
University. 127-140.
Beede, Burchard B.1984. Overland flow and sediment delivery: An e.xperime.nt with small subdrainages in southwestern ponderosa pine forests (Arizona, U.S.A.). Journal of
Hydrology. 72: 261-273.
Roundy, Bruce A. 1976. Influence of prescribe.d burning on
infiltration and se.diment production in the pinyon-juniper woodland. Unpubl. ]\1.S. Thesis. Reno, NV: University of Nevada.
p.
Bagnold, R. A. 1973. The nature of saltation and of bed-load
transport in water. Proceedings Royal Society Series. A:
473-504.
Bagnold, R. A. 1977. Bed load transport by natural rivers.
"Vater Resources Re.search. 13: 303-312.
Gifford, O. P.; Williams, G.; Coltharp, O. B.1970. Infiltration
and erosion studies in pinyon-juniper conversion sites in
southern Utah. Journal of Range Management. 23: 402406.
70
198