Geoscience Reference
In-Depth Information
Table 7.3 Infiltration rates measured in unsealed, sealed, crusted and vesicular soils, Fowlers Gap, western NSW, Australia
(unpublished data of the author).
Mean infiltration
Standard deviation of infiltration
Soil surface type
rate (mm/h)
test results (mm/h)
Number of tests
Unsealed and uncrusted soil
30.4
6.8
3
Biological crust in grazing exclosure
11.9
4.3
5
Inorganic seal in lower intergrove, banded shrubland
6.2
1.2
5
Vesicular horizon beneath fine stone mantle
0.8
n/a
1
rate and the rate of delivery of runon water from ups-
lope. Two implications arise from this. The first is that the
effect of a vesicular layer will almost certainly vary be-
tween stratiform rainfall, with relatively low rain rates and
convective rainfall, with higher rain rates and rain kinetic
energy. Therefore, the overall effect will be dependent
upon the kind of rainfall climate experienced at any par-
ticular site being considered. It is also probable that in
many areas this behaviour will result in the effects of a
vesicular layer varying seasonally. For example, for the
Mojave Desert, Hamerlynck, McAuliffe and Smith (2000)
suggested that vesicular horizons would have their great-
est effect in summer, by throttling the infiltration from
high rain rate convective storms, and be less significant in
more prolonged, lower intensity winter rains arising, for
example, from slow-moving frontal systems. The second
implication is that the effect of a vesicular layer on the
hydrologic response will probably depend upon its loca-
tion within the landscape. Upslope locations may receive
smaller volumes of runon water and downslope locations
a larger amount. Thus, the aggregate rate of delivery of
water to the soil surface will vary with topographic po-
sition and the fraction that can be taken in through soils
carrying a vesicular layer will vary similarly.
To do this, Table 7.3 presents results from the Fowlers
Gap Arid Zone Research Station in the drylands of far
western New South Wales, Australia. There are extensive
soil materials in this area that exhibit quite comparable
bulk properties, because the material is derived from a
blanket of Late Quaternary aeolian dusts set down over
this landscape. The dust materials were sourced from ma-
jor endorheic drainage systems of the continental interior.
In order to assess semi-natural surface properties, the in-
filtrability of biological crusts (including lichen crusts and
bryophyte crusts) were measured by cylinder infiltrome-
try within a grazing exclosure protected from hard-hoofed
animals for more than 30 years. Using identical apparatus,
infiltrability was determined from intergrove soils carry-
ing raindrop impact seals, from soils under a pavement of
fine stones with an extensive vesicular horizon and from
soils not affected by seals, crusts or vesicularity from
grove locations where soil shrink-swell phenomena mix
the soil and maintain a low bulk density and a friable
texture.
The unsealed and uncrusted soils exhibited a mean in-
filtrability of 30.4 mm/h. The soils containing a vesic-
ular horizon exhibited the lowest infiltrability (approx.
0.8 mm/h), or only 2.6% of the unsealed rate. Biologi-
cal crust sites exhibited a mean of 11.9 mm/h, or nearly
40% of the unsealed rate. Finally, sealed intergrove soils
showed a lower mean infiltrability of 6.2 mm/h, or about
20% of the unsealed rate. We can therefore put these soils
into their sequence of increasing infiltrability: vesicular
soils
7.6.1 Comparing the infiltrability of biological,
raindrop impact and vesicular surfaces
unsealed and
uncrusted soils. Finally, we can compare these rates with
the rain rates recorded at this location. The median rate
for rain events defined using a 6 hour minimum interevent
time (Dunkerley, 2008b) is 2.0 mm/h. The maximum event
rain rate from a 6 year rainfall record was 49.7 mm/h, but
only 5.3% of events have a rain rate exceeding 10.0 mm/h.
Therefore, vesicular soils can be expected to generate sur-
face ponding and overland flow in most rain events, while
<
sealed soils
<
biological crusts
<
We have seen that there is uncertainty attached to the ef-
fects of various soil surface characteristics on infiltrability.
Part of this undoubtedly relates to actual variability in the
field and between different kinds of surfaces (stable or
eroding, runoff or runon locations, surfaces having sandy
or finer-textured soils and so on) in the various drylands.
It may be useful to compare the three kinds of features
discussed in this chapter (inorganic seals, biological crusts
and vesicular horizons) within the one small dryland area.
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