Arid and semi arid areas constitute over 30% of the world land surface (Saco
et al., 2007). These areas function as tightly coupled ecological-hydrological
systems with strong feedbacks and interactions occurring across scales (Noy-meir,
1973; Wilcox et al., 2003). In semi arid ecosystems, its already well
established that hydrology exerts a profound influence over the abiotic components
of landscape primarily erosion (Weinwright et al.,
2000) and loss or redistribution of key plant-limiting nutrients such as
nitrogen (Schlesinger et al., 1999; Parson
et al., 2003).
Generally, the vegetation of semiarid and especially arid rangelands consist of mosaics or patterns composed of patches with high biomass cover interspersed within a low cover or bare soil components (interpatch). Soil hydrologic condition is the result of interaction between soil and vegetation. Infiltration rate and sediment yield integrate these factors and are good indicator of hydrologic condition.
A key condition for the development of these patterns is the emergence of a
spatially variable field infiltration with low infiltration rates in the bare
areas and high infiltration rates in the vegetated areas. The spatially variable
infiltration has been observed in many field studies and is responsible for
the development of a runoff-runon system (Saco et al.,
2007). Several field studies have reported much higher infiltration rates
(up to 10 times) under perennial vegetation patches than in interpatch areas
(Bhark and Small, 2003; Dunkerley,
2002; Ludwing et al., 2005). The enhanced
infiltration rates under vegetated patches are due to improve soil aggregation
macro porosity related to biological activity and vegetation roots (Tongway
et al., 1989; Ludwing et al., 2005).
The amount of water received and infiltrated into the vegetation patches, which
includes runon from interpatch, can be up to 200% of the actual precipitation
(Wilcox et al., 2003; Dunkerley,
2002). This mechanism triggers a positive feedback that increases soil moisture
and the vegetated patterns (Wilcox et al., 2003).
The redistribution of water from bare patches (source area, interpatch) to vegetation
patches (sink area) is a fundamental process with dry lands that may be disrupted
if the vegetation patch structure is disturbed. So vegetation patterns play
an important role in determining the location of runoff and sediment source
and sink areas (Cammeraat and Imeson, 1999; Wilcox
et al., 2003; Imeson and Prinsen, 2004). Consequently
infiltration rates are often observed to be different under different life forms
(Blackburn, 1975; Wood and Blackburn,
1981; Knight, 1984). Studies demonstrated the effect
of increasing cover of ground-story plant, particularly grasses, on increasing
infiltration rate and reducing runoff and erosion (Pressland
and Lenane, 1982).
There is extensive literature showing interactions between infiltration rate and rangeland vegetation, but only infiltration rate of different vegetation life forms in various rangeland sites in comparison with bare soil were considered and relatively few studies specified effect of various patches on infiltration rate.
Therefore, the present study was conducted to determine effect of different vegetated patches and interpatch on infiltration rates (with different time periods) in an arid rangeland ecosystem. The effect of patches and interpatch areas in delaying fixation time of infiltration rate was investigated too.
MATERIALS AND METHODS
The study area is located in Nodoshan arid rangeland ecosystems in the Yazd
Province in the center of Iran (31°4685 N, 53°4304
W). According to Emberger method arid frigid climate with warm summers and cold
winters prevails in the study area. The study site receives an average of 188
mm rainfall annual falling as rain and snow, concentrated in the period of autumn
and winter of the year. The rainfall erosivity caused by the frequent storms
of high intensity and short duration is high. Virtually no water exists on the
surface, except locally after infrequent, heavy rainfall. The area consists
primarily of sandy loam entisols with a low degree of development and moderate
depth (30-70 cm). The elevation of the study site ranges from 1500 to 1900 above
sea level. The Maximum and minimum mean temperatures of the hottest and coolest
month are 36 and -15°C, respectively. This study has been done in April,
The study area includes shrubland in gently sloping alluvial fans that are dominated by Artemiaia sieberi and Astragalus achrochlarus, both are native species that have expanded considerably in extent and density and each has its unique growth pattern and distribution. Some other plant species are: Astragalus candolleanus, Iris songarica, Acantholimun sp., Acanthphilum sp., Stachys inflate, Lactuca glaucifolia, Poa sinaica, Stipa barbata and Agropyron desertorum.
Different kinds of patches and interpatch were reconnoitered in the rangeland
study site. Plant species and interspaces between them were considered as microenvironments
that have different functions on rangeland hydrological processes, such as infiltration.
In this study area three kinds of patches were observed: shrub, grass, iris
and one kind of interpatch: bare soil that was included spaces between vegetated
patches. Shrub patches were included Artemisia sieberi, Artemisia
aucheri and Astragalus achrochlarus. Grass patches were included
stipa barbata and Agropyron desertorum. Iris patch was Iris
songarica. In all of these patches and interpatch, infiltration rates were
determined by using mono ring infiltrometer. ring diameter was 15 cm that is
optimum size for ring method. Infiltration rate was determined in 5 min periods
for 30 min as total infiltration rate. Infiltration rate in first 5 min period
was considered as initial infiltration (sorption). And last 5 min period was
considered as final infiltration (steady state flow). Various patches and interpatch
areas were considered as infiltration treatments, than Initial infiltration
rates, final infiltration rates and total infiltration rates were measured with
8 replications for every kind of patches and interpatch.
Analysis of variance and mean comparison (Bewick et
al., 2004) at the 95% confidence used to compare the effect of patches
and interpatch on infiltration rate. The Minitab statistical software mainly
Infiltration Rate Curves
Different responses of patches and interpatch areas in infiltration process,
during (30 min) time period are shown in Fig. 1. Results clearly
indicated that patches and interpatch had different responses at the start of
infiltrating process, So that the iris patch had highest infiltration rate (16
mm h-1) and the bare soil had lowest infiltration rate (5 mm h-1).
Infiltration rate of the shrub patch (13.5 mm h-1) was higher than
infiltration rate of the grass patch (9 mm h-1).
At the start of the second 5 min period, infiltration rate reduced in all of
patches and interpatch. The Highest reduction in slope of infiltration rate
curves, from first 5 min period to second, observed in the shrub patch and lowest
reduction in slope of infiltration rate curves, observed in the bare soil. The
slope of reducing infiltration rate curve in the iris patch was awhile higher
than the grass patch. Reduction of infiltration rates from first 5 min period
to second period was drastic in comparison with general reduction of infiltration
rates in 30 min.
|| Infiltration rate curves of different patches and interpatch
Highest reduction of infiltration rate in second period to fourth period observed
in bare soil and then in the iris patch. Reduction of infiltration rate in shrub
and grass patches were almost equal and less than reduction of infiltration
rate in iris patch. Reduction of infiltration rate in quintuplicate 5 min period
in the iris patch was highest in comparison with other patches and interpatch.
In this time period decrease of infiltration rates in the grass patch and bare
soil were similar and less than the iris patch. Infiltration rate in shrub patch
became stabilized at the last time period (25-30 min). Infiltration rate awhile
increased in shrub patch and approximately fixed in the other patches and interpatch.
Initial Infiltration Rate
Results showed significant differences in initial infiltration rates between
different patches and interpatch, but no significant difference between initial
infiltration rates of the iris and the shrub patches (Fig. 2).
Initial infiltration rate of the bare soil was lowest (5.24 mm h-1) and initial infiltration rate of the shrub and iris patches were highest.
Final Infiltration Rate
Results indicated that final infiltration rates of the different patches
and interpatch were significantly different (Fig. 3). Final
infiltration rate was highest in the iris patch (9.02 mm h-1) and
then in the shrub patch (7.01 mm h-1), the grass patch (4.68 mm h-1)
and the bare soil (2.32 mm h-1) decreased, respectively.
Total Infiltration Rate
Results obviously showed that differences in total infiltration rates, between
different patches and interpatch were statically significant (Fig.
Total infiltration rate in the iris patch was highest and in the shrub patch, grass patch and the bare soil decreased, respectively. The Iris patch recorded approximately 3 times higher average total infiltration rate (11.1 mm h-1) than on bare soil (3.29 mm h-1). The Shrub patch recorded approximately 2.5 times higher average total infiltration rate (8.16 mm h-1) than on bare soil and total infiltration rate of the grass patch (5.69 mm h-1) was about 2 times than on bare soil.
|| Initial infiltration rate means of different patches and
|| Final infiltration rate means of different patches and interpatch
|| Total infiltration rate means of different patches and interpatch
The results clearly showed that in study site, the areas treated with vegetated patches had higher initial, final and total infiltration rates than the bare soil, after 30 min. Infiltration rate decreased in iris, shrub, grass patches and bare soil, respectively.
Blackburn (1975), Wood and Blackburn
(1981), Knight (1984), Dunkerley
(2002), Ludwing et al. (2005) and Bhark
and Small (2003) confirmed infiltration rates are often observed to be different
under different life forms. Higher infiltration rates of the shrub patches in
comparison with the grass patches are in contradiction with results that indicated
by Pressland and Lehane (1982). This inconsistency is
related to shrub root system in arid environments. In limiting water ecosystems,
plant roots have wide extension especially in shrub life form and include higher
rate of plant biomass in comparison with grass life form. This improves soil
aggregate stability and soil porosity that infiltration rate is affected by
these items. Final infiltration rate in the iris patch was higher than other
patches that it can be related to its basal area bunch form. Thereby plant material
residue remained from pervious years, accumulate around the patch and it causes
increasing in humus, organic matter and soil aggregate stability that lead to
rising final infiltration rate. Existence of cryptogam cover reinforce infiltration
rate and in this patch cryptogam cover observed more than other patches and
As water moves into the soil profile it is influenced by the matric forces
of the soil (the force of attraction between soil and water molecules) and gravity.
The initial infiltration of water into a soil profile is dominated by a period
during which water is absorbed by the matric forces of the soil. Although the
difference in initial infiltration rates of the iris and shrub patches was not
significant, but high initial infiltration rates in these patches in comparison
with the bare soil is so important; becaus it maximizes total infiltrated water
into soil that indicated by Wilcox et al. (2003)
and Dunkerley (2002). Thereby, plant seeds and residual
materials do not loss and ecological patches will improve. Inversely, low initial
infiltration rate causes to decrease in the total infiltrated water into the
soil profile and increase in runoff and erosion.
Water infiltrating into a soil profile from saturated source, will reach a point where the rate of infiltration into the soil profile is constant and steady. At this point gravity and hydraulic conductivity has replaced the sorption as the dominant forces acting on the flow of water. This period of constant infiltration is expressed as the final rate and gives an indication of sub soil structure, water holding capacity and the drainage properties of the soil.
The low final infiltration rate of bare soil is related to the lack of soil biological crusts in interpatch. Inter spaces in arid rangeland ecosystems are not affected by microclimate of patches, thereupon there is no protection from splash erosion in bare soil areas. It leads to destroying soil aggregates and structure and reducing absorbed rain water. And is another major reason for low final infiltration rate. In this rangeland ecosystem there was no cryptogam cover in interpatch, which probably affect on decreasing final infiltration rate too.
Surface runoff can represent a serious loss of water from the ranch, resulting in significantly less sustainable forage production for livestock and/or wildlife. The erosive nature of runoff transports soil nutrients from the site. When erosion is severe, soil depth is reduced, which reduces the amount of water that can be stored in the soil profile. Reduced water storage within the soil profile results in the plants running out of water faster, thus increasing the frequency and severity of drought. Management can reduce runoff through manipulation of vegetated patch area.
Rangeland infiltration rates generally increase as total vegetated patches increases. Patch areas slows water movement across the soil surface, allowing more time for water to infiltrate before being lost down creeks and draws. Vegetated patches also protect the soil surface from raindrop splash. When raindrops hit unprotected soil surfaces, they tend to destroy soil structure, resulting in the pore spaces sealing and crusts forming. Stable soil pores allow water to move into the soil. Finally, patch areas provide organic matter to the soil, which maintains soil structure and aggregate stability, both of which positively influence rainfall infiltration rate.
This study demonstrated profound effect of vegetated patches on amount of infiltration
rate. Infiltration is a key factor in hydrological processes, so ecological
patches play a determinant role in hydrological processes that stated by Noy-Meir
(1973) and Wilcox et al. (2003). Because
of close relationship between patches and interpatch with infiltration rate
in the rangeland ecosystem, we can call them ecohydrological spaces. Although,
the iris patch has highest infiltration rates, Regarding to dominance shrubs
and their high infiltration rates in arid rangeland ecosystems we concluded
that shrub species are the best choice for restoration these areas that as ecohydrological
patterns have a prominent role on reducing runoff, redounded to lower transfer
of initial resources to out of ecosystem.