Soil erosion is one of the pervasive forms of land degradation around the world,
as well as Iran. For example, 12.5% of New South Wales of Australia is affected
by sheet and rill erosion, 23.4% by gully erosion, 2.9% by mass movement and
10.3% by scalding (Graham et al., 1998). Soil erosion importance is often
ignored by people, unless, they see a pervasive form of erosive evidence (Edward,
The challenge for soil conservation is to understand how erosion processes
occur and how the fertile soil to be utilized for minimizing soil erosion impacts.
The principle of conservation land use is to utilize the land according to its
potential but conserve it according to its need (Mctainsh and Boughton, 1993).
Although, this solution seems to be very simple, yet it is hard to achieve.
Accelerated soil erosion by human beings in compare with natural processes of
land denudation is responsible to increasing eroded material (Freebairn and
Soil erosion often leads to continuing destruction of fertile
top soil and so, fights with sustainable agriculture (Mahmoudzadeh, 1997).
Usually, poor and erodible soils in conjunction with low and highly variable rainfall, have made Australia particularly sensitive to soil erosion (Sanders, 1992). Ofcoure, mentioned characteristics which relate to arid regions partially, are also, seen in Iran as a part of arid region. Therefore, in Iran where, the same as Australia more than half of the country includes desertic and semidesertic areas, soil erosion is a high risk and different forms of it affects many parts of the country. An intensive rainfall event can easily detach 200 tones soil material, which can increase in areas with low vegetation cover. Unfortunately, in arid regions, usually, low vegetation cover accompanying the intensive rainfall events (Kardowani, 1988).
Therefore, necessarily, soil erosion harms might be decreased, so, to achieve
this goal, soil erosion should wholly be introduced and it intensity be
determined. To reach this important aim, a small pastured catchment with 44°,
59′, 10±-45°, 2′, 40± eastern longitude and 37°,
28′, 43±-37°, 51′, 28± northern latitude is selected
as a study site. The study site is one of the upstream small catchments of Emam
kandi catchment which in turn belongs to Urmia lake catchment area, in northwest
of Iran (Jihad Sazandagi of Western Azarbaijan, 1997). Todays, using small dams as sediment traps
is a known method of estimating sediment yield. Using this method leads to mean
annual sediment yield. However, sediment yields can be converted to soil loss
rates by using sediment delivery ratios of selected catchments. The resulted
mean annual soil loss rates are useful for farmers and land holders to plan
for most suitable strategy to control soil erosion.
Location of study site: All check dams which their sediments were measured for
estimating soilloss rates, are located on a small upland catchment area. Figure
1 shows the location of study site on the map of western Azarbaiajan and
Iran. That is, the study site belongs to hyrdrographic unit of A5.
Physiographical situation of study site: Evaluation of physiographical
and topographical situation of study area is necessary and includes the principals
of soil conservation study and is very important for watershed management purposes.
Therefore, in any planning for conservation programs including mechanical and
biological practices under a sustainable use of natural resources such an evaluations
||Location of study site on the map of western Azarbaiajan and
Some of the physiographical characteristics of hydrological
units of Emam kandi catchment area
Maximum and minimum altitudes in Emam kandi catchment area
are respectively 2230 and 1280 m
The most important physiographic characteristics are: surface area, perimeter,
main water ways length, slope steepness, catchment shape, altitude from sea
level, concentration time, hydrographic web and etc. Generally, Urmia lake catchment
area and particularly study area is mountainous. Eman kandi catchment area is
1526 hectars and is divided in to 6 hydrological units. Some of the physiographical
characteristics of hydrological units are shown in Table 1.
The study area is under native pasture and dominant species are: Bromus tectorum,
Verbascum aucher, Cynodon dactylon, Medicago sativa, Juniperus
excelsa and Salix alba (in the bottom of main water ways).
Climatological and hydrogical situation of study area: Climatological conditions particularly, rainfall and temperature are very important and effective on soil erosion trend. In additions, climatologic and topographic conditions highly affect the hydrologic condition, which has high importance in detachment and sediment transportation. Study site has semi-arid climate, with mean annual rainfall of 350 mm minimum annual temperature of -22°C and Max annual temperature of 38°C.
MATERIALS AND METHODS
Sediment yields: Study site as a small part of Urmia lake catchment
is located in western Azarbaiejan of Iran, with a longitude of 44°, 59′,
10±-45°,2′,40± east and a latitude of 37°, 48´,43±-37°,
51´, 28± north.
The method which is used in this study to determine
soil loss rate on a single pastured catchment with 75 hectars area and a sediment
trap dam in it outlet and some small sediment trap dams on main water
ways of the catchment, based on surveying the catchment area and the trapped
sediments behind the dams. That is, the small dams trap water and also sediment
loads. Meanwhile, the trapped sediments belong to whole catchment area, in other
hand, they are part of detached materials from the catchment surface area. So,
to determine soilloss rate, sediment yield is needed. To calculate, sediment
yield, catchment area, mass of trapped sediment might be estimated. Therefore,
volume of trapped sediment is calculated by multiplying the surface area of
reservoir by mean sediment depth. As, the sedimentation period also is known,
then sediment yield can be estimated by dividing the whole sediment mass by
production of sedimentation period (year) and catchment area (ha).
Figure 2 shows the base of this method.
One of the main criteria to use this method is the selection of suitable study sites. That is, the selected site should be located on upland areas of main catchment and also, this small catchment, might be has a small dam in it outlet without spillway runoff. Moreover, catchment area had homogeneous land use, that is, the same catchment land use had been maintained throughout the life of the dam. Dams only stored runoff from the upstream catchment (i.e., there were no inter-basin water transfers into or out of the catchment).
The method of Neil and Galloway (1989) was used to survey the dams. Sediment
cores were collected at a number of points in each dam. A gouge auger was used
to obtain sediment cores, which always penetrated the dam sediments into the
underlying in sito soil or saprolite. Dam sediments were readily discriminated
from the underlying materials on the basis of a lower bulk density, darker soil
colour and fine texture.
By using Walkley-Black method, the organic matter percent
was determined and hydrometer method was used to determine the soil texture
for all soil and sediment samples. Figure 3 demonstrates the
schematic position of sediment cores.
||Shows the schematic position of sediment cores
To use sediment yields for estimating the soil loss rates, Sediment Delivery
Ratio (SDR) and Sediment Trap Efficiency (STE) are needed. For selected small
dam with small catchment area, as the dam had no spillway runoff during it
sedimentation period, the STE considered to be 100% and SDR estimated from the equation of Walling (1983).
Predicted soil loss rates: Inside the measured sediment yields and calculated soil loss rates two models also were used: Modified Universal SoilLoss Equation (MUSLE) and the Pacific Southwest Inter Agency Committee (PSIAC), respectively after exclosure and before exclosure to predict soil loss rates.
RESULTS AND DISCUSSION
Measured soil loss: The volume of deposited sediments behind the constructed
check dams in catchment outlet and other segments of main waterways according
to the methodology of study are shown in Table 2.
Meanwhile, 653.93 m3 sediment has been trapped by an unsealed road
which is constructed in time of dam construction. That is, by considering bulk
density of trapped sediment with coarse material and light texture (1.3) sediment
mass is 850.11 tones, so, total trapped sediment mass is 1724.4 tones. As a
result mean annual sediment yield is:
The latest figure (5.75) by considering sediment delivery ratio (45%) which
obtained by using walling curve, gives soil erosion rate from catchment
area (12.77 t ha-1 year-1).
Predicted soil loss: To predict soil loss rates of study area semiquantitative
method of Pacific Southern InterAgency Comytee (PSIAC) inside with imperical
method of Modified Universal Soil Loss Equation (MUSLE) are used as fallows:
Table 3 shows the evaluated indices for PSIAC (De Vente et
||Active checkdams and trapped sediments
||Trend of using PSIAC indices before and after exclosure in
hydrologic unit of A5
1For calculating the sediment yield the fallowing
equation is used Q5= 38.77e(0.0353R), 2To
estimate the sediment delivery ratio wallings curve is used, 3By
using bulk density (1.3) the volume of sediment converted to mass
||Indices needed to use MUSLE (after exclosure)
|A = RKSLPC, A = 12.8
||Comparing measured and predicted soil loss rates
To use PSIAC, nine factors namely (surface geology, topography, landuse, ground
cover, climate, soil, upland erosion, channel erosion and run off) are evaluated
and used for study area, which resulted to soil loss rate of 26.54 t ha-1
year-1. The resulted soilloss rate refers to before exclosure.
Moreover, after exclosure using MUSLE and PSIAC models resulted to respectively
soil loss rates of 12.8 and 13.60 t ha-1 year-1. 12.8
t ha-1 year-1 is the mean for three selected slopes from
study area. Table 3 represents the trend of using PSIAC indices
before and after exclosure in hydrologic unit of A5. Table
4 shows the quantity of indices used to predict soil loss by MUSLE after
As it comes from Table 5, difference between the results
of using PSIAC and MUSLE and also, the measured result with predicted result
by PSIAC is very high and quite significant. It should be mentioned that measuring
sediment yield and predicting soil loss rate by MUSLE has been done in 2002,
but predicting soil loss rate by PSIAC were done in 1996.
Demonstration of pervasive role of vegetation cover in controlling
soil loss. 1) Measured soil loss after explosure, 12.77, 2) Predicted soil
loss by PSIAC before explosure, 26.54, 3) Predicted soil loss by MUSLE after
explosure, 12.80, 4) Predicted soil loss by PSIAC after explosure, 13.60
To control high soil erosion rete, the area exclosured from 1996 till now.
During exclosure, vegetation cover improved highly and so, soil erosion rate
decreased significantly, which is clearly seen in Table 5.
Most probably, the most important factor in decreasing soil erosion rate is
improved natural pasture in study area. Except, the difference between soil
loss rates before exclosure and after exclosure, other differences are not significant,
which represented by Fig. 4. So, Wilsons (1973: 348)
quote is an appropriate conclusion with this regard: The most important non-climatic
variable influencing Sediment yield is land use. The human influence on erosion
processes is so pervasive that attempts to study sediment yield variations are
likely to be unsuccessful unless land factors are considered.
My deep thanks goes to land owners and farmers of Emam kandi and all people who are working in State Department of watershed management.