Monitoring Soil Erosion in a Mountainous Watershed ...

Rural

and Environmental Engineering

No.43

(2002.8)

pp.

23•`30

Technical

Monitoring

Soil under

A. Nasir*,

Erosion

High

Report

in a Mountainous

Rainfall

UCHIDA

Zone

Kazunori**,

Watershed

in Pakistan

M. Shafiq***

and M. Khan***

Abstract A typical small watershed located in the northern part of Pakistan was selected for this trial study. Due to chronic mismanagement, the watersheds of Pakistan are steadily degrading. Environmental degradation of mountainous watersheds has become a matter of great concern. The surface runoff and soil loss from a given catchment / watershed area depends upon both the rainfall and the catchment characteristics. The objectives of this study are to quantify the amount of soil eroded under current land use and to predict soil loss according to different conservation measurements using rainfall and water sampling data. The degradation process may be mitigated through careful management of watershed resources to determine which soil and water conservation techniques to adopt based on rainfall data and other parameters. The maximum unit peak discharge in the study area ranged from 144 to 268 liter/sec/ha. However, in the past five years the erosion index (EI30)appreciably correlated with rainfall (R2 = 0.98). Annual soil loss varied from 2.8 to 18.6 ton/ha. This paper describes the effect of rainfall parameters on soil loss in the high rainfall zone, which may be utilized for the design and development of soil erosion control structures and models. Key words : Rainfall, Runoff Rainfall intensity, EI30, Soil loss, Pakistan

I. Environmental sidered

even

Pakistan, must

degradation in the field

versely

affected

resources,

of the

the agro-environment and salinity,

land and water.

and water

become

and food is mainly

Appropriate

global

development.

and pressing land

but is primarily

Soil erosion

management.

an urgent

and rural

land resources

yet the devastation

sion, waterlogging, land

has recently

of agricultural

with vast productive

be raised,

Introduction

and poor supply.

food water

Land

needs,

due to rainfall,

a significant

of rainfall

a year

and even

annual

rainfall

curs

mainly

portion

occurs between

a year during

as surface

is extremely the summer

July and

September,

runoff.

variable season

have

ad-

is not due to soil ero-

deforestation,

waste

con-

such as

productivity

management

use of the two main

will not only enhance

such as land, water, and vegetation. cutting of trees and removal of vegetation

countries

agricultural

degradation

upgrade natural resources Due to the indiscriminate within

and is now being

resource

due to inappropriate

land use

issue

In developing

natural

and inappropriate productivity

but also

cover , soil loss and floods The total accumulated precipitation over

and unpredictable

. About 60% of the mean storms . Soil erosion, which octo estimate in the watershed area due to

as high intensity

is not easy

* Graduate Student , Department of Agricultural & Environmental Engineering, Kobe University, Japan ** Professor/Chairman , Department of Agricultural & Environmental Engineering, Kobe University, Japan

***WRRI, National Agriculture Research Center, Islamabad, Pakistan (Manuscript Received Feb. 23, 2001, Accepted Dec. 21, 2001) REE No. 43 (2002)

A. Nasir,

24

UCHIDA

K. , M. Shafiq

& M. Khan

high rainfall intensity and subsequent high runoff (Shafiq, Nasir, et al., 1996).

Accumulation, duration, intensity, and aerial distribution of rainfall are the most important parameters affecting surface runoff and soil loss. Rainfall intensity influences both the velocity and volume of runoff. An intense storm can exceed the infiltration capacity by a greater margin than does a gentle storm; thus the total volume of runoff is greater for an intense storm even though the total precipitation of the two storms is the same. The infrequent combination of relatively high intensity and long duration of storms results in obvious amounts of runoff and subsequent massive erosion. These storms cause extensive and widespread erosion damage and devastating floods (Schwab et al., 1993). The hydrological cycle is the most important ecological process determining structural and functional dynamics in a watershed system. Information on the hydrological cycle in a watershed is very important for management strategies at the watershed level (Rai and Sharma, 1996). The degradation rate of the mountainous watershed environment can be lessened through careful management of watershed resources. To tackle the surface runoff and soil erosion problem, different soil/water conservation activities need to be carried out. Before designing any conservation technique it is necessary to have information on the probable quantity of flow, peak flow, and the temporal distribution of flow.

Apichart et al. (1980) stated that intense storms of short duration or light rainfall of long duration cause water erosion. Meyer (1981) found that rainfall intensity affected the rate of interrill detachment, splash, and runoff. Erosion (E) was related to rainfall intensity (I) in a power equation for a wide range of soil and cropping conditions represented by the equation (1)

E = aIb where,

E = erosion

rate in ton/ha/hr

a and b = coefficient The

rainfall

pattern

and exponent

has

also been

of best

shown

fit.

to greatly

influence

the total

amount

of runoff

and

soil loss.

Lee (1984) stated that soil in the loess plateau of North Western China was perhaps the most seriously eroded soil in the world. This was due to intensive rainfall in a concentrated period combined with the removal of grass cover and trees destroyed by farming activities. Alchin (1983) observed a high correlation between the amount of rainfall and the amount of runoff for individual events, and he further stated that rainfall events of relatively high amounts and intensity resulted in the highest percentage of runoff and soil loss. Therefore, In order ment

it is necessary

to manage

system.

to develop

a watershed

This requires

of its resources.

an effective

means

it is first necessary

an assessment

A soil erosion

of addressing

to develop

of the watershed

monitoring

mechanism

the soil loss problem.

an effective

conditions

watershed

in terms

is one important

manage-

of the degradation

criterion

for watershed

management.

II . Material 2.1

Study The

that

site

experimental

is 30

km

to the

latitude

33•‹30'•`33•‹45'

meters,

with

The

a total

catchment

falls in the range 40%.

and Methodology

catchment north

east and

catchment

area

consists

of 20-40%

area

selected

of

Islamabad,

longitude area

this

study

in the

gradient

12.64

ha

30% of gradient as shale

a small part

The

(Figure

and 20%

is

southern

72•‹30'•`72•‹45'. of

of about

The soil of this area is classified

for

Rural

of

the

the

Pothwar

Margala

elevation

Plateau

hills,

between

ranges

from

915

to 945

while

50% of the area

1).

less than 20%,

of the area

and sand

part of

stone.

has a slope The area

and Environmental

gradient

is a good

greater

than

representative

Engineering

No. 43, 2002

MONITORING

SOIL EROSION

Figure

Figure

2

IN A MOUNTAINOUS

WATERSHED

UNDER

1

Contour

map

of Satrameel

Watershed

Total

mean

monthly

rainfall

and percentages

HIGH RAINFALL

ZONE

and meteorological

for the

Satrameel

IN PAKISTAN

25

station

watershed

of more than 70% of Pakistani watersheds, most of which have similar topography , geology, vegetation, and climatic conditions (arid and semiarid). Most of the watershed is rough and mountainous, with a wide variation of relief. Storms are usually of short duration and high intensity especially during the Monsoon (July September) , which contributes more than 60% of the annual rainfall. Monthly rainfall percentages are shown in Figure 2 . 2.2

Climatic

Zones

of Pakistan

Pakistan can be divided into four climatic zones: humid, sub-humid , semiarid, and arid. The humid zone comprises the Murree, the northeastern parts of Rawalpindi , and some parts of the northwest Frontier Post (NWFP). The fields have been bench-terraced . The annual rainfall is about 1000 mm. The subhumid region comprises parts of the Attock, Rawalpindi, and Jhelum districts and parts of NWFP. Annual rainfall ranges from 500 to 1000 mm. The semiarid region comprises the area of Fateh Jang, Pindigheb, Talagang, and a belt from Mianwali to Chakwal. The annual rainfall in this area varies from 300 to 500 mm. The arid region comprises part of REE No. 43 (2002)

A. Nasir,

26

Figure Table

1

Rainfall

3

Climatic

recorded

from

zones rain

of Pakistan gauge

graph

based

UCHIDA

K.,

M.

Shafiq

& M.

Khan

on rainfall

and computation

of EI30 values

KE* =210 + 89 log (I)

Dera Ghazi Khan, Sargodha, and Jhang, receiving less than 300 mm/yr of rain (Chaudhry et al., 1986). The annual rainfall patterns for Pakistan are shown in Figure 3. 2.3

Measurement

techniques

Runoff volume was recorded at the outlet of the watershed with a 1.4-meter deep "H" type flume and "F" type Water Level Recorder, with a weekly graphic chart for recording flow depth with the movement of a float in the stilling well. The capacity of this flume is 2.4 m3/sec. The hydrographs from this flume were analyzed for peak flow rates and total volume. A recording rain gauge was installed near the flume in the catchment. The rainfall graphs were analyzed for each rainfall event for several parameters such as total rainfall, maximum 30-minute intensity (130), and Erosion Index (EI30). Soil loss depends partly on rainfall due to the energy of raindrops striking the surface with a kinetic energy (E) of the rain, and partly due to the entrainment of detached particles by further contribution of the rain to runoff water--, that is, rainfall intensity (I) . The product of El reflects how total energy and peak intensity are combined in each particular rainstorm, indicating the Rural

and Environmental

Engineering

No. 43, 2002

MONITORING

SOIL EROSION

IN A MOUNTAINOUS

WATERSHED

UNDER

HIGH RAINFALL

ZONE

IN PAKISTAN

27

ability of the rainstorm to cause erosion. It has been found that the maximum 30-minute intensity of rainfall is more highly correlated with soil loss than the intensity for any other interval (Wischmeier, 1959). The

EI30

was

computed

ET30 = KE •~

where, maximum

after

the

procedure

given

by

(Singh

et al.,

1991)

as

(2)

I30/100

EI30 = Erosion

index,

KE = Kinetic

energy

of storm,

130 = Rainfall

intensity

of storm

for

30 min.

The maximum 30-minute intensity is calculated by using rainfall records from the continuous recording gauge. Using equation 3, the value of KE can be found (Wischemier et al., 1969): KE= 210.3 + 89logI (3) where,

89 and

210.3

= constant,

I = Unit rainfall

of Storm

in cm/hr.

The relationship of soil loss to the EI30parameter is assumed to be linear, and the parameters of individual storm values are directly additive. The rainfall event of September 9, 1992, was selected as a typical sample for this calculation. In order to obtain seasonal values, the storm EI30 value for that length of period (storm) were summed up. I30values were classified into different groups as shown in Table 1. To calculate estimated soil loss in the study area, water samples were collected at different intervals during each storm. Concentration of sediment was obtained from the water samples by oven drying. The average sediment loads (g/l) were computed for the storm events and multiplied by the total volume of runoff from that particular storm to determine the erosion hazard (kg/ha). This data were then subjected to statistical analysis for frequency distribution, coefficient of variation, and regression. III.Results 3.1

and Discussion

Precipitation

The correlation relations between rainfall, peak discharge, and soil loss were carried out in a high-rainfall area (858-1527mm) at Satrameel (Rawal Watershed) for five consecutive years (1992-1996). 60% to 70% of rainfall occurred during the monsoon (July-September) season, as shown in Figure 2. During the study period it was observed that rainfall within the years and over the years was quite variable, with a coefficient of variation (CV) of 132% and 19%, respectively. The maximum rainfall per day ranged from 76 to 375 mm, and the maximum 30minute rainfall intensity (130)varied from 64 to 104 mm/hr. The 130of about 83% of all rainfall events was less than 50 mm/hr, 12% was in the range of 50 to 75 mm/hr, and only 5% was

Figure

4

Monthly

accumulated

rainfall

in the study

area REE No. 43 (2002)

A. Nasir,

28

Table

2

Hydrological

characteristics

of watershed

Figure 5 Relationship between rainfall and erosion index (EI30) in 1992

Figure

UCHIDA

during

6

, M.

monsoon

Relationship Percent

K.

Shafiq

& M.

season

between

and Peak

Khan

Runoff

Discharge

greater than 75 mm/hr. Monthly accumulated rainfall (mm) is shown in Figure 4. 3.2

Runoff During the monsoon

were

monitored.

age value surface

of 23%

runoff

and distribution Table

season

The maximum to 47%

within

and

in the value

study

of the incident over

of the rainfall

the years events.

period,

of surface rainfall may The

81 rainfall

runoff

ranged

during

be attributed

characteristics

events from

different

which

years.

to variations of rainfall

generated

54% to 92%, The

differences

in amounts, for each

year

runoff

with an averin the

intensities, are given

in

2.

3.3 Erosion Index (EI30)

The rainfall intensity is directly related to the amount of runoff produced by a specific storm. The rainfall data collected indicated that, for this watershed, the duration of the storms varied from 60 to 2200 minutes during the monsoon season. The cumulative annual values of the erosion index (EI30)for the study period ranged from 1111 to 2150 (MJ.cm/ha/hr). About 87 % of the erosion index was recorded during the monsoon season. Erosion index calculations in the study area were only computed for storms exceeding 12.5 mm of rainfall. Rain events of less than 12.5 mm, separated from other events by 6 hours or more, were omitted as insignificant unless the maximum 15-minute intensity exceeded 25 mm/hr. The relationship between rainfall and erosion index is shown in Figure 5.

Rural

and Environmental

Engineering

No. 43, 2002

MONITORING SOIL EROSION IN A MOUNTAINOUS WATERSHED UNDER HIGH RAINFALL ZONE IN PAKISTAN

Table

3

Relationship runoff

of surface

parameters

runoff

during

and soil loss with different

3.4

7

Relationships Regression

Relationship

between

of Rainfall

analysis

(b) Relationship between Rainfall and Soil Loss

Soil Loss

with Peak

been

itself

shown

has

for different

good

in excess

board

above

the flume

index

rainfall

in these

peak

correlation

of the flume and area

events,

and runoff

revealed mum

that the minimum

ranged

from

variables,

and

correlation

It is inferred shown

runoff

that

in Table

rainfall

capacity.

These

of the channel. in Table

in Figure 6. water samples soil loss ranged

1275 kg/ha/storm

of 2.8 to 18.6 ton/ha/yr. the rainfall

with

as shown

and peak discharge is shown The integrated and manual

This amount characteristics. analysis

soil loss

Discharge

and Rainfall

factors parameters

years as well as on an aggregate basis. The regression 3. It can be seen that the surface runoff has a better discharge

and

1992-1996

(a) Relationship between Peak Discharge and Soil loss

Figure

rainfall

29

good

out for individual

equations correlation

data were

1. The graphic

included

relationship

for soil loss data 2 kg/ha/storm

collected

due to the high erosion between during

runoff the study

to 11 kg/ha/storm

. The cumulative values in soil loss may be attributed

To investigate

the functional

out for individual

correlation

with

years

rainfall

percent period

and the maxi-

kg/ha/storm

of variation

was carried has

carried

for this data are given in Table with peak discharge . The peak amount and 130 values . In Figure 6 data have values have been estimated based on the free-

The

from

to 3510

was

are in the order to variability in

relationship

between

as well as on an overall

amount

and

peak

discharge

these basis . , as

3.

In Figures 7 (a) and (b) , the three points are away from others . The maximum rainfalls create less soil loss. The surface runoff occurs when rainfall exceeds a soil' s maximum saturation level and all surface depressional storage is filled up to field capacity . The rate of runoff flow depends on the ratio of rainfall intensity to the infiltration rate . In the study area the infiltration rate is relatively high due to the soil composition of this watershed , shale and sandstone, which has the capacity to absorb water at a higher rate than many other soil types . Hence if the rainfall intensity is high, then the runoff rate will also be high, and the corresponding hydrograph will REE No. 43 (2002)

30

A. Nasir,

show

a rapid

amounts

response

of soil,

to changes

as well

in intensity.

as transport

High

the organic

runoff

matter,

UCHIDA

rates

K. , M.

can detach

Fe, Al, Na,

Mg,

Shafiq

and

& M. Khan

transport

large

Ca, and K contents.

IV. Conclusion In the Rawal

watershed,

significant

amounts

of soil are lost annually

due to intense

poor or no land use planning. Unregulated forestry practices have exacerbated Using a data set covering five years, it has been shown that the rainfall within wide

range

rainfall.

of accumulation

Despite

an excellent it is possible will allow value

these

correlation to predict watershed

of a wide

and that on a year-to-year

variations

the data

with the input the effects

from

parameters.

of various

management

range

derived

remedial

techniques

there

flume

Using

in this watershed

of farming

basis

records

measures

and other

and

before

others

measures

presented

on soil erosion, agement

in this paper

which

strategies

can be used

is an important

for different

climatic

parameter region

as a reliable

designed

in

show

in this paper, to them.

nature

This

to assess

to stabilize

the

or reverse

technique is critical in It is hoped that the re-

tool for prediction

in the planning

sediment

committing

the soil loss. Analysis before commitment to a specific soil conservation cases like this where money must be spent to achieve the greatest effect. lationships

dried

variation

presented

of a similar

and

these conditions. a year has a very

is a considerable

the parameters

and

storms

of appropriate

of rainfall

effect

watershed

man-

of Pakistan.

Acknowledgement The authors cal review

express

their

deep appreciation

of James

Dave

Stedman

for the guidance

and criti-

of the manuscript.

References

Alchin, B.M. (1983) , "Runoff and Soil Loss on a Duplex Soil in Semi Arid New South Wales", Journal of Soil Conservation, Vol.39, No.2, pp.176-187. Apichart, A., M. Shahir Uzzaman and M. Enayet Ullah (1980) , "Rainfall and Evaporation Analysis", Division of Agri. and Food Engineering AIT, Bangkok. Chaudhry, M.A. and Shafiq, M. (1986), "Effect of Crop Management on Soil and Water Conservation", Journal of Agriculture Research PARC Islamabad, Vol.24, No.1, pp. 17-25. Lee, Hsiao-Tseng (1984), "Soil conservation in China's loess plateau", Journal of Soil and Water Conservation, Vol.39, No.5, pp. 306-307. Meyer, L.D. (1981), "How Rain Intensity Affect Inter-rill Erosion", Trans. ASAE, Vol.24, pp. 1472-1475. Rai, S.C. and Sharma, E. (1996), "Hydrological Analysis of an Agrarian Watershedof SikkimHimalaya", International Conference on Eco-Hydrology of High Mountain Areas, 24-28 March 1996. Katmandu. Schwab, G.O., Fangmeier, D.D., Elliot, W.J. and Frevert, R.K. (1993) , "Soil and Water Conservation Engineering", John Wiley and sons, Inc. New York. Shafiq, M., Nasir, A. and Ikram, M.Z. (1996) , "Rainfall Characteristics Under Medium and High Rainfall Zone of Pothwar", Journal of Science Technology and Development, Vol.15, No.4, pp. 41-45. Singh, G., Vankatarmanan, C., Sastry, G. and Joshi, B.P. (1991), "Manual of Soil and Water Conservation Practices (2ndEd.)", Published by Raju Primlani for Oxford & IBH Publishing Company Pvt. Ltd., New Delhi. Wischmeier, W.H. (1959), "A Rainfall Erosion Index for a Universal Soil Loss Equation", Proc. Soil Science Society of America, Vol.23, pp. 246-249. Wischmeier, W.H. and Mannering, J.V., (1969), "Relation of Soil Properties to its Erodibility", Proc. Soil Science Society of America, Vol.33, pp. 131-137.

Rural

and Environmental

Engineering

No. 43, 2002