Evaluation of Soil Erosion and Sedimentation Rates of Pangasugan Watershed in
Pangasugan Baybay City, Leyte
¹A Thesis manuscript presented in partial fulfillment of the requirements for graduation
with the degree of Bachelor of Science in Environmental Management from the College of
Forestry and Environmental Sciences Visayas State University, Baybay City, Leyte on
___________Contribution No._____. Prepared in the Institute of Tropical Ecology and
Environmental Management under the guidance and supervision of Dr. Victor B. Asio.
Wilbert Acebo Aureo
CHAPTER 1
INTRODUCTION
Nature and Importance of the Study
Soil is the most basic of all resources, is nonrenewable. Once lost, it is difficult to replace
within the foreseeable future. New soil formation, development of a biologically productive and
economically fertile soil from parent rock, is a slow process measured only in a geological time
scale. It takes hundreds to thousands of years to develop the equivalent of a 5-cm layer of fertile
soil. The equivalent of 1 cm or more topsoil may be lost in a single rainstorm. Literally speaking,
the soil formed over hundreds to thousands of years can be blown or washed away in a single
climatic event (Lal 1990). On the other hand, soil erosion is the detachment of soil materials
including rock fragment by an agent to an area of deposition. It is important to study rate of soil
erosion because of its adverse effects on the environment. These effects include: nutrient loss,
soil diversity loss, burying of crops, deposition of sediments, occurrence of floods and water
pollution. The problem of water induced soil erosion in the tropics has gained on increased
public attention in recent years. The reasons for soil erosion in the tropic regions, besides the
removal of natural vegetation, especially the forest, are due to land use systems which are not
adapted to ecological conditions, e.g. monocropping of annual crops and overgrazing. The
increasing population in these climatically favorable tropical mountain regions reinforces the
pressure in limited, non renewable resources. The rate of soil erosion is affected by: rainfall,
discharge rate, streamflow, vegetation, drainage pattern, land use system and soil erodibility (soil
type). Erosion in watershed is one of the major devastative processes which affected both hilly
and lowland environments. Erosion in nature is not destructive but because of man’s interference
and mismanagement of the land, it has been magnified to the extent that it caused destruction of
low lying agricultural areas, watershed, marine environments and even lives and properties.
(Dudal 1988) reported that the current rate of agricultural land degradation worldwide by soil
erosion and other factors is leading to an irreversible loss in productivity on about 20 million ha
of fertile land a year.
Sedimentation embodies the process of detachment, transportation, and deposition of
sediment by the erosive and transport agents including raindrop impact and runoff over the soil
surface (ASCE, 1975). Like erosion, sedimentation has serious environmental and economic
implications. Sedimentation decreases the capacity of reservoirs and chokes irrigation canals and
tributaries. Also, sediments are a major source of pollution and eutrophication (the aging of lakes
caused by water enrichment). (Judson 1988) estimated the river-borne sediments carried into the
oceans increased from 10 billion tons a year.
Agriculturally speaking, both erosion and sedimentation affect soil productivity through
their respective on-site and off-site effects. Erosion reduces on-site productivity by decreasing
the rooting depth and depleting nutrient and water reserves. Sedimentation lessens productivity
through off-site effects such as decreasing the capacity of water reservoirs and silting of
irrigation canals. Soil erosion and sedimentation are severe in temperate and tropical, whenever
the land is used beyond its capability by crop and soil management systems that are ecologically
incompatible.
Pangasugan watershed is located at Brgy. Pangasugan, Baybay City, Leyte where the
climate is more or less rainy. Pangasugan is generally built up by andesitic and basaltic
pyroclastic rocks (referred to as Pangasugan formation) which are mostly of Quaternary and
Tertiary origin. This rock formation is characterized by weak consolidation, lithologic
discontinuities, abundance of rock outcrops, and shearing due to the occurrence of the Philippine
fault line approximately at the center of the mountain range. Minor earthquakes are relatively
frequent in the area Asio (2010). All these geological characteristics indicate that the area is
unstable, thus expectation on the rates of soil erosion and sedimentation is high. Also,
Pangasugan watershed comprises different land use systems and is very suitable on studies
related to soil erosion and sedimentation. Moreover, studies on soil erosion are very minimal in
the region where in fact, soil erosion is one vital factor on determining the environmental status
of a watershed. Hence, the study will be conducted to determine the rates of soil erosion and
sedimentation and the factors associated. In the Philippines, extreme soil erosion is widely
observed because of less data on soil erosion rate. Thus the study is vital especially for the
records of VSU.
Scope and Limitation
The study will only focus on the determination and evaluation of soil erosion and
sedimentation rates and the factors associated on it for a period of three months. These factors
include: rainfall intensity, vegetation, slope or topography, streamflow, land use system and soil
erodibility (e.g. soil type). Also the study determines nutrient that was lost due to erosion
specifically Nitrogen and Phosphorus.
Objectives of the Study
1. To determine the rates of soil erosion and sedimentation of the Pangasugan watershed.
2. To evaluate the factors affecting the rates of soil erosion and sedimentation of the
Pangasugan watershed.
3. To determine the amount of nutrients ( N, P and K ) lost through soil erosion.
Time and Place of the Study
The study will be conducted at the Pangasugan watershed of Brgy. Pangasugan, Baybay
City, Leyte, on November to February, 2012. Study sites that are choosen were shifting
cultivation(kaingin), coconut monocropping and intact forest.
Review of Literature
Soil erosion is not just a problem of modern times, although in the past 50 years there has
been more awareness of its consequence, more understanding of the process involved and more
knowledge of its cause-effect relationships than ever before. Soil erosion began with the dawn of
agriculture, when people began using the land for settled and intensive agriculture (Lal 1990).
According to some estimates, soil erosion and other degradative processes have destroyed, over
the millennia, as much arable land as is now cultivated (Kovda, 1977). Soil erosion results in a
net loss of irreplaceable soil with constituents that are needed for crop production such as
nutrients and organic matter being washed away. The seriousness of soil erosion in our
watershed areas is attributed to some factors such as rainfall, slope, soil characteristics,
vegetation cover and system of cultivation practiced by farmer. The magnitude of soil loss in
cultivated sloping areas has reached to an alarming proportion. According to Paningbatan (1989),
soil loss rates were much higher than the acceptable tolerable loss of 3 t ha-1 year-1. Moreover, he
found that erosion on bareplots (no vegetation) with slopes of 27 to 29% ranged from 23 to 218 t
ha-1 year-1. David and Collado (1967), estimated the rill and erosion rates in Magat watershed
(located in Isabela south of Cagayan) basin) to reach as high as 239 in savannah, 264 in open
grassland and 587 t ha-1 year-1 in kaingin areas.
In Leyte Island, visual observation would indicate the severity of erosion problem in
hillylands due to exposure of subsurface layers, gravelly top soils and even the bedrock itself. In
terms of growth for eroded areas with vegetation, performance of plants in the area is very
improvished which is a big sign of infertility.
Rainfall is an active agent that directly affects soil erosion .Raindrop impact cause the
destruction of soil aggregates and clogging of the soil pores promoting runoff that carries along
the detached sediments. According to Paningbatan (1989), the Philippines generally experiences
highly erosive rainstorms with an average annual rainfall amount of 4.2 meters. When the rate of
rainfall exceeds the rate of infiltration, it causes surface runoff (Beasley 1972). Slope is another
factor to consider which either represses or triggers soil erosion. In sloping areas which is about
10M ha (Ly Tung and Balinda, 1993), soil erodibility is high, hence highly susceptible to
erosion. According to Schachtschavel et al (1982) as cited by Reigning (1992)), overland flow
and soil loss increases with increasing length of slope, however, this influence is less pronounced
compared to the steepness of the slope. Slope length is defined as the distance from the point of
overland flow to the point where either the slope gradient decreases enough that the deposition
begins or run off enters a well-defined channel that maybe part of a drainage network or a
constructed channel (Weschmeier and Smith, 1978). Vegetation provides protection to the soil
by absorbing the kinetic energy of raindrops giving little chance for water to exert destructive
impact to the ground (Lal 1988). Soil cover provided by natural vegetation or agricultural crops
reduces soil loss mainly because leaves intercept raindrops and thus reduce kinetic energy. This
reduces the splash effect of raindrops significantly and prevents disintegration of soil aggregates
(Scwertmann, 1981). According to Weschmeier and Smith (1987), a complete grass sod is the
most effective way to cover the soil and control erosion the effectiveness of any crop,
management system or protective cover also depends on how much protection is available at
various periods during the year, relative to the amount of erosive rainfall that falls during these
periods. In this respect, crops which provide a food, protective cover for a major portion of the
year (for example, alfalfa or winter cover crops) can reduce erosion much more than can crops
which leave the soil bare for a longer period of time (e.g. row crops) and particularly during
periods of high erosive rainfall (spring and summer). However, most of the erosion on annual
row crop land can be reduced by leaving a residue cover greater than 30% after harvest and over
the winter months, or by inter-seeding a forage crop (e.g. red clover). . Land use practice also
influences detachability of soil particles (Reining 1992). Thus removal of natural vegetation
coupled with massive tillage operations in sloping lands would loosen the soil, enhancing soil
removal during rainstorm. Soil erosion potential is affected by tillage operations, depending on
the depth, direction and timing of plowing, the type of tillage equipment and the number of
passes. Generally, the less the disturbance of vegetation or residue cover at or near the surface,
the more effective the tillage practice in reducing erosion. Climate, soil properties and
topography thus determine the potential erosion susceptibility of a location, whereas the actual
erosion susceptibility depends on soil cover and tillage and measures of soil erosion control. The
latter is prevaingly determined by man through the respective land use system
(Arbeitsgemeinscaft Bodenkunde 1982: Foster et.al. 1982c). Land use system in tropics,
especially in the mountain areas are characterized by small scale farming and mixed cropping.
MATERIALS AND METHODS
•
Selection of Study Sites
Potential study sites in different land use systems were selected based on topographic
map, land use map and geologic map. Three (3) study sites were selected during the actual
survey. The sites were 1. Shifting cultivation (kaingin), 2. Coconut monocropping and 3. Intact
forest.
•
Field Measurements and Sampling
•
Soil erosion
Three (3) erosion plots were established in each study site. In addition, a canal
that will serve as catchment was also been established in each plot for the
collection of eroded soil. Monitoring of soil erosion rate will be done once per
week for three (3) months. The eroded soil collected was brought to the laboratory
for nutrient analysis. In addition, three erosion bars will be installed in each plot
to measure soil erosion rates.
•
Sedimentation
Stream water samples will be collected three times a week from each sampling
site, one (1) liter bottles will be used. Three (3) replications per sampling period
per site. Collection will be done at the middle, and side portions of the stream.
Sediment load will be determined by letting the water evaporates using an oven
and the sediments will be collected and weighed.
Streamflow
For steamflow analysis a standard requirements of locating gauging sites was
followed. Three sites along the main stream channel in upper, middle and
downstream portions of the river was chosen. This includes the very slow flowing
section, moderately fast flowing section and a section having a faster flow. In
every gauging site measuring of the stream widths and so with the distance among
which the float (ping pong ball) shall travel was done. The distance must be
uniform for all three sites. At least three float-time trials was observed in every
site. Cross-sectional depth of the river under study was determined.
Measurements was done every 20 centimeter section across the river. Stream
discharge was computed using as follows;
Q – (A x V) .85
where: Q - is the stream discharge in cu.m. per second
A – average cross-sectional depth times stream width (grand mean
values)
V- grand mean velocity in meters per second.
0.85- velocity correction factor
•
Soil erodibility
Soil type, land use system and slope/topography will be determined on the actual
field survey or through existing primary data. Soil erodibility will be determined
following John et al. (2003).
•
Vegetation
Two quadrats (10m x 10m) of uniform size and shape in randomly selected
sampling areas each site of the vegetation will be laid out. Trees, undergrowth
up to 3 meters height and herbs and grasses will be counted. Computation of the
percent cover of the plants will be done.
F. Meteorological Data
Data on rainfall (mm) and rainfall intensity throughout the duration of the study
will be obtained from the records of PAGASA station, VSU, Visca, Baybay,
Leyte (macroclimate) and by using an improvised rain gauge (microclimate).
G. Laboratory Analysis
Soil samples will be brought to DASS laboratory for pH, Organic Matter, N and
P.
pH (Potentiometric Method)

Soil pH will be analyzed potentiometrically using a 1:2.5 soil and water ratio (ISRIC
1995). A 20 g sieved soil will be weighed and placed into a properly labeled plastic cup.
It will be added with 50 mL of distilled water and is stirred thoroughly so that a
suspension will be formed. The solution was stirred every 15 minutes for 1 hour before
reading in a precalibrated pH-meter (Mothrem).
Illustration:
soil
20g
s
oi
l
Add 50 mL distilled
water and stir
Stand for one hour
Stir and Determine
Organic Matter (Modified Walkley-Black Method)
Soil Organic Matter will be analyzed using a modified Walkley – Black method
(PCCAR 1980). Exactly 0.5 gram of sieved soil (0.425 mm sieve) will be weighed and
placed into a 500 mL Erlenmeyer flask. Using a volumetric pipette, the soil will be
oxidized by adding 10 mL of 1 N K2Cr2O7 and then swirled gently to dispense the
solution. At the fume hood, 10 mL of concentrated H2SO4 was added rapidly and swirl
the flask for 1 minute. Allow the mixture to react at the fume hood for one hour before
adding 200 mL of distilled water. Thereafter, add 4 drops of O-Phenanthroline indicator
before the final titration with 0.5 N FeSO4. Endpoint of the titration will be reached if a
clearly visible maroon coloration would be observed. Percent soil organic matter will be
computed using the formula:
S
1
% SOM = (1 − B) 0.069 w
Where:
SOM – Soil Organic Matter
S – Volume of Ferrous Sulfate used in the sample (mL)
B – Volume of Ferrous Sulfate used in Blank (mL)
W – Weight of the Soil Sample.
Total Nitrogen (Kjeldahl Method)
Total Nitrogen will be determined using kjeldahl method (USDA, 2004).
i. Digestion
A gram of soil will be sieved using a 0.425 mm sieve and place into a 100 mL digestion
flask. Add a gram of selenium reagent mixture to the soil and mix thoroughly by swirling.
Under the fume hood, add 6 mL of concentrated H2SO4 until the mixture condenses about
one-third of the way up to the neck of the flask. Rotate the flask at 20 minutes interval to
facilitate the digestion of the sample. Stop the digestion when frothing or charring ceases
leaving a white precipitate. Remove the flask from the digester and allowed to cool. Ii. Ii.
ii. Distillation
Slowly add 30 mL distilled water to the digest. Swirl the flask. Transfer the digest into a
Buchi distilling flask and add 50 mL of 40% NaOH, hold the flask at about 45 . Attach the
flask to the distillation set-up. For the receiver of the distillate, put a 125 mL Erlenmeyer
flask with 25 mL of 2% H3BO3 and three drops of mixed indicator just beneath the flask.
Titrate the distillate with 0.05 N H2SO4 until the color of the solution mixture changes from
green to pink. Prepare a blank solution and the same determination process will be done.
Compute for the total Nitrogen using the formula:
%N=
(a−b)×N×0.014
N – Nirmality of H2SO4
Where:
a – mL of H2SO4 in soil
Sample
b
– mL of H2SO4 in the
blank
W – weight of the soil
0.014 – meq. wt. of Nitrogen
W
× 100
Available Phosphorus

Available Phosphorus (ppm) will be determined following the Olsen method. Soils that
have pH above 7.0 will be extracted using NaHCO3 at pH 8.5 as extracting solution.
Weigh a 2-5 g sieved soil sample and place in a 50 mL Erlenmeyer flask. Add a 25 mL
extracting solution NaHCO3 and mix using a reciprocating shaker for 5 minutes within a
minimum of 180 oscillations per minute. After shaking, filter the mixture using
Whatmann #42 filter paper (optional). Collect the filtrate using a plastic receiver. Using a
volumetric pipette, add 2 mL Aliquot with reagent C (mixture of ammonium molybdate,
potassium antimony titrate, and ascorbic acid) and mix using a vortex mixer. 2 mL of the
working solution will be added with reagent C and shake. Prepare also a blank sample.
Percent transmittance will be read using a B-L Spectronic 20. Convert the value obtained
into Optical density (OD). The ppm P in the solution and in the soil will be calculated
using the formula:
ppm P in the solution = ODS × K
Where:
ODS – Optical Density of the sample
K – Slope of the standard curve for P
K=
ppm P standard
% Absorbance
ppm P in Soil = ppm P in solution ×
Where:
25
2.5
× dillution
25 – volume of extracting solution (mL)
2.5– weight of soil (g)
3. Data Analysis
Analysis of Variance (ANOVA) and linear regression will be analyzed after the
collection of the data.
LITERATURE CITED
Asio V.B. 2010, The Physical Environment of Mt. Pangasugan, Leyte, Philippines, SOIL
and ENVIRONMENT: soil and its relation to agriculture, environment, global
warming, and human health. Available @:
http://soil-environment.blogspot.com/2010/03/physical-environment-of-mtpangasugan.html
Flavio S. Anselmetti et al. 2007. Quantification of soil erosion rates related to Mayan
deforestation., also available at:
John R, H.P. Blume and V.B. Asio 2003. Student Guide for soil description,
Classification and Evaluation. University of Halley, Germany.
Lal R.: Soil Erosion in the Tropics-principles and management, McGraw- Hill Inc. R.R.
Donnelley and Sons Company: 1990
Nelson D.W and L.E. Sommers. 1982. Total Carbon, and Organic Matter. Pp. 539-579
Miller R.H. and Keeney D.R.In: Methods of Soil Analysis. Part 2. Chemical and
Microbiological properties. 2nd ed. Eds.
A.L. Page, x.
Pasa A.E., Watershed laboratory manual, College of Forestry and Environmental
Sciences, VSU.
Salas F.M., Environmental Chemistry manual, Dept. of Pure and Applied Chemistry,
VSU.
Reining
L.–Weikersheim;
Margraf,,.Erosion
in
Andean
Hillside
Farming;
characterization and reduction of soil erosion by water in small scale cassava
cropping systems 1992.
Wischmeier, N.H., D.D. Smith 1978, Predicting rainfall erosion looses-a guide to
conservation planning. U.S. Department of Agriculture, Agriculture Handbook
No. 537, Washington D.C.
Scwertmann, U. 1981. Die Vorausschatzung des Bodenabtrags in Bayern (Verfahren
Wischmeier and Smith). Bayer. St. Ministerium f. Ernahung, Landwirtschaft und
Forsten Munchen.
Kovda, V.A,Dregne, H.E.; Henning, D.; Flohn, H 1977: Status of desertification in the
hot arid regions. Climate aridity index map. Experimental world scheme of
aridity and drought probability. At a scale of 1:25,000,000, United Nations
Conference on Desertification. Nairobi (Kenya), 29 Aug 1977.
Paningbatan, E.P., A.R. Maglinao, L.A.Calanog and G.M. Huelgas. 1992.
Managementof sloping lands for sustainable agriculture in the Philippines. In:
Technical Reporton the Management of Sloping Lands for Sustainable
Agriculture in Asia,Phase 1, 1988-1991 (IBSRAM/ASIALAND).
Network Document No. 2
Photo Documentation
The Sites
•
Kaingin
•
Coconut Monocropping
•
Intact Forest