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

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:

% SOM=(1− SB )0.069 1

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 ×0.014W

×100

Where: N – Nirmality of H2SO4

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

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:

ppmP∈the solution=ODS×K

Where: ODS – Optical Density of the sample

K – Slope of the standard curve for P

K= ppmPstandard% Absorbance

ppmP∈Soil=ppmP∈solution× 252.5×dillution

Where: 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-mt-

pangasugan.html

Flavio S. Anselmetti et al. 2007. Quantification of soil erosion rates related to Mayan

deforestation., also available at:

http://www.ask.com/web?

qsrc=32

7 & o=102140 & l=dir & q=soil+eosion+and+sedimentation+rates & oo=102140

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