Evaluating effects of vegetation cover types on overland flow generation and soil erosion in luot mountain

ABSTRACT We examined the effects of three vegetation types: Cinnamomum parthenoxylon plantation, shrub and grass on overland flow and soil erosion.. We established three plots in the sa

MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT

VIETNAM FORESTRY UNIVERSITY

Faculty: Forest Resources and Environmental Management

Student: Chao Thi Yen Student ID: 1053020749

Class: K55 Natural Resources Management Course: 2010 - 2014

Advanced Education Program Developed in collaboration with

Colorado State University, USA

Supervisor: Dr Bui Xuan Dung

ABSTRACT

We examined the effects of three vegetation types: Cinnamomum parthenoxylon

plantation, shrub and grass on overland flow and soil erosion We established three plots in the same catchment but different vegetation cover types to measure the surface runoff and

soil erosion after each storm event The plots were labeled Plot 1 for Cinnamomum

parthenoxylon plantation, Plot 2 for shrubs and Plot 3 for grass In Plot 1, the canopy cover

was 90%; ground surface was covered 80% by small Cinnamomum parthenoxylon trees

and litterfall Ground surface in Plot 2 was covered by shrub and litterfall up to 90% and

40 - 80% was the surface cover of Plot 3 was grass The rainfall, surface runoff and sediment were collected after each storm event The total of monitoring storm was 10 storm events with the rainfall from 3 mm to 202 mm There was no clear difference of

surface runoff and sediment in Cinnamomum parthenoxylon plantation, shrub and grass based on ANOVA test The total amount of surface runoff in Cinnamomum parthenoxylon

plantation was 7 mm ranged from 0.01 mm to 5 mm This amount of surface runoff in shrub was 12 mm ranged from 0.03 mm to 7.3 mm and the total amount of surface runoff

in grass was 13 mm ranged from 0.01 to 7.0 mm The total amount of eroded soil in

, in shrub was 77 g/ ranged from 0.8 g/ to 30 g/ and 86 g/ ranged from 0.8 g/ to 26 g/ in grass When surface runoff increased, the sediment from soil erosion also increased The surface runoff in storms that bigger than 25 mm was 95 times higher than that in storms less than 25 mm; the amount of eroded soil in storms that bigger than 25 mm was 16 times greater than that in storms less than 25 mm

LIST OF TABLE CONTENTS

I INTRODUCTION 1

II OBJECTIVES 6

2.1 Hypothesis 6

2.2 Objectives 6

III STUDY SITE AND METHODS 7

3.1 Study site 7

3.2 Methods 8

IV RESULTS 13

4.1 Rainfall characteristics on Luot Mountain. 13

4.2 Surface runoff from Cinnamomum parthenoxylon plantation, grass and shrub. 13

4.3 Sediment from Cinnamomum parthenoxylon plantation, grass and shrub. 17

4.4 Relationship between sediment and surface runoff. 20

4.5 Amount of surface runoff and sediment from big storms compared to small storms. 21 V DISSCUSION 24

5.1 Vegetation cover and surface runoff relationship. 24

5.2 Vegetation cover and sediment. 26

5.3 Sediment and surface runoff relationship. 28

5.4 Effect of storm sizes on surface runoff and soil erosion. 29

VI CONCLUSION 30

VII REFERENCES 31

LIST OF FIGURES

Figure 1 (a) Location and topography of study site (b) detail of study site and sample plots (c) plot 1 (d) plot 2 (e) plot 3. 7

Figure 2 (a) Plot 2 (b) Plot 1 9

Figure 3 (a) Weighing soil samples (b) Soil drying 10

Figure 4.1 Rainfall characteristic measured at the VFU Luot Mountain weather station. 13

Figure 4.3 Surface runoff from Cinnamomum parthenoxylon plantation, grass and shrubs

16

Figure 4.4: The response of sediment in Cinnamomum parthenoxylon plantation, grass and shrub cover to each storm (a) precipitation, (b) sediment in each storm. 19

Figure 4.5 Sediment from Cinnamomum parthenoxylon plantation, grass and shrub. 19

Figure 4.6 The relationship between sediment and surface runoff in different vegetation cover types: (a) in Cinnamomum parthenoxylon forest (b) in shrub (c) in grass. 20

Figure 4.7 Amount of sediment, surface runoff, surface runoff coefficient from small storms and big storms: (a) sediment, (b) surface runoff, (c) surface runoff coefficient. 22

LIST OF TABLES

Table 1 Results of surface runoff analysis between three vegetation cover means: Cinnamomum parthenoxylon plantation, grass and shrub. 14

Table 2 Surface runoff from Cinnamomum parthenoxylon plantation, grass and shrub. 14

Table 3 Percentage of surface runoff in Cinnamomum parthenoxylon, shrub and grass (%). 17

Table 4 Results of surface runoff analysis between three vegetation cover means: Cinnamomum parthenoxylon plantation, grass and shrub 18

Table 5 Sediment from Cinnamomum parthenoxylon plantation, grass and shrub by each storm event. 18

Table 6 Amount of surface runoff and sediment in different storm sizes. 23

Table 7 Percentage of throughfall and interception in Cinnamomum parthenoxylon forest (%) 24

Table 8 The response of surface runoff and sediment to the change of coverage in grass

27

I INTRODUCTION

Soil erosion occurs when soil is removed through the action of wind and water at a greater rate than it is formed (National Department of Agriculture) The causes of soil erosion include natural, animal and human activities such as climate change, overgrazing, overcultivation, forest clearing, mechanized farming, road systems Soil erosion has effects

on agriculture, forestry, and on human life Soil erosion results in land infertility and leads

to desertification as well as devastating flooding Erosion results in the degradation of a soil„s productivity: it reduces the efficiency of plant-nutrient use, damages seedlings, decreases plants rooting depth, reduces the soil water-holding capacity, impedes permeability, increases surface runoff, and decreases infiltration rate (Zuazo et al., 2011) Soil erosion also causes water pollution by increasing turbidity, concentration of heavy metals and the complexity and the uncertainty of the non-point source pollution Assessment and quantification of pollutant loads are very difficult and inaccurate The Food and Agriculture Organization (FAO; - a branch of United Nations) estimates that the global loss of productive land through erosion is 5-7 million ha/year" (National Department of Agriculture) Zuazo et al (2011) estimated global soil loss to erosion to be

26 billion Mg (an average of 16 Mg ) Many authors have showed that 5-12 million hectares of land (0.3-0.8% of the world„s arable area) are rendered unsuitable for agriculture each year due to soil degradation Oldeman et al (1991) concluded that human-induced soil degradation has affected nearly 2 billion hectares, or 15% of the earth„s total land area since the middle of the twentieth century Water and wind erosion worldwide has accounted for about of 1,094 and 548 million hectares, respectively"(NOVA Science Publishers 16) Zuazo et al (2011) estimated that soil loss from global farmlands is currently running at a rate of more than 6 t

Overland flow is an important hydrological process that occurs either when rainfall intensity exceeds the infiltration rate of the soil (i.e Hortonian overland flow) or when the soil is saturated and depression storage capacity is exceeded (Dung, 2001) The direct impact of raindrops on the soil surface break down soil structures (aggregates) and disperse the aggregate material transporting by surface runoff easily Light aggregate materials such as fine sand, silt, clay, and organic matter could be removed by the rain splash and surface runoff, greater raindrop or greater surface runoff amounts might be required to move the larger sand and gravel particles

The soil lost from surface runoff is usually greatest and most noticeable during short-duration, high-intensity thunderstorms and less noticeable during long-lasting and less intense storms Surface runoff can occur when there is excess water on a slope that cannot be absorbed into the soil or trapped on the surface The amount of surface runoff will increase if infiltration decreases due to plant cover, precipitation, soil compaction, soil roughness, and topography (steepness and length of slope) According to Romkens (2001) total sediment yield in the initially smooth surfaces was smaller than that in the initially medium-rough and rough surface conditions, while the sediment yield of the latter two roughness conditions were very similar for corresponding steepness of slope and rainstorm intensity regimes Total sediment yield from the initially smooth surface of the 8% and 17% slope steepness cases were larger for the decreasing rainstorm intensity sequences as compared to the increasing rainstorm intensity sequences Sediment concentration in surface runoff during prolonged rainfall on an initially dry soil surface increases rapidly, and then decreases gradually

The length and steepness of slope are two essential features of topography relating to soil erosion and surface surface runoff Topography steepness is a significant factor

of respective increases in volume and velocities of surface surface runoff The determination of slope-steepness factors is necessary for measuring soil erosion

Soil texture also influences surface surface runoff and soil erosion Soil erodability is

a factor to estimate the ability of soils to resist erosion, based on the physical characteristics of each soil Soils with faster infiltration rates, higher levels of organic matter and improved soil structure usually have a greater resistance to erosion Sand, sandy loam and loam texture soils tend to be less erodible than silt, very fine sand, and certain clay texture soils Zuazo et al (2011) also indicated that silty soils tend to be the most erosive, soils that have a relatively high content of clay tend to be the least erosive soils Soils have a mixture of sand silt and clay, and in many soil the ratio is very similar However, even with soils with similar ratios of sand, silt and clay may have drastically different soil erodability Soil with good soil structure will allow more water infiltration, thereby reducing surface runoff water and erosion

Erosion is affected by the condition of the soil surface, the slope of the land, and how much vegetation covers the soil surface Vegetation cover is the most significant factor to determine the severity of erosion process Plants and litter cover play an important role in soil surface protection or soil erosion prevention “Plants slow down water as it flows over the land (surface runoff) and this allows much of the rain to soak into the ground Plant roots hold the soil in position and prevent it from washed away Plants break the impact of

a raindrop before it hits the soil, thus reducing its ability to erode Plants in wetlands and

on the banks of rivers are particular importance as they slow down the flow of the water and their roots bind the soil, thus preventing erosion”(National Department of Agriculture) Miyata et al also emphasized that ground vegetation cover is an important factor in controlling overland flow and inter-rill soil erosion, which consists of the detachment and transport of soil particles Raindrop impact causes mechanical breakdown of soil

aggregates and soil detachment (Miyata et al,2009) Soil erosion being from the simulated storm was greatly reduced by vegetative cover, declining from 30-35 t ha-1 at 0% vegetative cover to 0.5 t ha-1 at 47% cover, and reductions in erosion at lower levels of vegetative cover were greater than predicted by the cover/erosion relationship used in the USLE" (Zuazo et al., 2011)

The combination of root system and canopy cover is also useful for reducing soil erosion Roots of trees increase number of macro-pore so they promote the infiltration rate and reduce the surface runoff, whereas the canopy cover acts as a roof to cover the land surface and intercept rainfall In forested zones, surface runoff and erosion are drastically reduced by the presence of trees; the leaf litter also acts as protection of the soil (Descroix

et al, 2001) According to Descroix et al (2001), the amount of surface runoff coefficient without tree was 0.23 %, without tree but with litter was 0.085 %, and with tree was 0.028% The amount of sediment also differed from the site of with and without trees In particularly, amount of sediment in the site without trees was 133 g/ , without trees but with litter was 30 g/ , and with trees was 1.1 g/

Three- quarters of Vietnam is hilly with steep slopes Precipitation of Vietnam is high from 1800 to 2000 mm/year and the 4-5 month rainy season accounts for 80% of total rainfall With a large amount of rainfall concentrating in some rainy month in the year, the amount of surface runoff accumulates and creates a big surface runoff flow with high intensity Vietnam's forest cover is about 39%, mostly is poor forest with low coverage (surface cover) because of deforestation and soil cultivation In recent years, farmland area was enhanced due to loss of grass and shrubs area because of herbicides These are the main reasons for soil degradation in Vietnam Total amount of soil loss in Northwest of Vietnam was 119.2 tons/ha in 1962, this amount have reached 134.0 tons/ha in 1963

alluvium to the oceans The Red River in Northern Vietnam accounts produces 1000 g/

of water (Phuong et al, 2012) From 1983 to 1994, approximately 1.3 millions of hectare forest was destroyed for timber and cultivation leading to the rate of soil erosion and overland flow increased dramatically Soil degraded rapidly in the Northern of Red River, where about 700 thousand of hectares of soil were degraded The percentage of lost soil is

1 -2%/year Vietnam Soil Science Congress has estimated that 80 thousand tons of soil will

be lost, damaging 15 billion Vnd (nearly $ 7000) each year More than 50% of natural ground will be degraded Soil erosion and degradation threat to the economic development

of Vietnam with a higher and higher rate (Phuong et al, 2012)

Soil erosion causes a lot of damage and one of the primary factors affecting erosion and surface runoff is vegetation cover Research regarding the importance of especially the importance of shrubs and grass in protecting soil surface is limited In Vietnam, cultivating of medical herbs and food for cattle are the primary topics on interests This is the reason of overgrazing in grasslands Many researchers and stakeholders emphasize the importance of forest in providing fresh air, scenic landscapes, and keeping water, but the role of forest in preserving ground surface is seldom mentioned Differences soil erosion

in Cinnamomum parthenoxylon forest, grass and shrubs as well as the importance of cover

types for erosion controlling is necessary for providing good data and good information of erosion so that we can know how can manage the erosion by using vegetation cover types

We therefore can decide which cover type is the best for managing the soil erosion, which

one should be protected and developed Cinnamomum parthenoxylon was planted

everywhere in Vietnam which has a high canopy cover and economic value, especially land surface in Vietnam is covered almost by grass and shrubs so a research on the importance of these three types of vegetation cover in preserving ground surface is of great interest

II OBJECTIVES 2.1 Hypothesis

Different cover types have a significantly different overland flow (amount of surface

runoff) and soil erosion Overland flow and soil erosion of Cinnamomum parthenoxylon plantation is highest and is lowest in grass Shrub has higher overland flow and soil erosion

than grass but lower than plantation

2.2 Objectives

The specific objectives of this research are:

1 To determine the characteristic of precipitation on Luot mountain

2 To examine effects of different cover types: Cinnamomum parthenoxylon plantation,

grass and shrubs on overland flow generation

3 To evaluate soil erosion characteristics: amount of sediment and soil erosion rate in

Cinnamomum parthenoxylon plantation, grass and shrubs

4 To evaluate the relationship between surface runoff and sedimentation in Cinnamomum

parthenoxylon plantation, grass and shrub

5 To examine the effect of rainfall on total surface runoff and sediment

III STUDY SITE AND METHODS 3.1 Study site

Figure 1 (a) Location and topography of study site (b) detail of study site and sample

plots (c) plot 1 (d) plot 2 (e) plot 3

The research was conducted on Luot Mountain which is a part of Vietnam Forestry

University campus This has large area of Cinnamomum parthenoxylon plantation, grass,

Source: Dzung et al, 2012

(a)

(b)

and shrubs The mountain has moderate mountainous terrain with two small mountains, the upper mountain is 133 meters above sea level and the lower mountain is 99 meters above sea level The average slope is 15 - 20 º Luot Mountain has tropical monsoon climate The average temperature is from 23.9◦C with the lowest mean temperature of 17.1◦C in January and the highest mean temperature of 28.5 ºC in June and July Average relative humidity is 81.5% with the highest humidity being 85.5% in March and the lowest humidity being 78

% in December Annual precipitation is 1647 mm/year The highest monthly precipitation

is in July and August with more than 300 mm and the lowest is 22 mm in December

3.2 Methods

Installing monitoring plots

Using GPS to measure the distance between 3 points that will be established sample plots

Overland flow and soil erosion were monitored at bounded monitoring plots (1 m

wide and 2 m slope length) on the three types of vegetation cover: Cinnamomum

parthenoxylon plantation, shrub and grass We established one monitoring plot for each

vegetation cover type; the size of plot is 2 The plots were labeled Plot 1 for

Cinnamomum parthenoxylon plantation, Plot 2 for shrubs and Plot 3 for grass In Plot 1,

the canopy cover was 90%; ground surface was covered 80% by small Cinnamomum

parthenoxylon trees and litterfall Ground surface in Plot 2 was covered by shrub and

litterfall up to 90% and 40 - 80% was the surface cover of Plot 3 was grass We took photos of the plots and then used photoshop to calculate the canopy cover

The border of plot was built by plastic We buried the plastics at least 8 cm to make sure that the plastic can stand during heavy rain and the height of the border was 25 cm to prevent rain splash The plot was perpendicular to contour line At the down slope end of

each plot, a plastic sheet was inserted between the O-horizon and the soil matrix to transport overland flow and sediment to a collection gutter [Miyata et al., 2007], which led

to a big can that we used to collect surface runoff and sediment after each storm event The gutter was made by a big plastic pipe so that it had enough room for surface runoff and sediment transferring during some very big storms We used nylon to cover the gutter to prevent rain splash and rainfall from outsides After we have finished establishing the plots, we set up a rain gauge for each plot to measure the rainfall The rain gauges were positioned perpendicular and far from trees to avoid interception from the overlying canopy

Figure 2 (a) Plot 2 (b) Plot 1

In Plot 3, we first cut all shrubs covering grass and then we removed all trees surround the plot to avoid raindrop and to make sure only grass in this plot We took photos of this plot in different period to calculate the change of cover ratio over time

Erosion factor, surface runoff and sediment measurement

Time of observation was from 17 July 2014 to 30 August 2014

(a) Precipitation: Precipitation was measured by America plastic rain gauge The total

amount of precipitation and the time of the start and end of each rainfall event was recorded We measured 10 storms; an inter-storm period was defined as a period of at least

6 hr without rain Because the amount of overland flow decreased quickly after

precipitation ceased, a 6-hr period without precipitation was sufficient to distinguish storm events (Dzung et al.2011) The second storm therefore was not affected by the first storm After each storm, we collected rainfall in every rain gauge and then compared to the rainfall in Vietnam Forestry University's weather station We also compared the rainfall

between three plots to know the throughfall of Cinnamomum parthenoxylon forest

(b) Surface runoff: We collected the amount of surface runoff from the collecting cans

We used the vitro with 1000 ml and 50 ml to measure surface runoff When the amount of surface runoff was more than 100 ml, we used 1000 ml vitro to measure; but when the amount of surface runoff was less than 100 ml, the 50 ml vitro was preferred

(c) Sediment measurement: Sediment was also collected from cans after every storm

event After storms we waited until sediment had been accumulated to the bottom of cans, and then we poured the amount of surface runoff to the vitro slowly, after that we used labeled bottle to contain the sediments and bring them to Laboratory for drying process The collected sediment samples were weighed before and after drying; these samples were dried at 105º C

Figure 3 (a) Weighing soil samples (b) Soil drying

(d) Soil properties: We identified soil characteristics by soil porosity and bulk density To

measure soil porosity and bulk density we did some following steps:

- Step 1: We cut surface vegetation and cleaned ground surface

- Step 2: We used a wood to gentle hammer to the bulk density ring until its level with the soil surface

- Step 3: A shovel was used to dig around the ring after that we used a knife to cut the redundant soil

- Step 4: Soils were labeled in plastic bags

- Step 5: We brought the samples to Laboratory to weigh and dry the soil

Soil porosity and bulk density were calculated by following formulas:

Determine the Bulk Density:

Bulk Density (p) = Mass of oven dried soil / Total volume

Porosity: we calculated soil porosity when bulk density was known by below formula:

P = 1 - where: BD is bulk density and PD is particle density

Because we only knew bulk density, so we can assume particle density is equal to 2.65 g/ ( )

(e) Vegetation cover: After finished establishing the plots, we took photos of each plot

and then using photoshop software to calculate the cover of each

(g) Slope: We used Compass to measure slope in each plot The slope gradient of Plot 1

was 20.5 º, Plot 2 was 18.5º and 17º was steepness of Plot 3

(h) Surface runoff coefficient:

Surface runoff coefficient = (total amount of surface runoff/ total of precipitation)*100

IV RESULTS 4.1 Rainfall characteristics on Luot Mountain

Figure 4.1 Rainfall characteristic measured at the VFU Luot Mountain weather

station

Total amount of rainfall on Luot Mountain from January to October was 1517 mm Average precipitation was 152 mm/month The highest monthly precipitation was in August with nearly 500 mm and the lowest is 1.2 mm in June

Our monitoring time was in July and August that were two of three months having

highest precipitation on the study site

4.2 Surface runoff from Cinnamomum parthenoxylon plantation, grass and shrub

The difference in surface runoff generation was testing using a ANOVA test There

is no significant difference in amount of surface runoff in Cinnamomum parthenoxylon

plantation, grass and shrub cover (table 1)

0 100 200 300 400 500

Table 1 Results of surface runoff analysis between three vegetation cover means:

Cinnamomum parthenoxylon plantation, grass and shrub

P -value shows us how the samples different In this case, P-value was 0.7 and bigger

than 0.05 so amount of surface runoff in Cinnamomum parthenoxylon plantation, grass and

shrub had no significant difference Table 2 shows the amount of surface runoff in different vegetation cover means overtime, we can see the change of surface runoff in these different cover types

Table 2 Surface runoff from Cinnamomum parthenoxylon plantation, grass and shrub

Figure 4.2 The response of surface runoff in Cinnamomum parthenoxylon plantation, grass and shrub cover to each storm (a) precipitation; (b) Surface runoff during each storm event; (c) Surface runoff coefficient during each storm event

0 50 100 150 200 250

Cinamomum Shrub Grass

Cinamomum Shrub Grass

Surface runoff coefficient during each storm event