Evaluating effects of acacia plantation forest on surface runoff and erosion in luong son headwater of vietnam

Comparison of the runoff generation to forest cover change by bare land and four different Acacia tree-aged .... ABSTRACT To assess soil losses and overland flow in Acacia plantations of

MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT

VIETNAM NATIONAL UNIVERSITY OF FORESTRY

FINAL THESIS EVALUATING EFFECTS OF ACACIA PLANTATION FOREST

ON SURFACE RUNOFF AND EROSION IN LUONG SON HEADWATER

OF VIETNAM

Major: Natural Resources Management

Faculty: Forest Resource and Environmental Management

Advanced Education Program Developed in collaboration with Colorado State University, USA

Supervisor: Adjunct Associate Professor Dr Bui Xuan Dung

Hanoi, 2018

ACKNOWLEDGEMENT

First and foremost, to be able to conduct this research, I would like to express my sincere respect to my supervisor - Assoc Prof Dr Bui Xuan Dung for his enthusiastic and patient support with invaluable comments In addition, I appreciated the support of other lectures during the time I analyzed the data

Not only that, many thanks are due to my friends when I started to collect and analyze data They always give me the support whenever I needed In terms of difficulties, the transport

to study sites was hard without my companions

Lastly, I express my gratitude to local people who own the Acacia plantation model for allowing us to conduct this research in this site They also informed me about the weather which extremely support me to get the data

Hanoi, October 2018

iii

TABLE OF CONTENTS

ACKNOWLEDGEMENT ii

TABLE OF CONTENTS iii

LIST OF FIGURES v

LIST OF TABLES vii

ABSTRACT viii

CHAPTER I: INTRODUCTION 1

CHAPTER II: GOAL AND OBJECTIVES 4

2.1 Goal 4

2.2 Objectives 4

CHAPTER III: STUDY SITE AND METHODS 5

3.1 Study site 5

3.2 Methods 6

3.2.1 Installing monitoring plot 6

3.2.2 Rainfall and soil physical characteristics 8

3.2.3 Runoff and soil erosion measurement 11

3.2.4 Vegetation observation 12

3.2.5 Topographic survey 12

3.2.6 Data analysis 13

CHAPTER IV: RESULTS 14

4.1 Soil physical factors, vegetation and precipitation characteristics on study sites 14

4.1.1 Soil physical factors and vegetation on study sites 14

4.1.2 Precipitation characteristics 15

4.2 Runoff generation characteristics on study site 18

4.3 Soil erosion on study site 24

CHAPTER V: DISCUSSION 29

5.1 Surface runoff at bareland and different ages of Acacia 29

5.2 Soil erosion at bareland and different ages of Acacia 34

CHAPTER VI: CONCLUSION 41

CHAPTER VII: RECOMMENDATION 41

REFERENCES 44

APENDIX 49

v

LIST OF FIGURES

Figure 3.1 The map of study site 6

Figure 3.2 Location of five plots 7

Figure 3.3 The model of plot and experiment conducted 8

Figure 3.4 Rain gauge 8

Figure 3.5 Measured poroxity 10

Figure 3.6 Determined amount of runoff 11

Figure 3.7 Took eroded soil and dried and in the Lab 11

Figure 3.8 GLAMA and Canopy Cover Free application 12

Figure 3.9 GPS and Compass application 12

Figure 3.10 Create box-plot by R-studio 13

Figure 3.11 Box-plot graph 13

Figure 4.1 Storm events 15

Figure 4.2 Precipitation, surface runoff and runoff coefficient at different ages of Acacia 18

Figure 4.3 Precipitation accumulation and runoff accumulation at different age of Acacia 20

Figure 4.4 Comparison of the runoff generation to forest cover change by bare land and four different Acacia tree-aged 21

Figure 4.5 Correlation between precipitation and surface runoff at different age of Acacia 22

Figure 4.6 Precipitation and soil eroded at different ages of Acacia 24

Figure 4.7 Precipitation and soil erosion accumulation at different ages of Acacia 25

Figure 4.8 A comparison of soil erosion to forest cover change by bare land and four different Acacia tree-aged 26

Figure 4.9 Correlation between soil erosion and precipitation, surface runoff 27

Figure 4.10 Annual soil erosion at different ages of Acacia 28

Figure 5.1 Runoff coefficient of species on other study 30

Figure 5.2 The soil erosion annually at different Acacia ages 34

Figure 5.3 Annual soil erosion of vegetation types on other studies 36

Figure 6.1 Effect of Acacia plantation in different ages 42

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LIST OF TABLES

Table 4.1 Outline of five plots of different vegetation cover types 14

Table 4.2 Precipitation, API7 and rainfall intensity on study sites 16

Table 4.3 Vegetation on study site 17

Table 4.4 Surface runoff, runoff coefficient analysis between five plots 19

Table 4.5 Independent samples t-test for the effect of surface runoff to Acacia’s age 22

Table 4.6 Soil erosion analysis between five plots 25

Table 4.7 Independent samples t-test for the effect of soil erosion to Acacia ages 26

Table 5.1 TCVN 5299: 2009 - Method for determination of soil erosion by rainfall 36

Table 5.2 The soil erosion annually on other studies 37

ABSTRACT

To assess soil losses and overland flow in Acacia plantations of different ages and evaluating erosion risk to surface cover by investigating the characteristics of surface runoff generation and soil erosion derived from bare land and Acacia Plantation forest model in Luong Son headwater of Vietnam Five plots (10 m2/plot) at different vegetation cover conditions (bare land, 4-month-old Acacia trees, 1-year-old Acacia trees, 2-year-old Acacia trees and 4-year-old Acacia trees) was set up and monitored over 75 storm events from September 2017 to August

2018 in Truong Son Commune, Luong Son district, Hoa Binh Province The main findings included: (1) Runoff coefficient was the highest at bare land (7.87%) and the lowest 4-year-old Acacia trees (0.40%); (2) The highest amount of soil loss accumulation during the observed time was induced from bare land plot (103.75 kg), the intermediate from 4-month-old Acacia (27.52 kg) 1-year-old Acacia (12.61 kg), 2-year-old Acacia (11.99 kg), and the lowest at 4- year-old Acacia (14.19 kg); (3) Both the amount of runoff and soil erosion has a strong relationship with vegetation cover and precipitation (p<0.00)

Key words: Acacia plantation model, bare land, field study, headwater, runoff generation, soil erosion, vegetation cover conditions

Most countries in the world suffer from the effects of soil erosion According to the results announced by Eswaran et al (2001), the manufacturing capabilities of some areas in the world will be reduced to 50% because of erosion and desertification In South Asia, cereal production fell by about 36 million tons per year due to water erosion equivalent to $5.4 billion;

At global level, the surface of the earth loses 75 billion tons of land annually, equivalent to the economic value of 400 dollars billion In per capita, every citizen on Earth loses approximately

70 USD/year (Eswaran et al., 2001) In Vietnam, having 80,000 tons of soil erosion, damaging

15 billion VND each year (Phuong et al, 2012), especially Hoa Binh is one of the provinces of Vietnam most affected by erosion due to the geological structure had several fault systems and structural elements such as anticlines, synclines, grabens, accumulated address exists in the territorial demarcation Hoa Binh is a mountainous province in the Northwest of Vietnam In Hoa Binh province, according to the investigation, at 2012, the topsoil is eroded due to the precipitation loss of over 34.5 million tons of soil per year, in which the hills and mountains

are eroded over 1 million tons per year, less than 90,000 tons/ha per year under 900 meters, over 84,000 tons/ha/year (Viet Lam, 2012)

Soil erosion is determined by various factors such as rainfall, rainfall intensity, soil properties, topography and vegetation (Vo Dai Hai, 1996, Nguyen Van Dung and Tran Duc Vien, 2005) Among the above factors, vegetation is considered an important contributing factor in soil protection, reducing surface runoff and erosion (Pham Van Dien, 1998; Vo Dai Hai, 1996; Castillo et al., 1997, Canton et al., 2001, Vo Dai Hai, 2006; Miyata et al., 2009) For decades, authors at home and abroad have done many researches about vegetation cover related

to soil erosion (Nguyen Trong Ha, 1996, Nguyen Quang My, 2005, Hudson, 1981, Zakharov, 1981) There are many studies which found that plants can trap runoff then reduces the amount

of soil eroded (ex Etafa Emama and Morgan, 1995; Morgan, 2007) In general, natural forest land has ability to penetrate and retain water well due to its high-water consumption, strong roots rooted deep into the soil, while natural forest also had a thick mat of thick soils, from which soil erosion was significantly reduced (Bonell, 1998; Descorix et al., 2001) The coefficient of surface runoff where no tree is 0.230% and where having tree is 0.028% The amount of sediment also varies from where having trees and without trees, sediment where no tree is 133 g/m2 and where having tree is 1.1 g/m2 (Descroix et al., 2001) The previous studies also showed that in species of different plants, the ability to regulate water and to reduce erosion

is different (Vo Dai Hai, 1996, Chao Thi Yen, 2014; Bui Xuan Dung, 2016) For a long time, plants have been an effective way to combat erosion as well as being widely disseminated as a means of soil conservation (Morgan, 1986)

Acacia mangium Willd., also known as mangium, is a species of plants indigenous to Northern Queensland (Australia), found in Indonesia's Irian Jaya, Maluku (Doran and Skelton, 1982) This is a fast-growing species which widely used for various purposes such as timber, firewood, agroforestry, land improvement (Turnbull et.al, 1983) From the economic and social benefits of Acacia, Acacia plantations are expected to increase annually (Ministry of

3

Agriculture and Rural Development, 2012) However, the lack of a database reflects the relationship between Acacia plantations and the formation of surface runoff and erosion in Vietnam, leading to difficulties and challenges in the development of plantation forest models

to achieve the best environmental performance In Vietnam, about 24% of the forest area is planted forest, of which Acacia mangium is a popular crop, bringing high economic value (Ministry of Agriculture and Rural Development, 2012) Acacia mangium is one of the grown plants in Luong Son, Hoa Binh According to decision on approving and publishing forest inventory results of Hoa Binh province 2016 (People's Committee of Hoa Binh province, 2016), 92% of forest type in this province is Acacia forest Acacia grows well at slope from 15o – 25o, with elevation bellow 400 m and soil depth greater than 50 cm This type of forest is mainly planted in headwater Prior to severe erosion headwater situation, the Department of Science and Technology and the Center for Environment - Territories (CEGTeP) have proposed measures to promote the greening of barren land and hills In total 150 ha of forest trees in Truong Son communes, Luong Son district, Hoa Binh province, there are 136 ha of Acacia forest with different ages (Bui Duc Thuan, 2018) However, with the 7-year life cycle of Acacia, before starting new cycle, people have to clear cutting and burning to make ground preparation and reforestation Does soil protection at Acacia different ages differ? Though there have been numerous studies on the ability to protect soil of Acacia mangium, but still a lot of information has not been clearly elucidated Namely, the effect of degrading surface flow as well as effective against soil erosion through the development stages of Acacia mangium To further clarify this

issue, I have conducted a study on: “EVALUATING EFFECTS OF ACACIA

PLANTATION FOREST ON SURFACE RUNOFF AND EROSION IN LUONG SON HEADWATER OF VIETNAM” From there, step by step quantify these relationships to

develop Acacia plantation models, not only for the research area but also to expand into areas similarities, and to provide the basis for science and technology Aim for further research to develop solutions to regulate water and protect soil resources

CHAPTER II

GOAL AND OBJECTIVES

2.1 Goal

Finding solutions to design model of acacia plantations at different ages, aim to mitigate

as well as prevent the effects of surface runoff on soil erosion

2.2 Objectives

➢ Hypothesis

The vegetation cover plays an extremely significant role in protecting soil and reducing surface runoff Amount of surface runoff and soil erosion in Acacia mangium plantation forest

is much lower than bare land

The specific objectives of this research are:

To determine some soil physical factors and vegetation characteristics of the study site

in Luong Son, Hoa Binh

To evaluate amount of surface runoff in Acacia mangium plantation and bare land

To evaluate soil erosion characteristics in Acacia mangium plantation and bare land

1871 people in 1999 and a population density of 61 persons/km² (Figure 3.1)

About topography, Luong Son district in the midland - where the transition between the delta and mountainous, so the terrain is very diverse The low mountainous terrain is approximately 200-400 m in height, formed by magma, limestone and terrigenous sediments, with a dense network of rivers and streams (Figure 3.2)

Luong Son climate is tropical monsoon, with cold winters - less rainfall; hot summer - heavy rain The average temperature of the year is 22.9 - 23.30°C The average rainfall is from 1520.7 to 2255.6 mm/year, but unevenly distributed during the year and even during the season

is very erratic The average precipitation is 276 – 322 mm/month Each year, there are at least

2 typhoons that affect the area, the wind velocity is about 30 m/s The rainfall is unevenly distributed, mainly occurs on in some months during the rainy season, it can generate huge amount of runoff, causing flood and seriously landslide and erosion (Linh, 2017)

Figure 3.1 The map of study site: a) Location of Hoa Binh province on Viet Nam map; b)

Location of Luong Son district on Hoa Binh map; c) Location of Truong Son commune on

Luong Son map

3.2 Methods

3.2.1 Installing monitoring plot

I took five plot samples with bare land and four ones of different ages of Acacia tree The first plot was for bare land, the second for 4-month-old Acacia trees, the third for 1-year- old Acacia trees, the fourth for 2-year-old Acacia trees and the last for 4-year-old Acacia trees Standard plots were protected and had clear boundaries The area of each plot is 10 m2 (2.5m x 4m), with monitoring time from September 2017 to August 2018 The data in 2017 of four plots was from Scientific Research of me and my associates I kept continue to collect, established plot 5 and combine data from April 2018 to August 2018

7

Figure 3.2 Location of five plots: a) Contour line map of location;

b) Bare land plot; c) 4-month-old Acacia trees plot; d) 1-year-old Acacia trees plot;

e) 2-year-old Acacia trees plot; f) 4-year-old Acacia trees plot

The border of plot was built by aluminum plates There was buried at least 10 cm deep

to ensure that it can withstand heavy winds and heavy rains Aluminum plates was 30 cm high

to prevent rain splash, held and reinforced to stand upright by steel wires and bamboo piles

a )

a)

b )

c)

d)

The plots were perpendicular to the contour line At the down slope end of each plot, an aluminum trough was inserted connecting to a plastic pipe to transport overland flow and sediment to the buckets used to hold water and soil after each storm Nylon was used to cover the trough in order to prevent rain splash and rainfall from outside After finished establishing plot, the rain gauge was set beside these plots to measure the precipitation The rain gauges were set far from the tree canopy to avoid interception from overlying canopy

Figure 3.3 The model of plot and experiment conducted

3.2.2 Rainfall and soil physical characteristics:

a Precipitation:

America plastic rain gauge was used to measure the total rainfall (mm) Precipitation was recorded each rainfall event in amount of water

coming to the rain gauge from the start to the end of the storm

The number of the storm was 75, an inter-storm period was defined as a period of at least 6 hours without rain (Yen, 2014) Because

the amount of overland flow felt down rapidly after precipitation

Fences to prevent soil

from inserting to plot

Troughs to catch water and soil

Plastic fence to prevent water from inserting to troughs

Pipe: lead water and soil into the bucket Bucket to hold water and soil

Figure 3.4 Rain gauge

i: daily number of days to calculate precipitation index (before) (API) (I € 1-n)

i, j must satisfy condition 0 ≤ i - j ≤ 10

n: number of days in whole observation period

Pj: The corresponding rainfall of rainy-day j If there is much rain on the jth day, it will be equal

to the total number of rains on that day Any Pj that satisfies the condition 0 ≤ i - j ≤ 10 will be accrued into the API of the i the date in accordance with the above equation Outside this area, the rainy day will not affect the API of the ith day

b Soil properties

• Dry Bulk Density Tube was used to collect soil in order to determine bulk density

i) Dry Bulk density (D) is the weight of a unit volume of a loose material (such as a powder

or soil) to the same volume of water (g/cm3) Calculated by using the formula:

D = 𝑀

𝑉

In which:

D: Dry Bulk density (g/cm3)

M: Weight of dry land in its natural state (g)

V: The volume (cm3)

ii) Porosity of the soil is the ratio of the pores in the soil compared to the volume of soil The porosity of the soil is determined by the particle density and the Dry Bulk density of the soil Porosity is calculated by using the formula:

X% = 1−𝐷

𝑑 * 100

In which:

d: is the particle density (g/cm3)

D: is the bulk density (g/cm3)

Because I only knew bulk density, so I could assume particle density is equal to 2.56 g/cm3 (Liesch, 2013)

Soil moisture content (%): Determination of soil moisture following steps

Step 1: Weigh the aluminum box, (W1) (g)

Step 2: Weigh soil and aluminum box, I got W2 (g)

Step 3: After 24 hours drying in an oven at a temperature of 105⁰C, weight soil and aluminum and I got W3 (g) Calculated according to the following formula:

W% = 𝑊2−𝑊3

𝑊2−𝑊1* 100 Soil depth was measured by measuring tape, I excavated the soil profile (surface cut straight from the ground down to the bare rock layer.) then used the tap to have the depth of soil

Figure 3.5 Measured poroxity

Sediment also came with surface runoff to the buckets,

so after each storm, when soil settled down to the bottom

of the bucket then the water was taken to cylinder to

measured, the soil was left in the bucket would be collected

then bring to the laboratory as well as the soil from troughs

and pipes in each plot The soil then was dried in laboratory

Figure 3.7: Took eroded soil and dried and in the Lab

and weighted to determine the amount of soil erosion

3.2.4 Vegetation observation

Canopy and vegetation cover were

determined by using GLAMA and Canopy

Cover Free application The picture was taken

from the canopy and the vegetation cover

(standing in the central of each plot) and

inserted to the program to process then

recorded the results

The diameter of trees was determined by caliper and height of the trees was measured

by blume-leiss

measured by using GPS and compass application

Figure 3.9 GPS and Compass application

Figure 3.8 GLAMA and Canopy Cover Free

application

• Median

• 2 hinges = Lower Quartile – Q1 (25%) and Upper Quartile – Q3 (75%)

• Fences = 1.5 x Interquartile range (IQR) (Benjamini, Y., 1988)

• Whiskers

• Outliers: unusual observations, extremely out of the population (Faraway, 2014)

Figure 3.10 Create box-plot by R-studio

Figure 3.11 Box-plot graph

library(readxl)

> SR2 <- read_excel("~/R/SL/SR2.xlsx")

> View(SR2)

> prec = c(SR2$Precipitation)

> boxplot(prec)

> library("ggplot2", lib.loc="~/R/win-library/3.4"

Using code to import data and make box-plot

Median Q1 (25%)

Q1 (75%) IQR = Q3 – Q1

Upper Extreme = Q3 + 1,5 x IQR

Lower Extreme = Q1 + 1,5 x IQR

Outliers

Whiskers

CHAPTER IV: RESULTS

4.1 Soil physical factors, vegetation and precipitation characteristics on study sites

4.1.1 Soil physical factors and vegetation on study sites

The area of each plot was 10 m2 (4 m long and 2.5 m wide) Five plots had approximately the same height: plot 1 with bare land had the elevation of 63 m; plot 2 with 4-month-old Acacia trees was at 65 m; plot 3 with 1-year-old Acacia trees was at 64 m, plot 4 with 2-year-old Acacia trees was at 65 m and plot 5 with 4-year-old Acacia tress was at 67 m above sea level The soil depth of five plots was 0.68 m, 0.85 m, 1.02 m, 1 m and 1.15 m, respectively The slope of 5 plots was 22o; 23o; 23o, 24o, and 26o, respectively The ground vegetation cover of five plots also had different percentage, which of plot 1, plot 2, plot 3, plot 4, and plot 5 was 0.5%, 69.2%, 45.9%, 36.5%, and 30.5% respectively The porosity of soil at five plots was 25%, 38%, 39%, 40% and 38% respectively (Table 4.1)

Table 4.1 Outline of five plots of different vegetation cover types

Vegetation cover type Bare

land

old trees

4-month- old trees

1-year-2-year-old trees

4-year-old trees

4.1.2 Precipitation characteristics

The total observed data was 75 storm events Based on study site's data, the amount of rainfall ranged from 0.5 mm to 197 mm.

Figure 4.1 Storm events

Luong Son, Hoa Binh belongs to the third sub-zone of the North Vietnam which has two seasons: rainy season and dry season Monitoring time occurred in the both dry (September

2017 to November 2017) and rainy season (April 2018 to August 2018), it could be seen that rainfall in the dry season was relatively small, while the opposite was witnessed by the rainy season During 2017 to August 2018, there was a fluctuation in the amount of precipitation The lowest rainfall was 0.5 mm on 29/10/2017, the highest storm was 197 mm on 19/07/2018 Over a period, the average precipitation was 33.71 mm.

Table 4.2 Precipitation, API7 and rainfall intensity on study sites

No Date Precipitation

Rainfall intensity (mm/hr)

Table 4.3 Vegetation on study site

(*): Average diameter breast height of trees; (**): Average height of trees

Parameters Plot 1 Plot 2 Plot 3 Plot 4 Plot 5

4.2 Runoff generation characteristics on the study site

Figure 4.2 Precipitation, surface runoff and runoff coefficient at different ages of Acacia

In all five plots, the surface runoff and runoff coefficient were proportional to precipitation (Figure 4.2) It was clear that the amount of surface runoff and their coefficient shared the same changed with amount of rainfall, although that in plot 1 was the highest, while the lowest was plot 5 Surface runoff (± Standard Deviation: SD) of plot 1 (bare land) ranged from 0 - 35.25 ± 7.06, averaged 4.47 mm/storm This number was 5 times as many as than that

19

in plot 2 (4-month-old trees), which ranged from 0 – 6.89 ± 1.55, averaged 1.02 mm/storm Plot

3 (1-year-old trees) had surface runoff (± SD) ranged from 0 – 4.10 ± 0.96, averaged 0.49 mm/storm Surface runoff (± SD) of plot 4 (2-year-old trees) ranged from 0 – 4.06 ± 0.94, averaged 0.48 mm/storm And surface runoff (± SD) of plot 5 (4-year-old trees) ranged from 0 – 1.55 ± 0.36, averaged 0.26 mm/storm The runoff coefficient (± SD) of bare land ranged from

0 – 19.6 ± 6.38%, averaged 7.87%, while the runoff coefficient of plot 2, 3, 4, 5 ranged from 0 – 5.51 ± 1.40% - averaged 1.81%, 0 – 3.47 ± 0.82% - averaged 0.77%, 0 – 3.46 ± 0.82% - averaged 0.73%, 0 – 1.01 ± 0.32% - averaged 0.40%, respectively (Table 4.5, Figure 4.2) This showed the surface runoff and runoff coefficient of bare land was greater than Acacia planted plot

With the same result as total surface runoff, over 75 storm events, runoff coefficient also was the highest at plot 1, intermediate at plot 2, the second-lowest at plot 3, the third-lowest at plot 4 and the lowest at plot 5 The relationship between runoff coefficient and precipitation during the plot 1, 2, 3, 4 and 5 displayed average values of 7.87%, 1.81%, 0.77%, 0.73% and 0.40%, respectively

Table 4.4 Surface runoff, runoff coefficient analysis between five plots

Figure 4.3 Precipitation accumulation and runoff accumulation at different age of Acacia

Total rainfall accumulation observed after 75 storm events were 2528.35 mm (Figure

4.3) Meantime, the runoff accumulation from five plots was 335 mm, 76.24 mm, 36.92 mm, 35.76 mm, and 14.33 mm, respectively

From early April to mid-June 2018, the runoff accumulation increased slightly due to the rainfall in that period was not too large However, during the later part to August, the

rainfall was much higher, resulting in a quick jump of runoff acuumulation in all five plots, while plot 1 went up steeper The ablilty to generate surface runoff of plot 1 was about 5

times higher than that in plot 2; approximately 9 times greater than that in plot 3 and plot 4;

20 times as much as in plot 5 (Figure 4.3.)

0 500 1000 1500 2000 2500 3000 3500

4000 0

Figure 4.4 Comparison of the runoff generation to forest cover change by bare land and

four different Acacia tree-aged

Fig 4.4 compares the surface runoff to forest cover change by bare land and four different ages of Acacia trees Overland flow wrote off with increasing age of Acacia trees As suggested by the statistical tests (Table 4.5), the median of the overland flow and runoff coefficients with changes in forest cover in the bare land were significantly higher than the plots covered by different ages of Acacia (α = 0.05) The trend of the five plots during the study was almost in the upper extreme part However, it could be seen that the runoff coefficients favor older tree coverings; for instance, with 95% confidence level, in plot 5 - 4-year-old Acacia trees, runoff coefficient fell significantly and hit the bottom value.

Table 4.5 Independent samples t-test for the effect of surface runoff to Acacia’s age

Figure 4.5 Correlation between precipitation and surface runoff at different age of Acacia

There was a slight difference in the correlation coefficient R2of each plot Results of plot 1, 2 and 5 had higher R2surrounding 0.9, showing the strong relationship of precipitation

23

and surface runoff with p < 0.00, although the latter was much greater (plot 5 – R2 = 0.965) Meanwhile, this relationship was medium in plots 3 and 4, R2 was about 0.7 (Figure 4.4.) Results from the majority of monitoring plots show that Acacia forest cover can decrease runoff (Figure 4.5) Independent sample t-test suggest a significant negative relationship (α = 0.05) between Acacia developed stages and its runoff

Surface runoff and precipitation at bare land and difference ages of Acacia had strong relationship This correlation had the statically significant with p = 0.00 (Fig 4.5)