Runoff generation and soil erosion from different age of acaia plantation forest in truong son commune, luong son district, hoa binh province, viet nam master thesis in forest science

RUNOFF GENERATION AND SOIL EROSION FROM DIFFERENT AGE OF ACACIA PLANTATION FOREST IN TRUONG SON COMMUNE, LUONG SON DISTRICT, HOA BINH PROVINCE, VIETNAM MASTER THESIS IN FOREST SCIENCE

RUNOFF GENERATION AND SOIL EROSION FROM

DIFFERENT AGE OF ACACIA PLANTATION FOREST IN

TRUONG SON COMMUNE, LUONG SON DISTRICT, HOA BINH

PROVINCE, VIETNAM

MASTER THESIS IN FOREST SCIENCE

MINISTRY OF EDUCATION AND TRAINING MINISTRY OF AGRICULTURE AND RURAL

DEVELOPMENT

VIETNAM NATIONAL UNIVERSITY OF FORESTRY

- -

CHIN KOLYAN

RUNOFF GENERATION AND SOIL EROSION FROM

DIFFERENT AGE OF ACACIA PLANTATION FOREST IN TRUONG SON COMMUNE, LUONG SON DISTRICT, HOA BINH

ABSTRACT

Vegetation cover is the key factors that protect the soil, reduce surface runoff and soil erosion Present days, citizens are paying more attention to the industrial

plantation in Vietnam, especially Acacia magium plantation as it takes part in

improving the socio-economic condition To propose a solution to mitigate the impact of Acacia plantation, understanding about runoff generation and soil erosion are needed To determine the characteristic of runoff generation and soil erosion derived at the different ages of Acacia plantation forest in Luong Son headwater of Vietnam, four plots (15m2) were set up Of those, 2 plots (up-hill and down-hill) in 1-year-old Acacia plantation and 2 plots (up-hill and down-hill) in 5-years-old Acacia plantation The main finding includes: (1) runoff coefficient at Acacia 1-year-old down and up was (0.36% - 0.46%) with the average 0.41% And Acacia 5-years-old, down and up was (0.35% - 0.39%) with the average 0.37% It shows the slightly different between the locations of two years due to the different ground cover However, t-test result showed that the difference between locations in years one and year-5 are not significant (P-value = 0.31 and P-value = 0.96 respectively) T-test also indicated that there was no significant difference in term of an runoff at two different ages of Acacia plantation with (P-value = 0.95 > 0.05); (2) soil erosion

is significantly different between two years of Acacia plantation (P-value = 0.004 < 0.05) Total soil erosion in year-1 and year-5 were 21.84 ton/ha/6months and 14.20 ton/ha/6months, respectively that is classified as strong erosion (level IV) base on TCVN5299:2009; (3) both runoff and soil erosion at 2 different ages of Acacia plantation forest has strongly depended on the amount of precipitation (R2 range from 0.52-0.85, with P-value = 0.00) This suggests that the amount of runoff and soil erosion at Acacia 1-year-old was a greater amount of soil erosion because of fewer vegetation covers we need to be more concerned and apply suitable management for reducing the negative impact of Acacia plantation forest at the headwater of Vietnam

TABLE OF CONTENT

ABSTRACT i

TABLE OF CONTENT ii

ABBREVIATIONS iv

LIST OF TABLES v

LIST OF FIGURES vi

CHAPTER I: INTRODUCTION 1

CHAPTER II: LITERATURE REVIEW 5

2.1 Runoff generation and soil erosion globally 5

2.2 Runoff generation and soil erosion in Vietnam 10

CHAPTER III: STUDY SITE AND METHODS 13

3.1 Study site 13

3.2 Methods 15

3.2.1 Plots design for an experiment 15

3.2.2 Measure runoff 19

3.2.3 Measure soil erosion 20

3.2.4 Measure precipitation 21

3.2.5 Soil moisture measuring 21

3.2.6 Measure porosity of the soil 22

3.2.7 Soil texture 23

3.2.8 Vegetation observation 25

3.2.9 Topographic survey 25

3.3 Data analysis 25

CHAPTER IV: RESULTS 26

4.1 Runoff generation at 2 different ages of Acacia plantation 26

4.2 Soil erosion at 2 different ages of Acacia plantation 30

4.3 The relationship among runoff, soil erosion, and precipitation 35

4.4 Suggested solution 39

CHAPTER V: DISCUSSION 41

5.1 Runoff generation at two different ages of Acacia plantation forest 41

5.2 Soil erosion at two different ages of Acacia plantation forest 44

CHAPTER VI: CONCLUSION 49

ACKNOWLEDGMENT 50 REFERENCES

APPENDIX

ABBREVIATIONS

SPSS Statistical Package for the Social Sciences

LIST OF TABLES

Table 3.1 Observation plots characteristic at the study site 17

Table 3.2 Soil texture at study site follow FAO classification 2006 17

Table 4.1 The summary result from SPSS (Appendix table1) 30

Table 4.2 The summary result from SPSS (Appendix table 1) 34

Table 4.3 Summary of correlation analysis between runoff, soil erosion, and both rainfall and soil moisture characteristics 38

Table 4.4 TCVN5299: 2009 Method for determination of soil erosion by rainfall 39 Table 5.1 Comparison the runoff coefficient with other studies (the unit for first two columns are %/6months while the rests are a %/1 year) 43

Table 5.2 Compare the soil erosion among Acacia plantation forest year-1, year-5 (ton/ha/6months) with other land types (ton/ha/year for the rest) 47

LIST OF FIGURES

Fig 2.1 Soil erosion effect by human impact in Madagascar, (Source: US news) 8

Fig 2.2 Soil erosion by water globally (ton/ha/year) 9

Fig 2.3 Soil erosion in Lai Chau after heavy rain in June 2018 12

Fig 3.1 The map of the study site: a) Location of Hoa Binh province on Viet Nam map, b) Contour line map of four plots location; c) Acacia 1-year-old; d) Acacia 5-years-old 14

Fig 3.2 Picture a) Contour line map shows the location of four plots, b) Model illustrates elevation, slope, and distance of 4 plots at the study site 16

Fig 3.3 Picture plot 1-down and 1-up of Acacia 1-year-old and, plot 5-down and plot 5-up of Acacia 5-years-old planation forest at study site 16

Fig 3.4 The model of plot and experiment conducted, a) gutter, b) plastic tube, c) aluminum sheet, d) plastic container 18

Fig 3.5 A measured amount of runoff 19

Fig 3.6 Took eroded soil to dried and weighted in the Laboratory 20

Fig 3.7 US standard plastic rain gauge 21

Fig 3.8 (a) Bulk density tube, (b) took a soil sample, (c) soil sample keep in zipper plastic bag, (d) measured wet soil 23

Fig 3.9 (A) Removed fraction, (B) rock and roots, (C) soil after grind and (D)pipette 24

Fig 4.1 Runoff generation and precipitation at the study site 26

Fig 4.2 Runoff coefficient from four plots 27

Fig 4.3 Runoff accumulation and precipitation from four plots 28

Fig 4.4 Runoff generation box chat plot 29

Fig 4.5 Precipitation and soil erosion from four plots 31

Fig 4.6 Soil erosion accumulation from four plots 31

Fig 4.7 Soil erosion box chat plot 33

Fig 4.8 The model illustrates the result of runoff and soil erosion at two different

ages of Acacia plantation forest from April to September 2018 34

Fig 4.9 Correlation between precipitation and runoff from four plots 35

Fig 4.10 Correlation between soil erosion and precipitation from four plots 36

Fig 4.11 Correlation between soil erosion and runoff from four plots 37

CHAPTER I: INTRODUCTION

Runoff and erosion are considered the main cause of soil and water resources degradation (Clement et al., 2006; Yang et al., 2003) Runoff is a process of water surface flow, other sources flow over the earth’s surface Runoff is one of the main factors that support for non-point source pollution It’s also the main cause of sediment delivery to stream and river (Braskerud, 2002) However, sometimes runoff can start before all surface depressions have been filled (Moore and Larson, 1979)

Erosion is the action of the surface process of soil due to a water drop and wind, under the impact of gravity on earth (Ellison, 1945) Previous researches indicated that runoff and erosion have a close relationship with precipitation (Ramos‐ Scharrón and MacDonald, 2007) Soil erosion is a natural phenomenon but this phenomenon is becoming more serious (Hudson, 1995) It not only directly affects the agroforestry production activities but also affects the environment and the life of the downstream communities as land degrades rapidly in all aspects: chemistry, physics, and biology (Joseph, 2005) The characteristics of runoff and erosion will have a direct impact on soil quality, stream health and water quality In the rapid developing speed of the world, the environmental problem has emerged as one of the most important challenges that the world together must face Industrialization and agriculture activities have left behind serious impact on the natural condition

Globally Soil erosion has been an environmental concern in such countries as China and those bordering the Mediterranean Sea for millennia (Morgan, 2009) The potential of soil loss estimated is about 0.38mm/year The most seriously affected region in the world in Southeast Asia It nearly 60% of present soil erosions are induced by human activity, global warming, the increasing trend of precipitation and population (Yang et al., 2003) Erosion happens quite frequently in Asia, Africa and South America with the soil mass from 30 to 40 tons per hectare for every year (Barrow, 1991) In Cambodia, the degradation of soil and water quality will lead to

serious consequences for human life in the natural ecosystem as well as ecology, social and economic aspect Erosion is the most serious because of the sediments transport from China to the Mekong basin In 1997 during the flood season on lower forest and floodplain in Cambodia, 84.6 million ton of soils were washed from the lancing Jiang to the lower Mekong (Kelin and Chun, 1999) The annual sediment load of the basin was estimated around 67 x 106 tons/year at Chiang Saen (Hården and Sundborg, 1992) In recent years, in the mountainous areas in Vietnam has lost

a huge amount of soil due to erosion According to land use analyzed, Vietnam has

about 25 million for steep land, with huge potential for erosion, about 10 ton/ha/year (Vinh TQ and Minh HT, 2009) According to systematic monitoring from 1960 until now, there is 10-20% of area affected by erosion from moderate to strong (Xiem NT and Phien Thai, 1999) Runoff generation and soil erosion from mostly occurs at headwater area (Miyata et al., 2009) Headwater systems are the areas of originated water within a channel network, are characterized by interaction with the hydrologic, geomorphic and biological process It varies from hill slopes to downstream channels (Hack and Goodlett, 1960)

Runoff generation and soil erosion from headwater area are often governed by several factors such as climate, human activities and animal, terrain, rainfall intensity, topology, soil characteristics, and especially vegetation cover (Huang et al., 2002; Miyata et al., 2009) Vegetation cover is the key factor that protects the soil and it reduces runoff and soil erosion (Castella et al., 2006; Descroix et al., 2001a; Pham Van Dien, 2006) In a natural forest, undisturbed perennial forestlands generally produce the least amount of runoff and soil erosion among all land use systems Soil erosion from undisturbed forest soil normally ranges from 0.02 to 1.2

Mg ha (Wagenbrenner et al., 2006) Normally in the forested zone, the amount of runoff and erosion are negligible because of well-vegetated forest land has high infiltration rate because it was protection by trees, understory vegetation cover, and leaf litter, it should be noted that well-managed forest land can maintain higher storage capacity (Descroix et al., 2001a) According to (MacDonald et al., 2001), in

a natural forest, overland flow only presented during large storms When the forest

is being extracted, it can produce more surface runoff as well as soil (Imeson and Vis, 1982) The amount of sediment varied according to the status of vegetation cover, in particular, sediment from bare land was 133g/m2, area without a tree but have litter was 30g/m2 and the forest area was 1.1g/m2 (Descroix et al., 2001b) In the clear-cutting or disturbed forest land, the amount of Runoff and soil erosion are significantly higher, because of the lacks of vegetation cover such as grass, shrub Runoff and soil erosion is also dependent on the canopy cover (Nanko et al., 2008) Forests with more canopy layers, the ability to retain water and soil is higher, forest with one canopy layer, the amount soil erosion will be three times higher than the forest with three canopy layers The different ages of the tree in plantation forest also impact on the process of runoff generation and soil erosion in different ways Previous studies have also shown that the ability to regulate water and reduce erosion is varied with different tree species (Dung B.X, 2011) And for plantation forest different age of trees planting even though the same types of forest plantation but it could be different the amount of runoff and soil erosion, because of it different vegetation cover

In Vietnam, about 24% of the forest area is planted forest, in which Acacia

mangium is a popular crop, which brings high economic value (Ministry of

Agriculture and Rural Development, 2012) Acacia mangium is a native species in

northern Queensland (Australia), found in Iran Jaya, Maluku, Indonesia (Doran and Skelton, 1982) This is a fast-growing species, which is widely used for various purposes such as timber, firewood, tannery, and agroforestry and soil improvement From the economic and social benefits of acacia, the acacia plantation area is expected to increase every year The area of plantation forest tends to increase annually (Ministry of Agriculture and Rural Development village, 2012) However, the lack of a database reflects the relationship between Acacia plantations and the generation 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 the mountainous area of Vietnam, due to the sloping hilly terrain combined

with large annual precipitation, surface runoff and erosion are serious issues in the

management of land and water resources Behind, the indigenous people are tending

to growth more industrial plantation – especially Acacia as it can improve their

livelihood Although there have been many studies on the protection of soils from

Acacia mangium, it is still a lot of unclear information Specifically, the effect of

different ages of Acacia mangium in term of reducing the surface flow and against

soil erosion remain unknown To further clarify this issue, and present the solutions

I conducted my final thesis “Runoff generation and soil erosion from a different

age of Acacia plantation forest in Truong Son commune, Luong Son district,

Hoa Binh province, Vietnam”

Afterward, step by step quantify these relationships to develop Acacia

plantation models not only for the research area, but also for another similarity area,

and to provide the basis for science and further research to develop solutions to

regulate water and protect land resources Here, we hypothesize that vegetation

cover significantly plays an important role in protecting soil and reducing the

amount of runoff and soil erosion For the main goal of this study is a strong

characterize the process of runoff generation and soil erosion from a different age of

Acacia plantation forest in Truong Son commune, Luong Son district, Hoa Binh

Province, Vietnam Hence the objectives of this study were to:

1) To determine runoff generation at 2 different ages of Acacia plantation forest

1-year-old and 5-years-old

2) To determine the amount of soil erosion at 2 different ages of Acacia

plantation forest

3) To propose some suggested solution to reduce the negative impact of runoff

and soil erosion from Acacia plantation forest

CHAPTER II: LITERATURE REVIEW

2.1 Runoff generation and soil erosion globally

The soil is one of the most important things to support every element in the ecosystem In addition, Altieri, (1999) stated that 90% of all human foods are produced on land is a process from the natural ecosystem Presently the ecosystem was changed by many driven factors such as socioeconomic forces, land use change, and decreasingly of land cover It has to be more challenging for researchers to know the crucial factor of the problem For instance, the decreasingly soil quality because of human activities and natural disaster is unknown In the world, twenty-three percent of the land surface has been affected by soil erosion it about 5-10 million hectares were affected every year (Bui Xuan Dung, 2017; Van

De et al., 2008) In Asia the population grew rapidly, land-use changes and degraded land it’s the cause of soil erosion (Ha et al., 2012; Lal, 2004) For the highest percentage of degraded land is about 31% in the history of Asia, followed

by Africa at 27% (Oldemal, R.L, 1994) In developing countries, the cost of land degradation may be more than 15% of the Gross National Product (Barbier and Bishop, 1995) In Western Europe, there was a growing realization from the 1970s that soil erosion could have a major effect on soils, even on lowland arable areas (Morgan, 2009) According to Yang et al (2003) stated that watershed management must be done carefully because it should control the runoff and erosion which are two of the most serious problems in the world

Runoff and soil erosion are often not only the primary consequences and symptoms of land mismanagement but contribute to negative downstream off-site impacts such as flooding pollution and siltation of water bodies and reservoirs (Bruijnzeel, 2004) Previous researches Maglinao and Leslie, (2001) said that erosion is regarded as a major type of environmental damage in South-East Asia Erosion is the phenomenon of surface erosion under erosive forces of water or wind Soil erosion is a natural phenomenon, but this phenomenon is happening more and more serious (Ellison, 1944) Soil erosion from land areas is widespread

land degradation at the global scale in term of loss soil fertility, water quality and adversely affects all natural and human-managed ecosystems, including agriculture and forestry Its effects are pervasive, and its damages are long lasting (Pimentel and Kounang, 1998) It is generally hypothesized that increased exploitation of land resources in headwater catchment areas, even as small as km2 < 1, with the associated fragmentation of native forest vegetation, can result in increased sediment discharge and elevated nutrient loads that act to reduce water quality and availability to downstream users (Bruijnzeel, 2004)

There have been some attempts to study the dynamics of the natural resources

in the Himalayas and other mountainous areas of the world Sharma et al (2001)evaluated the fluxes of overland flow, sediment output, and nutrient losses Eighty percent of the sediments delivered to the world’s oceans each year came from Asian rivers and amongst these Himalayan rivers are the major contributors (Chorley, 1969) In mountain zones, overland flows are influenced by surface features, slope value and vegetation affect runoff and erosion according to (Nguyen Trong Ha, 1996) The rainstorms often generate substantial surface runoff with the potential for accompanying soil and nutrient losses (Wells, 1981) These changes in rainfall patterns and intensities may result in greater amounts of overland flow and a concomitant increase in the quantities of C and N removed from hill slopes (Edwards and Owens, 1991) Furthermore, Negi et al (1998) studied three high-altitude forested watersheds in Nandadevi Biosphere Reserve of Central Himalayas This study indicated a significant role of tree physiognomy on the relative partitioning of total rainfall into infiltration and overland flow McDowell and Sharpley, (2002) studied the effect of flow path length on phosphorus (P) loss

in overland flow with and without localized dairy manure application at two sites withinan agricultural watershed in Pennsyl vania, USA Most early studies Malmer, (1996) as well as current work, focus on presented experimental data on hydrological effects and nutrient losses of forest plantation in an experimental catchment in Sabah, Malaysia Mandal et al (2012)compared soil nutrient datafrom

different land uses across Amazonia Patterns were identified regarding nutrient concentration and variation with the type of land use (McGrath et al., 2001)

Runoff occurred after a storm event and it is one of the main sources of point pollution at stream ecology Previous researches indicated that runoff has a close relationship with precipitation (Ramos‐Scharrón and MacDonald, 2007) Van

non-De et al (2008) the major cause of runoff is not only rainfall but also depend on other factors such as slope, gradient, terrain, and under vegetation story considered

as a method of runoff prediction (Hudson, 1995) For example, research has provided evidence for the result from field studies describing the effect of slope on runoff are contradictory (Chaplot and Le Bissonnais, 2000; Fox et al., 1997) Sharma et al (1983) stated that some case increases the amount of runoff Moreover, Poesen, (1984) claimed that runoff decreases On the other hand Mah et

al (1992) as slope gradient increase runoff, it not significantly different As has been previously reported in the literature the concept of Horton, (1933) developed that overland flow or surface runoff occurs when rainfall rates exceed soil infiltration rates The coefficient of surface runoff where no tree is 0.23%, where no tree but having carpet is 0.085%, and were having a tree is 0.028% (Descroix et al., 2001b) Previous studies have emphasized, in the catchment area, the saturation overland flow and subsurface flow were dominant processes in rainfall-runoff processes (Bonell, 1998) In additionally Bonell, (1998), in the forested zone or a natural forest, the amount of runoff is negligible because of the structure of the tree, leaf litter, and understory vegetation cover

Fig 2.1 Soil erosion effect by human impact in Madagascar, (Source: US news) Vegetation cover are plays an important role to control erosion by canopy, roots system, litter component for reducing the power of runoff and soil erosion from the environmental condition (Zuazo et al., 2004) In the same fashion, numerous studies have reported vegetation have influence reducing erosion, sediment processes with spatial scales Xu, (2006) and eco-environmental changes For globally during the past, concerning to re-establishing perennial vegetation cover that is a good way to protect environmentally, reduces runoff and soil erosion (Davie and Lant, 1994) In other words, Wilcox et al (2003) quote that related to eco-hydrology the effects of vegetation scale on runoff and erosion will be different depending on the degree of disturbance However, many researchers have done about vegetation cover related

to soil erosion (Nguyen Trong Ha, 1996; Xu, 2006; Zheng, 2006; Zuazo et al., 2004) Additionally, there are many studies found that the stems of plants can trap runoff then reduces the amount of soil eroded (Dung B.X, 2011; Ligdi and Morgan, 1995)

Fig 2.2 Soil erosion by water globally (ton/ha/year) Source: Nachtergaele et

al (2010)

In summary: Runoff and soil erosion are growing globally, soil performs a range of key functions, including the production of food, the storage of organic matter, water and nutrients, the provision of a habitat for a huge variety of organic matter, water and preserving a record of past human activity fortunately, there is a strong correlation between runoff, soil erosion, and vegetation cover Thus, increase the vegetation cover can control soil erosion and runoff

2.2 Runoff generation and soil erosion in Vietnam

Soil erosion is a hazard traditionally associated with agriculture in the tropical and semi-arid area and important for long-term effects on soil productivity and sustainability Morgan, (2009) reported that it is a problem of wider significance occurring additionally on land devoted to forestry, transport, and recreation Erosion also leads to environmental damage through sedimentation, pollution and increased flooding In watershed management, the most common and critical problem directly impact on soil quality, hydrology, stream health and water quality are runoff and soil erosion According to the previous study of Lal, (1998) quote that in southeast Asia the major type of environmental damage with dramatic consequences in term

of water quality and soil fertility is erosion on sloping land In Vietnam since the mid-eighties, agricultural start to the development and the population growth of the peoples also increased frequently In addition due to rapid human population growth, the cropping areas have expanded to more marginal lands such as mountains and the fallow periods have been shortened or even abandoned (Clement

et al., 2006)

Natural forest was destroyed by illegal logging to get the timber production and then they convert forest to become agriculture land or forest plantation Plantation holds an important part in water regulation via its impact on the hydrological process Among other factors such as soil, precipitation, slope, road length, plantation also controls the amount of runoff and soil erosion By way of example Castella et al (2006) from the mid-70s until 1998, villagers in Vietnam cultivated cassava, arrowroot, taro, maize, eucalyptus plantation on the upland After that in

2005 five agro-system were dominated such as Acacia mangium plantation, cassava

planted, bracharia ruzziensis planted, Venicia Montana planted and Eucalyptus plantation (Orange et al., 2007) As a matter of fact, Dien, (1996) quote that different land use system directly affects the physical structure and biochemical characteristics of the soil In order to estimate soil and nutrient losses under different land use management on sloping land under intense cultivation in Southern

Vietnam Van De et al (2008) using the Revised Universal Soil Loss Equation (RUSLE) standard it can apply many land use situations for prediction the amount

of soil erosion Example erosive farming practice, climate with intense rainfall and sloping topography are often cited as the main cause of soil erosion such as reduction in biodiversity, loss of topsoil, flash floods frequent occurrence during rainy season and droughts during the dry season Moreover, Tran and Le, (1999) concentrated 80% of intense rainfall during the rainy season, increased the runoff process and high soil erode ability make upland fields prone to erosion and diffuse pollution of river networks

Ngo et al (2015) reported that different land use type varied strongly water yield was 376.9 mm in 2005 and 72.0 mm in 2010 Base flow also increased 189.4

mm and 63.9 mm, respectively Moreover, while surface runoff increase strongly water yield was also increasing Similarly, sediment yield from soil erosion increased from 101.3 ton-ha-year to 148.1 ton-ha-year during (1995-2005) was a similar trend to runoff All sediment delivery to rivers or streams can cause flooding and reservoir sedimentation as well as negative effects on water quality and aquatic resources (Chappell et al., 2004; Gomi et al., 2006) As an illustration in northern

Vietnam, soil loss varies greatly with land use and location In 1999, soil losses by

water erosion (runoff erosion) on upland areas were estimated at about 3.022 billion ton/ha/year for the whole 22 million ha area that represents approximately 70% of the total country area (Tran and Le, 1999) The amount of the total soil loss in a 250

ha forested land and watershed covered by agricultural ranged from 16.3 to 172.2 g /m2/year in Vinh Phuc province (van Keulen et al., 2013) As well as according to

Ha et al (2012) soil losses were reported in Hoa Binh province is about 14 to 150 g/m2/year Other studied reported soil losses of up to 1305 g/m2/ year in Hoa Binh Province (Podwojewski et al., 2008) Additionally for maize fields soil erosion up to 17,000 g/m2/year in Son La province (Tuan et al., 2014) One case study said that the annual soil loss recorded before 2002 through bed load measurements have decreased from 3.6 t ha−1 year−1 to 0.1–0.3 t ha−1 year−1 in 2004 (Orange et al.,

2007) The highest amount of soil losses will lead to increasing the need for chemical fertilizers and decrease land productivity (Lal, 1998) Not only that problems but socioeconomic also decreases, including regional poverty, food insecurity, lower household incomes and so on (Ananda and Herath, 2003)

Fig 2.3 Soil erosion in Lai Chau after heavy rain in June 2018 Photo:

Prokeraia.com

In summary: The study on runoff generation and soil erosion in Vietnam is poorly known while Vietnam is the most impacted by soil loss and flooding It is the cause of food security, poverty of the country Recovering of the forest is important to protect the soil that indirectly protects the national food security and reduce the poverty

CHAPTER III: STUDY SITE AND METHODS

3.1 Study site

The planted Acacia forests in Chanh village, Truong Son commune, Luong Son district, Hoa Binh province were chosen to be the monitored area The study site belongs to Truong Son commune which is located in a mountainous area The coordinate is 20°51'N 105°27'E The study area belongs to Lam Son Forestry Company (Figure 3.1) The total area of this commune is 3060 ha, in which forest account for 2610 ha with the total area of Acacia plantation forest is up to 1359.86

ha occupied 13.9% of total manage areas of Lam Son company (Hoa Binh forest company, 2016)

The study site belonging to Hoa Binh province, with the basic characterized of geography as a hilly area situated between mountains and the red river plain The altitude ranges from 0 to 1,510 m above sea level The elevation decreases from the northwest to the southeast Located at Hoa Binh province, the study site is also influenced by the monsoon weather characterized by hot, rainy, and dry seasons According to observations over the last decade, the coldest month was January with

an average temperature of 14.9 °C, and the hottest month being July with an average temperature of 26.7 °C The rainy season is normally from May to October with both a high frequency and intensity of rainfall In August and September, rainfall peaks at values from 300 to 400 mm per month The rainfall during this period accounts for 84–90% of the yearly rainfall The frequency and intensity of the rainfall are concentrated over a short period where rainstorms and super rainstorms are major contributions to the landslide hazard in the area (Bui et al., 2012) Generally, average precipitation range from 1400-1900mm per year (Ngo et al., 2015) Due to the hilly terrain and a large amount of precipitation, runoff, erosion, and landslide become a serious issue in Truong Son commune Behind, indigenous people mostly earn a living from agroforestry activities, so they also disturbed to the natural forest and with the poor management and monoculture of

only Acacia or Eucalyptus plantation forest is the problem which leads to the sharp increase in the amount of runoff and soil erosion

Fig 3.1 The map of the study site: a) Location of Hoa Binh province on Viet Nam

map, b) Contour line map of four plots location; c) Acacia 1-year-old; d) Acacia

3.2 Methods

3.2.1 Plots design for an experiment

Four plots were installed at two Acacia plantation forest at different ages, in detail: plot 1 was set up in down-hill is called plot 1-down and second plot up-hill is called plot 1-up at 1-year-old Acacia plantation forest; the third plot is set up in down-hill is called plot 5-down and fourth plot is set up up-hill is called plot 5-up at 5-years-old Acacia plantation forest At each forest stage, in order to see the amount

of runoff and erosion at different elevations, one plot was set up at the upper-hill part and the other one was located at the down-hill part (Fig 3.2) For the general information from four plots in detail at (table 1&2)

-a-

Fig 3.2 Picture a) Contour line map shows the location of four plots, b) Model

illustrates elevation, slope, and distance of 4 plots at the study site

Table 3.1 Observation plots characteristic at the study site

Parameters

1-year-old Acacia 5-years-old Acacia

Silt (%) 0.002-0.05

Clay (%)

< 0.002 Soil types

Fig 3.4 The model of plot and experiment conducted, a) gutter, b) plastic

tube, c) aluminum sheet, d) plastic container Each plot was 3.0 m wide, 5.0 m in length and was bordered at the upper side, the left side and the right side by an aluminum sheet, which was 13 m in length and the height, was 0.4 m The aluminum sheet was buried 0.1 m deep in to the soil, and

to make sure that it could firmly stand even in heavy storm condition with a large amount of runoff and strong wind, steel wires and bamboo sticks were propped up surrounding the aluminum sheet, with 0.3 m height above the soil, the aluminum sheet could help to prevent runoff from the upper area entered the plot as well as the impact of rain splash At the down end side of the plot, an aluminum gutter was installed to catch the water and soil from the plot The aluminum gutter was 3.0 m

in length, 0.2 m wide and 0.2 m in height, noted that, at the side where the gutter meets the plot, the length of the sheet was longer, so that it could be buried into the

plot to ensured that runoff accumulated at the end of the plot would move to the gutter but not leached out The gutter was connected with a container, which had a volume of 180 L, by a plastic tube To get the accurate result, the gutter and the container was covered above to make sure the rain did not fall inside, (Fig 3.4)

Field observation and calculation:

Runoff and soil erosion from 4 plots was measured for 55 storm events An storm period was defined as a period of at least 6 hours without rain Because the amount of overland flow decrease quickly after precipitation cased, a 6 hours-period without precipitation was sufficient to distinguish storm events (Dung B.X, 2011)

inter-3.2.2 Measure runoff

After each storm event using a graduated cylinder to measure directly

amount of runoff from 4 plots different ages of Acacia Plantation on the top and

bottom of the hill that contains in the container (Fig 3.5) The amount of runoff will

be converting from litter to mm by taking the amount of runoff divide by the plot area Runoff coefficient was calculated by the formula:

Runoff coefficient = x 100%

Fig 3.5 A measured amount of runoff

Total Runoff Depth Total Storm Precipitation

3.2.3 Measure soil erosion

Let the soil settle down at the bottom of the container, after taking out all the water as well as the soil left in the container, gutter and the plastic tube taken to the laboratory to dry (at 105oC for 24 hours) and weight in order to determine the amount of soil erosion (kg) from each plot To convert soil loss from (kg) to (mm) consider the bulk density of soil is 2.65 g/cm3 And then divide the amount of soil loss by bulk density and then keep divide by the area of the plots

Fig 3.6 Took eroded soil to dried and weighted in the Laboratory

3.2.4 Measure precipitation

Rainfall was monitored by using US standard plastic rain gauge The rain gauge was installed in an open area near the forest Precipitation was recorded each rainfall event in the amount of water coming to the rain gauge from the start to the end of the storm

Fig 3.7 US standard plastic rain gauge

3.2.5 Soil moisture measuring

Using the Antecedent precipitation index for 7 days (API7) to determine the soil moisture for each storm event was also figured out The first rainfall monitored was on 22 April 2018, and to find out the API7 for that day, the rainfall was first monitored since 15 April 2018

 API7 was calculated by the formula:

API7 = ∑

- API7: Antecedent precipitation index for 7 days

- i : daily number of days to calculate precipitation index (before API)

3.2.6 Measure porosity of the soil

To determine the porosity of soil at each plot, soil samples were taken by using Bulk density tube and keep soil in the zipper plastic bag to keep the moisture of the soil And then bring soil to analyze in the laboratory The 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 (Fig 3.8)

- Porosity was 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 we only knew the bulk density, so we can 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, we got W2 (g)

Step 3: After 24 hours drying in an oven at a temperature of 105⁰C, weight soil and aluminum and we got W3 (g)

- Calculated according to the following formula:

W% =

Fig 3.8 (a) Bulk density tube, (b) took a soil sample, (c) soil sample keep in

zipper plastic bag, (d) measured wet soil

3.2.7 Soil texture

In order to determine soil type the first took a soil sample from each plot and then brings to testing in the laboratory for a quantitative determination of the particle sizes (Jahn et al., 2006) The following are some of the things which were done in the soil laboratory: the first removed the fraction > 2 mm (rock, leaf, roots

or organic material) by sieving or by hand and then dried soil sample, (Fig 3.9) After that grind, the soil for separate particles the total weight of the fine earth will

be accurately measured The fine earth will be passed through a series of sieves with

a mesh of different size, down to about 0.1 mm in diameters (Jahn et al., 2006) Fill the pipette with soil about a quarter full and add chemical (Borax 5 ml) to help the clay settle down Keep pipette in a place where it can sit undisturbed for 24 to 48 hours after that different component of your soil will settle into layers and then the result of soil was showed With this result using textural triangle method of US

(b) (a)

Department of Agriculture (USDA) to check the number of soil types: silt, sand, and clay at the study site

Fig 3.9 (A) Removed fraction, (B) rock and roots, (C) soil after grind and (D)

pipette

(D) (C)

(B)

3.2.8 Vegetation observation

Grown canopy and understory vegetation cover were determined by using GLAMA and Canopy Cover Free application Took the picture from the canopy and the vegetation cover (standing in the center of each plot) and inserted to the program for processing and then the results will show on the screen

3.2.9 Topographic survey

GPS was used to take the coordinate system, measure the altitude, slope of each plot and make the map of the study site

3.3 Data analysis

The data was analyzed and processed by using Microsoft Excel and SPSS

Using SPSS to compare the difference in term of runoff generation and soil erosion

we use T-test with confidence 95% To check the relationship among runoff, soil and precipitation we use correlation and linear regression function in SPSS

CHAPTER IV: RESULTS

4.1 Runoff generation at 2 different ages of Acacia plantation

There were 55 storm events has been collected from 6 months It was started from April to September 2018 Based on the result showed that the amount of lowest rainfall range from 2.25 mm on 20-Jun-18 and the highest storm was 117.50

mm on 19-Jun-18, with (mean 33.81±22.57 mm), (Fig 4.1)

Fig 4.1 Runoff generation and precipitation at the study site The runoff generation is varied from upper plots to down plots and from Acacia plantation 1-year-old to 5-years-old The runoff generation in 1-year-old was slightly different from plot 1-down and plot 1-up ranged from 0.00-1.01 mm (mean 0.16±0.18 mm/15m2/storm) and 0.00-1.13 mm (mean 0.20±0.20 mm/15m2/storm), respectively (Fig 4.1) While 5-years-old, the runoff generation get a similarly between two plots, plot 5-down ranged from 0.00-1.36 mm (mean 0.17±0.27 mm/15m2/storm) and plot 5-up range from 0.00-0.71 mm (mean 0.18±0.18 mm/15m2/storm), (Appendix table1)

Fig 4.2 Runoff coefficient from four plots With the same results as total runoff, over 55 storm events, runoff coefficients

at 2 different ages was showed the results, at Acacia year-old plots down and

1-up were range from 0.00-1.3% (mean 0.36±0.28%) and 0.00-1.4 % (mean 0.46±0.34%) with the average 0.41%, respectively And Acacia 5-years-old, plot 5-down and plot 5-up were range from 0.00-1.5% (mean 0.35±0.33%) and 0.00-1.1% (mean 0.39±0.27%) with the average 0.37%, respectively Thus result was showed that there were not significantly different between 2 different ages of Acacia plantation forest 1-year-old and 5-years-old in term of runoff, with the (P-value = 0.95 > 0.05), (Fig 4.2)

In the process of runoff it not only different age condition which has affected to surface flow but it also from the other factors like the location different between upper and down of the hill also have a direct impact to the process of runoff For Acacia plantation forest at 1-year-old and 5-years-old at the upper-hill, plot 1-up is greater than plot 5-up And for the down-hill plot 1-down is slightly lower than plot

5-down These among factors impacted on the location such as steep slope, elevation, root systems, and the density of trees and the porosity of the soil from each plot For the highest runoff related to the location impact of this study is plot 1-up, range from 0.00-1.13 mm (mean 0.20±0.20 mm/15m2/storm) with the mean of runoff coefficient is 0.46%

Fig 4.3 Runoff accumulation and precipitation from four plots The total amount of rainfall accumulation of 55 observed storm events was 1887.4 mm The amount of runoff accumulation in 1-year-old of Acacia plantation forest at plot 1-down range from 0.17-8.84 mm (mean 3.84±2.60 mm) and plot 1-up range from 0.21-10.90 mm (mean 4.85±3.24 mm) For 5-years-old Acacia plantation at plot 5-down range from 0.11-11.11 mm (mean 5.10±3.49 mm) and plot 5-up range from 0.18-9.72 mm (mean 4.72±2.98 mm), (Fig 4.3)

The ability to generated surface runoff the highest at plot 5-down, but it slightly different from 1-up (1.02 times), 5-up (1.14 times) and 1-down (1.26 times)