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

MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT VIETNAM FORESTRY UNIVERSITY STUDENT THESIS EVALUATING EFFECTS OF VEGETATION COVER TYPES ON OVERLAND FLOW GENERATION AND SOIL EROSION IN LUOT MOUNTAIN Major: Natural Resources Management (Advanced Curriculum) Code: D850101 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 Hanoi, November 2014 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 for Cinnamomum parthenoxylon plantation, Plot for shrubs and Plot 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 was covered by shrub and litterfall up to 90% and 40 - 80% was the surface cover of Plot 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 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 mm ranged from 0.01 mm to 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 Cinnamomum parthenoxylon plantation was 36 g/ , in shrub was 77 g/ 0.8 g/ to 26 g/ ranged from 0.8 g/ ranged from 1.1 g/ to 30 g/ and 86 g/ to 16.4 g/ ranged from 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 II OBJECTIVES 2.1 Hypothesis 2.2 Objectives III STUDY SITE AND METHODS 3.1 Study site 3.2 Methods 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 (a) Location and topography of study site (b) detail of study site and sample plots (c) plot (d) plot (e) plot Figure (a) Plot (b) Plot Figure (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 Results of surface runoff analysis between three vegetation cover means: Cinnamomum parthenoxylon plantation, grass and shrub 14 Table Surface runoff from Cinnamomum parthenoxylon plantation, grass and shrub 14 Table Percentage of surface runoff in Cinnamomum parthenoxylon, shrub and grass (%) 17 Table Results of surface runoff analysis between three vegetation cover means: Cinnamomum parthenoxylon plantation, grass and shrub 18 Table Sediment from Cinnamomum parthenoxylon plantation, grass and shrub by each storm event 18 Table Amount of surface runoff and sediment in different storm sizes 23 Table Percentage of throughfall and interception in Cinnamomum parthenoxylon forest (%) 24 Table 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 humaninduced soil degradation has affected nearly 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 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 affecting sediment yields Soil erosion increased with slope length and steepness as a result 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/ with litter was 30 g/ , without trees but , 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 (Vietnam Soil Science Congress) Every year, streams and rivers carry 200 million tons of 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 -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 Figure 4.6 showed us the relationship between sediment and surface runoff in three different vegetation cover types In this case, the relationship between sediment and surface runoff in three types of vegetation cover was similar When the surface runoff increased the amount of eroded soil also increased The best model representing for the relation of surface runoff and sediment is linear function with the coefficient of determination in Cinnamomum forest = 0.870, = 0.798 in shrub and = 0.682 in grass A single outlier as a large storm event indicates potentially higher differences between plots and vegetation cover when precipitation is high Having only 10 events in this study limits the data used for our analysis and may prevent us from identifying tends in sediment erosion at each plot Because the difference in erosion between the study plots is greater for the single large event, differences in erosion between vegetation type could be more important for large storms than our data allow us determine Additional data collected in a long-term study may provide additional insight into the effect of large storms on erosion associated with different vegetation types 4.5 Amount of surface runoff and sediment from big storms compared to small storms We divided storms to two means: storms bigger than 25 mm and smaller than 25 mm The amount of surface runoff coefficient and sediment differed from big storms and small storms showing in figure 4.7 and table 6: 21 (a) Sediment (g/cm2) Amount of sediment from small storms and big storms (g/cm2) 40.0 < 25 mm 30.0 > 25 mm cinamomu m shrub 20.0 10.0 grass 0.0 Storm size (mm) 10 12 Surface runoff from small storms and big storms (mm) (b) Runoff (mm) < 25 mm > 25 mm cinamomum shrub grass 0 (c) Storm size (mm) 10 12 Runoff coefficient from small storms and big storms (%) Runoff coefficient (%) >25 mm < 25 mm cinamomum shrub grass 0 Storm size (mm) 10 12 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 Table Amount of surface runoff and sediment in different storm sizes n Surface runoff (mm) Surface runoff coefficient (%) Sediment (g/m2) 25 mm storms 31.6 5.70 35.0 4.8*10^-7 11.3 187 2.5*10^-5 P - value 0.004 There is a significant difference in surface runoff and sediment in small storm events and big storm events (Table 6) P-value is smaller than 0.05; it means the rainfall has a deal effect on sediment and surface runoff Total of Surface runoff, surface runoff coefficient and sediment in 25mm storms Total of surface runoff amount in >25 mm storms was 105 bigger than in 25 mm storms was 6.1 times bigger than in