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Hydrology is the force that drives wetland soil development and biotic process, regulates sediment deposition, plant productivity and organic matter accumulation. Hydrology is probably the single most important determinant of the establishment and maintenance of specific types of wetlands and wetland processes. This studying purpose is to describe general hydrologic properties, and to provide an overview of the processes that control wetland hydrologic behavior. Based on these properties, the study will describe and calculate the wetland hydrology of specific types of Wetland in Vietnam to deeply understand about hydrology of specific types of Wetlands in Vietnam
VNU University Of Science - 334 Nguyen Trai –Thanh Xuan – Hanoi Faculty of Environment- Environmental Technology Group working: Wetland Topic: Describe/Calculate the wetland hydrology of different types of Wetlands in Vietnam Group : Cù Thị Hiền, Nguyễn Thị Hương, Nguyễn Văn Luân, Nguyễn Ngọc Khánh, Vũ Thị Bích Ngọc Instructors : Nguyễn Thị Loan Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page Hanoi, 10th May 2013 Abstract Hydrology is the force that drives wetland soil development and biotic process, regulates sediment deposition, plant productivity and organic matter accumulation Hydrology is probably the single most important determinant of the establishment and maintenance of specific types of wetlands and wetland processes This studying purpose is to describe general hydrologic properties, and to provide an overview of the processes that control wetland hydrologic behavior Based on these properties, the study will describe and calculate the wetland hydrology of specific types of Wetland in Vietnam to deeply understand about hydrology of specific types of Wetlands in Vietnam I Introduction The purpose of this study is to describe, calculate and discuss the general hydrologic properties that make wetlands unique, then to provide calculation of specific different wetlands in Vietnam that classified base on the Vietnam wetland classification system Firstly, this studying gives a general definition of hydrology to have a hydrologic overview Hydrology is all hydrologic characteristics of area that are periodically inundated or have soils saturated to the surface at sometimes during the grow season Hydrology is probably the single most important determinant of the establishment and maintenance of specific types of wetlands and wetland processes.(Mitsch and Gosselink (1993)) Therefore, discussion and calculation wetland hydrology are very important for understanding how much water is stored, how all components determine amount of water in the each types of wetland in Vietnam, and lead to the balance of inflows and outflows If the need arises to obtain more specific information, the reader is advised to refer to wetland books or articles, including those referenced within this study Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page II Study Methodology To calculate and describe hydrology of different wetland in Vietnam, this study will firstly give all components that affect indirectly or directly the water budget and water balance in wetlands with formulas and description respectively Besides, it is very important to contribute a complete equation describing the balance of all components that is known as water budget Next, this study will show the way to classify Vietnam wetland system based on the Vietnam wetland classification system, then giving specific wetlands in Vietnam relative to each systems, subsystems and classes and leading to calculate and describe hydrology of different chosen wetlands are final purposes in this study Moreover, in order to describe and calculate correctly, data collection relating to hydrological properties is so essential for each chosen Vietnam wetland including: Da river, Tra O marsh, Ba river delta, Ba Lat mouth, Can Gio mangrove forest Because the information and data are not enough to calculate the water budget of Can Gio mangrove forest, this wetland just is described about hydrological factors This study methodology is clear and simple to get approaches of study purpose General Hydrologic conditions and their simple determinations, such as hydroperiod, water budget and so on, in wetland studies can contribute to a better understanding of wetland function The wetland hydroperiod is the seasonal pattern of water level of a wetland It defines the rise and fall of a wetland’s surface and subsurface water For wetlands that are not subtidal or permanently flooded, the amount of time that a wetland is in standing water is called the flood duration, and the average number of times that a wetland is flooded in a given period is known as the flood frequency The hydroperiod is not the same each year but varies statistically according the climate and previous conditions The balance of inflows and outflows of water through a wetland defines the water budget and determines the amount of water stored within the wetland The major components of a wetland’s Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page water budget include precipitation, evapotranspiration, flooding in riparian wetlands, surface flows, groundwater, tides in coastal wetlands The water level fluctuations are also determined by level of development, changes in activities and so on Hydrological components 2.1 Precipitation Wetlands occur most extensively in regions where precipitation, a term that includes rainfall and snowfall, is in excess of losses such as evapo-transpiration and surface runoff Some of precipitation will be retained by the vegetation cover that is called interception (I) The total amount of precipitation that actually reaches the water’s surface or substrate of a wetland is called the net precipitation (Pn) and is defined as Pn = P – I (1) 2.2 Surface Water Surface Runoff Wetlands are subjected to surface inflows of several types Overland flow is nonchannelized flow that usually occurs during and immediately following rainfall or as tides rise in coastal wetlands and so on A wetland influenced by a drainage basin may receive channelized stream-flow during most or all of the year Wetlands are often an integrated part of a stream or river Surface inflow from a drainage basin into a wetland is usually difficult to estimate without a lot of data The direct runoff component of stream-flow refers to rainfall during a storm that causes an immediate increase in stream-flow An estimate of the amount of precipitation that results in direct runoff from an individual storm can be determined from the following equation: Si = RPPAw (2) where Si = direct surface runoff into wetland (m3 / storm event) RP = hydrologic response coefficient P = average precipitation in watershed (m) Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page Aw = area of watershed draining into wetland (m2) - The value of RP, which represent the fraction of precipitation in the watershed that becomes - direct runoff, for example, range from 4-18% for small watersheds in the eastern U.S In some cases, peak runoff or flood peak may be important An useful way to predict peak runoff for only small watersheds less than 80 in size, is calculation by using the following equation: Si(pk) = 0.278CIAw (3) where Si(pk) = peak runoff into wetland (m3 / s) C = rational runoff coefficient I = rainfall intensity (mm / h) Aw = area of watershed draining into wetland (km 2) The coefficient C, which ranges from to1, depends on the upstream land use Concentrated urban areas have a coefficient ranging from 0.5 to 0.95, and the rural areas have lower coefficients that greatly depend on soil type, with sandy soils lowest (C = 0.1-0.2) and clay soils highest (C = 0.4-0.5) Stream-flow The water that flows down river courses is obtained from either surface runoff of rainfalls and snowmelt from catchments In order to obtain the water discharge in wetland from channel, flow rates must be measured Most measurements for flow rates depend on following equation: Q = AV (4) Where Q = discharge or volume per unit time A = cross-sectional area of flow V = mean velocity of flow through the cross-section 2.3 Groundwater Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page Groundwater inflow results when the surface water (or groundwater) level of a wetland is lower hydrologically than the water table of the surrounding land (called a discharge wetland) When the water level in a wetland is higher than the water table of its surroundings, groundwater will flow out of the wetland (called a recharge wetland) It is increasingly recognized that groundwater plays an important role in the water balance, water quality and ecology of surface water bodies Along with that realization, there has been much progress in recent years in the detection and measurement of groundwater discharge to surface water (and of groundwater recharge from surface water) These include physical methods such as stream gauging and seepage meters, chemical and isotope methods for hydrograph separation and for determining groundwater inflow, thermal methods for detecting groundwater flow, and continuing advances in hydro(geo)logical instrumentation However, conceptual, theoretical and technical challenges remain Water movement at the margins of surface water generally involves a complex and constantly changing interaction between regional groundwater flow, local saturated and unsaturated subsurface flow, surface water fluctuations, root uptake and evapo-transpiration 2.4 Evapo-transpiration Several direct measurement techniques can be used in wetlands to determine evapo-transpiration The classical reference method is the measurement of evaporation from a water-filled pan , usually by measuring the weight loss, by measuring the volume required to replace lost water over a period of time, or by measuring the drop of water level Water Budget and Water Balance The hydroperiod, or hydrologic state of a given wetland, can be summarized as being a result of the following factors: The balance between the inflows and outflows of water The surface contours of the landscape Subsurface soil, geology, and groundwater conditions Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page The first condition defines the water budget of the wetland The second and the third define the capacity of the wetland to store water The general balance between water storage and inflows and outflows, illustrated in Figure 1, is expressed as ∆V/∆t = Pn + Si + Gi – ET – So – Go ± T (5) where V = volume of water storage in wetlands ∆V/∆t = change in volume of water storage in wetland per unit time, t Pn = net precipitation Si = surface inflows, including flooding streams Gi = groundwater inflows ET = Evapotranspiration So = surface outflows Go = groundwater outflows T = tidal inflow (+) or outflow (-) Each of the terms in equation (1) can be expressed in terms of depth per unit time (e.g., cm / year) or in terms of volume per unit time (e.g., m3 / year) The average water depth, d, at any one time, can further be described as d=V/A (6) where A = wetland surface area Figure Generalized water budget for a wetland with corresponding terms as in equation Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page The Vietnamese Wetlands Classification System The following Table illustrates this classification structure Here, the term “system” refers to a collection of wetlands that share the influence of the salinity of water and marine factors Therefore, wetlands are divided into systems: coastal and inland wetlands We further divide “system” into subsystems based on origin of wetlands, due to human activities or natural processes Thus, a system is divided into two subsystems: Natural wetlands and Artificial wetlands The class is determined based on the hydrological condition As a result, a subsystem is divided into two classes: permanent and nonpermanent saturation Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page List of name of chosen different wetlands in Vietnam System Inland Wetland Sub-system Natural wetland Coastal Wetland Artificial wetland Natural wetland Class Permanent Nonpermanent Nonpermanent Permanent Name Da River Tra O Marsh Rice field( Ba river Delta) Ba Lat Mouth Can Gio Mangrove Forest III Result of study Da River The Black River sub-basin in Vietnam have territorial catchment area is 25,200 km The total amount of water flowing into the basin from outside: 26 million m3 per year, annual rainfall across the basin: 50 million m3 per year, the total evaporation over an area of 24 million m3 per year, total local flow and outside is about 52 million m3 per year, 25 million m3 per year for outflow It can be said the water resources in the Black River sub-basin is rich, the rivers and streams of the Black River sub-basin with high slopes create the largest hydropower potential in Vietnam's rivers, accounting for 35% of total potential hydropower of the country The water budget can be shown by = Pn + Si + Gi – ET – So Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page 10 = 50x106 + 26x106 + 26x106 – 24x106 – 25x106 = 53x106 (m3/year) Tra O Marsh Tra O Marsh is located in Phu My- Binh Dinh, and is built up a remarkable area of wetland ecology for Binh Dinh Tra O mash has an abundant water resource, provides water for My Thang, My Loi, My Duc commons in Phu My district for agriculture and a large amount of aquatic products for local people Tra O Marsh has a large scale of precipitation, average amount of precipitation is around 1900mm/year Precipitation in months of growing season (September, October and November) accounted to be P = 1233 mm and take about 65% of rainfall in this year Month Precipitatio n Evaporation Flow 43 10.5 6.1 27.7 75.3 61.7 75.3 73.4 -32.3 -51.2 -67.4 -45.7 97.8 92.1 5.7 10 11 12 146 67.5 112 122 34 -54.5 115 142 -27 249 84.3 164 560 73.1 487 424 61.8 362 152 69.4 82 Phu My is located near the East sea, so it has a large scale of precipitation and also a high amount of inflow in growing season In growing season: F= (164 + 487 + 362) =1013( mm) Then the storage balance can be illustrated by: = P-ET+F = 1233- 219.2+ 1013 = 2026.8 (mm/3months) = 675.6 (mm/month) Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page 11 With this high amount of adding water in water budget, it is easy to cause flood in growing period Opposite with growing season, in the dry season (January, February and April), the precipitation is very low with P= 59.6 mm (only 3.14% of total precipitation of a year) and the outflow is significant with F= -150.9 mm Besides, the temperature in this season is lower and humidity is higher than this in dry season, lead to reduce evaporation rate Therefore, the change in water budget has the minus number = P-ET+F = 59.6 – 150.9 -213.3 = - 334.6 mm/3months = -111.56 mm/ month That explained why Tra O Marsh has area of 1.200 in dry season and spreading out to 1600 in growing season Although Tra O Marsh has a long dry season (from January to August), but Tra O receives quietly a high amount of precipitation and run-off flow, so the water storage average in a year is still quite high = P-ET+F = 1900- 1042.4 + 856.6 = 1712.8 mm/year Ba Lat Mouth BaLat estuary lies in the area governed both by flood and tides, and the area essentially controlled by the tidal regime One of the main hydrological characteristics of the river mouth is the behavior of the river water during the dry and flood seasons From current data during the flood and dry seasons of 2000,it appears that the outflow lasts longer than the inflow, especially during the flood season(outflow 19h,inflow–6h;respective maximum velocities : 1.2m/s to 0.4m/s) The interface between saline and freshwater depends on the river flow, the tidal cycle and the river morphology Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page 12 During the dry season, the out flow (river flow plus ebb-tide flow) were 0.45m/s during spring tides and 0.25m/s during neap tide the inflow were 0.25m/s during spring tides and 0.0 during neap tides During the flood season , outflow velocity reach 1.2m/s ,and the outflow is permanent during both neap and spring tides The tide at the BaLat mouth is varying from 1.9 to 2.6m Average water discharge through the BaLat mouth is about 300m /s in dry and 3500m3/s during flood seasons Each year, the Red river discharge about 137×10 m3 of water and distributes 25% of the flow of the Red River discharging into the sea via the Ba Lat mouth There for the annual discharge of Ba Lat mouth is 137×109 × 25% = 3.425 × 1010m3 Rice field Delta is in Ba river‘s downstream This is the largest delta in Central South Area of about 500km Rice field is an artificial wetland, so the inflow and outflow are stopped in order to control water budget in rice field The income water is depend on rainfall and irrigation, and the outcome are regulated by evapotranspiration and percolation through soil structure (DP) The following table shows all water budget components in the Ba river Delta Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page 13 Water budget in rice field is largely depend on irrigation and percolation, the rainfall is not the main source of hydrology as other wetlands The change in water budget can be illustrated by following equation : = Pn +Ir – ET - DP = (18.15 ×106 + 520.7 ×106 – 134.6 ×106 – 333.85 ×106) = 70.40×106 m3/3.5months = 20.11×106 m3/month Can Can Gio Mangrove Forest Gio mangrove forest lies entirely within the Can Gio district of Ho Chi Minh City It covers an area of 75740 in 2012 Mangrove areas account for 56.7% of the total area (42944.6 ha) • Precipitation: Precipitation (P) in Can Gio is the lowest in the Ho Chi Minh City area, with an average range of 1300 – 1400 mm per annum In 2012, the average rainfall is 1336 mm a year • Humidity and evapo-transpiration The humidity in Can Gio is about 4% - 8% higher than in other areas of Ho Chi Minh City During the rainy season, humidity ranges from 79% - 83%, with a maximum of 83% in September In the dry season, humidity ranges from 74%-77%, with a minimum of 74% in April Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page 14 The average extent of evaporation is mm/day, and 120.4 mm/month Evaporation is at its highest in June (173.2 mm/month) and at its lowest in September (83.4 • mm/month) Tidal inflows and outflows The Can Gio mangrove forest lies in a zone with a irregular semi-diurnal tidal regime (i.e two times ebb and flow tides per day) Tidal amplitudes range from about m at mean tide to m during spring tides It has been observed that the two daily high and low tides differ in height Maximum tidal amplitudes, in the region of 4.0 – 4.2 m are the highest observed in the whole of Viet Nam Tidal amplitude decreases with distance north (i.e inland), relating to the proximity of the Bien Dong Sea (South China Sea) High tides reach their maximum between September and January, at 3.6 - 4.1 m in the southern and 2.8 - 3.3 m in the northern regions of Can Gio In Can Gio, the maximum high tide occurs in October or November, and the minimum in April or May According to lunar calendar, between the 29th and 3rd day of the month and between the 14th and 18th day of the month, the whole of the Can Gio mangrove forest is flooded at high tide (twice a day) On the 8th and 25th day of the month (lunar calendar) the low tide is at its minimum IV Conclusion Hydrology is probably the single most important determinant of the establishment and maintenance of specific types of wetlands and wetland processes Depending on the topography, vegetation, climate, sources of water… wetlands in Vietnam is divided into 38 types based on physiography and composition of substrate features, geomorphologic, geologic features and origin of wetland Types of wetland are different in term of hydrology characteristics All the calculations in the study base on the equation of water storage in wetland = Pn + Si + Gi – ET – So – Go ± T Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page 15 Depend of the differences of hydrological statistics in different wetlands, the equation can be calculated with different factors and have different units Therefore, the results show hydrological status of given wetlands that are water budget and water stored capacity of wetlands Basically, Vietnam lies in tropical weather zone, and has a long coastal area leading to have a large scale of precipitation and the precipitation is greater than evapotranspiration The change in water budget is highly greater than zero Besides, Vietnam has a higher relief in the western and the Eastern Sea in the east, the water-flow and tidal are quite the main water sources for some wetlands V References “Hydrology and morphology of two river mouth regions (temperate Vistula Delta and subtropical Red River Delta)” Zbigniew Pruszak1 Pham van Ninh2 Marek Szmytkiewicz1 Nguyen Manh Hung2 Rafał Ostrowski1,∗ “The Vietnamese Wetlands Classification System ” Mai Trong Nhuan, Nguyen Thi Thu Ha, Tran Dang Quy, Nguyen Thi Ngoc, Do Thi Thuy Linh, Nguyen Thi Minh Ngoc, Nguyen Thi Hong Hue, Pham Bao Ngoc “Định hướng qui hoạch sử dụng nước lưu vực Đầm Trà Ô, tỉnh Bình Định” Nguyễn Thanh Sơn- Khoa khí tượng Thủy văn Hải dương học Đại học khoa học Tự Nhiên- ĐHQG Hà Nội “ Wetland Hydrology” The Regional Training Course on SUSTAINABLE USE AND MANAGEMENT OF COASTAL WETLANDS – 20 November 2007 Faculty of Environment and Resource Studies, Mahidol University, Thailand - Kobkaew Manomaipiboon Calculate/describe the wetland hydrology in different types of Vietnam wetlands Page 16