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Unit 2 Hydrology_Lecture 2 - Wetland and Water Budget

Unit 2 Hydrology_Lecture 2 - Wetland and Water Budget -...

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Unformatted text preview: Wetlands and Water Quality Unit 2: Hydrology Lecture 2: Wetland Water Budget In the last lecture we learned how Hydropattern can be used to characterize and quantify the influence of flooding on flora, fauna and biogeochemical processes within the wetland. But what drives hydropattern? If the signature hydropattern of a wetland is in part measured by changes in water level over time, than factors regulating the total amount of water in a wetland must also be the factors regulating hydropattern. To understand what regulates water level in a wetland we need to look at the components that make up the water budget. Wetland Water Budget V/t = Qi - Qo + Si - Go + Gi + PnA - EA V/ Precipitation Evapotranspiration Channeled flow in Surface flow in Channeled flow out Groundwater flow in Groundwater flow out 1 Components of the Water Budget change in storage = inflows outflows Inflows: Precipitation Overland flow Upstream flow Groundwater influx Outflows: Evapotranspiration Downstream flow Infiltration to Groundwater System Storage: Open Water In Porous Media In the most simplistic terms if inflows are greater than outflows water levels go up and flooding occurs. If outflows are greater than inflows, water levels go down and eventually standing water and saturated soils are drained. Precipitation P = I + TF + SF I - interception TF - through fall SF - stem flow 2 Interception 8 - 35% (13% deciduous; 28% coniferous) Net Precipitation (Pn) Pn = P - I = TF + SF Pn = Net Precipitation P = Precipitation I = Interception TF = Throughfall SF = Stemflow Evapotranspiration Evapotranspiration = Evaporation + Transpiration Evaporation is the water that vaporizes from the water or soil directly to the atmosphere. Transpiration is the water that moves from the soil or water into vegetation and then is released to the atmosphere, typically through stomates within leaves. 3 Evaporation Rate E = c f(u) (ew- ea) E = rate of evaporation c = mass transfer coefficient f(u) = function of wind speed, u ew = vapor pressure at surface, or saturation vapor pressure at wet surface ea = vapor pressure in surrounding air Factors Regulating Evapotranspiration Vapor pressure Temperature Wind Solar radiation Vegetation Elevation Humidity Determining Evapotranspiration Estimation methods for ET Dalton's law Dalton' Thornthwaite Equation Penman Equation Direct Measurement (Diurnal Method) Other experimental approaches 4 Comparison of ET methods ET measured from water level Assumptions: constant recharge midnight to 4:00am, negligable ET at midnight, water level at or near root zone 5 Surface Inflows: Surface overland flow (Si) Non-channelized sheet flow Non- Channelized flow (Qi or Qo) Streams flowing into wetland Streams flowing out of wetland Surface Overland Flow Si (total volume estimate) Si = Rp . P . Aw Si = surface runoff to wetland, m3/event Rp = hydrologic response coefficient, represents fraction of precipitation that becomes surface runoff (4-18% small watershed, see table 4-3 (44Mitsch and Gosselink) P = average precipitation in watershed, m Aw = area of watershed draining into wetland m2 6 Surface Overland Flow (peak flows, the Rational Runoff Method) flows, Si(pk) = 0.278 CIAw Si(pk) = peak runoff (m3/sec) C = rational runoff coefficient I = rainfall intensity (mm/hr) Aw = area of watershed draining into wetland, km2 Channelized Water Flow: Q Qi or Qo = AxV Qi = channelized flow into the wetland, m3/sec Q0 = channelized flow out of the wetland, m3/sec Ax = cross sectional area of the channel m2 V = average velocity, m/sec 7 Groundwater-Surface Water Exchange in GroundwaterWetlands (Darcy's Law) (Darcy' G = kas G = flow rate of groundwater (volume per unit time) k = hydraulic conductivity or permeability of soil (length per unit time) a = groundwater cross-sectional area perpendicular crossto the direction of flow s = hydraulic gradient (slope of water table or piezometric surface) Wetland Water Budget V/t = Qi - Qo + Si - Go + Gi + PnA - EA V/ Precipitation Evapotranspiration Channeled flow in Surface flow in Channeled flow out Groundwater flow in Groundwater flow out 8 Total Water Budget Depending upon the degree of confidence in the measurement, quantifying each of these components of a water budget can be a complex, technologically intensive and costly. However, even ballpark numbers allow us to estimate the relative contribution of each hydrologic component. Wetland Water Budget P I ET Pn So Si V/t T Gi Go Example Water Budgets alluvial swamp, Illinois P = 105 I = 31 ET = 72 Flood = 5300 Pn = 74 So = 232 Si = 229 V/t = 0 Gi = 22 Annual water budget, units = cm/yr Go = 21 9 Example Water Budgets bog, Massachusetts Pn = 145 ET = 102 V/t = +19 So + Go = 24 Annual water budget, units = cm/yr Example Water Budgets rich fen, North Wales P=102 ET=49 Si+Gi = 38 V/t = -9 So =100 Annual water budget, units = cm/yr Example Water Budgets mangrove swamp, Florida P = 121 ET =108 V/t = -54 So =90 Tin = 1228 Tout = 1177 Go = 28 Annual water budget, units = cm/yr 10 0% 100% Gr ou nd wa ter 33% Bogs, Pocosins 67% on iio t t ita iip ec re Pr 67% Fens, Seeps 100% 0% 33% 67% Riverine, Fringe 33% 0% 100% Surface Flow Wetland Classification of Hydrologic Inputs (effecting trophic condition) Ombrotrophic principal input from precipitation Phreatotrophic principally input from groundwater Rheotrophic principally input from surface water Minerotrophic rheotrophic and or phreatotrophic input term relates more to chemical signature signature Groundwater "Recharge" Wetland Water moves from the wetland towards the water table which is lower in the surrounding landscape. Leaching environment, tends to lower nutrient and carbonate concentrations Wetland recharges the groundwater 11 Groundwater "Discharge" Wetlands Water moves from the groundwater into the wetland Tends to be an enriching environment with accumulation of carbonates, higher nutrients Fluctuating water tables can cause wetland to shift back and forth between discharging and recharging wetlands Groundwater "Flow-through" "Flow- Wetlands Water moves through wetland at surface of exposed water table Flow-through wetlands are often connected with outflows Flowof one becoming the inflow of the next. Water supply to the lower wetland is often delayed until the upper one fills Groundwater "Perched" Wetlands Low conductivity soils below wetland reduce infiltration and can cause water within the wetland to become disconnected "perched" from the groundwater. Often a transient condition during the beginning of a wet season or shortly after a rain event. 12 Edwin Romanowicz 13 ...
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