MICHAEL S. ROLBAND, P.E., P.W.S.

NICOLE FOMCHENKO, E.I.T.

CALCULATING A WETLANDWATER BUDGET

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OUTLINE

\ Introduction

\ Wetland Models

\ Water Budget ModelâPrecipitation

âSurface Water Runoff

âGroundwater Inflow (Infiltration)

âEvapotranspiration

âGroundwater Outflow (Exfiltration)

âSurface Water Outflow

Wetland Models

\ Novitzki (1979, 1989) outlined 5 basic sets ofhydrologic conditions that cause wetlands. Creating a wetland requires one toreplicate one of these sets of characteristics:

1. Surface water depression wetlands2. Ground water depression wetlands3. Surface water slope wetlands4. Ground water slope wetlands5. Extensive wetlands on flat plains

Surface Water Depression

Ground Water Depression

Surface Water Slope

Ground Water Slope

Water Budget Model

\ Model of water depths and potential draw downfor proposed wetland constructionâMonthly estimates represented graphically

F Water Depths vs. Time

\ Reliable Water SourceâPredict water inflows and outflows

âAll units converted to water depth over wetlanddesign

âAll inputs conservatively estimated

âAll outputs generously estimated

W a t e r B u d g e t - T y p i c a l Y e a r

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Oct. Nov. Dec. Jan. Feb. Mar. A p r . May June July A u g Sept.

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Initial Fill

In f i l t ra t ion

Runoff

Precip.

PET

Exf i l t ra t ion

O u t f l o w

Water

Elevation:

Graphical Water Budget

Water Budget ComponentsMASS BALANCE

âINFLOWS - OUTFLOWS = ∆STORAGEF Inflows = Precipitation + Surface Water +

Groundwater Inflows

F Outflows = Evapotranspiration + Surface WaterOutflows + Groundwater Outflows

l all components measured in inches

l volumes divided by area of wetland to obtain depthof water in inches.

F ∆Storage = change in water level depth (inches)

InflowsOutflows

Water Budget Components

âVariables:F Precipitation

l Actual Rainfall - Amount/Distribution

F Surface Water Runoff

F Groundwater Inflow (infiltration)l assume zero to be conservative

F Potential Evapotranspiration

F Groundwater Outflow (exfiltration)l soil permeability (k)

F Surface Water Outflow

F Change in Storage or water level

F Watershed Changes Overtime

Water Budget Components

\ Input/OutputType Typical Data Sources

1. Precipitation Historical Rainfall2. Infiltration Site Measurements3. Direct Surface TR-554. Overbank Flooding TR-55, HEC-2, Stage5. Evapotranspiration Pan Data, Thornthwaite6. Exfiltration Soil Testing7. Spillway Outflow Weir Formula

Typical Water Budget ModelS = P + RO - ET - GWO - SWO

Precipitation

\ Daily precipitation data obtained from nearestweather station - Historical RainfallâPrecipitation selected from wet, dry, and typical years

F Dulles or Washington National Airportsl http://205.156.54.206/er/lwx/iad/cliiad.htm

l http://tgsv5.nws.noaa.gov/er/lwx/dca/clidca.htm

F VA State Climatology Officel 804-924-0548 http://www.people.virginia.edu/~climate/

F National Climatic Data Centerl 804-924-0548 http://www.ncdc.noaa.gov/

S = P + RO - ET - GWO - SWORunoff

âSurface Water RunoffF SCS Runoff Curve Number Method - TR-55

F Q = (P - Ia)2 / [(P - Ia) + S] Eq. 1l Q = runoff

l P = rainfall

l S = potential maximum retention after runoff begins

l Ia = initial abstraction

• units are inches

F Ia = 0.2S Eq. 2l surface depressions, interception, evaporation, and

infiltration

l minimum precipitation to cause runoff

S = P + RO - ET - GWO - SWORunoff

âSubstituting Eq.2 into Eq.1 gives:

âQ = (P - 0.2S)2 / (P + 0.8S)F P = daily precipitation

F S = maximum retention potential

F S = (1000 / CN) - 10l related to soil and cover conditions of the watershed through

the CN

l determined by rainfall-runoff plots

F Influenced by:l rate of infiltration at soil surface

l rate of transmission in the soil profile

l water-storage capacity of the profile

S = P + RO - ET - GWO - SWORunoff

\ CN = Curve Number - ranges from 0 - 100âhydrologic soil group (A - D) (Exhibit A-1)

F A (high infiltration) - D (low infiltration)

âcover type (Tables 2-2)F vegetation, bare soil, impervious surfaces, residential, etc.

âhydrologic condition (Tables 2-2)F poor, fair, and good

âantecedent moisture condition (AMC I - III)F based on 5 day rainfall totals

l I - dry periods

l II - average moisture periods

l III - wet periods

l Assume II

S = P + RO - ET - GWO - SWORunoff

\ Calculate CNF 1. Delineate Watershed

F 2. Locate hydrologic soil groups (A - D) from Soil Survey.

F 3. Divide watershed into different cover types (woods,paved, etc) and conditions (good, fair, or poor).

F 4. Overlay watershed, soils, and cover type maps andcalculate area of each subgroup.

F 5. Determine CN for Each subgroup (Tables 2.2 a-d)

F 6. Calculate weighted CN (CNw)l CN x Area for each subgroupl CNw = CN x Area / ∑Areas

S = P + RO - ET - GWO - SWORunoff

\ Calculate Runoff:F 1. Calculate S = (1000/CNw) - 10

F 2. Using Daily Precipitation Data (P) calculate:l Q = (P - 0.2S)2 / (P + 0.8S) (inches of runoff over watershed)

l Note that minimum size storm event that creates runoff = Pmin= 0.2S

F 3. Volume RO = Q x Net Contributing Watershed Area

F 4. Depth RO contributing to constructed wetland = VolumeRO / (Constructed Wetland Area + Existing Pond/WetlandArea)

F 5. Repeat steps 2 - 4 to for each daily storm event

F 6. Sum daily RO depths to determine RO depth for month.

S = P + RO - ET - GWO - SWORunoff

âLimitationsF Ia = 0.2S = 20% of

maximum retention

F developed bymeans of rainfalland runoff data fromexperimental smallwatersheds (< 10acres).

F highly variableF Source:

F USDA, 1972. NationalEngineering Handbook,Section 4, Hydrology,Chapter 10. Estimation ofDirect Runoff From StormRainfall.

S = P + RO - ET - GWO - SWORunoff

\ Limitationsâ CNs describe average conditions.

â Time is not a factor, therefore rainfall duration nor intensityare accounted for.

â Does not take into account runoff from snowmelt or rain onfrozen ground.

â CN is less accurate when runoff is < 0.5 inches.

â Applies only to direct subsurface runoff not subsurface flowor high groundwater levels that contribute to runoff.

â If CN < 40 use another method.

â Safety Factor - The opposite of Typical Civil EngineeringSF’s

Sunrise Valley Nature Park Runoff ComparisonReston, VA

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Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

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SCS TR-55 Measured Inflow

âAMC IIF Source:

F Wetland Studies and Solutions, Inc., 1998. Evaluation of a FlexibleFabric/Bentonite Pond Liner through The Analysis of a Water Budget for aConstructed Pond/Wetland System for Walt Disney Imagineering.

S = P + RO - ET - GWO - SWOPotential Evapotranspiration (ET)

\ Thornthwaite ET methodF empirical equation

F mean monthly temperature (T) degrees Celsius

F ET = 1.6(10T/I)a

F I = heat index = ∑ (T/5)1.514 (sum 12 months)

F a = 0.49239 + 0.01792 (I) - 7.71x10-5 (I2) + 6.75x10-7 (I3)

\ orâObtain ET values from nearest weather station

âPercentage of Pan Evaporation

âWeighing Lysimeter

S = P + RO - ET - GWO - SWOGroundwater Outflow (exfiltration)

\ Darcy’s Equation q = -k dh/dlâq = groundwater seepage rate (cm/day)

âk = hydraulic conductivity of soil

âdh/dl = vertical hydraulic gradient, assume = 1

\ Laboratory testing for Kâcollect soil samples for testing

\ Estimate K based on soil textureâSand = >1x10-3 cm/sec (>0.002 ft/min)

âSilt = 1x10-3 to 1x10-5 cm/sec (0.002 to 2x10-5 ft/min)

âClay = < 1x10-6 cm/sec (<2x10-6 ft/min)

S = P + RO - ET - GWO - SWOSurface water Outflow

\ Outflow ControlâExcess Runoff is your Safety Factor

âAlways provide a way to spill excess water and adjustwater levels

âControlled with berms, dikes, dams, spillways, andweirs

Water Level = Initial Fill + P + RO -ET - GWO - SWO

Water budget model components and water level prediction for pond/wetland system.

1995/96 Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug Sept. Year

INPUT:

Initial Fill 2.28 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.28

Infiltration 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Runoff 3.79 1.18 0.06 1.16 0.17 0.37 0.47 0.57 0.79 1.21 2.05 6.35 18.18

Precipitation 6.51 4.75 2.05 5.61 2.62 3.52 3.69 7.07 4.88 5.89 4.16 7.73 58.48

OUTPUT:

PET -2.23 -0.39 -0.03 0.00 -0.10 -0.46 -2.16 -3.06 -5.36 -5.28 -4.90 -3.67 -27.65

Exfiltration -0.10 -0.10 -0.10 -0.10 -0.10 -0.10 -0.10 -0.10 -0.10 -0.10 -0.10 -0.10 -1.22

Outflow -0.38 -5.44 -1.98 -6.66 -2.59 -3.33 -1.90 -4.48 -0.20 -1.71 -1.20 -10.28 -40.13

Water Level * 9.85 9.85 9.85 9.85 9.85 9.85 9.85 9.85 9.85 9.85 9.85 9.85

* expressed in depth (inches) over baseline elevation which is approximately the average distance from weir invert

to average elevation of soil substrate in wetland.

Graphical Water Budget

Constructed We tla nd Mitiga tion Site

Oct. 1995- Sept. 1996

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Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug Sept.

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Inf iltration

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Precip.

PET

Exf iltration

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WaterElevation: