eeos 350: quantitative hydrogeology

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EEOS 350: Quantitative hydrogeology Lecture 2 Water balance

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EEOS 350: Quantitative hydrogeology. Lecture 2 Water balance. The Hydrologic Cycle. Fluxes (flows) Stocks (storage). Source: www.cet.nau.edu/Projects/ SWRA/research.html. Control Volume (CV). For mass (or water) balance analysis, need to establish the size (scale) of the problem. - PowerPoint PPT Presentation

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Page 1: EEOS 350: Quantitative hydrogeology

EEOS 350: Quantitative hydrogeology

Lecture 2

Water balance

Page 2: EEOS 350: Quantitative hydrogeology
Page 3: EEOS 350: Quantitative hydrogeology

The Hydrologic Cycle

Source: www.cet.nau.edu/Projects/ SWRA/research.html

Fluxes (flows)Stocks (storage)

Page 4: EEOS 350: Quantitative hydrogeology

Control Volume (CV)

• For mass (or water) balance analysis, need to establish the size (scale) of the problem.

• Need to be able to quantify the inputs and outputs that cross the CV boundaries and the storage terms within the CV.– For water balance, the CV = watershed.– What are the inputs and outputs that cross the

boundaries of the CV?

Page 5: EEOS 350: Quantitative hydrogeology

Inflows

1. Precipitation (P)

2. Groundwater inflow (GWin)

3. Artificial import; e.g., pipes and canals (Hin)

4. Note: there is no Qin….the watershed is defined such that all Q is generated within the boundaries and flows through the outlet.

Page 6: EEOS 350: Quantitative hydrogeology

Outflows

1. Transpiration from plants (T)2. Evaporation from surface water bodies

(E) E+T=evapotranspiration (ET)3. Runoff of surface water (Q)

4. Groundwater outflow GWout)

5. Artificial export (Hout), ie. export water from basin, i.e. Deer Island effluent.

Page 7: EEOS 350: Quantitative hydrogeology

Storage terms

1. Surface water bodies (lakes, rivers, wetlands)2. Soil moisture in unsaturated zone3. Ice and snow (significant storage term in

northern climates in winter)4. Depression storage (puddles, ephemeral

wetlands)5. Water intercepted by vegetation6. Ground water below water table (saturated

zone)

Page 8: EEOS 350: Quantitative hydrogeology

Water Balance

tSGWQET

GWP

out

in

)(

)(

Looking at just the natural water cycle (ignoring human components):

(At the watershed scale, we often assume that GWin = GWout)

inputs

outputsChange in storage

Page 9: EEOS 350: Quantitative hydrogeology

Simplified Water Balance

At an annual timestep (average of long-term yearly totals), we often assume that there is no change in storage:

tSQETP

TEQP

QTEP

0

Under what conditions might this assumption be wrong?

Page 10: EEOS 350: Quantitative hydrogeology

Computing an annual water balance for the Lamprey River

Q

P

Watershed area =212 mi2

Catchment area forstream gage atPackers Fall =183 mi2

Precipitation gageat Durham.

Page 11: EEOS 350: Quantitative hydrogeology

Water balance data• Annual precipitation from the Durham NH rain

gage.– Dimensions: [L/T]– Units: inches per year (in/yr)

• Streamflow from the USGS gage at Newmarket.– Dimension: [L3/T]– Units: cubic feet per second (cfs).

all terms in the water balance must be in the same dimensions and units!! Now what?

Page 12: EEOS 350: Quantitative hydrogeology

Water balance computations

• All water balance terms should be in [L3/T]• Time step is annual (by definition of our

problem), so time unit = year.• Typical volume unit = ft3 or m3 or km3.

– How do we convert Q from ft3/sec to ft3/yr?– How do we convert P from in/yr to ft3/yr?

• First, let’s compute long-term average annual values for P & Q.

Lamprey_P&Q.xls