Watershed Hydrology and Water Resources

STEP July, 2007: Hydrology – Page No. 1

Watershed Hydrology and Water ResourcesScience Teacher Education Program

(STEP)

Presented by

Amy Tidwell

Water and Environmental Research Center/Institute of Northern Engineering

University of Alaska Fairbanks

July 2007


Watershed Hydrology and Water Resources

Watershed HydrologyWatershed Delineation (Exercise)Water BudgetPrecipitationEvapotranspirationInfiltrationRunoffGroundwater (Demonstration/Activity)Wetlands (Hand out)Climate Change Considerations

Water ResourcesWater Resources Planning and Management (Hand out)Water SupplyWater as a HazardWater Management: Health, Safety, and the EnvironmentClimate Change and Water Resources

Additional ResourcesOnline Activities (time permitting)

Outline


Watershed Hydrology


Watershed Hydrology

We will apply some of what you’ve learned about the global hydrologic cycle to watersheds. As you will see, a watershed is a logical accounting unit in hydrology and water resources.

What is a watershed?

Where is a watershed? And how large is a watershed?

Delineating watersheds• Topographic maps, contour lines and slopes– where does the water flow?• Begin with a point of interest, usually along a stream.• Trace the outline of the watershed, beginning at one side of the stream, by following the steepest

slope (gradient). Recall that the steepest gradient occurs at a right angle to contour lines.• Then begin tracing the outline from the other side of the stream until your second trace meets up

with the first.• Check your work: Consider a rain drop falling over your delineated watershed. Pick several

points around and even outside of your watershed and trace the downhill flow path of the rain drop. Is it consistent with your drawn watershed boundary?

Watersheds


USGS, 2001


Where,

P = precipitationET = evapotranspirationI = InfiltrationG=GroundwaterQ= Runoff

ΔWatershed Storage = P – ET – G – R

Hydrologic Cycle and the Water Budget

P

I Q

ET

G

Watershed Hydrology

The Hydrologic Cycle and its Role in Arctic and Global Environmental Change, 2001.


Precipitation


Precipitation

• Precipitation is the primary driver for the land phase of the hydrologic cycle

• Total precipitation over land gets partitioned into different components: some soaks into the ground (infiltration), some evaporates from the surface of leaves and soil, some is taken up into plant roots and released back into the atmosphere (transpiration), some is stored at the surface as snow/ice.

• For a given watershed, how precipitation is partitioned depends on a number of environmental factors:

– Temperature– Soil moisture– Intensity of rainfall– Vegetation (seasonal effects)

• Furthermore, the state of precipitation (liquid/solid) is very important for seasonal (and sometimes interannual) partitioning.

Introduction


• C

USGS, 2001


Precipitation

•Precipitation measurements are point data; however, models often require the amount of rainfall over the watershed or area of interest.

•As a result, several methods have been developed to determine the average rainfall over the watershed - called the mean areal precipitation (MAP).

MAP = Total Precipitation VolumeWatershed Area

•Examples of methods include:– Arithmetic Average (simple average of

all stations)– Thiessen Polygon (weighted average based

on area of influence)– Hypsometric (weighted average based on

basin topography and location of stations)

G1G6

G2

G7

G5

G3

G4

Mean Areal Precipitation


Precipitation

• In the United States the National Weather Service (www.nws.noaa.gov) has a network of precipitation gages

– 278 primary stations - staffed full time by paid technicians (~20 AK)– 8,000 cooperative stations - mostly volunteer stations (~70 AK)

• Historical data for these stations may be downloaded at the National Climate Data Center website (www.ncdc.noaa.gov)

Where to Obtain Precipitation Data (and Other Surface Observations)


Precipitation

Precipitation gage networks

Primary Stations

Dingman, 2002


Precipitation and Rainfall Climatology

Precipitation gage networks

Cooperative Stations

Dingman, 2002


Evapotranspiration


Evapotranspiration

• Evaporation (E) occurs when water is converted into water vapor. This may occur from an open water surface or through exfiltration of soil moisture.

• Methods for estimating evaporation include:– Water budget– Energy budget– Mass transfer techniques– Pan evaporation measurements

• Transpiration occurs when water vapor is lost to the atmosphere through small openings in the leaves of plants.

• Potential Evapotranspiration (PET) is a combined estimate of the maximum potential evaporation + transpiration over an area. When there is limited water (open surface or soil moisture) actual rates of evapotranspiration (ET) are less than the potential rate.

Overview

TranspirationTranspiration

TranslocationTranslocation

AbsorptionAbsorption

soil surface

Evaporation + Transpiration = Evapotranspiration


Mass Balance for Water body:ΔV = P + SWin + GWin − E – SWout – GWoutSolve for E

E = P + SWin + GWin – SWout – GWout − ΔV

Where,E = EvaporationP = PrecipitationSWin = Surface Water InflowSWout = Surface Water OutflowGWin = Groundwater InflowGWout = Groundwater OutflowΔV = Change in storage

• The water balance method is computationally simple. However, gathering the data for implementation of this method may be difficult.

• Each of the quantities in the equation above are measured or estimated, which results in uncertainty. Thus the calculation of evaporation includes the sum of the errors related to each component.

Evapotranspiration

Estimating Evaporation: Water Balance Method

SWoutSWout SWinSWinΔVΔV

EE

GWinGWin

PP

GWoutGWout


L = net long wave radiation input

Ta = temperature of atmosphere, in ºCTs = temperature of surface, in º Cσ=Stefan-Boltzmann const= 4.90x10-9 [MJ/m2dayK4]

εw= effective emissivity of water = 0.97εat= effective emissivity of atmosphere

va = wind speed [km/day]ra = relative humidity = ea/e*a

e*a = saturation vapor pressure at the air temperature = [kPa]

Where,Δ with Ta in ºC

γ = psychrometric constant = ca =heat capacity of air=1.0x10-3 [MJ/kgK]P = pressure [kPa]λv = latent heat of vaporization [MJ/Kg]

= 2.50 – 2.36x10-3 •Ts, T in ºCK = net short-wave radiation input

= Io • (0.803-0.34c-0.458c2) • (1- a)Io = solar insolation at the top of the atmosphere [MJ/m2day]a = albedoc = Cloud cover

KE = coefficient reflecting the efficiency of vertical transport of water vapor by turbulent eddies of the wind = 1.69 • 10-5 • AL

-0.05

AL = water surface area [km2]

ρw = density of water = 1000 [kg/m3]

EvapotranspirationEstimating Evaporation: Penman Method

Required input data1. AL – used in KE2. P or Altitude 3. Ts – used in λv and L4. Ta – used in Δ , L and ea

*

5. ea6. va7. c – used in K8. Io – used in K9. a – used in K

Required input data1. AL – used in KE2. P or Altitude 3. Ts – used in λv and L4. Ta – used in Δ , L and ea

*

5. ea6. va7. c – used in K8. Io – used in K9. a – used in K

)()1()( *

γλρλργ+Δ

−++Δ=

••

•••••••

vw

aaavwE revKLKE

⎟⎟⎠

⎞⎜⎜⎝

⎛

++=

••

3.2373.17

exp2)3.237(3.2508

aTaT

aT

)622.0()( vPac λ••

⎟⎟⎠

⎞⎜⎜⎝

⎛ •• + 3.237

3.17exp611.0aT

aT

44 )2.273()4.273( +−+= ••••• swaatw TT σεσεε

)22.01(2.273

72.1 27/1

CT

e

a

a •+•⎟⎟⎠

⎞⎜⎜⎝

⎛+

•=


Infiltration


Dingman, 2002

Infiltration

Soil Properties

Sand grains

Clay particles

Dingman, 2002

• The properties of a homogeneous soil matrix include:

– Porosity, φ =

– Water content, θ =

– Field capacity, θfc = water contentat which further drainage due togravity is negligible

– Permanent wilting point, θpwp = watercontent at which plants are unable toextract additional water

• If a soil is saturated and then allowed to drain, its water content will decrease indefinitely in a quasi-exponential manner, with the drainage rate negligible within a few days to a week

tot

void

VV

MineralsWaterAirVolumeWaterAirVolume

=&,

&

ott

w

VV

MineralsWaterAirVolumeWaterVolume

=&,


Dingman, 2002

Infiltration

• Ground-water zone: Saturated, positive pressure; in absence of ground-water flow pressure is hydrostatic , where p is the pressure, z is the height above the datum, and γw is the specific weight of water

• Tension-saturated zone (capillary fringe):Saturated zone above the water table due to capillary rise through the pore spaces; pressure is zero at the top of the water table and negative in the capillary fringe

• Intermediate zone: Water enters as percolation from above and leaves by gravity drainage

• Root zone: Layer from which plant roots can extract water, bounded by the surface above and an indefinite and irregular lower bound; water enters by infiltration and leaves via transpiration and gravity drainage

Hydrologic Horizons

2121 ;)()( zzzzzp w >−⋅= γ


Infiltration

• Infiltration is the process by which water arriving at the soil surface enters the soil column. The maximum rate that a soil can accept water is called the infiltration capacity, f(t)*.

• At a given point the infiltration rate, f(t), changes systematically with time and is influenced by:– The rate at which water arrives from above, w(t), or the depth of ponding on the surface, H(t)– The hydraulic conductivity of the soil, Kh

*

– Antecedent soil moisture

• Three general conditions during infiltration may be distinguished– No ponding: In this case the infiltration rate equals the water-input rate and is less than or

equal to the infiltrability

– Saturation from above: Ponding is present because the water-input rate exceeds the infiltrability in which case the infiltration rate equals the infiltrability

– Saturation from below: Ponding is present because the water table has risen to or above the surface in which case the infiltration rate is zero

The Infiltration Process

)()()(,0)( * tftwtftH ≤==

)()()(,0)( * twtftftH ≤=>

0)(,0)( =≥ tftH


Runoff


Runoff

Definitions• Watershed response to an input event is characterized by

stream discharge at a single point that defines the outlet of the watershed

• A graph of water input vs. time can be constructed from spatially averaged precipitation measurements and is called a hyetograph

• A graph of stream discharge vs. time is a streamflow hydrograph

• A storm hydrograph is the time trace made by an observer at a fixed point of a flood wave moving downstream

Basic Aspects of Stream Response

Rai

n (d

epth

/tim

e)D

isch

arge

(vol

ume/

time)

Hyetograph

Hydrograph

Dingman, 2002

STEP July, 2007: Hydrology – Page No. 26Dingman, 2002

Runoff

Streamflow• Streamflow is a spatially and temporally integrated response

determined by– Spatially and temporally varying input rates (precipitation, snow

melt, glacial melt)– Time required for each drop of water to travel from where it

strikes the watershed surface to the stream network (determined by length, slope, vegetative cover, soils, and geology of hillslopes)

– Time required for water to travel from its entrance into the channel to the point of measurement

• Flow may enter the stream at the surface, from overland flow and channel precipitation, and as subsurface flow, from groundwater and interflow

• Flow in the stream takes the form of a flood wave that moves downstream through the stream network

Basic Aspects of Stream Response


Dingman, 2002

Runoff

Effective Rainfall• Only a fraction of water input to the watershed actually appears in the response hydrograph, with

the remainder leaving the watershed as– Evapotranspiration– Streamflow that is realized too long after

the input event to be associated with that event (baseflow)

– Groundwater outflow (other than baseflow)• Depending on the type of model, it is often

necessary to estimate the effective rainfall from the hyetograph of water input

• There are several approaches used for this estimation as shown here

– a) Losses equal to a constant fraction of water input for each time period

– b) Losses equal a constant rate throughout event– c) Losses given by an initial abstraction followed by a constant rate– d) Losses given by an approximation to an infiltration-type curve

Response Hydrographs


Runoff

Hydrograph Separation• Event flow is streamflow resulting from the effective rainfall

• Hydrograph separation divides the hydrograph into a portion attributed to event flow and a portion attributed to baseflow

• Gaging station measurements of streamflow cannot distinguish event flow from flow originating from a previous event

• Therefore, graphical hydrograph separation is often used as a convenient delineation in order to analyze and model event responses and the factors influencing them

• Graphical separation does not actually identify flow from different sources

Response Hydrographs

After Linsley and Franzini, 1979


Flow statistics of three rivers near the headwaters of the Yukon River.

USGS, 2001


Runoff

US Geological Survey•http://water.usgs.gov/ Select Alaska Select “Real Time Data Table”

Select station Select data product (Daily data is usually best)

Where to Obtain Streamflow Data (and Groundwater Data)


Runoff

Simple Runoff Model

Rational Method• Regional equations suitable for assessing the impact of developing on peak discharge are not

generally available for small watersheds

• One widely used method, intended for use on small watersheds, is the Rational Method, which relates the peak discharge of an area, qp(ft3/s), to

– Drainage area, A (acres), – Rainfall intensity, i (in/hr),– Runoff coefficient, C

• Rainfall intensity is obtained from an intensity-duration-frequency (IDF) curve using a specified return period

• Primary use of Rational Method: design problems for small urban areas (small drainage areas, short times of concentration)

q = CiA


Runoff

• The model states consist of the contents of various conceptual reservoirs identified in the upper and lower soil zones

• Water fills and spills over in a cascade of reservoirs based on parameters that represent average soil characteristics in each reservoir

• This movement of water between compartments is governed by the precipitation rate, the capacities of each reservoir, evapotranspiration, and the rates at which water can transfer between compartments (infiltration, interflow, or percolation)

• While an infinite number of layers could be established, the goal of parameterization is to use no more than necessary to effectively describe the physical system

Sophisticated Model: Sacramento Soil Moisture Accounting Model


Runoff

Sophisticated Runoff Model


Groundwater


Groundwater

Basic Groundwater Characteristics

Cone of depression

Ocean

Pumped well

Unsaturated zone

Perched water table

Spring

Lake

Perched aquifer

Spring

Marsh

Influent stream(seepage from stream)

Effluent stream(seepage into stream)

Ground water flow

Water table

Artesian well

Bedrock

Piezometricsurface

Water table

Zone of saturation

Confined (artesian) aquifer

Confining layer

Unconfined aquifer

Infiltration

Percolation

Saltwater intrusion

Snow

Cone of depression

Ocean

Pumped well

Unsaturated zone

Perched water table

Spring

Lake

Perched aquifer

Spring

Marsh

Influent stream(seepage from stream)

Effluent stream(seepage into stream)

Ground water flow

Water table

Artesian well

Bedrock

Piezometricsurface

Water table

Zone of saturation

Confined (artesian) aquifer

Confining layer

Unconfined aquifer

Infiltration

Percolation

Saltwater intrusion

Snow

•The groundwater portion of the hydrologic cycle is rather complex– Water enters at the surface (infiltration),– Redistributes under forces of gravity, energy gradients, capillary rise, and evapotranspiration– Water percolates to lower water reservoirs (aquifers)– Groundwater flows under the influence of energy gradients– Groundwater may flow into or receive recharge from surface water bodies– Pumping of groundwater alters the region around the well by drawing down either the water

table or the piezometric surface (cone of depression)

Reproduced from McCuen, 1998


Bouwer, 1978

Groundwater

Definitions•Water that enters the soil is considered soil moisture while in the unsaturated zone and is called groundwater once in the saturated zone.•Within the saturated zone water occupies all pore space and is under hydrostatic pressure

•Aquifer- groundwater-bearing formations sufficiently permeable to transmit and yield usable quantities of water•Unconfined Aquifer- permeable underground formation having a surface at atmospheric pressure•Confined Aquifer- confined (or artesian) aquifers form between layers of very low permeability material

– If the layers are essentially impermeable they are called aquicludes– If the layers are permeable to transmit water vertically to or from the confined aquifer, but not permeable enough for lateral transport, they are called aquitards

Basic Groundwater Characteristics


Wetlands

STEP July, 2007: Hydrology – Page No. 38USGS, 2001

Handout: What are wetlands and why are they important?


Watershed Hydrology

• Ways that climate change might affect hydrology: (class suggestions- recall the components of the water budget)

• Example from the Nile basin

• How are Alaska and the Arctic different from lower latitudes?

• Evapotranspiration- temperature, soil moisture, vegetation

• Glacial fed streams

• Continuous permafrost regions

• Discontinuous permafrost

• Groundwater

• Storm frequency/intensity

Climate Change Considerations

30-Year Monthly Mean Flow

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

1 2 3 4 5 6 7 8 9 10 11 12

Month

Flow

(mcm

)

Base MaxBase MinBase MeanFuture MaxFuture MinFuture Mean

Blue Nile


0

1000

2000

3000

4000

5000

6000

1 2 3 4 5 6 7 8 9 10 11 12

Month

Flow

(mcm

)


Atbara


0

500

1000

1500

2000

2500

3000

1 2 3 4 5 6 7 8 9 10 11 12

Month

Flow

(mcm

)


Sobat


USGS, 2001


Water Resources


Handout: The development of Dryville

Water Resources Planning and Management

Introduction to Water Resources


Purpose Economic Development

Environmental Quality

Social Well-Being

Hydrologic Analysis

Public water supply X X WS, D, Q

Industrial water supply X X WS, D, Q

Irrigation X X WS, D, Q

Hydroelectric power X X WS

Navigation X X WS

Waste transport and treatment X X X WS, Q

Recreation X X WS, Q

Wildlife habitat X X WS

Reduction of flood damages X X F

WS = water supply; D = drought; Q = water quality; F = flood magnitude-frequency.

Goals

Water Resources Planning and Management

Hydrologic analysis for water resources management can be categorized according to the following assessments:1. Present and future supply of water available from surface and/or ground water sources;2. Present and future quality of surface and/or ground water;3. Present and future frequency with which human activities will be subject to floods; and4. Present and future frequencies of low streamflows and drought.

Relationships among water resources management goals, purposes, and types of analyses:

Role of Hydrologic Analysis

Purpose Economic Development

Environmental Quality

Social Well-Being

Hydrologic Analysis

Public water supply X X WS, D, Q

Industrial water supply X X WS, D, Q

Irrigation X X WS, D, Q

Hydroelectric power X X WS (D, F)

Navigation X X WS

Waste transport and treatment X X X WS, Q

Recreation X X WS, Q

Wildlife habitat X X WS (Q, F)

Reduction of flood damages X X F

WS = water supply; D = drought; Q = water quality; F = flood magnitude-frequency.

Goals

After Dingman, 2002


Water Supply

• Water Users

• Water Uses

• Infrastructure

• Resource allocation and competing objectives

Water in Our Communities

Environmental needs-users, uses, minimum requirements?


Water as a Hazard


USGS, 2001


Water as a Hazard

There’s not enough…• Storage: Reservoirs, tanks, groundwater

– Redistribute water for year round availability, according to demand– Mitigate the effects of drought (period of consecutive dry years)

• Emergency supply: fire flows require greater water volumes and much higher pressures

There’s too much…• Storms and runoff• Seasonal high flows • Excessive wet years

(storage plays an important role here as well)• Wetlands??

It’s dirty…• Sediment and mineral loads (water quality, water treatment)• Naturally occurring and introduced contaminants (suitability, pollution)• Flooding and water quality (contamination)

Planning, Management and Risk


Water as a Hazard

Impacts of Water Management

Activity Magnitude-Frequency Timing Duration Rate of Change

Damming Reduced variability (WS, FC); Reduced peak flows (FC)

Altered (WS, FC, HP)

Reduced periods of inundation (FC)

Rapid fluctuations (HP)

Diversion Reduced flows; Reduced variability Altered

Urbanization and drainage Increased variability; Increased peak flows

Reduced periods of floodplain inundation due to stream entrenchment

Levees and channelization May increase downstream peak flows

Reduced periods of floodplain inundation

Groundw water pumping Reduced low flows

Deforestation Increased variability; Increased peak flows; Reduced low flows

WS = water supply; FC = flood control; HP = hydropowerAfter Dingman, 2002


Water Management: Health, Safety, and the Environment

• Point source versus Non-point source (NPS)

• Types of pollutants:– Oxygen-demanding material– Nutrients– Pathogens– Suspended solids– Toxic metals and organic compounds– Heat

• Typical water quality concerns for types of water resources:

• Pollution management: “control the discharge of pollutants so that water quality is notdegraded to an unacceptable extent below the natural background level.” Davis and Cornwell

• To do so, we need to: Measure pollutants, predict impacts of pollutants, know background (natural) water quality, decide on acceptable water quality level for a water body.

Pollution

Water Resource pH Dissolved

SolidsSuspended

SolidsDissolved Oxygen

Organics and Petrleum

Compounds

Pathogenic Organisms Excess Heat

Precipitation X X

Ground Water X X X

Streams X X X X X X X

Lakes X X X X

"X" Indicates that a given type of water-quality constituent is typically of concern in a given type of water resource. After Dingman, 2002


Water Management: Health, Safety, and the Environment

Biological Oxygen Demand (BOD): dissolved oxygen consumed during the oxidation of an organic compound. Consumption of dissolved oxygen poses a threat to fish and other higher forms of aquatic life that must have oxygen to live.

Pollution

Davis and Cornwell, 1998


Climate Change and Water Resources

• Tools for understanding potential consequences of change (for better or worse)– Global Climate Models (GCMs), historical climate records– Hydrologic Models (& other environmental models)– Water Resources Systems Models– Verification requires observations and an understanding of uncertainty

• Climate Change Impact Assessments:– Climate change and water supply– Climate change and infrastructure– Climate change and ecology– Sustainability– …

• What are the appropriate time and spatial scales for these assessments?– How do we experience the change?– At what scales do we have confidence in projections and models?

• Hand-out: “Climate Change and Wetlands”

Potential Impacts of Climate Change


Water Science

The Water Source Books by the Environmental Protection Agency http://www.epa.gov/safewater/kids/wsb/index.html

Water Science for Schools by the US Geological Surveyhttp://ga.water.usgs.gov/edu/

Watershed Game at the Bell Museum site http://www.bellmuseum.org/distancelearning/watershed/watershed2.html

Rivers 2001 by the National Geographic Society http://www.nationalgeographic.com/geographyaction/rivers/ga22.html

Alaska Wildlife Curriculum Teacher’s Guide by the Alaska Dept of Fish and Game http://www.wildlife.alaska.gov/index.cfm?adfg=education.awc

Additional Resources


References

Davis, M. and D. Cornwell, 1998. Environmental Engineering. 3rd Ed. McGraw-Hill, Boston, 919 pp.

Dingman, L., 2002. Physical Hydrology. 2nd Ed. Prentice Hall, Upper Saddle River, New Jersey, 646 pp.

McCuen, R., 1998. Hydrologic Analysis and Design. 2nd Ed. Prentice Hall, Upper Saddle River, New Jersey, 814 pp.

Thompson, S., 1999. Water Use, Management, and Planning in the United States. Academic Press, San Diego, 371.

Watershed Hydrology and Water Resources

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