Hillslope hydrology and Hillslope hydrology and intro to groundwaterintro to groundwater

(with many slides borrowed from Jeff McDonnell/Oregon State)

Where does our water come Where does our water come from? Oceanic Sources of from? Oceanic Sources of Continental PrecipitationContinental Precipitation• Global evaporation: 500,000 km3/yr of waterGlobal evaporation: 500,000 km3/yr of water

– 86% oceans, 14% continents86% oceans, 14% continents• 90% of water evaporated from oceans goes 90% of water evaporated from oceans goes

back to oceanback to ocean– 10% to continents10% to continents– 2/3 of the 10% is recycled on the continents2/3 of the 10% is recycled on the continents– 1/3 of the 10% runs off directly to ocean1/3 of the 10% runs off directly to ocean

• Isotopic analysis to determine source of Isotopic analysis to determine source of waterwater– relative proportions of isotopes of hydrogen and relative proportions of isotopes of hydrogen and

oxygenoxygen

Sources: EOS, 7 June 2011 and Gimeno et al., 2010b

JJA

DJF

Implications of Climate Implications of Climate ChangeChange• Changing atmospheric circulation patterns Changing atmospheric circulation patterns

=> changing precipitation patterns=> changing precipitation patterns• Convergence and transport from regions Convergence and transport from regions

of high water vapor => extreme floodsof high water vapor => extreme floods• Absence of moisture transport => Absence of moisture transport =>

extreme droughtextreme drought• Regions getting water from multiple Regions getting water from multiple

oceanic source regions are less oceanic source regions are less susceptible to shifts in circulation patternssusceptible to shifts in circulation patterns

How does the water come from How does the water come from the ocean?the ocean?• ““Atmospheric river” (Zhu and Newell, 1998)Atmospheric river” (Zhu and Newell, 1998)• 90% of the poleward atmospheric water vapor 90% of the poleward atmospheric water vapor

transport through the midlatitudes is transport through the midlatitudes is concentrated in 4-5 narrow bandsconcentrated in 4-5 narrow bands– <10% of the Earth circumference<10% of the Earth circumference

• Transport 13-26 km3/day of water vaporTransport 13-26 km3/day of water vapor– =7.5-15 times Qavg of Miss. R at New Orleans=7.5-15 times Qavg of Miss. R at New Orleans

• Land interactionsLand interactions– forced up/over mountainsforced up/over mountains– cool, condense, produce precipitation (rain or snow)cool, condense, produce precipitation (rain or snow)

• Major source of precip in coastal regionsMajor source of precip in coastal regions

Fig. 1. Analysis of an atmospheric river (AR) that hit California on 13–14 October 2009. (a) ASpecial Sensor Microwave Imager (SSM/I) satellite image from 13–14 October showing the AR hitting the California coast; color bar shows, in centimeters, the amount of water vapor present throughout the air column at any given point if all the water vapor were condensed into one layer of liquid (vertically integrated water vapor).

Source: EOS, 9 August 2011

General Water CycleGeneral Water Cycle

Water Balance: accounting of water conservation of water volume

Input (I) – Output (O) = S (changes in storage)Inputs: rain and snowOutput: stream discharge (Q), evapotranspiration (ET), groundwater/infiltrationStorage: Soil moisture, groundwater, snow, ice, lakes

Hewlett (1982)

Over long periods (> 1yr), changes in storage can be neglectedS(t) = 0

Groundwater flow is very small compared to the other termsQgw(t)

Q(t) = A[ R(t) – ET(t)]

For example,CONUS average annual precipitation: 76 cmQ = R – ET23 = 76 – 53 (cm)

S(t) = A R(t) – Q(t) – Qgw(t) – A ET(t)

Water BalanceWater Balance

storage

rainfall

stream discharge

groundwater discharge

evapotranspiration

A, drainage basin area

HYDROGRAPH

also infiltration

A whole litany of controls A whole litany of controls on runoff or discharge (Q) on runoff or discharge (Q) generationgeneration

Broad conceptual controlsBroad conceptual controls

• Rainfall intensity or Rainfall intensity or amountamount

• Antecedent conditionsAntecedent conditions

• Soils and vegetationSoils and vegetation

• Depth to water table Depth to water table (topography)(topography)

• GeologyGeology

Overland Flow OccurrenceOverland Flow Occurrence

• On road surfaces and other impermeable areasOn road surfaces and other impermeable areas– bedrock outcrops, city parks, lawnsbedrock outcrops, city parks, lawns

• On hydrophobic soils (fire and seasonality)On hydrophobic soils (fire and seasonality)

• On trampled and crusted soils On trampled and crusted soils

• On low permeable soils On low permeable soils – Silt-clay soils without macroporesSilt-clay soils without macropores

• On saturated soils (SOF) On saturated soils (SOF) – Riparian zoneRiparian zone– Waterlogged soilsWaterlogged soils

Overland flow generationOverland flow generation

• Runoff occurs whenRunoff occurs when– R > IR > I– Or in words, rainfall Or in words, rainfall

intensity exceeds intensity exceeds the infiltration ratethe infiltration rate

FE 537

Oregon State University

Horton Overland FlowHorton Overland Flow

Qho(t) = w(t) - f(t)

where: w(t) is the water input rate f(t) is the infiltration rate

Fig. 5.3

A different form of overland flowA different form of overland flow

R > I

Runoff PathwaysRunoff Pathways

InfiltrationCapacity

R a i n f a l l

Saturation OF

BedrockAquifer

Percolation

RegolithRegolith Subsurface Flow

Saturation

Aquifer Subsurface Flow

Hortonian OF

Percolation

Slide from Mike Kirkby, University of Leeds, AGU Chapman Conference on Hillslope Hydrology, October 2001

Storm Precipitation

Soil Mantle Storage

Baseflow

Channel Precip.+

Overland Flow

Overland Flow

InterflowSubsurfaceStormflow

Saturation Overland Flow Hortonian Overland Flow

Basin Hydrograph

Re-drawn from Hewlett and Troendle, 1975

Troendle, 1985

Dominant processes of Dominant processes of hillslope response to hillslope response to rainfallrainfall

Horton overland flow dominates hydrograph; contributions from subsurface stormflow are less important

Direct precipitation and return flow dominate hydrograph; subsurface stormflow less important

Subsurface stormflow dominates hydrograph volumetrically; peaks produced by return flow and direct precipitation

Arid to sub-humid climate; thin vegetation or disturbed by humans

Humid climate; dense vegetation

Steep, straight hillslopes; deep,very permeable soils; narrow valley bottoms

Thin soils; gentle concave footslopes; wide valley bottoms; soils of high to low permeability

Climate, vegetation and land use

Topograp

hy and soils

Variable source concept

(Dunne and Leopold, 1978)(Dunne and Leopold, 1978)

The old water paradoxThe old water paradox“…“…streamflow responds promptly to streamflow responds promptly to

rainfall inputs, but fluctuations in rainfall inputs, but fluctuations in passive tracers are often strongly passive tracers are often strongly damped. This indicates that storm damped. This indicates that storm flow in these catchments is mostly flow in these catchments is mostly ‘old’ water”‘old’ water”

Kirchner 2003 Kirchner 2003 Hydrological ProcessesHydrological Processes

Runoff Generation Mechanisms

The Water Cycle: More DetailThe Water Cycle: More Detail

Infiltration Infiltration

• ““the entry of waters into the ground”the entry of waters into the ground”• rate and quantity of infiltration = f(rate and quantity of infiltration = f(

– soil typesoil type– soil moisturesoil moisture– soil permeabilitysoil permeability– ground coverground cover– drainage conditionsdrainage conditions– depth to water tabledepth to water table– intensity and volume of precipintensity and volume of precip

PorosityPorosity

• Ratio of void volume to Ratio of void volume to total volumetotal volume

• V = Va + Vw + VsV = Va + Vw + Vs• Voids are spaces filled with Voids are spaces filled with

air and waterair and water• Range of porosity valuesRange of porosity values

– granular mass of uniform granular mass of uniform spheres with loose spheres with loose packing, n=47.6%packing, n=47.6%

– granular mass of uniform granular mass of uniform spheres with tight packing, spheres with tight packing, n = 26%n = 26%

– unconsolidated material unconsolidated material like sandstones and like sandstones and limestones, n = 5-15%limestones, n = 5-15%

• Vv = Va + VwVv = Va + Vw

V

V

V

VV

V

Vn ssv

p

1

Volumetric water content

total

water

V

V

“Hillslopes consist of soils and regolith overlying rock.Both have a definable porosity.”

At saturation,

pn

Horton’s eqn.Horton’s eqn.

tcc effftf 0

f = infiltration rate at some time t, cm/hr or in/hrfo = initial infiltration rate at time zerofc = final constant infiltration capacity, analogous to soil permeabilitybeta = recession constant, hr-1

Rate of Infiltration (velocity of Rate of Infiltration (velocity of flow through unsaturated flow through unsaturated media)media)• Green/Ampt eqn.Green/Ampt eqn.

zhKtf s /

f = infiltration rate or velocity, (in/hr)Ks = hydraulic conductivity, (in/hr)h = pressure head, (in or ft)z = vertical direction, (in or ft)

Infiltration is a function of time because as the ground/soil becomes more saturated, there is less infiltration

Calculate the steady state water discharge at the base of a hillslope. The hillslope is 150 m long, the rainfall rate is 7 mm/hr and the rain has been falling for long enough that the hydrology of the slope may be taken as steady, with a uniform steady infiltration rate of 1.5 mm/hr.

Provide the answer both in m3/s per m length of the bounding stream, and in cubic ft per second (cfs) per linear foot of channel.

S(t) = A R(t) – Q(t) – Qgw(t) – A ET(t)

At steady state the inputs of water to the hillslope must equal the outputs

Q = L[ R – I]Q (R I )L (0.007 0.0015)

m

hr

1hr

3600s150m 2.3x10 4 m3 / s

Q 2.3x10 4 m3

s

1 ft

0.304m

3

2.3x10 4 (35.3) 8.1x10 3cfs

1 cf = .3 m3

GROUNDWATER

TABLE 3.1 Range of Porosity TABLE 3.1 Range of Porosity

Soil TypeSoil Type Porosity, Porosity, pptt

Unconsolidated depositsUnconsolidated deposits

GravelGravel 0.25 - 0.400.25 - 0.40

SandSand 0.25 - 0.500.25 - 0.50

SiltSilt 0.35 - 0.500.35 - 0.50

ClayClay 0.40 - 0.700.40 - 0.70

RocksRocks

Fractured basaltFractured basalt 0.05 - 0.500.05 - 0.50

Karst limestoneKarst limestone 0.05 - 0.500.05 - 0.50

SandstoneSandstone 0.05 - 0.300.05 - 0.30

Limestone, dolomiteLimestone, dolomite 0.00 - 0.200.00 - 0.20

ShaleShale 0.00 - 0.100.00 - 0.10

Fractured crystalline rockFractured crystalline rock 0.00 - 0.100.00 - 0.10

Dense crystalline rockDense crystalline rock 0.00 - 0.050.00 - 0.05

Source: Freeze and Cherry (1979).Source: Freeze and Cherry (1979).

n = Sy + Sr

Specific yield (effective porosity): measure of gw that drains by gravity; storage characteristics of aquifer

Specific retention: measure of gw that doesn’t drain under gravity

http://www.uiowa.edu/~c012003a/14.%20Groundwater.pdf

Hydrograph of streamflow

Hyetograph of rainfall

Initially, there is little runoff => b/c more rain goes into infiltrationLater, there is more runoff => less infiltration due to saturated ground

GROUNDWATER

655.5

656

656.5

657

657.5

658

658.5

0 50 100 150 200 250 300 350 400

Days

Ele

vati

on

(m

sl)

Ground surface

O D F A J A

Large seriesof storms

Cold dry

Series ofsmall storms

Snow

Midwinter melt

Snow

Spring melt

Summer

Large storm

GROUNDWATER

Impermeable rock

Ground water

Salt water

Stream channel

Ocean

Land surface

Vadose zone

Wet-season water tableDry-season water table

Well

Capillary fringe

Infiltration or seepage recharge (unsat. flow) Ground water (sat. flow)

Phreatic zone

GROUNDWATER