Hillslope hydrology and intro
to groundwater
(with many slides borrowed from Jeff McDonnell/Oregon State)
Where does our water come from?
Oceanic Sources of Continental
Precipitation
• Global evaporation: 500,000 km3/yr of water
– 86% oceans, 14% continents
• 90% of water evaporated fromJJAoceans goes
back to ocean
– 10% to continents
– 2/3 of the 10% is recycled on the continents
– 1/3 of the 10% runs off directly to ocean
• Isotopic analysis to determine source of water
– relative proportions of isotopes of hydrogen and
oxygen
DJF
Sources: EOS, 7 June 2011 and Gimeno et al., 2010b
Implications of Climate Change
• Changing atmospheric circulation patterns =>
•
•
•
changing precipitation patterns
Convergence and transport from regions of high
water vapor => extreme floods
Absence of moisture transport => extreme
drought
Regions getting water from multiple oceanic
source regions are less susceptible to shifts in
circulation patterns
How does the water come from the
ocean?
• “Atmospheric river” (Zhu and Newell, 1998)
• 90% of the poleward atmospheric water vapor transport
through the midlatitudes is concentrated in 4-5 narrow
bands
– <10% of the Earth circumference
• Transport 13-26 km3/day of water vapor
– =7.5-15 times Qavg of Miss. R at New Orleans
• Land interactions
– forced up/over mountains
– cool, condense, produce precipitation (rain or snow)
• Major source of precip in coastal regions
Fig. 1. Analysis of an atmospheric river (AR) that
hit California on 13–14 October 2009. (a) A
Special 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 Cycle
Hewlett (1982)
Water Balance: accounting of water
conservation of water volume
Input (I) – Output (O) = DS (changes in storage)
Inputs: rain and snow
Output: stream discharge (Q), evapotranspiration (ET), groundwater/infiltration
Storage: Soil moisture, groundwater, snow, ice, lakes
A, drainage basin area
Water Balance
stream discharge
HYDROGRAPH
storage
evapotranspiration
S(t) = A R(t) – Q(t) – Qgw(t) – A ET(t)
rainfall
groundwater discharge
Over long periods (> 1yr), changes in storage can be neglected
S(t) = 0
Groundwater flow is very small compared to the other terms
Qgw(t)
Q(t) = A[ R(t) – ET(t)]
For example,
CONUS average annual precipitation: 76 cm
Q = R – ET
23 = 76 – 53 (cm)
also infiltration
A whole litany of controls on
runoff or discharge (Q)
generation
Broad conceptual
controls
• Rainfall intensity or
amount
• Antecedent
conditions
• Soils and vegetation
• Depth to water
table (topography)
• Geology
Overland Flow Occurrence
• On road surfaces and other impermeable areas
•
•
•
•
– bedrock outcrops, city parks, lawns
On hydrophobic soils (fire and seasonality)
On trampled and crusted soils
On low permeable soils
– Silt-clay soils without macropores
On saturated soils (SOF)
– Riparian zone
– Waterlogged soils
Overland flow generation
• Runoff occurs when
–R>I
– Or in words, rainfall
intensity exceeds the
infiltration rate
FE 537
Horton Overland Flow
Qho(t) = w(t) - f(t)
where:
w(t) is the water input rate
f(t) is the infiltration rate
Oregon State University
Fig. 5.3
A different form of overland flow
R>I
Runoff Pathways
R
Hortonian OF
a i n
f a l l
Infiltration
Capacity
Percolation
Saturation OF
Saturation
Regolith
Regolith Subsurface Flow
Percolation
Bedrock
Aquifer
Aquifer Subsurface Flow
Slide from Mike Kirkby, University of Leeds, AGU Chapman Conference on Hillslope Hydrology, October 2001
Storm Precipitation
Saturation Overland Flow
Hortonian Overland Flow
Channel Precip.
+
Overland Flow
Soil
Mantle
Storage
Overland
Flow
Subsurface
Stormflow
Baseflow
Interflow
Basin Hydrograph
Re-drawn from Hewlett and Troendle, 1975
Troendle, 1985
Dominant processes of
hillslope response to
rainfall
Thin soils; gentle
Arid to sub-humid
climate; thin vegetation
or disturbed by humans
concave footslopes;
wide valley bottoms;
soils of high to low
permeability
Variable source
concept
Subsurface stormflow
dominates hydrograph
volumetrically; peaks
produced by return
flow and direct
precipitation
Steep, straight
hillslopes; deep,very
permeable soils;
narrow valley
bottoms
Topography and soils
Horton overland flow
dominates
hydrograph;
contributions from
subsurface stormflow
are less important
Direct precipitation
and return flow
dominate hydrograph;
subsurface stormflow
less important
Humid climate;
dense vegetation
Climate, vegetation and land use
(Dunne and Leopold, 1978)
The old water paradox
“…streamflow responds promptly to
rainfall inputs, but fluctuations in
passive tracers are often strongly
damped. This indicates that storm
flow in these catchments is mostly
‘old’ water”
Kirchner 2003
Hydrological Processes
Runoff Generation Mechanisms
The Water Cycle: More Detail
Infiltration
• “the entry of waters into the ground”
• rate and quantity of infiltration = f(
– soil type
– soil moisture
– soil permeability
– ground cover
– drainage conditions
– depth to water table
– intensity and volume of precip
Porosity
“Hillslopes consist of soils and regolith overlying rock.
Both have a definable porosity.”
• Ratio of void volume to total
volume
• V = Va + Vw + Vs
• Voids are spaces filled with air
and water
• Range of porosity values
– granular mass of uniform
spheres with loose packing,
n=47.6%
– granular mass of uniform
spheres with tight packing, n
= 26%
– unconsolidated material like
sandstones and limestones, n
= 5-15%
• Vv = Va + Vw
Vv V  Vs 
Vs
np 

 1
V
V
V
Volumetric water content
Vwater

Vtotal
At saturation,
np  
Horton’s eqn.
f t   f c   f 0  f c e
 t
f = infiltration rate at some time t, cm/hr or in/hr
fo = initial infiltration rate at time zero
fc = final constant infiltration capacity, analogous to soil permeability
beta = recession constant, hr-1
Rate of Infiltration (velocity of flow
through unsaturated media)
• Green/Ampt eqn.
f t   Ks Dh / Dz 
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]
1 cf = .3 m3
Q  (R  I )L  (0.007  0.0015)
3
m 1hr
150m  2.3x10 4 m 3 / s
hr 3600s
m 3  1 ft 
4
3
Q  2.3x10

  2.3x10 (35.3)  8.1x10 cfs
s 0.304m
4
GROUNDWATER
TABLE 3.1 Range of Porosity
Soil Type
Porosity, pt
Unconsolidated deposits
Gravel
0.25 - 0.40
Sand
0.25 - 0.50
Silt
0.35 - 0.50
Clay
0.40 - 0.70
Rocks
Fractured basalt
0.05 - 0.50
Karst limestone
0.05 - 0.50
Sandstone
0.05 - 0.30
Limestone, dolomite
0.00 - 0.20
Shale
0.00 - 0.10
Fractured crystalline rock
0.00 - 0.10
Dense crystalline rock
0.00 - 0.05
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
Hyetograph of rainfall
Hydrograph of streamflow
Initially, there is little runoff => b/c more rain goes into infiltration
Later, there is more runoff => less infiltration due to saturated ground
l
GROUNDWATER
658.5
Large seriesSeriesof Midwinter Spring
of storms
small storms melt
melt
Cold drySnow
658
657.5
E
657
656.5
Snow
L
Summer
GROUNDWATER
W
et-seasonw
atertable
D
ry-seasonw
atertable
C
apilla
W
ell
Landsurface
S
treamchannel
P
hreaticzone
G
roundw
ater
GROUNDWATER