Transcript powerpoint here

Hydrologic interactions between streams
and their subsurface hyporheic zones
CENTRAL QUESTION
What is the hyporheic zone?
It derives from the Greek meaning
under (hypo) the current or flow (rheo).
The hyporheic zone
is a portion of the groundwater interface in streams where a
mixture of surface water & groundwater can be found (Bencala 1993)
The hyporheic zone
In the bed and banks of streams, water and solutes can
exchange in both directions across the streambed. This
process, termed hyporheic exchange, creates subsurface
environments that have variable proportions of water from
ground water and surface water.
Figure from Winter et al. 2000
The hyporheic zone
The bi-directional exchange of water mixes water & solutes
from surface and subsurface environments. An empirical
perspective from hydrologists recognizes a hyporheic zone
as the subsurface zone receiving at least 10% of water by
input from the stream (>10% channel water).
Definition after Triska et al. 1989. Figure from Winter et al. 2000
Some important concepts
Length and
timescale of the
interaction help
to distinguish
hyporheic
exchange from
the much larger
(and longer
term) channel
and groundwater
interactions
Figure from Winter et al. 2000
Some important concepts
• Hyporheic flowpaths leave and return to the stream many
times within a single study reach; the exchange of surface
water back and forth between the active channel and the
subsurface is rapid. Within several kilometers, stream
water in relatively small channels is often completely
exchanged with the porewater of the hyporheic zone
• This repeatedly brings stream water into close contact
with geochemically and microbially active sediment.
Figure from Harvey & Wagner 2000
Chemical gradients at the interface
Steep concentration gradients in dissolved constituents exist at the
groundwater/surface water interface, due to mixing of chemicallydifferent waters and chemical transformations
Zonation of metabolic activity in hyporheic zone
• Hydrologic
exchange regulates
sources of oxygen,
carbon, &
nutrients to
organisms and
plants
• This directly
controls trophic
structure and
primary
productivity in
streams
•Hydrologic fluxes potentially
can enhance ecosystem
production on both sides of the
groundwater/surface water
interface, by supplying solutes
that are needed in biological
metabolism.
hyporheic zone is important to aquatic biota
The term hyporheic zone was originally used by Orghidan
(1959), who described the interface as a new groundwater
environment containing distinctive biota. Higher than
expected abundances of aquatic insects were found in
sediments where concentrations of oxygen were high.
hyporheic zone can be a source of nutrients:
example from Sycamore Creek, Alaska
In a study of the ecology of Sycamore
Creek near Phoenix Arizona, hydrologic
exchange fosters recovery of desert
stream ecosystems following storms, by
stimulating recycling of the nutrients
from buried organic matter.
hyporheic zone at Sycamore Creek, Arizona
Photo courtesy of Stuart Fisher, Arizona State University
Following a storm, oxygen advection into hyporheic zone
stimulates aerobic decomposition of buried algae. Released
nutrients are delivered to the surface channel by return
flows from hyporheic flow paths. Nitrate in surface channel
stimulates new primary productivity by algae.
After Valett et al. 1994
Abundance of algae in streambed sediments, as indicated by
concentration of chlorophyll a, was markedly greater in areas of
upwelling (where groundwater moved upward through the
sediments) than in areas of downwelling.
After Valett et al. 1994
Hyporheic zone: under and adjacent to stream with
active exchange of water
a
Permafrost
Permafrost
Permafrost
In dry valley streams the “hyporheic zone” is observed as a
wetted zone adjacent to stream
Dry valley streams flow through porous alluvium with high rates of
“hyporheic exchange”.
Experimental enrichment with nitrate
and phosphate: 1.5 hrs
Sampling Green Creek during
experiment
Stream algal mats win! Added nutrients are taken up and do
not reach the lake.
hyporheic zone feedbacks to terrestrial landscape:
example from southeast Alaska
• Pacific salmon spend most of their lives growing at sea before
returning to fresh water to spawn and die in their natal streams,
carrying marine-derived nitrogen in their body tissues. Returning
salmon provide a seasonal food source for birds & mammals, and
nutrients from decaying salmon carcasses are incorporated into
freshwater biota at various trophic levels.
• From this nutrient subsidy, growth rates are significantly increased
in trees near spawning streams.
• MDN be transferred from spawning streams to riparian forests by
flooding which deposits salmon carcasses on stream banks &
through the transfer of dissolved nutrients from spawning by
hyporheic exchange.
Flooding
Bank erosion
Shading
Hyporheic
Sediment deposition
DOC. inputs
Sediment, nutrient
filtration
Bank stabilization
MDN fertilization of
riparian vegetation
Enhanced riparian growth
Changes in spp. composition,
diversity, soil chemistry
MDN carried
upstream
Materials carried downstream
Slide courtesy of Bob Naiman, University of Washington
Where does hyporheic exchange take place?
example from southwest Alaska
• Lateral nutrient transfers occur in the hyporheic zone &
in adjacent riparian floodplains along spawning reaches.
• Hyporheic storage & re-release of MDN is an important
mechanism by which nutrients are retained over winter
within stream ecosystems and subsequently made
available to primary producers the following growing
season.
hydraulic head gradients in a well network
Slide courtesy of Tom O’Keefe & Rick Edwards, University of Washington
DO gradients illustrating hyporheic exchange
Slide courtesy of Tom O’Keefe & Rick Edwards, University of Washington
Photo courtesy of Bob Naiman, University of Washington
Loss of water from stream reach to the
hyporheic zone commonly related to
meanders in streambed
Figure from Winter et al. 2000
Where does hyporheic exchange take place?
example from St. Kevin Gulch, CO
A study of a third order gravel bed
stream in the Colorado Rocky
Mountains, characterized by pools &
riffles, indicates geomorphic controls on
where hyporheic exchange takes place.
Inflow of water from the hyporheic zone to the stream
was greatest at the downstream end of riffles.
After Harvey & Bencala, 1993
Stream water flows into the subsurface beneath and to the
side of steep sections of streams (riffles), & ground water
enters streams most readily at the upstream end of deep pools
From Harvey & Bencala, 1993
important points about controls on where
hyporheic exchange takes place
• Hyporheic exchange caused to a large extent by the
irregular topography of the streambed, which creates
pools & riffles characteristic of mountain streams.
• Channel irregularity is an important control on the
location of groundwater inflow to streams and on the
size of the hyporheic zone in mountain streams
because changes in slope determine the length &
depth of hyporheic flow paths.
Harvey & Bencala, 1993
Loss of water from stream reach to the
hyporheic zone commonly related to abrupt
changes in slope of the streambed
Figure from Winter et al. 2000
How extensive can a hyporheic zone be?
• Depending on the type of sediment in the
streambed and banks, the variability in slope of
the streambed, and the hydraulic gradients in the
adjacent ground-water system, the hyporheic zone
can be as much as several feet in depth and
hundreds of feet in width.
• The dimensions of the hyporheic zone generally
increase with increasing width of the stream and
permeability of streambed sediments.
Example of large hyporheic zone:
Willamette River, OR
• A large, 9th order stream
with permeable, coarsegrained, alluvial deposits in
the channel & floodplain
• HZ width = hundreds of m
• Nitrogen coming in from
regional groundwater was
removed in the HZ
• HZ gains/losses in flow
over reaches of 1-2 km were
on the order of 5% of
streamflow
CENTRAL QUESTION
What are some common methods used
to quantify hyporheic flowpaths?
Subsurface measurements in piezometers
Water table
Unconfined
aquifer
Water levels in piezometers are located at the
water table. This level is a measure of the
hydraulic head, or potential at that point.
upwelling & downwelling can be inferred
with piezometer monitoring data
Horizontal flow direction
Vertical flow direction
Hydrometric measurements at Rio Calavares, NM
Photo courtesy of Michelle Baker, Utah State University
Injection & transport of
a solute tracer in the stream
A tracer is released,
and measurements
of its passage are
made at a location
downstream. What
will a graph of
tracer concentration
over time look like
at the downstream
monitoring point?
Injection & transport of
a solute tracer in the stream
The concentration will rise,
reach a plateau, and then
decline as the pulse passes
the point of monitoring. The
solute disperses from its
point of release due to the
force of the current
(advection) and diffusion and
turbulent mixing throughout
the stream.
Dye tracer illustrates water storage at channel margins:
1) sides are initially dye-free
Photo courtesy of Jud Harvey, USGS, Reston, VA
Dye tracer illustrates water storage at channel margins:
2) all parts of channel have dye
Photo courtesy of Jud Harvey, USGS, Reston, VA
Dye tracer illustrates water storage at channel margins:
3) sides retain dye longer than center channel
Photo courtesy of Jud Harvey, USGS, Reston, VA
Transient Storage: the temporary retention of solutes in
slowly moving or stationary water, and the eventual
movement of solutes and water back into the stream channel.
Five storage zones observed in a small mountain
stream (Bencala & Walters 1983):
• turbulent eddies generated by large-scale bottom
irregularities
• large but slowly moving recirculating zones along the
sides of pools
• small but rapidly recirculating zones behind flow
obstructions, especially in riffles
• side pockets
• flow in and out of beds of coarse substrate.
modelling the injection & transport of
a solute tracer in the stream
As a first approximation, this curve can be achieved with a
basic equation describing advection & dispersion, taking into
account stream dimensions & water velocity.
Change in solute concentration over time =
∂ C / ∂ t = - U ∂ C / ∂ x + D ∂ 2 C / ∂ x2
– 1st term describes downstream advection and is
proportional to water velocity, u.
– 2nd term describes mixing of the solute randomly throughout
the mass according to a dispersion coefficient D
modelling the injection & transport of
a solute tracer in the stream
More complicated models are needed to account for
additional variables such as groundwater and tributary
inputs, channel storage and subsurface flow.
Inclusion of terms for transient storage is necessary for the
description of solute dynamics in small streams and their
hyporheic zones.
OTIS: One-dimensional
Transport with Inflow
and Storage—
Conceptual Model
(OTIS, Runkel 1998)
Represents stream transport,
inflow, outflow, & exchange
modelling the injection & transport of
a solute tracer in the stream
Passage of a the tracer, including transient storage is given by:
• ∂ C/ ∂ t = -U ∂ C/ ∂ x + D ∂ 2/C/ ∂ x2 + α(Cs - C)
advection
dispersion
exchange
• ∂ Cs/ ∂ t = - α A/As (Cs – C)
exchange
Where As is the cross-sectional area of a hypothetical storage
zone. The rate of dispersion of solute in or out of this zone is
proportional to the difference between solute concentration in
the storage zone (Cs) and the water column (C) and an exchange
coefficient (α).
Adding a transient storage term permits the model to
account for significant features of the of a solute pulse that
the earlier equation is unable to mimic. Specifically,
measured passage of a tracer pulse usually shows the
rising shoulder of the actual pulse to be more gradual and
the descending tail to be prolonged relative to the
River by
- Transport
Study equation.
symmetrical curve Santiam
generated
the earlier
Rhodamine WT (ug/L)
3222m
4.0
3.0
2.0
1.0
0.0
12
13
14
Time (hr)
Data
Transient Storage
Convection-Dispersion
15
The storage zone component of the model is an
abstraction. In contrast to cross-sectional area (A) of the
stream channel, which can be measured directly, storage
zone area (As) is determined by fitting the model to
observed solute dynamics.
Nonethelesss, storage zones exist and are numerous.
Model estimates of As provide a useful index of the size
of the transient storage effect.
These models are
empirically useful
descriptions of observed
dynamics, in which
transient storage clearly
takes place.
Example: Rhodamine WT
tracer Experiment in
Clackamas River, OR
(Laenen & Risley 1997).
Downstream data are better
when simulated with
hyporheic exchange
considered in the modeling.
Hyporheic zone as a sink for nutrients:
Example w/ modelling to interpret field data
from Little Lost Man Creek, CA
• A study of a coastal mountain stream in northern
California indicated that transport of dissolved oxygen,
dissolved carbon, and dissolved nitrogen in stream water
into the hyporheic zone stimulated uptake of nitrogen by
microbes and algae attached to sediment.
• A model simulation of nitrogen uptake indicated that
both the physical process of water exchange between the
stream and the hyporheic zone and the biological uptake
of nitrate in the hyporheic zone affected the
concentration of dissolved nitrogen in the stream.
Kim et al. 1992
Nitrate injected into stream was
stored and taken up by algae and
microbes in the hyporheic zone.
After Kim et al. 1992
Channel friction factor =
8gds/u2 (where g is gravitational
acceleration, d is stream depth, s
is streambed slope, and u is
stream velocity).
The ratio of storage zone area to stream cross sectional area As/A
exhibits a strong positive relation with channel friction factor.
(From Harvey & Wagner 2000)
references
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Bencala K (1993). A perspective on stream-catchment connections. Journal of the North American Benthological
Society 12(1):44-47.
Eagleson PS (2002). Ecohydrology: Darwinian expression of vegetation form and function. Cambridge University
Press, 443pp.
Harvey JW & KE Bencala (1993). The effect of streambed topography on surface-subsurface water exchange in
mountain catchments. Water Resources Research 29(1):89-98.
Harvey JW & BJ Wagner (2000). Quantifying hydrologic interactions between streams and their subsurface hyporheic
zones. pp. 3-44 in: JB Jones & PJ Mulholland, eds., Streams and Ground Waters. Academic Press.
Helfield JM & RJ Naiman (2001). Effects of salmon-derived nitrogen on riparian forest growth and implications for
stream productivity. Ecology 82(9):2403-2409.
Hinkle SR, JH Duff, FJ Triska, A Laenen, EB Gates, KE Bencala, DE Wentz, & SR Silva (2001). Linking hyporheic
flow and nitrogen cycling near the Willamette River – a large river in Oregon, USA. Journal of Hydrology 244:157-180.
Nuttle WK (2002) Eco-hydrology’s past and future in focus. Eos 83(7 May): 205.
O’Keefe TC & RT Edwards (2002). Evidence for hyporheic transfer and removal of marine-derived nutrients in a
sockeye stream in southwest Alaska. American Fisheries Society Symposium 33:99-107.
Valett HM, SG Fisher, NB Grimm, & P Camill (1994). Vertical hydrologic exchange and ecologic stability of a desert
stream ecosystem. Ecology 75:548–560.
Rodriguez-Iturbe I (2000). Ecohydrology: a hydrologic perspective of climate–soil–vegetation dynamics. Water
Resources Research 36:3–9.
Runkel (1998). One-dimensional transport with inflow and storage (OTIS): A solute transport model for streams and
rivers. US Geological Survey, Water Resources Investigations Report 98-4018.
Triska FJ, VC Kennedy, RJ Avanzino, GW Zellweger, & KE Bencala (1989). Retention and transportation of nutrients
in a third-order stream in northwestern California: hyporheic processes. Ecology 70:1893-1905.
Kim BKA, AP Jackman, & FJ Triska (1992). Modeling biotic uptake by periphyton and transient hyporheic storage of
nitrate in a natural stream. Water Resources Research 28(10): 2743–2752
Winter TC, JW Harvey, OL Franke, & WM Alley (1998). Ground water and surface water: A single resource. US
Geological Survey, Circular 1139, 87 pp.