Hydrology Introduction and Field Exercise:
Groundwater Section:
A recent focus of watershed hydrology has been on the interactions of surface and subsurface
water through riparian and hyporheic zones. The stream channel connection with the adjacent area is
an integral part of watershed function where water exchanges between the stream, the hyporheic zone,
and the riparian area. These areas are often indistinct and variable in direction, time, and space.
Groundwater contributions to the stream vary significantly based primarily on topographic and geologic
indices and also interacts with the stream at different scales.
In any particular watershed what are the local controls on connections between the subsurface water
and stream channel? These controls, as noted above, are variable in geographic areas but are generally
thought to be the stream topography and the step-pool morphology that is often present in mountain
streams. The step-pool mechanism provides the hydraulic gradient and force that connects the surface
and subsurface pools.
Watershed 1 (WS01) description:
The watershed is 1 km2 and was clear cut in the mid-60’s as part of the paired watershed study
with WS02 as the control watershed and WS03 as a minimal impact watershed with a small road system
cut into the area. These studies were designed to look into logging practices effects on water quantity
and quality. Long-term stream chemistries, sedimentation rates, and streamflow measurements from
this and other studies has helped shape present logging practices within the NW.
The encompassing riparian area averages 13.7m in width across the valley floor with a
downstream elevation gradient of 0.13 m/m. In 1997 WS01 was instrumented with 6 transects of water
table wells and piezometers, running the width of the valley floor. Each well is screened over the
bottom 50 cm with piezometers screened over the bottom 5 cms. The wells are within the riparian zone
and the piezometers are placed within the channel itself. The transects stretch out over a 29m length of
stream channel at various hydrologically important zones, i.e. the toe of the bounding hillslopes, on the
stream bank, and at the edge of the stream bank on each side of the channel (figure 1).
Groundwater flow basics:
DARCY’S LAW:
Q = -k*(dH/dL)*A
Where:
Q = Discharge (m3/s)
k = hydraulic conductivity (m/s)
dH/dL = hydraulic gradient (-)
A = area (m2)
Figure 1: WS01 with well transects highlighted. Upstream is at the top of the map. Transects go
from C (downstream) to H (upstream), and 1 to 7 (left to right or north to south).
Surface water section:
As noted in the previous section, surface and subsurface waters are not easily delineated and
have variable interactions over the course of small spatial and temporal scales. The inputs from
groundwater flow and hyporheic zones play a significant role in temperature mediation, biological
activity, and chemical reactions within the stream channel. The mechanisms relating groundwater
inputs to surface water vary. Bedrock geology, surface topography, bedrock topography, sediment
thickness, and local hydraulic conductivities all play significant roles in routing flowpaths. To gauge
spatial variability in groundwater inputs salt dilution is a common technique.
Salt dilution involves the conservation of mass. A know flux rate (injection) of salt is added to
the stream at an upslope location. At some point downstream after the salt has become thoroughly
mixed we measure the salt concentration and calculate discharge.
Q = q/BC
where:
Q = stream discharge (m3/s)
q = salt input mass (kg/s)
BC = Background Concentration (kg/m3)
*note: The EC probe will give measurements in units called uS/cm (microSiemans per cm). To
convert this relationship into a volumetric weight (mg/l): conversion factor equals ~0.5
We will inject a known salt solution at the upstream location and once steady state is reached, we will
traverse the stream channel and record changes in salt concentration along the stream reach. Steady
state conditions will allow us to determine the groundwater and hillslope inputs along the stream
channel.
Field Exercises:
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Penn State Group introduction
Study overview
Locate transects
Introduction to field equipment
o Electrical Conductivity meter
o Water level sounder
Break into groups
o Salt dilution group (5)
o Water level sounder group (5)
o Stream measurements (5)
Calculations to Perform:
1) Create a map of the water table equipotentials and flow lines. Quantify the horizontal hydraulic
gradients (dH/dL) within the riparian zone
2) Using Darcy’s law estimate the down-valley and lateral flow velocities corresponding to field
observations of water table measurements. *(k ~ 9.2 m/day)
3) Calculate stream discharge using the stream salt dilution data. Quantify longitudinal stream
discharge.
Questions to Answer:
1) How does stream channel morphology affect groundwater flowpaths? How does near stream
hillslope topography affect groundwater flowpaths?
1) After looking at the longitudinal stream discharge data, does discharge increase/decrease
linearly? Or with some other consistent pattern? Why?
2) How does topography affect stream discharge? Riparian zone? What are the causes for
increase/decrease in discharges?
3) Why do we care about surface/subsurface interactions at the watershed scale? At the basin
scale?
Flow Direction
N
H
G
F
E
E
D
D
C
Tower