Transcript Document

Watershed Hydrology, a Hawaiian
Prospective; Groundwater
Ali Fares, PhD
Evaluation of Natural Resource
Management, NREM 600
UHM-CTAHR-NREM
Presentation Outline

Infiltration
– Definition & theory
 Green-Ampt, Horton & Philip equations
– Infiltration measurement
– Infiltration as affected by:
 Soil type
Aquifers
Aquifer Properties
Water flows from area of high
energy to low energy

A unit of water has
energy due to 3
factors:
– Elevation
– Pressure
– Velocity (not important
for groundwater)

Total Energy is
expressed as feet of
“head”
 Head is equivalent to a
column of water so
many feet high
(convertible to
pressure by
multiplying by the
weight of water)
Darcy’s Law


a) French engineer
experimented with water
flowing through sand in mid1800s
b) He found that the amount
of flow was
– i) proportional to the area
through which the flow
occurred, and was
– ii) proportional to the hydraulic
gradient (the change in head
per length of flow path), and
– iii) related to the nature of the
material through which the
flow was occurring
Hydraulic Conductivity
Darcy’s K, a velocity, is
known as Hydraulic
conductivity
 Darcy’s K is actually a
function of

–
–
the liquid as well as
the porous media (sand,
gravel, clay, etc)
Darcy’s Law can be
used to predict total
flow if the hydraulic
gradient and other
factors are known
 Transmissivity:
hydraulic
conductivity times
the thickness of the
aquifer (Kb)

Subsurface classifications
Water content of an Unconfined
Aquifer


porosity sets the
maximum volume
available for water
below the water table
porosity (n) = Specific
yield (Sy - the water the
aquifer will release) +
Specific retention (Sr the water not released)
Surface water & Groundwater
Interactions
A “gaining” stream:
receives water from the
groundwater (the
groundwater discharges to
the surface)

–
The water table is higher
than the stream bed
A “losing” stream
recharges the groundwater

–
The water table is lower than
the stream bed
Effect on the Water Table of
Pumping from a Well



a “Cone of Depression” is created
in the water table (potentiometric
surface) when a well is pumped at
a sustained rate
This can result in a shift in
direction of flow as the
potentiometric surface is changed
sub-surface flow patterns are a
function of basin thickness, water
table gradient, and geologic
complexity ( mixed hydraulic
conductivities)

Darcy’s Law can be
used to predict the
drop in level of the
water table due to
pumping from a well,
or system of wells
Drop Down

Under steady state
conditions at a given
pumping rate, and if K is
known
 Theis in the 1930s
predicted the drawdown
in the water table at any
given time while the
Cone of Depression is
expanding
 Values of W(u) can be
looked up in Table 9.1 for
calculated values of u
Capture Zone of Wells


The zone of influence of
a well is determined by
the slope of the
potentiometric surface
(water flows down
“energy”)
Dimensions of the
capture zone can be
calculated if the
characteristics of the
aquifer are known
(thickness, K, hydraulic
gradient, etc.)