Worksheet
ksat
Physical and selected chemical methods of soil analysis,
IHLW, SS2018
Hydraulic Soil Conductivity
Determination of saturated hydraulic conductivity,
ksat
–
Lab method with falling head and rising water level
Sample preparation
•undisturbed soil sample in steel core
•extend the sample upside with another empty steel core of the same size,
connect the two cylinders with rubber band
•the soil sample needs to be water saturated before the measurement –
place the sample into the measurement pan onto a two-layer basis of wire
mesh and textile mesh (textile above wire), fill the pan stepwise with de-
aerated water, with each step, water level should rise 0.5 cm, each step
lasts at least three hours, last water level is equal to soil sample height
Now the water level inside and outside of the sample are nearly equal. To
measure hydraulic conductivity, we will rise the outer water table and establish
a gradient of hydrostatic pressure. To equilibrate both water levels, water will
flow from the sample bottom through the soil and the inner water table will
rise. The speed of this rise is determined by the conductivity of the sample.
The outer water table is kept constant by a Marriott bottle.
Measurement
•prepare measurement form, timer and further measuring equipment
•fill the pan rapidly with de-aerated water to 0.5 cm below the upper border
of the upper cylinders
•take readings, record time
t
, inner water table
h
i
,t
, and outer water table
h
o
,t
, at the beginning of the experiment every 10 s, later in dependence of
the rising speed; at least six readings are to be recorded before outer and
inner water table equal
Results
•calculate values for each adjacent pair of readings using Eq. (1)
•the average of all part results is the saturated hydraulic conductivity
ksat
ksat=L
t2−t1
ln hi ,1−ho ,1
hi ,2−ho ,2
(1)
Equipment
•water pan on horizontal work surface
•soil core, identical to sampling core
•rubber band
•Mariott bottle
•caliper and reference bar (or similar)
Why do we measure?
To know how fast water flows through a
given volume of soil is important if we
want to estimate infiltration during rain
events, groundwater recharge, loss of
water into deep layers out of reach for
plants, ...
Saturated hydraulic conductivity is the
most significant parameter which
determines water balance quantities. It is
one of the most frequently measured soil
physical properties as measurements are
easy and cheap.
Some background information
Sample saturation before the actual
measurement is crucial, because water
needs to flow through the full cross
section of the sample. If we would rise
the water level from zero to the
maximum, water would flow rapidly
through the larger pores while the smaller
pores remain filled with air. Hence, water
is flowing through an area smaller than
the sample cross section.
Due to high spatial variability and the fact
that fully saturated state does not occur in
reality, we can interpret ksat only as a
relative value to compare different soils or
to get an idea about the magnitude of soil
conductivity. To obtain a value
representative at field scale, a high
number of replicates has to be analysed.
hi,2
hi,1
datum
soil sample
ho,t
rubber band
nylon fabric
+ wire mesh
water
basin
steel
cores
L