cylinders were capped and returned to the lab.
Campbell Scientific (Logan, Utah) CR10 datalog-
The total volume of sediment was weighed, dried
gers. The average hourly measurements were
for 24 hours at 105C and reweighed, and the dry
stored on Campbell Scientific SM716 storage
bulk density of the sediment ρb was estimated:
modules. Each datalogger was installed on an in-
strument tripod with a 3-m center mast to keep
ρb = massdry soil/volumecylinder .
the datalogger above flooding tide levels (Fig.
17). These tripods were self supporting and did
not require any stakes to be driven into the
This method slightly underestimated the bulk
ground for anchoring. The dataloggers were
density since the cores shrank slightly upon dry-
powered with 12-V rechargeable batteries, which
ing.
were trickle charged with a MSX5 5-W solar pan-
el attached to the top of the center mast.
Tensiometers
Soil temperatures were monitored using Mod-
A series of SoilMoisture (SoilMoisture Equip-
el 107B soilwater temperature probes, which uti-
ment Corp., Santa Barbara, California) Series
lized the Fenwal Electronics UUT51J1 thermistor
2725 tensiometers were installed at six sites to
probe, which has an accuracy of 0.4C over the
provide another measure of near-surface sedi-
range 33C to +48C. Probes were placed 5, 10,
ment moisture. Tensiometers provide a direct an-
20 and 30 cm deep at each site.
alog readout of soil suction in centibars. The ten-
Sediment moisture was monitored at 5, 10, 20
siometers consist of a sealed hollow plastic tube
and 30 cm deep at each site using Model 257 (Wa-
filled with water with a porous ceramic tip at the
termark 200) soil moisture sensors, which esti-
bottom. As the sediment surrounding the buried
mate the soil water potential in the range of 02
ceramic tip dries out, water is drawn out through
bar. The output from the sensors is the ratio of ex-
the ceramic tip, creating a suction within the hol-
citation voltage to signal voltage, from which re-
low tube. The resulting suction is read by an at-
sistance is calculated. Resistance is functionally
tached dial gauge, which is graduated from 0 to
related to soil water potential. A sensor in saturat-
100 centibars (kPa) of soil suction. The tensiome-
ed sediment will give a resistance near zero; as
ters were installed at 25 and 10 cm deep (the min-
sediment dries, resistance increases. These sen-
imum depth for the model of tensiometer) at
three sites within the intermittent ponded area,
sors have internal gypsum tablets, which reduce
two sites on the mudflat and one site on the river
the error associated with changing salinity.
levee.
Periodically samples were collected for deter-
mination of the gravimetric moisture content.
Samples were collected down to 30 cm using an
Air-entry value
Using a standard Tempe pressure cell (Soil-
Oakfield Model B 2-cm-diameter soil sampling
Moisture Equipment Corp., Santa Barbara, Cali-
tube. Recovered soil cores were split into subsam-
fornia), a moisture retention curve (Hillel 1982)
ples, placed into plastic bags, labeled with site
number, depth and date, and returned to the field
was determined for a sediment sample collected
laboratory for determination of gravimetric wa-
south of the transect between sites 3 and 4 (Fig.
ter content.
16) in the intermittent ponded area (at site 883
The gravimetric water content θg of a sample
discussed in the next section). This curve was
of moist soil was measured by weighing the
used to estimate the air-entry value, which gives
moist soil sample, drying it at 105C for 24 hours
an indication of how far the water table must
to remove the water, and reweighing it.
drop before surface sediments become unsaturat-
ed. It also gives an estimate of the tensiometer
θg = (mass wet soil massdry soil)/mass dry soil.
reading corresponding to the onset of unsaturat-
ed conditions as sediments dry. To generate the
curve, air pressure was applied to a core of satu-
Bulk density measurements
Sediment samples were collected from each
rated sediment placed on a porous ceramic plate
site to determine the bulk density. A 35-cm-deep
in the Tempe pressure cell. Air pressure was in-
pit was dug at each site; then brass cylinders (i.d.
crementally increased, and the amount of water
= 5.39 cm, length = 3.0 cm ) were pressed into the
forced out of the sediment was recorded. The air-
undisturbed sediment of the side wall of the pit at
entry value corresponds to the pressure at which
5, 10, 20 and 30 cm deep. The sediment was dug
capillary forces are overcome and air begins to
out around the outside of each cylinder, and the
penetrate the sediment matrix.
44
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