Geology Reference
In-Depth Information
last hay cutting was low, reflecting the fact that
in many years drought impacted upon hay yield
late in the growing season.
There are many variations of tillage sequences
that could have been used, but the sequences and
equipment chosen in these four scenarios encom-
pass the broad range of management practices
that might occur in this area, from the most erod-
ible to the least erodible. The types of machinery
used in the model are frequently used in this area
and were selected from the WEPP database.
WEPP allows the user to develop a user database
for local conditions. This includes the parameteri-
zation of plants, machinery, and initial conditions
for the local situation. WEPP also includes graphi-
cal and text outputs helpful in evaluating existing
or user-developed plants, machinery and local con-
ditions, as well as dates of various operations.
A one-year portion (Year 170) of the 1000 years
of daily output is shown in Table 9.6. The WEPP
daily output is a text file that can easily be trans-
ferred to a spreadsheet for extensive analyses.
Most WEPP output files are text files that can
also be imported into spreadsheets.
Similar outputs, but for months and years, can
also be selected. The year 170 was selected for pres-
entation because it contained the largest one-day
sediment yield, 77 t ha −1 for the moldboard plough
system on 12 October. Table 9.7 shows the maxi-
mum daily sediment yield for each management
system, and the sediment yield for all systems for
the day, month and year in which a maximum
occurred. These were gleaned from the daily out-
put for each of the 1000 year runs for the baseline
climate condition for each management system.
The maximum sediment yields never occurred
on the same day (or even the same year) for any of
the management systems. Also, while for the
moldboard and chisel plough systems maximum
loss occurred towards the end of a wet period, for
the no-till and grass systems the maximum losses
occurred as relatively isolated events.
Generally, a few storms produce the majority of
soil erosion and sediment yield (Edwards & Owens,
1991; Ghidey & Alberts, 1996). This is also true
for this application (Fig. 9.4). Fewer than 10% of
the storms having sediment yield (about one storm
a year) produced about 50% of the total sediment
yield. A major contributing factor to the high ero-
sion rates for the moldboard and chisel plough sys-
tems was that corn harvest was on 30 September
in all even-numbered years, with moldboard or
chisel ploughing occurring on 1 October in prepa-
ration for planting wheat on 16 October. The
severe erosion occurred after that date.
The severe erosion for the moldboard plough
occurred in the period with the highest 6-day
rainfall in the 1000-year period, a total of 534.1 mm
of rainfall on 8-13 October. This Cligen-generated
rainfall amount was nearly identical in amount
and timing to a 6-day rainfall amount on 29
September to 4 October 1986, when 533 mm of
rainfall was recorded at Fort Scott. This event
caused major flooding on the Marmaton river in
Fort Scott, only a few miles from the location of
this watershed. This was an extensive storm, with
9.5 Model Results
Model results needed to make decisions about
management of this small agricultural watershed
include average annual soil loss and sediment
delivery, sediment characteristics, return period
estimates, location of erosion and deposition, and
an estimate of the impact of global climate
change. Additionally, information about perform-
ance of potential management systems during
very unusual erosion events might be helpful in
determining best management systems.
9.5.1
Soil loss and sediment delivery
The standard summary watershed model output in
its standard format is shown in Table 9.5 for the
chisel plough management system. Values given
are average annual values of precipitation volume,
runoff volume, soil loss and sediment yield for
each hillslope, channel, and for the entire water-
shed. Dimensions of fields and channels are also
given, as well as the overall sediment delivery ratio
(ratio of sediment delivered from watershed to that
detached on the watershed - including erosion on
hillslopes and detachment in channels). Sediment
and water yields for impoundments would also be
available if impoundments were present.
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