Jeffrey G. White responded:
>Let's put Karl's figures into perspective. What proportion of that 175
>sq. mi. watershed IS cropland, because the watershed as a whole is doing
>quite well, soil erosion-wise. That 22,000 tons spread over 175 sq. mi.
>is an erosion rate of about 0.2 tons/acre, which is quite low.
>"Natural" or "non-accelerated" rates of erosion are usually cited as
>between 0.1-0.2 tons/acre. The tolerable soil loss "T-values" that
>Karl mentioned (i.e., the maximum erosion that can take place on a given
>soil without degrading its long-term productivity) commonly range from 2
>to 5 tons/acre for US soils.
We need to be careful when comparing in-field sheet and rill erosion (which
the T-value is based on) with sediments delivered to streams, rivers, and
lakes. Most of the time when you hear quotes about erosion rates on fields
it is only the sheet and rill (near surface) erosion that is being referred
to since it is key to soil fertility. Delivery rates are invariably less as
much of the soil is deposited near where it erodes. When structures are in
place like sediment basins and buffer strips, the delivery rate may
approach 0 (or 0.2 tons/acre). Never-the-less, the soil that is lost (or
moved around) within in the field could dramatically effect the quality of
The sheet and rill erosion rate does not account for the formation of
gullies and stream-bank cutting which could deliver massive amounts of
sediments. Thus two watersheds, each with the same rate of sheet and rill
erosion, could have massively different sediment delivery rates as one
system produces huge gullies and causes stream-bank erosion while the other
So, you want to reduce delivery rates. First you reduce sheet and rill
erosion. Then you look at gully formation and streambank erosion during the
large rain events which causes almost all the delivery. If it is still too
much, then you have to figure out how to slow that water down. That could
mean changing what is actually planted on the land so that it catches and
holds more water (high residue crops, cover crops, permanent crops,
permanent conservation cover) or putting in more buffers, catch basins, and
dams. Making those choices is when life gets interesting.
"Since the achievement of our independence, he is the greatest
patriot who stops the most gullies." --Patrick Henry
Eric G. Hurley
829 Douglas Ave #3
Ames IA 50010-6221
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