**TECHNICAL NOTES**

**USDA-Natural Resources Conservation Service**

**Boise, Idaho**

TN AGRONOMY NO. 31 April 24, 1989

Prepared by Floyd G. Bailey, State Conservation Agronomist, SCS, Boise,
Idaho.

**Measuring and Estimating Concentrated Flow**

**and Ephemeral Gully Erosion on Cropland**

When runoff water from several individual rills within a small
watershed, begins to converge into concentrated flows, soil erosion
occurs that is not estimated by the Universal Soil Loss Equation (USLE).
This erosion H referred to as [concentrated flow erosion]{.underline}
when it covers broad surface areas without eroding deep gullies and
[ephemeral]{.underline} [gully erosion]{.underline} when gullies are
eroded. Concentrated flow and ephemeral gullies are oblitera­ted each
year by subsequent tillage operations and do not show up in the
following cropping seasons.

[Measuring Concentrated Flow and Ephemeral Gully Erosion]{.underline}

Concentrated flow and ephemeral gully erosion can be measured in the
watershed or field when the average width, depth and length of the gully
are known. The amount of soil eroded away can be estimated by using the
formula:

CFEG = [W x D x L x SW]{.underline}\
2000 lbs/ton

Where:

CFEG = Concentrated Flow and Ephemeral Gully Erosion in tons\
W = Average width in feet**\
**D = Average depth in feet\
L = Length in feet\
SW = Soil weight in pounds per cubic foot. Soil weight can be calculated
from the bulk density\
for an individual soil map unit. Bulk density is found on the SOILS-5,
soil survey reports and CAMPS Soil Data Base. Bulk density is in grams
per cubic centimeter. The formula for converting grams/cc to lbs/cu.ft.
= 62.44 x grams/cc = lbs/cu.ft. Bulk density does not include rock
fragments. When significant amounts of rock are present, the soil weight
must be adjusted. Rocks usually weigh about 2 times as much as soil.

2000 = Pounds per ton.

**EXAMPLE:**

> **A 40 acre field has two ephemeral gullies (A and B) and one
> concen­trated flow (C) with the following dimensions:**
>
> **A** --- **6 inches wide, 6 inches deep and 1320 feet long**
>
> **B** --- **8 inches wide, 5 inches deep and 660 feet long**
>
> **C** --- **36 inches wide, 3 inches deep and 451 feet long. The soil
> weight is 90 pounds per cubic foot.**

**Question: How many tons of soil have eroded in the ephemeral gullies
and concentrated flow?**

**Solution:**

**Gully A = .5 ft. x .5 ft. x 1320 ft. = 330 cu. ft.**

**Gully B = .66 ft. x .41 ft. x 660 ft.** = **179 cu. ft.**

**Gully C** = **3 ft. x .25 ft*.* x 451 ft.** = **[338]{.underline} cu.
ft.**

**TOTAL 847 cu. ft.**

**847 cu. ft. x 90 lbs/cu. ft.** = **76,230 lbs. divided by 2000
lbs./ton = 38 tons.**

Where the answer is needed for a field or watershed representing the
average tons per acre, the formula can be modified to:

CFEG = [W x D x L x SW]{.underline} OR [W x D x L x SW]{.underline}

[2000 lbs./ton]{.underline} 2000 lbs/ton x A

A

> Where:

A = acres in the field or watershed

**EXAMPLE:**

**Using the same previous sample, what is the average soil loss in
tons/acre for the 40 acre field?**

**Since we already determined 38 tons of soil has eroded, all we have to
do is divide that amount by the acres in the field [38]{.underline} =
.95 tons/acre.**

**40**

> [Estimating Long---Term Average Annual Concentrated Flow and Ephemeral
> Gully Erosion]{.underline}

Estimates of the amount of soil loss occurring over time from
concentrated flow and ephemeral gully erosion can be made by developing
a volumetric measure of the amount of area that has been voided by the
gullies and allocating the amount back out to the area of the field or
watershed and the time interval involved.

When estimating the amount of long-term concentrated flow and ephemeral
gully erosion, the soil conservationist has to look at the whole field
or watershed, and observe the runoff patterns developed over time. These
runoff patterns show up on the landscape as shallow, round-edged gullies
or depressions where water flows when runoff occurs. Annual tillage has
worked soil back into the gully smoothing the eroded area into more of a
rounded depression on the landscape. As tillage has worked soil into the
voided area, it has widened the gully. The resulting cross-sectioned
shape is usually trapezoidal. Aerial photographs can be helpful in
locating these trapezoidal depressions caused by long-term concentrated
flow and ephemeral gully erosion.

The depressions voided by concentrated flow and ephemeral gully erosion
can be measured to gain an estimate of the volume of soil that has
eroded away. The measurements are made on a cross section for average
width and average depth of the depression. The length can be measured
from beginning at the upper end of the field or slope where the
depression begins and measuring downslope to the end of the depression.
The object of the measurement is to get a representation of the volume
of soil that has been eroded, forming the depression over time. The time
period for the depression to form must be known or estimated. This is
usually the length of time the area has been cultivated.

The formula for estimating long-term average annual concentrated flow
and ephemeral gully erosion for a trapezoidal depression is:

> CFEG = [TW **+** BW x .5 x D x L x SW]{.underline}
>
> 2000 lbs/ton x acres x years

Where: TW = depression top width in feet

BW = depression bottom width in feet

**EXAMPLE:**

> **A 40 acre field has a water flow depression where concentrated flow
> and ephemeral gully erosion occurs. The depression is 16 feet wide
> across ­the top and 4 feet wide across the bottom. The depression is
> 1.5 feet deep at the center. The length of the depression is 1285 feet
> from where it starts to where it leaves the field. The soil weighs 90
> pounds/cu. ft. This field has been farmed the same for the past 60
> years.**
>
> **Question: What is the estimated long-term average annual
> concentrated flow and ephemeral gully erosion from the field?**

**Solution:**

**[16 ft + 4 ft x .5 x 1.5 ft x 1285 ft x 90 lbs/cu.ft]{.underline}** =
**[1,734,750 lbs]{.underline}.** = **. 36 T/ac./yr.**

**2000 lbs/ton x 40 acres x 60 years 4,800,000 T/ac/yr.**

When a field has more than one watershed with a representative
concentrated flow or ephemeral gully depression, the amount of erosion
for each can be calculated, then added together to gain the total
long-term average annual erosion from concentrated flow and ephemeral
gully erosion for the field.

In project activities where broad-based planning estimates are used, the
soil conservationist can make an estimate of concentrated flow and
ephemeral gully erosion occurring by developing an estimate of the
long-term average concentrated flow erosion per acre per year on
representative sample fields within a treatment unit or watershed, and
expand the data to the total acreage within the treatment unit or
watershed. This will provide representative erosion rates with
sufficient accuracy for broad area resource planning.