POLYACRYLAMIDE AND GYPSIFEROUS MATERIAL EFFECTS
ON RUNOFF AND EROSION UNDER SIMULATED RAINFALL
J. R. Peterson, D. C. Flanagan, J. K. Tishmack
ABSTRACT. An indoor laboratory study was conducted to compare the dry versus sprayed application of polyacrylamide
(PAM) and the use of different gypsiferous materials (gypsum and a class C ponded fly ash) on runoff and sediment yield. The
six treatments incorporated in this study were: control, Nutra–Ash (NA), gypsum (G), sprayed PAM plus NA (PAMW+NA),
sprayed PAM plus G (PAMW+G), and granular PAM plus G (PAMD+G). Simulated rainfall at an intensity of 70 mm hr–1 was
applied for 2 hours to a silty clay loam soil packed into erosion pans. Only one of the two liquid PAM treatments (PAMW+G)
significantly reduced runoff (35%), while both liquid PAM treatments (PAMW+G and PAMW+NA) significantly reduced
sediment yield (74% and 77%) compared to the control.
Sprayed PAM was more effective than granular application in terms of total runoff, but there was no statistical difference
with regard to total sediment yield. Differences between the effects of sprayed and granular PAM are explained by the
mechanisms by which they reduce erosion. Sprayed PAM, in combination with gypsum, increases infiltration during the first
part of a rainfall event until sufficient rainfall has occurred to break down the PAM–treated aggregates, at which time runoff
rate and sediment yield rate approach those of the control. Runoff and sediment yield rates from the granular PAM application
were initially similar to those from the control. However, as time increased, sediment yield reached a maximum and then
decreased without a corresponding decrease in runoff. This likely occurred because the PAM particles became activated
during the rainfall and acted as a mortar to stabilize the soil matrix.
Gypsum was a better source of electrolyte than a class C ponded fly ash, commercially known as Nutra–Ash (NA), likely
due to its greater solubility. Addition of PAM decreased soil erodibility and may be a viable erosion control practice for soils
susceptible to flow detachment. Choice of application method should be based on the expected amount and severity of
precipitation before vegetation establishment. These results indicate that sprayed PAM, in combination with gypsum or
Nutra–Ash, provides immediate erosion control, but its effectiveness decreases over time, as indicated by steadily increasing
sediment yield rate. Dry PAM application was not as effective in the beginning of the experiment, but after sufficient rainfall
it became “activated” and sediment yield continuously decreased over the remainder of the experiment.
Keywords. Polyacrylamide, PAM, Soil amendments, Gypsum, Soil erosion, Runoff.
A
griculture is a leading source of pollution to
streams, rivers, and lakes in the United States.
According to the US EPA (1996), agricultural
pollution affects 25% of streams and 19% of lakes
and contributes to 70% of all identified water quality
problems in streams and 49% in lakes. Sediments are the
most common pollutant affecting rivers and streams and
contribute up to 51% of water quality problems in rivers and
Article was submitted for review in November 2001; approved for
publication by the Soil & Water Division of ASAE in April 2002.
Use of trade names does not imply an endorsement by Purdue
University or the USDA–ARS.
The authors are Joel R. Peterson, ASAE Student Member, Graduate
Research Assistant, Department of Agricultural and Biological
Engineering, Purdue University, and National Soil Erosion Research
Laboratory, West Lafayette, Indiana; Dennis C. Flanagan, ASAE Member
Engineer, Assistant Professor, Department of Agricultural and Biological
Engineering, Purdue University, and National Soil Erosion Research
Laboratory, West Lafayette, Indiana; and Jody K. Tishmack, Ash
Management Coordinator/Continuing Lecturer, Department of
Agricultural and Biological Engineering, Purdue University, West
Lafayette, Indiana. Corresponding author: Joel R. Peterson, National Soil
Erosion Research Laboratory, Purdue University, 1196 Soil Building, West
Lafayette, IN 47907–1196; phone: 765–494–5158; fax: 765–494–5948;
e–mail: [email protected].
streams and 25% in lakes (US EPA, 1996). The problem of
erosion is not only an agricultural one. Erosion rates at
construction sites can far exceed those of agricultural land,
approaching 163 Mg ha–1 yr–1, which is about 18 times
greater than maximum tolerable rates from agricultural lands
(Daniel et al., 1979).
Soil erosion on upland areas is caused by the impact of
raindrops, which leads to interill erosion, which in turn
breaks down soil structure and leads to detachment by
concentrated water flows (rill erosion). Infiltration can be
hampered by sealing of the soil surface, which can result in
reduced infiltration rates and increased runoff and erosion.
Surface sealing may also interfere with seedling germination
(Shainberg and Levy, 1994). Seal formations are caused by
two primary mechanisms: (1) physical disintegration of soil
aggregates by raindrop impact, and (2) physicochemical
dispersion and migration of clay particles in the 0.1 to 0.5 mm
depth, where they can clog pores (Levy et al., 1992;
Shainberg and Levy, 1994). Seal formations can be prevented
by introducing mulch or some other cover to protect the soil
surface from raindrop impact, introducing electrolytes at the
soil surface to reduce chemical dispersion of clay particles,
and/or stabilization of aggregates at the soil surface by
polymeric soil conditioners (Stern et al., 1991).
Transactions of the ASAE
Vol. 45(4): 1011–1019
2002 American Society of Agricultural Engineers ISSN 0001–2351
1011
One such soil conditioner is polyacrylamide (PAM).
Polyacrylamide is a water–soluble, synthetic organic polymer high in molecular weight that primarily interacts with the
clay fraction of soils (Seybold, 1994) and has been proven to
be superior to other polymers in erosion control applications
(Shainberg and Levy, 1994). Polyacrylamide stabilizes the
soil through two major mechanisms: (1) adsorption of the
polymer by clay causes a physicochemical charge at the clay
surface, reducing repulsive forces among clay particles, and
(2) the polymer acts as a bridge between soil particles in an
aggregate by binding bridged particles together (Ben–Hur,
1994). Polyacrylamide can be synthesized in cationic,
nonionic, or anionic forms, although anionic PAM has been
found to be the most effective for erosion control (Shainberg
and Levy, 1994). Shainberg et al. (1990), Levy et al. (1992),
and Smith et al. (1990) reported that the benefits of PAM were
enhanced by the introduction of an electrolyte source that
helps to create a cation bridge for the polymer to adsorb to the
soil. Typically, the electrolytes are introduced by application
of a gypsiferous material.
Several types of amendments have been used to enhance
the electrolyte concentration of the soil solution. These
include phosphogypsum (PG), gypsum, and different types of
ash from coal–fired power plants. Phosphogypsum is a
byproduct of the phosphate fertilizer industry (Shainberg et
al., 1990). Fluidized bed combustion bottom–ash (FBCBA)
and flue gas desulfurization (FGD) are byproducts from
coal–fired power plants. Norton et al. (1993) studied the
effectiveness of two types of FGD material, an FBCBA
material, and PG on infiltration and erosion in a laboratory
setting. The effectiveness of the different materials was
related to particle size and solubility (ability to release
electrolytes). They concluded that gypsiferous materials
with greater solubilities were more effective, and that the
coarser the material, the less soluble it was due to surface area
constraints.
The effect of PAM on interill and rill runoff and soil loss
was examined by Flanagan et al. (1997a, 1997b). Treatments
included a liquid PAM solution applied at 20 kg ha–1 and then
exposed to both deionized and tap water rainfall, a control
under both deionized and tap water rainfall, and an FBCBA
surface amendment under deionized rainfall. They found that
interill soil loss was not significantly different between
treatments. Final infiltration rates for the two PAM surface
treatments were not significantly different from other
treatments in the rill plots, but the authors noted that the PAM
treatments did appear to reduce aggregate breakdown and
enhance infiltration rates. The FBCBA significantly increased infiltration on the interill plots compared to the tap
water or deionized water treatments. The FBCBA also
significantly reduced sediment concentrations and sediment
discharge rates on the rill subplots on initially dry soil. For the
rill subplots, the 20 kg ha–1 PAM treatments reduced
detachment compared to the control. Flanagan et al. (1997b)
recommended using FBCBA in conjunction with PAM.
Mitchell et al. (1996) reported that measured runoff from
PAM–treated plots was not significantly different from that
from the control plots using PAM application rates of 1.1
(10 Mg mol–1 MW) and 17.6 kg ha–1 (0.25 Mg mol–1 MW).
The ineffectiveness of PAM was attributed to low application
rates for both PAMs, but particularly for the second PAM
because of its low MW. Mitchell et al. (1996) did not apply
a gypsiferous material, which most likely lessened the
1012
effectiveness of the PAM in their study. Because of the high
viscosity of PAM in solution, adequate dilution with water to
spray the PAM solution may require excessive amounts of
water, causing runoff during application or requiring several
applications. Mitchell et al. (1996) suggested that dry
application of PAM should be studied.
Chaudhari and Flanagan (1998) conducted a natural
rainfall experiment on a steep slope (34% to 37%) to evaluate
the effects of PAM on runoff, erosion, and seedling
emergence. Treatments were a control, PAM sprayed at 80 kg
ha–1, and PAM plus gypsum applied at 80 kg ha–1 and 5 Mg
ha–1, respectively. Both the PAM and PAM plus gypsum
treatments significantly reduced runoff and sediment yield
over the control.
Stern et al. (1991) evaluated the effectiveness of PG (5 Mg
ha–1), mulch, and two rates of PAM (5 kg ha–1 and 20 kg ha–1)
on reducing runoff and soil loss. Measured runoff from the
mulch and PAM treatments were significantly less than that
from the control and PG treatments.
The cost of PAM application can be small in comparison
to traditional erosion control measures such as mulch.
Chaudhari and Flanagan (1998) reported the cost of traditional mulch, adhering to Indiana Department of Transportation
guidelines, was $1900 ha–1 compared to $160 ha–1 for PAM.
Benefits of gypsiferous material and PAM on runoff and
erosion have been well documented. However, the method of
PAM application (dry vs. sprayed) and the interaction of PAM
with different gypsiferous materials have received little
attention. Our hypotheses are that sprayed PAM significantly
reduces total runoff and sediment yield compared to dry PAM
application, and that total runoff and sediment yield from
treatments of different gypsiferous material are not statistically different. The objectives of this study were to compare
the dry versus sprayed liquid application of PAM and the use
of different gypsiferous materials on runoff and sediment
yield.
MATERIALS
The six treatments incorporated in this study were:
control, Nutra–Ash (NA), gypsum (G), sprayed PAM plus
NA (PAMW+NA), sprayed PAM plus G (PAMW+G), and
granular PAM plus G (PAMD+G). Application rates of each
material are presented in table 1. The application rate of
gypsum was chosen based on research conducted by
Chaudhari and Flanagan (1998). The application rate of
Nutra–Ash (NA) was designed to provide the same amount
of calcium as gypsum. The soil used was a silty clay loam
(20% sand, 42% silt, 38% clay, 3.0% OM, 390 ppm Mg, 4600
ppm Ca, CEC 26.6 meq 100 g–1) taken from the floodplain
of the Wabash River by Purdue University facilities, which
uses the soil for various landscaping projects around campus.
This soil was used because of its relatively high clay content,
which was expected to show differences in runoff and erosion
between PAM–treated and untreated soil and because the soil
was locally available in quantity. Estimated saturated
hydraulic conductivity of the soil is 0.29 cm hr–1 based on soil
texture (Saxton et al., 1986).
The PAM used was anionic Percol 336, a commercially
available material (Allied Colloids Inc., Suffolk, Va.), having
32% charge density and a high molecular weight, in
comparison to the study conducted by Mitchell et al. (1996),
TRANSACTIONS OF THE ASAE
Table 1. List of treatments and associated application rates.
Application Rate (kg ha–1)
Gypsum
Abbreviation
Treatment
C
G
PAMW+G
PAMD+G
NA
PAMW+NA
Control
Gypsum
PAM (wet) + gypsum
PAM (dry) + gypsum
Nutra–Ash
PAM (wet) + Nutra–Ash
5000
5000
5000
PAM Nutra–Ash
40
40
40
8042
8042
of 20 Mg mol–1. Nutra–Ash is a reclaimed Class C fly ash
marketed as a liming/fertilizer supplement. It was manufactured from ponded class C fly ash that has been mined,
crushed, and sieved to produce various sized products.
Commercial fertilizer–grade gypsum was obtained from
Shoals, Indiana. A fertilizer/lime analysis of each gypsiferous material is given in table 2.
Erosion pans used in the study measured 32 cm wide Ü
45 cm long Ü 20 cm deep. Holes placed in the bottom of the
box allowed for free drainage. A programmable rainfall
simulator, described by Foster et al. (1982), was used in the
experiments. Adjusting the frequency of nozzle oscillation
with a programmable logic controller permits one to set the
rainfall intensity. The simulator troughs used 80100 VeeJet
spray nozzles (Spraying Systems Co., Wheaton, Ill.), which
were operated at a nominal water pressure of 41.4 kPa. The
troughs were suspended from the ceiling at a height of
approximately 2.5 m above the surface of the erosion pans.
Kinetic energy from this simulated rainfall is about 75% that
of natural rainfall in a 64 mm hr–1 event (Meyer and McCune,
1958). Deionized water, with an electrical conductivity of
approximately 15 mS cm–1, was used to approximate the
electrolyte level of natural rainfall.
METHODS
Soil was gently crushed and sieved to pass through an
8 mm opening and then allowed to air dry. Sand was placed
in the lower 5 cm of each erosion pan to promote percolation
through the box. The remaining 15 cm of each box was
packed with soil, in three 5–cm layers, to achieve a 1.35 g
cm–3 bulk density. Soil amendments were then added
according to the treatment specified (table 1). Polyacrylamide in solution was applied at a rate of 40 kg ha–1 using a
common household plant sprayer at a concentration of
405 mg L–1. The same amount of water (1.4 L) was also
Table 2. Fertilizer/lime analysis of the mineral
amendments used in the study.
Nutra–Ash
Gypsum
Parameter
% calcium (Ca)
% magnesium (mg)
% CCE[a]
Moisture (105°C)
% passing U.S. #8 sieve
% passing U.S. #20 sieve
% passing U.S. #60 sieve
Bulk density (g cm–3)
[a]
As
Received
16.0
2.7
48.1
6.8
As
Received
22.4
1.4
11.7
0
87.1
46.6
27.5
1.3
CCE = calcium carbonate equivalent.
Vol. 45(4): 1011–1019
Dry
Basis
17.2
2.9
51.6
Dry
Basis
22.4
1.4
11.7
97.6
68.2
24.6
1.3
applied using the same sprayer to those treatments not
requiring sprayed PAM to minimize differences in treatments
due to the effects of antecedent moisture content. The
granular PAM (PAMD) was applied at a rate of 40 kg ha–1 to
dry soil and then wetted with 1.4 L water. Erosion pans were
allowed to air dry for 24 hours. Slope was set at 17%.
Simulated rainfall was applied using deionized water at an
intensity of 70 mm hr–1 for 2 hours after runoff initiation on
any one of the boxes. Samples were collected every 5 minutes
after runoff initiation. Three or four erosion pans were used
at a time with treatments assigned randomly. Treatments
were replicated three times. Runoff rate was determined by
measuring the mass of runoff collected in each 1 L bottle
collected during 5 minutes. Sediment concentration was
determined gravimetrically. Percolate through the bottom of
the erosion pans was not analyzed.
Differences in total runoff, total sediment yield, and final
runoff and sediment yield rates between treatments were
identified using the least significant difference (LSD)
method at a significance level of 0.95. To assess the
effectiveness of a treatment over time, the relative effectiveness, R(t), of a treatment at time t is defined as:
T (t) − C(t )
R(t ) = i
∑ (Ti − C)
(1)
where
Ti (t)
= sediment or runoff collected for treatment i
during time increment
C(t) = sediment or runoff collected for the control
during time increment
Ti – C = difference between total sediment or runoff
collected.
A sequential dilution dissolution analysis was performed
on the gypsiferous soil amendment materials (G and NA) in
order to better understand their effect on electrolyte levels
over the course of the experiment. Laboratory analyses were
conducted by A&L Laboratory, Inc. Fifteen grams of
material was placed in a 500 mL plastic bottle. Then 300 mL
of ultra–pure water was added, and the bottle was mechanically shaken for 10 minutes. At the end of 10 minutes, an
11.0 cm Whatman No. 1 filter was placed in a Buchner
funnel, under suction, and the contents of the sample bottle
were filtered. All solids were recovered from the filter paper,
including the filter itself, and then were placed back into the
mixing bottle. Another 300 mL of ultra–pure water was
added, and the procedure was repeated a 2nd and 3rd time.
Thus, three filtrate samples, representing 10, 20, and
30 minutes were obtained. The filtrate was analyzed for Ca,
Mg, Na, K, S, B, Cu, Fe, Mn, and Zn. Samples were analyzed
using inductively coupled plasma–mass spectrometry (ICP)
water matrix standards.
Linear regression was used to determine the relationship
between sediment yield rate and runoff rate using the
equation:
qs = aqw + b
(2)
where qs is the sediment yield rate, and qw is the runoff rate.
The data used were the runoff/sediment yield rate data pairs
collected at each time increment for each replicate by
treatment from the beginning of each experiment until
steady–state runoff had been achieved.
1013
RESULTS AND DISCUSSION
GYPSIFEROUS MATERIAL ANALYSIS
Nutra–Ash was less soluble than gypsum, and this resulted
in a lower concentration of calcium in solution (table 3). Both
materials maintained fairly uniform Ca concentrations over
the course of the dissolution analysis.
A rapid decrease in the concentration of a given ion is
often referred to as a “first flush,” where easily dissolvable
minerals are quickly depleted when water first contacts a
material. Class C fly ash normally contains a number of
soluble calcium–containing minerals when the ash comes in
contact with water. Since Nutra–Ash had been exposed to
moisture for several years while in an ash landfill, it likely did
not contain as many soluble minerals as fresh ash. Grinding
of the material may have exposed new surface area that had
slightly higher solubility than the otherwise cemented or
hydrated matrix.
RUNOFF
Treatments in which PAM was applied as a liquid solution
(PAMW) to the soil surface and allowed to dry reduced
runoff. Only the PAMW+G treatment had significantly less
(a = 0.05) runoff than the control, a reduction of 35%
(table 4). The PAMW+NA treatment was just outside the
statistical range of significance.
Comparing the two electrolyte treatments, there was no
statistical difference in total runoff between the G and NA
treatments. The amount of calcium applied in the G and NA
treatments was equal; however, the dissolution analysis
indicated that much higher concentrations of Ca, and hence
electrolyte, would be present for the G treatment.
Table 3. Constituent concentrations by ICP in solutions
(20:1) extracted using sequential dissolution.
Nutra–Ash (ppm)
Gypsum (ppm)
Parameter
10
min
20
min
30
min
Calcium (Ca)
Magnesium (Mg)
Sodium (Na)
Potassium (K)
Sulfur (S)
Boron (B)
Copper (Cu)
Iron (Fe)
Manganese (Mn)
Zinc (Zn)
660
<1
920
34
530
20
0.1
2.1
<0.1
<0.1
700
<1
340
17
350
14
0.1
3.6
<0.1
<0.1
660
<1
200
10
370
10
<0.1
8.0
<0.1
<0.1
10
min
20
min
12,500 12,300
18
61
11
7
28
18
9070
8870
1.4
1.0
2.2
1.9
2.0
2.4
0.1
<0.1
0.2
0.2
30
min
12,300
62
<1
11
8740
0.6
1.8
1.7
<0.1
0.1
Table 4. Total average runoff volume, average sediment yield, and
respective percent reductions over the control. The same letter
following a treatment indicates no significant difference using
the least significant difference method at a = 0.05.
Total Runoff
Total Sediment Yield
Treatment
Control
Gyp
PAMW+G
PAMD+G
NA
PAMW+NA
1014
mm
% reduction
of control
kg m–2
% reduction
of control
88.1 a
81.7 a
57.1 b
81.9 a
84.9 a
77.5 a
7.3
35.2
7.0
3.7
12.0
1.31 a
0.87 b
0.35 c
0.48 c
1.02 b
0.30 c
33.6
73.6
63.2
21.9
77.0
The addition of PAM to the electrolyte source resulted in
a further significant decrease in runoff when comparing
PAMW+G versus G. Total runoff for the G treatment was
81.7 mm, which declined to 57.1 mm with the addition of
PAMW (table 4). During the first 35 minutes, the slope of the
line for the PAMW+G treatment was less in comparison to
the other treatments (fig. 1). At about 35 minutes, the
PAMW+G line slope increased sharply and then increased at
a fairly uniform rate. This suggests that the effects of the
PAMW+G treatment were very pronounced early in the
experiments but became slightly less so as the experiment
progressed beyond 40 minutes. Because the gypsum was
crushed and sieved, there was some amount of fines created
that were more soluble than the larger particles and would
have solubilized quickly. This would result in the early
release of a substantial amount of cations when the PAM was
sprayed, thus providing ample divalent cations to enhance
the cation bridging process. This is substantiated by the
dissolution analysis, which shows high Ca concentration in
the first 30 minutes for G compared to NA. However, after
approximately 35 minutes continued raindrop impact likely
began to break down soil aggregates, leading to increased
surface sealing, which led to reduced infiltration and
increasingly greater runoff rates.
The effect of application method of PAM (PAMD+G and
PAMW+G) was examined, and there was a significant
difference in treatment effect with respect to total runoff
volume but no significant difference in final runoff rates
(tables 4 and 5). The runoff rate for PAMD+G reached a
relative steady state within the first hour of the simulation,
while PAMW+G did not (fig. 1). Therefore, we conclude that
the sprayed PAM, in the presence of gypsum, is able to better
stabilize the soil surface. However, the experimental results
also suggest that runoff rates may eventually approach those
of the control, either due to saturation of the soil surface layer
or through eventual degradation of the soil surface structure
by continued raindrop impact.
SEDIMENT
All treatments significantly reduced total sediment yield
compared to the control. These reductions ranged from 22%
for the NA treatment to 77% for PAMW+NA (table 4, fig. 2).
Comparing the gypsiferous compounds, total sediment yield
was significantly different between the G and NA treatments
(table 4). In all cases, the combination of PAMW and the
gypsiferous material resulted in a significant reduction in
total sediment yield (table 4).
The G treatment had a significantly lower final sediment
yield rate than the control, while the rate for NA was not
significantly different from that for the control (table 5). The
Ca concentration for G was nearly constant throughout the
dissolution analysis. Results from the dissolution analysis,
along with results plotted in figure 3, where the fraction of
each treatment’s sediment reduction has been plotted versus
time, support the supposition that the Ca concentration for the
G treatment remained fairly constant through the experiment. The relative effectiveness of G, in terms of sediment
yield, is fairly uniform with time after 35 minutes (fig. 3).
This is also demonstrated in figure 2, where the lines for the
control and G treatments are nearly parallel.
The NA treatment behaved noticeably different from
other treatments in terms of relative effectiveness. Between
TRANSACTIONS OF THE ASAE
Control
G
PAMW+G
40
60
PAMD+G
NA
PAMW+NA
60
50
Runoff Rate (mm⋅hr –1 )
40
30
20
10
0
0
20
Time (min)
80
100
120
140
Figure 1. Average runoff rate (mm hr–1) versus time (min) for each treatment.
Table 5.Final average runoff rate, average sediment yield rate, and
respective percent reductions over the control. The same letter
following a treatment indicates no significant difference using
the least significant difference method at a = 0.05.
Final Runoff Rate
Final Sediment Yield Rate
Treatment
mm hr–1
% reduction
of control
kg m–2 hr–1
% reduction
of control
Control
Gyp
PAMW+G
PAMD+G
NA
PAMW+NA
44.4 ab
42.7 ab
40.7 b
45.5 ab
47.8 a
45.6 ab
3.8
8.4
–2.6
–7.7
–2.7
0.61 a
0.36 b
0.31 b
0.18 c
0.57 a
0.27 bc
40.2
49.8
71.1
6.5
55.0
20 and 45 minutes into the experiment, the NA treatment
realized its maximum relative effectiveness, after which the
effectiveness dropped off dramatically. From table 3 one can
see that the Ca concentration for the NA treatment remained
fairly constant through the duration of the dissolution
analysis, albeit substantially less than the G treatment. In
figure 2, the increase in sediment yield rate with time for the
NA treatment is approximately linear for the first 50 minutes
of the experiment, after which a steady–state sediment yield
rate, approximating the sediment yield rate of the control, is
attained. Thus, the greatest reduction in sediment yield for
the NA treatment as compared to the control occurred in the
in 20 to 45 minute range. Apparently, the contribution of Ca
from the NA treatment was minimal after the first 45 to
50 minutes of the experiment. All of the PAM treatments
mirrored each other in terms of relative effectiveness (fig. 3).
That is to say, regardless of absolute differences in sediment
yield, the PAM treatments behave similarly over time with
respect to sediment yield.
While the PAMW+G treatment provided significantly less
total runoff than did the PAMD+G treatment, there was no
Vol. 45(4): 1011–1019
statistical difference between the two treatments in terms of
overall sediment yield. Final sediment yield rates were
significantly less for the PAMD+G treatment than for
PAMW+G, although there was no difference in final runoff
rates. This result may be due to the migration of dry PAM
granules into pore spaces, due to either raindrop impact or the
application process. When the PAM particles first became
activated during wetting, they provided little benefit in terms
of infiltration compared to the control. The soil may become
adsorbed to the activated PAM granules, or the PAM may act
as a mortar to limit erosion (Peterson et al., 2001).
RUNOFF AND SEDIMENT YIELD RELATIONSHIP
The relationship between sediment yield rate and runoff
rate can be used as an indicator of soil erodibility (Huang and
Bradford, 1993). Soil erodibility is a measure of the increase
in sediment yield rate per unit increase in runoff rate. Linear
regression was used to determine the relationship between
sediment yield rate and runoff rate. It was common for runoff
to reach steady state within the first hour, at which time a
maximum sediment yield rate was measured. The sediment
yield rate would subsequently decrease while the runoff rate
would remain constant, an indication that the detachment of
soil was being limited.
Reductions in sediment yield due to the addition of
amendments can be attributed to a decrease in soil erodibility
or an increase in infiltration (i.e., reduction in shear by
flowing water). The definition of erodibility can vary slightly
depending on whether the rill or interill process is being
considered. Rill erodibility is a measure of the susceptibility
of soil to become detached by concentrated flow and can be
defined as the increase in soil detachment per unit increase
in shear stress of clear water (Flanagan and Nearing, 1995).
Interill erodibility is the rate at which sediment is delivered
to rills as a function of rainfall intensity and runoff rate
1015
Control
G
PAMW+G
PAMD+G
NA
PAMW+NA
0.9
0.8
Sediment Yield Rate (kg⋅m
–2
⋅hr –1 )
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
20
40
60
Time (min)
80
100
120
140
Figure 2. Average sediment yield rate (kg m–2 hr–1) versus time (min) for each treatment.
G
PAMW+G
PAMD+G
NA
PAMW+NA
14%
Percent Reduction in Total Sediment Yield
12%
10%
8%
6%
4%
2%
0%
0
20
40
60
–2%
80
100
120
140
Time (min)
Figure 3. Percent of average total reduction in sediment yield compared to control as a function of time.
(Flanagan and Nearing, 1995). What has been measured in
this study is sediment yield, not detachment, but sediment
yield is proportional to soil detachment under these experimental conditions. Shear stress is also proportional to flow
rate. Thus, an estimate of soil erodibility can be made by
regressing sediment yield rate on runoff rate.
A linear model best represented the relationship between
sediment yield rate and runoff rate for each treatment. Both
1016
wet PAM treatments and the PAMD+G treatment were
represented by a combination of two linear functions. The
control, G, and NA treatments were represented with a single
linear function. Huang and Bradford (1993) stated that, under
net detachment conditions, sediment yield rate (qs ) is a linear
function of runoff rate (qw ). Under depositional conditions,
the relationship can vary between linear and quadratic,
depending on the importance of the ratio of deposition rate
TRANSACTIONS OF THE ASAE
to erosion length scale. When deposition rate is great, qs
should be a quadratic function of qw (Huang and Bradford,
1993).
Figure 4 shows plots of sediment yield rate versus runoff
rate for each treatment, along with the accompanying best–fit
lines. The models for the best–fit lines and model coefficients
are provided in table 6. The sprayed PAM treatments can be
viewed as having two distinct linear segments: a lower region
where the amendments have reduced the erodibility of the
soil, and an upper region where the PAM has failed, resulting
in an increased erodibility that approaches that of the control.
1
0.8
0.6
(kg⋅m –2 ⋅hr –1)
Sediment Yield Rate
0.7
PAMW+G
0.8
PAMD+G
0.7
Linear (PAMW+G)
0.6
Linear (PAMD+G)
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0
0
10
20
30
40
50
60
(b)
0.9
0.5
0
0
10
20
30
40
50
60
Runoff Rate (mm⋅hr –1)
1
0.9
(c)
NA
PAMW+NA
Linear (NA)
Linear (PAMW+NA)
0.8
0.7
0.6
(kg⋅m –2 ⋅hr –1 )
Sediment Yield Rate
1
(a)
G
Control
Linear (G)
Linear (Control)
0.9
With the PAMW+G treatment (fig. 4b, table 6), the change
in slope from the lower to upper region does not appear to be
great; however, the difference between the two slopes was
significant at a = 0.05. The failure point is best exhibited in
figure 4c for the PAMW+NA treatment. The erodibility
(2.55 Ü 10–3 kg mm–1 m–2) was significantly less than the
control (2.02 Ü 10–2 kg mm–1 m–2) until failure of the PAM
occurred at a runoff rate of 37 mm hr–1. At this point, the
erodibility increased dramatically for the treated soil and was
not significantly different from the control.
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
40
50
60
Runoff Rate (mm⋅hr –1 )
Figure 4. Sediment yield rate versus runoff rate for each treatment.
Treatment
Table 6. Results of regression analysis on sediment yield rate (qs ) as a function of runoff rate (qw ) for each treatment.
a[a]
95%
95%
b
r2
(kg mm–1 m–2)
Upper Limit[b]
Lower Limit
(kg m–2 hr–1)
Control
Gyp
PAMW+G[c]
PAMD+G
NA
PAMW+NA
2.02E–02
1.69E–02
7.75E–03
1.37E–02
1.27E–02
2.91E–03
1.44E–02
2.55E–03
2.07E–02
2.20E–02
1.94E–02
8.76E–03
1.84E–02
1.50E–02
4.73E–03
1.59E–02
3.66E–03
2.50E–02
1.84E–02
1.44E–02
6.75E–03
8.91E–03
1.03E–02
1.08E–03
1.28E–02
1.43E–03
1.64E–02
–1.53E–01
–1.30E–01
–4.39E–02
–2.79E–01
–6.99E–02
1.94E–01
–7.49E–02
4.02E–03
–7.04E–01
0.963
0.901
0.843
0.573
0.937
0.417
0.929
0.468
0.674
[a] Model coefficients found by linear regression using the model: q = aq + b.
s
w
[b] Confidence intervals of a calculated using two–sided t interval with α = 0.05.
[c] For treatments with more than one model, the first row indicates the model fitting the lower region, while the second row represents the model fitting the
upper region.
Vol. 45(4): 1011–1019
1017
The reduction in erodibility attributed to the addition of
PAM is similar to that found by Chaudhari (1999), where the
reduction in erodibility over the control using a PAM and
gypsum treatment was 77% for a silt loam soil. This study
saw reductions of 61.6% and 87.4% for the PAMW+G and
PAMW+NA treatments, respectively. These values were
calculated based on the erodibility of the lower region, before
failure, as shown in figure 4. The erodibility reduction
suggests that PAM application may be beneficial in such
applications as temporary protection of soil in grassed
waterways or other applications where concentrated flow
may be present.
The PAMD+G treatment did not have a failure point for
the duration of these experiments, but it did have a point at
which it became effective. Figure 4b shows that the
erodibility was initially 1.27 Ü 10–2 kg mm–1 m–2, and at a
runoff rate of 28 mm hr–1 the erodibility decreased to 2.91 Ü
10–3 kg mm–1 m–2. The initial erodibility was not significantly different from that of the G treatment, while the erodibility
of the upper region was significantly less than the erodibility
of the lower region of the PAMW+G treatment. This supports
the earlier statement that the dry PAM was initially inactive
and after a sufficient amount of rainfall became active.
The results discussed heretofore have been based on the
2–hour duration of this experiment. Clearly one would reach
a different set of conclusions had the experiment been
conducted for only one hour. The total precipitation amount
over the course of the experiment was 140 mm, which is close
to the 12–hour, 100–year rainfall event for West Lafayette,
Indiana. The 70 mm of rain in the first hour represents the
1–hour, 100–year event for West Lafayette, Indiana. Based
on these experimental results, sprayed PAM application
would be expected to initially provide better erosion control
over a dry application. However, for longer–term erosion
control, or a longer duration precipitation event, a dry PAM
application may provide the same level of erosion control as
the sprayed application. If the goal were to protect the soil
surface from initial large runoff events prior to establishment
of permanent vegetative cover, then the liquid spray
application would be recommended.
SUMMARY AND CONCLUSIONS
A laboratory study using simulated rainfall was conducted
to compare the dry versus sprayed application of PAM and the
use of different gypsiferous material on runoff and sediment
yield. The six treatments incorporated in this study were:
control, Nutra–Ash (NA), gypsum (G), sprayed PAM plus
NA (PAMW+NA), sprayed PAM plus G (PAMW+G), and
granular PAM plus G (PAMD+G). Deionized water was
applied at a target intensity of 70 mm hr–1 for two hours to a
silty clay loam soil.
Only one of the two liquid PAM treatments significantly
reduced runoff (35%), while both liquid PAM treatments
significantly reduced sediment yield (74% and 77%)
compared to the control. Only the PAMW+G treatment
significantly reduced total runoff. Since the PAMW+G
treatment resulted in significantly less runoff than the
PAMW+NA treatment, it appears that the PAM was better
able to adsorb to the soil particles in the presence of G.
All amendment treatments significantly reduced total
sediment yield. Both gypsiferous material treatments signifi-
1018
cantly reduced total sediment yield but were not different
from each other. All PAM treatments significantly reduced
total sediment yield but were not different from each other.
Thus, the sprayed PAM application performed marginally
better compared to dry PAM application in terms of runoff
reduction but had no additional benefit with regard to total
sediment yield for the storm intensity and duration used in
this study.
Only the PAMW+G treatment significantly reduced final
runoff rate compared to the control. One explanation is that
after sufficient rainfall enough of the PAM–treated aggregates broke down, thus reducing the infiltration rate of the
soil.
All of the PAM treatments significantly reduced final
sediment yield rate. This suggests that enough of the
aggregates in those treatments were stable so as not to be
transported in runoff, or reduced runoff through increased
infiltration such that flow shear stress was reduced. There
were no statistical differences in total sediment yield between
PAMD+G and PAMW+G.
An analysis of erodibility indicates that the mechanism by
which each treatment reduces soil erosion is different. The
decision as to which application method to use should be
based on local climate conditions (i.e., expected amount and
severity of precipitation before vegetation establishment).
Gypsum was a better source of electrolyte than a class C
ponded fly ash, commercially known as Nutra–Ash (NA),
based upon its greater solubility.
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TRANSACTIONS OF THE ASAE