Contact Person : Dr. D. J. Mulla
Producers from the Minnesota River Agricultural Team (MNRAT),
an advisory task force created by the Minnesota Department of
Agriculture (MDA), have been asking whether or not the improved
land practices adopted by producers are improving the quality
of water in the Minnesota River. The answer to this question has
been difficult to provide because of the difficulty in separating
climatic effects on erosion and flow from management effects.
Results from this study indicate that producers have reduced the
amount of sediment and phosphorus entering the Minnesota River.
Significant reductions in sediment and phosphorus loads were observed
during the month of June from 1974-1994 after compensating for
seasonal and annual variations in climate. The magnitudes of the
reduction at the Minnesota River near St. Peter were an annual
decrease of 2666 metric tons in total suspended solids (2.9%/yr
reduction) and 3 metric tons of total phosphorus (1.5%/yr reduction).
The majority of these reductions at St. Peter could be attributed
to reductions in loading of total suspended solids and total phosphorus
originating in the Blue Earth and Le Sueur watersheds. The increased
adoption of conservation tillage practices in the Blue Earth and
Le Sueur watersheds in comparison to adoption rates in other watersheds
is at least partly responsible for reductions in sediment and
phosphorus loadings.
Methods
The study analyzed monthly average total suspended solid (TSS)
and total phosphorus (TP) loads during the months of March through
August from 1974-1994 at the mouths of the Pomme de Terre, Chippewa,
Yellow Medicine, Redwood, Cottonwood, Watonwan, Blue Earth (below
its confluence with the Le Sueur and Watonwan rivers), and the
Minnesota River at St. Peter (Fig. 1).
Data for the study were obtained from the Minnesota Pollution
Control Agency, the U.S. Geological Survey, and the Metropolitan
Waste Control Commission.
The first step in the analysis was to separate the effects of
climatic fluctuations on sediment or phosphorus load. The Palmer Drought Severity Index (PDSI) was
used as an indicator for changes in the water budget. PDSI values
are widely used by climatologists to classify monthly climatic
records according to the extent of moisture deficiency or excess
due to patterns in precipitation, evapotranspiration, recharge,
and runoff. PDSI values greater than 3 indicate very wet months,
while PDSI values less than -3 indicate very dry months.
PDSI values were found to be correlated with monthly flow, and sediment or phosphorus load. Fig. 2 shows the relationship between PDSI, flow, and phosphorus load for the month of May between 1974 and 1994 at the sampling site located in the Blue Earth River near Mankato. It is apparent from Fig. 2 that the 80's and 90's have wetter climates than the late 60's and 70's. Linear regression between the PDSI and the logarithms of sediment or phosphorus load was used to model the effects of climate on sediment or phosphorus load.
The PDSI regression model for sediment or phosphorus load was
subtracted from the monthly values of sediment or phosphorus load.
The residuals from the model are the amounts of sediment or phosphorus
load attributable mainly to non-climatic effects. Trends in these
residuals were evaluated using a Mann-Kendall Tau statistical
test, which is widely used by hydrologists to detect trends in
long-term water quality monitoring data.
Total Suspended Solids
Trend analysis of raw monthly data before removing climatic effects
showed significant increases in TSS loading for all study sites
from March to August. Typical of these are results for June, shown
in Table 1 and Fig. 3. Annual
increases during June ranged from 3,086 metric t/yr in the Minnesota
River at St. Peter (3.3%/yr of median load) to 6 t/yr in the Pomme
de Terre watershed (1.3%/yr increase in median load). Particularly
large annual increases in load (between 8.1 and 9.7%/yr) were
observed for tributaries draining the Coteau region, namely; the
Yellow Medicine, Redwood, and Cottonwood watersheds.
Table 1. TSS Load and Trends in the Minnesota River and its
Tributaries (June 1974-94).
* Monitored downstream of confluence between the Blue Earth, Watonwan, and Le Sueur rivers.
+ Monitored near St. Peter.
After removing climatic effects (Fig. 4),
trend analysis showed generally small annual changes in all months
except June. The differences in annual change before and after
removing climatic effects were striking (Table 1, Fig. 3
and Fig. 4). Most of the apparent
increases in loading which have occurred since 1974 can be explained
by increasingly wetter climatic patterns in the last decade.
During June, trend analysis after removal of climatic effects
showed annual increases in TSS loading of from 1.3 to 3.0% for
the Chippewa, Yellow Medicine, Redwood, and Cottonwood watersheds.
Small decreases in TSS loading were observed in the Pomme de Terre
and Watonwan watersheds. Substantial annual decreases in TSS loading
(-4.6%/yr) were observed in the Blue Earth watershed downstream
of its confluence with the Le Sueur and Watonwan rivers. Since
the decreases for the Watonwan watershed were not as great, it
appears that most of the reductions in TSS loading from the greater
Blue Earth watershed are occurring in the upper Blue Earth and
Le Sueur watersheds.
After removal of climatic effects, significant reductions in TSS
load were observed in the Minnesota River near St. Peter (Table
1, Fig. 4). The annual magnitude
of these reductions was 2,666 t/yr, which is a reduction of roughly
2.9%/yr relative to the median load of the Minnesota River in
June. The reduction in the Minnesota River is slightly greater
than the annual reduction which occurs in the Blue Earth and Le
Sueur watersheds (2,215 t/yr). Thus, it appears that the bulk
of the improvement in the Minnesota River can be attributed to
reductions in TSS originating in the Blue Earth and Le Sueur watersheds.
To understand why the reductions in TSS loading occur mainly in
the Blue Earth and Le Sueur watersheds, it is instructive to review
the results of the 1995 crop residue transect survey conducted
by the NRCS. This survey includes data for the percent of cropped
land in each major watershed of the Minnesota River basin having
more than 15% of the soil surface covered by crop residue after
soybeans (Table 2).
Table 2. Percent of land with corn planted after
soybeans having greater than 15% crop residue.
Results from the 1995 crop residue survey show that the watersheds
with the greatest percentage of land managed by conservation tillage
are the Le Sueur, Watonwan, Pomme de Terre, and Blue Earth watersheds.
From Table 1, these are also the watersheds showing reductions
in loading of TSS after removal of climatic effects. Thus, it
is reasonable to conclude that reductions in loading of TSS are
partly attributable to increased adoption of conservation tillage.
Other improved land management practices have been implemented
over the last 20 years, including early spring planting of crop
and enrollment of land in the Conservation Reserve Program (CRP).
Total Phosphorus
Results of the trend analysis for total phosphorus are quite consistent
with the results for total suspended solids. Trend analysis of
raw data prior to removal of climatic effects (Table 3, Fig. 3)
showed significant increases in phosphorus loading for all months.
The largest percentage annual increases in raw data occurred for
the Yellow Medicine, Cottonwood, and Watonwan watersheds. After
removal of climatic effects, trend analysis showed significant
reductions for phosphorus loading only during the month of June
(Table 3, Fig. 4).
Table 3: TP Loads and Trends in the Minnesota
River and its Tributaries (June 1974-94).
*Monitored downstream of confluence between the Blue Earth, Watonwan, and Le Sueur rivers.
+ Monitored near St. Peter.
After removing climatic effects, trend analysis for the month
of June showed annual reductions in total phosphorus loadings
of 3.0 metric tons in the Minnesota River at St. Peter, an annual
reduction of 1.5%. Reductions in June were also observed for the
greater Blue Earth watershed and the Pomme de Terre watershed.
The reductions from the greater Blue Earth watershed were 3.7
t/yr, which is an annual decrease of 3.8%. Because the loadings
of total phosphorus from the Watonwan watershed did not change
significantly over time, the reductions in phosphorus loading
from the greater Blue Earth watershed can be attributed to changes
occurring in the upper Blue Earth and Le Sueur watersheds. As
noted in Table 2, these changes include increased adoption of
conservation tillage, which is probably partly responsible for
reductions in phosphorus loading from the Blue Earth and Le Sueur
watersheds.
Summary
An increasingly wetter climate in the last few decades has masked
reductions in delivery of sediment and phosphorus to the Minnesota
River due to improved crop and land management practices. While
the wetter climate has actually increased sediment and phosphorus
loads due to more frequent erosion-causing rainstorm events, producers
have adopted better crop and land management practices that reduce
erosion. These practices include earlier crop planting dates,
conservation tillage, and cropland retirement (CRP). The effectiveness
of erosion control practices in reducing sediment and phosphorus
loads in the Minnesota River appears to be greatest during June,
when both crop canopy growth and conservation practices provide
dual protection for the soil surface.
After compensating for climatic effects, significant reductions
in sediment and phosphorus loads were observed in the Minnesota
River only during June. The main contributors to these reductions
were the upper Blue Earth and Le Sueur watersheds. Most other
watersheds showed small increases in phosphorus and sediment loading
during June after removal of climatic effects. In most cases,
these increases were in watersheds having low rates of adoption
for conservation tillage.
The results of this study show the importance of promoting adoption
of conservation tillage. There is a great variation in the adoption
rates for improved soil conservation practices throughout the
Minnesota River basin. For instance, in 1995 the percent of county
acres with corn planted into 15% surface residue cover ranges
from 6 to 51% with an average of 28%. Clearly, increased adoption
of soil conservation practices can further accelerate the reduction
in sediment loads to the Minnesota River. Efforts to accelerate
the adoption of conservation practices should focus first on the
most erodible land (Fig. 5) and
the steep lands near rivers (Fig. 6).
Our previous report on pollutant loadings from major watersheds of the Minnesota River basin noted that two-thirds of the sediment and phosphorus in the Minnesota River at Fort Snelling was produced in the Lower Minnesota, Le Sueur, and upper Blue Earth watersheds. Based on these findings, and the significant reductions in sediment and phosphorus loadings in the Le Sueur and upper Blue Earth watersheds, we recommend focussing greater attention on reducing sediment and phosphorus loads from the Lower Minnesota watershed, while continuing efforts to reduce loads from the Blue Earth and Le Sueur watersheds.
