The overall purpose of
the water quality component of PWS was to
define water quality concentrations and mass loadings between
conventional and conservation farm systems. The specific objective
relating to the water quality evaluation component of the PWS are:
through water quality
monitoring, quantify loading rates of phosphorus and soil loss at
three scales (plot, microbasin and watershed) in the short-term
(single rain event) and long-term (seasonally and annually); and
quantify the benefits
to receiving waters of conservation system adoption at the watershed
Surface Water Quality
microbasins were instrumented with flow monitoring and water quality
sampling devices to facilitate the accurate estimation of mass loadings
of water quality parameters. In-stream water control devices (v-notch
and rectangular weirs) were installed to better define low flow
estimates. Water quality shelters were installed at each watershed
outlet (two per study area, six in total) to house automated water level
monitoring, meteorological, and water quality sampling equipment.
Continuous water level data was used in conjunction with flow velocity
determination to derive flow-discharge curves for each watershed outlet.
The resultant continuous flow record was ultimately used for calculating
relationships between flow and water quality concentrations for water
quality loading determination.
A total of twelve
microbasins (four in each of Essex, Kettle Creek and Pittock Watersheds)
were instrumented with automated water level monitoring equipment and
manual water quality sampling which were operated during non-winter
months. Hydrologic control structures (v-notch weirs and Parshall flume)
were installed at each microbasin along with a stilling well for water
level-flow estimation purposes. Water quality samples were collected
during rainfall events.
techniques were employed to evaluate water quality at the plot scale at
critical times of the year when soil conditions may vary due to farm
management or seasonal influences (ie. post-fall tillage 1990-91; spring
pre-plant 1991-92; and post harvest 1991). Water quality samples were
collected in triplicate from three field plots at benchmark sites in
both the test and control watersheds to determine water quality loads.
One pathway for
phosphorus loss is through subsurface soils and groundwater transport.
To define the magnitude of this pathway, groundwater monitoring wells
were installed in each watershed to determine the amount of phosphorus
was transported through the subsurface in the soluble phase. One
groundwater monitoring well was installed near each microbasin of each
study area. Groundwater samples were collected approximately monthly
through the monitoring phase of the study.
Water Quality Loading
Water quality loads were
determined from continuous water level/flow data and discrete water
quality sample concentrations. A least squares technique was employed to
determine the best fit method or relationship between water quality
parameters and streamflow. From the least squares best fit equations,
continuous water quality loading estimates were determined for all
stations throughout the study period. Results
In-stream total suspended
solids (TSS) and total phosphorus (TP) concentrations at the six
watershed outlets are quite variable both temporally and with respect to
location (i.e. test or control sub-watershed). TP concentration at the
watershed outlets follows similar patterns to TSS, since a large
proportion of the total phosphorous content is sediment bound. In the
Essex watershed, the test sub-watershed exhibits higher TSS
concentrations than the control throughout the study period except for
the first six months of 1992. In general however, TSS concentrations
consistently decreased in the test sub-watershed over the time
conservation practices had been phased in. The control sub-watershed did
not exhibit this trend. In the Kettle Creek watershed, TSS
concentrations were again higher in the test than in the control except
in 1991 when average annual TSS concentration was higher in the control.
Neither the test nor the control sub-watershed exhibited a consistent
increase or decrease in TSS over the study period. TSS concentrations at
the Pittock watershed were lower at the test outlet than at the control
except for during the first six months of 1992, when both the test and
control TSS concentrations were very low. Overall, the lower Pittock
test TSS concentrations are positive evidence of the potential water
quality enhancement of conservation practices.
As with concentrations of
water quality parameter concentrations, loads are quite variable in
time, especially in the fall season, and with respect to location. In
the Essex watershed, annual unit area TSS loads where similar for the
test and control sub-watersheds in every year of the study. In Kettle
Creek, the test sub-watershed outlet had higher unit area TSS loads than
the control except during 1991 - a very dry year. A partial explanation
for low Kettle Creek control TSS loads is the sediment trap in the low
lying, wetland area situated in the control sub-watershed. In the
Pittock watershed, TSS loads were not consistently higher in either of
the sub-watersheds and therefore inconclusive.
Loadings of TP were
similar in pattern to TSS loads, since a large portion of the TP is
delivered in a sediment bound form.
General trends between
water quality and conservation farm benefits were more apparent at the
microbasin scale as confounding spatial factors tended to decrease with
a corresponding decrease in size or scale of monitoring.
In Essex, TSS
concentrations consistently decreased in the test microbasins while
showing no trends in the control microbasins. The same decreasing trend
was observed for one of the test microbasins in Kettle Creek while the
other microbasins (test and control) was inconsistent. This decrease in
TSS in test microbasins is evidence of potential positive effects of
land conservation practices on water quality. The lack of flow events in
the Pittock control microbasins makes comparisons less conclusive.
Microbasin TP concentrations follow a very similar pattern to TSS
Overall, unit area
microbasin loads show no distinct trends with respect to time or
location (test or control). Again, the rarity of significant microbasin
flow events makes it difficult to make firm conclusions and underscores
the need for further study.
Correlation analysis of
lumped microbasin loading data did however, provide for a comparison of
test and control loads for both Essex and Kettle Creek watersheds. In
both cases, TSS and TP loads were higher in the test at low flows but
were higher in the control at moderate to high flows. This indicates a
possible advantage to land conservation practices in the enhancement of
water quality during critical large rainfall/flow events when a very
large proportion of the total annual soil loss usually occurs.
The rainfall simulation
component of the PWS water quality evaluation produced the most obvious
and predictable results. This is due to the controlled nature of these
tests in terms of area, land management, rainfall intensity and time,
and precise measurement of runoff, soil moisture, slope and residue.
Test plots had
significantly lower loads of TSS and TP, often due to lower TSS and TP
concentrations in test plot runoff particularly in Essex. This was
particularly true for the Pre-Plant period which generated the highest
unit area loads by far and for the Post-Tillage period which generally
produced the next highest loads of the three time periods examined.
The Post-Tillage and
Pre-Plant periods bracket a significant period of the year usually from
early fall to early summer and represent an erosion prone period with
minimal live plant cover. Results show that during this period
conservation tillage reduces TSS and TP loadings.
The watershed scale
proved to be the most complex in terms of inherent variability, of
climatic, soil, runoff and erosion process factors. However, even with
the inherent variability at this monitoring scale, at four of the six
subwatersheds there was an indication that an increase in percent
cover decreased water quality loads.
The positive effects of
conservation practices in the test watersheds were most evident during
the November through April period when the percent cover was much
higher than the control watersheds.
In Essex test
sub-watershed, average and median yearly TSS concentration decreased
consistently since the onset of the project. In contrast, Essex
control sub-watershed average and median concentrations were variable.
Other watersheds did not show this trend but may with time.
Dry weather of the
study period has not provided for a comparison of test versus control
during years with higher than normal precipitation.
During high flows (in
the November to April periods) the high percent cover in the test
microbasins of Essex and Kettle Creek proved to be effective in
reducing TSS loads when compared to control microbasins (low percent
loads were very low in both test and control. Due to the lack of
moderate to high flow events, conclusions could not be drawn from the
resulted in significantly reduced loadings of TSS and TP in general
with some exceptions. These exceptions include Post-Tillage periods
wherein some cases, test plots produced significantly more runoff due
to soil surface differences between test and control plots.
produced the highest loadings in all areas due to much higher TSS and
TP concentrations and generally higher runoff volumes. Post-Harvest
measurements are of little value since in conventional systems the
condition is short lived and crop residue levels are similar in test
and control plots.
Crop live and dead
cover is an effective level of protection against interrill (sheet)
erosion as determined in intense simulated rainfall measurements.
measurements are an effective means of evaluating conservation farming
systems and identifying the effect of various independent factors
(i.e. cover, soil moisture, tillage practices).