Ontario - Canada Logos
SWEEP Banner

SWEEP Report #74

Evaluation of Conservation Systems: Water Quality
(PWS Report #6)

Researchers: 

Beak Consultants Ltd., Guelph, Ont.

  View / Download Final Report  [1549 KB pdf] (Appendices C, D, & E included)

 

 

List By Number | List By Sub-Program

Completed: September, 1994

Executive Summary

Objectives

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 scale.

Methods

Surface Water Quality Monitoring

Watershed Scale

Watersheds and 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.

Microbasin Scale

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.

Plot Scale

Rainfall simulation 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.

Groundwater Monitoring

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 Estimation

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

 

Parameter Concentrations

Watershed Scale

Parameter Concentrations

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.

Parameter Loads

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.

Microbasin Scale

Parameter Concentrations

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 concentrations.

Parameter Loads

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.

Plot Scale

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.

 

Conclusions

Watershed Scale

  • 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.

Microbasin Scale

  • 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 cover).

  • Pittock microbasin 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 Pittock microbasins.

Plot Scale

  • Conservation tillage 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.

  • Pre-Plant periods 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.

  • Rainfall simulation 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).

 

 

 

List By Number | List By Sub-Program | LSP Report List
SWEEP Home

 

Created: 05-28-1996
Last Revised: Thursday, May 19, 2011 07:46:27 PM