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SWEEP Report #73

Evaluation of Conservation Systems: Hydrology
(PWS Report #5)

Researchers: 

R. R. Walker and R. W. Tossell, Beak Consultants Ltd, Guelph, Ont.

View / Download Final Report  [2525 KB pdf]  (265 p, with appendices)

 

 

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Completed: May, 1994

Executive Summary

Objectives

The purpose of the hydrology component of PWS was to monitor runoff and stream flow to determine if the implementation of conservation farm systems had an affect on the overall basin hydrology. The specific objectives relating to the hydrology component of the PWS are:

  • through environmental monitoring, establish continuous records of watershed water balance in order to investigate hydrologic process;

  • determine the flow component of runoff necessary for estimating mass flux of water quality parameters; and

  • investigate the relationships between soil conservation systems and hydrologic response at the plot, field and watershed scales.

Methods

Water Quantity 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 per study area) 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.

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.

 

Results and Discussion

Watershed Scale

Runoff
  • Essex control unit area runoff slightly exceeded test runoff over the period of study.

  • Kettle Creek test unit area runoff exceeded control runoff during the wettest seasons.

  • Pittock test unit area runoff was significantly greater (nearly 50%) than that of the control throughout the study period.

Hydraulic Yield
  • Essex and Kettle Creek test and control watersheds compare closely with respect to hydraulic yield throughout the period of study.

  • Pittock test had generally higher hydraulic yields than control.

Microbasin Scale

Runoff
  • Frequency and extent of microbasin flow was quite variable in all microbasins.

  • Generally control microbasins produced less runoff than test microbasins, especially during periods of tillage.

Plot Scale

Rainfall Simulation Results
  • Time to ponding (TTP) values, indicating soil porosity, showed no distinct trends when comparing test and control sites, possibly due to spatial variability of infiltration rates of soils.

  • Time to runoff (TTR) values, reflecting soil storage capacity, were not significantly different for test or control sites, again possibly due to high spatial variability of soil properties .

  • Total runoff volume was generally higher for test sites than for control.

  • Runoff volume was lowest for control sites following tillage, likely due to increased surface roughness.

  • The difference between test and control runoff volume is most pronounced during pre-plant and post-tillage, when the difference in surface cover is most pronounced.

 

Conclusions

The following are general conclusions drawn from the PWS concerning hydrology.

  • At the watershed scale, the effects of conservation tillage on runoff volume are site specific:

  • Essex: runoff is higher from the test watershed due to higher resultant cover in the last 2 years of the study.

  • Kettle Creek: runoff is higher from the test watershed during the periods of high annual rainfall.

  • Pittock: the test watershed produced significantly higher unit area runoff volumes throughout most of the study period.

  • At the microbasin (field) scale, there is no clear pattern of dependency between unit area runoff and conservation farm management in Kettle or Pittock. In Essex, microbasins generally indicate that increased cover resulting from conservation systems correlates with increased runoff.

  • At the plot scale, responses of runoff to the presence of conservation tillage increased cover corresponds to increased runoff in Kettle and Pittock, but in Essex the reverse is true.

  • During rainfall simulation, test runoff exceeded control when all data were analyzed together.

  • During rainfall simulation, control runoff was lowest during the post-tillage period, most likely due to high surface storage.

  • More replication is recommended for plot scale studies, such as rainfall simulation experiments, to reduce the effects of spatial heterogeneity of soil characteristics.

  • At the microbasin scale, greater duration of study is required so that more events are available to be used in time trend analysis on individual microbasins. Currently, direct comparisons between different microbasins are confounded by differences in physical and agronomic factors, despite all efforts to avoid such differences when choosing microbasins.

  • At the watershed scale, extension of the duration of study to create a larger database would help to more clearly define the effects of tillage and residue on runoff.

 

 

 

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Created: 05-28-1996
Last Revised: Thursday, May 19, 2011 07:37:36 PM