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SWEEP Report #69

Study Area Selection, Description and Climate
(PWS Report #1)

Researchers: 
R. R. Walker, Beak Consultants Ltd., Guelph, Ont., and J. Sadler Richards, Ecologistics Ltd., Waterloo, Ont.

Executive Summary

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Completed: September, 1994

Executive Summary

Objectives

The objectives of the study components documented in this report were as follows:
  • select three matched pairs of agricultural watersheds in the Lake Erie drainage basin suitable for conversion to conservation farming systems;
  • compile physical and social information on these watersheds to allow for agronomic and environmental analysis; and
  • monitor meteorology as a primary causative factor in crop performance and erosion - transport processes.

Climate

Installations

The three pairs of sub-watersheds were monitored in a similar manner. Each pair of sub-watersheds had one main weather station located in one of the sub-watersheds. The main station included automatic instrumentation for monitoring the following parameters at indicated frequencies:
  • rate of rain or snow (15 minutes);
  • wind speed (5 minutes);
  • air temperature (15 minutes);
  • solar radiation (15 minutes);
  • potential evaporation (daily); and
  • volume of rain (daily).

In addition to the main stations, each test sub-watershed outlet had secondary meteorological stations where the following measurements and measurement frequencies occurred:

  • rate of rain (hourly);
  • air temperature (hourly);
  • water temperature (hourly);
  • soil temperature at 3 depths (hourly); and
  • relative humidity (hourly) at Kettle Creek only.

In addition to these two stations, each pair of sub-watersheds had a set of three or four standard rain gauges spatially distributed to address the heterogeneity of precipitation.

All instruments functioned, with minor upsets, over the period January 1989 to June 1992.

Temperature

In general, all study areas experienced a cooler than normal 1989 and warmer than normal temperatures in 1990 and 1991. Most winter periods were warmer than normal resulting in frequent winter rains and minimum snowpack. Growing season air temperatures did not differ from the normal values significantly. In all, air temperatures were within 1 degree C of normals in all areas on an annual basis.

Temperature data were used to calculate Corn Heat Units (CHU) for all sub-watersheds. CHU were found to be below average in all study areas in 1989 and 1990 while 1991 values marginally exceeded the normals. Essex displayed the highest corn heat units, followed by Kettle Creek and then Pittock in all years. This was also the order of recorded total solar radiation, discussed below.

Soil temperatures at depths of 5, 50 and 150 cm were reported for each sub-watershed and compared to air temperature. The shallow 5 cm temperature probe was shown to clearly relate to air temperature as expected with a typical annual range encompassing 26 to 27 C. The deepest probe was below the frost line and displayed a typical annual range covering approximately 10 C.

Precipitation

Similar total precipitation trends were evident in each of the study areas. All areas had much lower than normal annual precipitation in 1989, 1991 and the first six months of 1992. All study areas had higher than normal precipitation in 1990.

In general, winter precipitation was lower than normal while growing season totals were near normal. In conjunction with warmer than normal temperatures, winter periods were generally characterized by minimal snow cover and frequent mid-winter thaws. This fact has implications in terms of winter erosion potential and spring runoff volumes discussed in (PWS) Reports #5 and #6 (SWEEP Report #73 and SWEEP Report #74).

Significant differences were noted on a storm-by-storm basis and in the long term between study areas and between single event rain volume measured at different locations within the same study area.

Extreme hourly and daily precipitation is reported for the study period. Maximum hourly and daily precipitation were 66.5 mm/hr and 95.6 mm/day, both in Kettle Creek.

The total number of significant precipitation events, exceeding 60 mm/day was one in each study area. The number of precipitation events exceeding 40 mm/day was 12 in total. The frequency of extreme high precipitation events and associated erosion-runoff events was low during the course of this study.

Wind

Wind speeds recorded during the study were slightly below normal in all areas. Wind speed affects the rate of potential evaporation and snowmelt. The lower than normal wind speeds have not likely been a significant factor in the study results.

Solar Radiation

Solar radiation has direct affects in terms of crop productivity, potential evaporation and rate of snowmelt.

Essex received the most solar radiation through the study period, Kettle Creek received the second most and Pittock receiving the least. This trend corresponds with differences in latitude and is expected. Total solar radiation received was near normal in all areas.

Watershed Selection

Methodology

The Pilot Watershed Study was designed to be conducted in the Lake Erie watershed within small sub-watersheds. The sub-watersheds were to be approximately 400 ha in size and representative of broader physiographic units. Agriculture within these sub-watersheds was to be carried out on a commercial scale and typify the predominant crops and livestock systems found in southwestern Ontario. It was particularly important that the selected sub-watersheds be representative of the landscapes that were contributing phosphorus to Lake Erie.

The five phases of the basin selection screening exercise are presented in Figure 2.1. Figure 2.1 illustrates the progression from a general to a more detailed analysis as the focus moved from large candidate watershed areas to specific sub-watersheds. Along the way, selection criteria and exclusionary factors were applied. Initially these were based primarily on physical attributes that were readily evaluated with published information applied at a smaller scale.

Each subsequent phase introduced a greater level of detail and expanded the range of criteria from the strictly physical watershed criteria to encompass socio-economic factors that had a bearing on the final selection. The mapping analysis was completed at a larger scale and the data inputs were drawn increasingly from primary data collection efforts. The final phase focused entirely on farm operator characteristics and relied, for these, on inputs from the local farm communities.

By the end of April, 1988 the watershed and the sub-watershed designations had been finalized. The final selections were
 

WATERSHED TEST CONTROL
Essex 5th Concession drain 2nd Concession drain
Kettle Creek Madter drain Holtby drain
Pittock Webber drain Goring drain
 

Conclusions

The virtue of the basin selection approach was that it eliminated most of the unsuitable watershed and sub-watershed areas at an early stage before detailed and costly data collection efforts were initiated. Sub-watersheds that were carried into the final screening phases generally satisfied the physical criteria and further distinctions among them could be made based on farm operator attributes.

It was not enough to assure physical homogeneity across the control and test study areas as would be required under a typical experimental situation. The human dimension was of paramount importance, for without the cooperation of farm operators in each study the PWS would not be feasible.

The screening process had to locate paired sub-watersheds that exhibited physical similarity, that were representative of major agricultural regions in southwestern Ontario and that were farmed by individuals who, for the most part, were likely to cooperate with the study team. These demanding requirements necessitated an innovative methodology that enabled the screening of a very large number of candidate areas while still allowing for a degree of site specific investigation that would enable an assessment of local farm communities. The phased approach to screening fulfilled these requirements while making best use of available study resources.

The three selected Pilot Watershed Study areas represented typical farming communities in southwestern Ontario. Cash cropping and livestock operations predominated. Interest in the Study was high in all areas, with only two out of 78 farmers declining to participate in any way. Similar numbers of farmers operated in each area and the majority of farms were larger commercial operations. Cultural practices were conventional, though experimentation with conservation practices had occurred in all the sub-watersheds.

From a socio-economic perspective, the three pairs of sub-watersheds fulfilled project requirements in that they contained commercial farm operators who were familiar with conservation issues, but were generally committed to conventional farming methods at the time of the selection. This enabled a more critical examination of a pro-active approach to encourage the use of soil conservation practices.

From a physical standpoint, the sub-watersheds are considered to be as closely paired as could be expected given the various constraints and objectives that were in effect during the selection process. Differences among sub-watershed parts in shape, size and soils caused somewhat similar hydrologic responses. These differences necessitated a more critical approach in the analysis of monitoring data and encouraged greater reliance on field scale investigations and modelling analysis to help interpret monitoring results from the watershed outlets. 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.

Parameter Concentrations

Microbasin Scale
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 particulary 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).

 

 

 

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Created: 05-28-1996
Last Revised: Thursday, May 19, 2011 07:21:35 PM