An Interim Review of The South Tobacco Creek’s Twin Watersheds Zero and Conventional Tillage Comparison

Bill Turner

The Deerwood Soil and Water Management Association (DSWMA) was established in 1985, by a group of local farmers in south central Manitoba, who were concerned with soil and water management. The initial area of concern was local and applied for the most part to the Manitoba Escarpment and the land area immediately below this erosive topography landscape.

In 1991 DSWMA formed an official agreement with the federal and provincial governments to look more directly at land and water management, applied conservation practices and their related environmental impacts. To achieve this, the activities were focused on the South Tobacco Creek watershed, the escarpment watershed area west of Miami, Manitoba.

The South Tobacco Creek watershed area of approximately 29 square miles has been extensively monitored for 10 years. This includes individual farmland use, water flow rates and quality parameters from Miami to the upper reaches of the watershed. As well, a Twin Watersheds study was established on two small (5.6 and 4.2 ha) adjacent drainage areas in the watershed. The objective of this Twin Watershed (TWS) study is to evaluate the differences between conventional and zero tillage.

The Twin Watersheds occurs within the Pembina Hills Upland, which is typically characterized by undulating to hummocky morainal landscapes with moderate to strong slopes and associated sloughs. The surface deposits consist mainly of glacial till derived from shale (Cretaceous), limestone (Paleozoic) and granitic (Precambrian) origin. They are moderately to strongly calcareous and have a dominant loam to clay loam texture. The thickness of till over the bedrock varies from 1 – 10 m. The soils at the Twin Watersheds on NE 29-4-7 w1 are classified as dominantly Orthic Dark Gray (Dezwood series). (Michalyna et al. 1988)

The Twin Watersheds (TWS) study was initially monitored and operated as conventional tillage for a period of 4 years, to characterize natural flow rates and water quality parameter differences between the 2 small watersheds. In the fall of 1996 the east watershed field was not tilled in preparation for spring initiation of zero-tillage. Since the inception of this study the monitored parameters include; runoff flow rates and water quality parameters during spring snowmelt and rainfall events, air temperature, rainfall amounts and intensity, snow trap and moisture equivalence, and all land use activities. To help improve the understanding of the water budget for the TWS, the installation of soil probes and manual sampling for soil moisture have recently been initiated. As well, an earthworm survey was completed.

The clay till at the TWS site has not preformed, as we would typically have expected a zero till field to respond on its 6^th year. For this reason the STC partners would like to continue the zero and conventional tillage comparison for several more years. For this reason the information presented must be taken as interim, and not as final conclusive information.

This South Central Escarpment portion of Manitoba as a general rule receives approximately 25 percent of its moisture from snowfall. The total annual precipitation varies from 498.6 mm (23.1 inches) to 498.9 mm (19.6 inches) below the escarpment. This creates different challenges in water management than you might expect to address in dryer south-western areas of Manitoba.

The initial 4 years (93-96) of comparing the 2 watersheds while both were under conventional till indicates pre-melt snow depth were quite similar. The west watershed having an easterly slope collected slightly more snow in snow bank depth.

The 6 year pre-melt snow average after the east watershed was converted to zero till, shows the standing stubble provided an increase in snow collection, surpassing the average for the conventional till.

To demonstrate the variability in available moisture from the snow pack and field moisture retention, a comparison is made between the zero and conventional tillage. 1999 is the 3^rd year of zero till.

The March 1999 pre-melt snow course survey (available moisture in m³/acre) shows the zero till having considerably more available moisture per acre. Although the moisture is available there was not a large difference in the retained moisture on the zero till field.

After completion of the 1999 March snowmelt and the field sites were clear of snow, an April 5 snowstorm occurred. As evident the snow was wet and heavy with very little drifting. This provided very similar potential moisture availability for both fields. This snowmelt resulted in a higher m³ /acre retained on the tilled field verses the zero till field.

The TWS-W (dots) is the permanent conventional till field. The TWS-E ( lines) is managed as conventional till in 1993 – 1996, with spring 1997 being the first year of zero till comparison.

To provide an overview of the interim snowmelt information, a graph was created from an interim linear progression analysis graph. In the pre-tillage change period at 10 to 20 mm of snowmelt runoff, both fields responded quite similar, with only a slight difference upwards to near 40 mm of runoff.

Looking at the post-tillage change period comparison of runoff, the standing stubble on no till has quite significantly more runoff than the conventional tillage in the lower runoff events. Where as in the higher snowmelt runoff events the 2 watersheds respond more similar.

To demonstrate the variability in rainfall runoff/retention, the 1999 and 2000 rainfall percent runoff results are presented.

The initial 1999 rainfall of 38 mm in May was the only rain event for the year in which the conventional till passed a higher percentage from the field. There were 2 July rain events, which produced low level runoff from the zero till field, while virtually no runoff occurred on the conventional till. The last July event saw approximately the same percentage of runoff from the 24 mm rainfall event. Due to the May 22 event the conventional till passed a much higher percent loss from the field during the rain season, although the zero till was more persistent in producing runoff.

The initial 2000 rainfall of 55 mm produced a minimal rainfall percent runoff from both fields. Aug 11 rainfall runoff was the only event in which the conventional till percent loss exceeded the zero till loss. The zero till total loss slightly exceeded the conventional till total. Similar to 1999 the zero till was more persistent in producing runoff.

The TWS-W (dots) is the permanent conventional till field. The TWS-E ( lines) is managed as conventional till in 1993 – 1996, with spring 1997 being the first year of zero till comparison.

To provide an overview of the interim rainfall runoff information, a graph was created from an interim linear progression analysis graph. In the pre-tillage change period of 1993 to 1996 the 2 watersheds responded quite similar when comparing mm of rainfall runoff.

During the post-tillage change 1997 to 2001, the Twin watersheds East (TWS-E (ZT)) while under zero till generated an increasing loss of rainfall runoff towards the higher runoff rates, in comparison to the Twin Watersheds West (TWS-W (CT)) under conventional till.

To further assist in the understanding of the Twin Watershed soil condition and its impact on hydrology an earthworm survey was taken in Oct. 2002. As well a nearby "Check Field" of zero and conventional till was also surveyed for comparison. Both zero till fields have been down for 6 years.

There were 2 recommended process for earthworm recovery followed. A comparison was made between an application of Formaldehyde solution and hand sorting.

Hand sorting produced 83 percent of the recovered worms. At 17 percent recovery the formaldehyde solution did not provide an accurate earthworm survey.

Twin Watersheds Survey Results	Check Field Results
Zero till field – 3 of 5 sites surveyed had earthworms, numbers varied between 8 per m²to 32 per m²at a depth of 15 cm (6 inches).	Zero till field – 3 of 3 sites surveyed had earthworms, numbers varied between 7 per m²to 21 per m² at a depth of 15 cm (6 inches).
Conventional till field – no earthworms were found at the 5 surveyed sites at a depth of 15 cm (6 inches).	Conventional till field – 3 of 3 sites surveyed had earthworms, numbers varied between 5 per m²to 19 per m² at a depth of 15 cm (6 inches).

Here the total annual sediment loading at the Twin Watersheds is expressed as kg per hector, comparing spring snowmelt (S) and rainfall ( R ) runoff loading.

The TWS-W (dots) is the permanent conventional till field. The TWS-E ( lines) is managed as conventional till in 1993 – 1996, with spring 1997 being the first year of zero till comparison. (Note: 1996 TWS-E spring snowmelt flow rates are questionable, snow depth was very similar to TWS-W.)

1993 was the only spring runoff producing majority of the annual sediment load, for the balance of years, rainfall runoff was responsible for majority of the sediment loss.

Note the low kg/ha. of sediment loading in 1997 snow melt runoff, as compared to the 5 year high for rainfall runoff for that year. Sediment is often assumed to be the major transporter of phosphorus (see phosphorus graph, 1997)

Here the total annual phosphorus loading at the Twin Watersheds is expressed as kg per hector, comparing spring snowmelt (S) and rainfall ( R ) runoff loading for each year.

The TWS-W ( dots) is the permanent conventional till field. The TWS-E (lines) was managed as conventional till in 1993 – 1996, with spring 1997 being the first year of zero till comparison. (Note: 1996 TWS-E spring snowmelt flow rates are questionable, snow depth very similar to TWS-W.)

Similar to1993 sediment loading, 1993 spring snowmelt runoff produced majority of the annual phosphorus loading, in comparison to the 1993 rainfall runoff. In 1994-1996 the difference between annual snowmelt and rainfall runoff phosphorus loading was somewhat similar. 1997 produced the highest spring melt phosphorus loss, exactly the reverse of that year’s sediment loss for snowmelt/rainfall comparison.

Annual sediment losses in kg per hector are compared between the Twin Watersheds and the Miami station at the bottom of the escarpment watershed near Miami Mb.

The TWS-W ( dots) is the permanent conventional till field; the TWS-E (lines) was managed as conventional till in 1993 – 1996, with spring 1997 being the first year of zero till comparison. The (diamond) graphs are the results from the 29 square mile watershed at the Miami Station.

The total sediment load expressed in kg per hector for each year shows the Twin Watersheds both responding quite similar, with the Twin Watersheds West (TWS-W) slightly higher most years. This was predicted by Michalyna et al. 1988, in part to the steeper slopes associated with this field. During the 1993 – 1996 period when both fields were managed under conventional till they produced a low of 34 to a high of 89 kg/ha. of sediment. Keeping in mind the total area of these fields was under conventional tillage practice. In 1997 TWS-W produced 113 kg/ha, a 109% increase over this field’s previous 4 -year average. And TWS-E being direct seeded into standing stubble produced 96 kg/ha, a 195% increase over this field’s previous 4 -year average.

The Miami station annual sediment loading varies significantly over this same 4 -year period at 64 – 399 kg/ha. The 1997 total kg/ha of sediment were 1196, a 380% increase over the previous 4 –year average. Also keeping in mind that the Miami watershed land use is approximately 71% under annual crop production, of which 11-15% is forage or pasture, 6% is non-rotated forage, 19% is natural tree cover, and 4% in creeks, ponds and road ways. It appears that a good portion of sediment loading at Miami may be contributed by other sources that solely land under annual crop production.

In reviewing the relationship between runoff water flow rate and sediment loading (non-filterable residue), the 2 main channel monitoring stations at Miami and Highway 240 (HWY 240), there appears to be a direct relationship. This is identified by the solid lines in the 2 graphs connecting the increasing sediment (non-filterable residue) in mg/liter, with the increasing flow rates.

Where as in comparison to the Twin Watersheds - West conventional till field the relationship between sediment loading and increasing flow rate does not appeared to be directly linked, as does the main channel. The sediment loading in the upper portions at field level is more directly linked to other factors than flow rate. It appears a significant portion of the sediment in the main channel at HWY 240 and Miami is stream bank erosion.