Data on rainfall and runoff were collected from field edges and in streams in each of the three subwatersheds in the Root River watershed from 2010-2015 under the Field to Stream Partnership.

The partnership, led by Minnesota Department of Agriculture staff in Preston, finished phase one of its two-phase program. Phase one was data collection. Here are some of the numbers gathered during that time.

Rainfall and runoff

Diverse landscape. Three small subwatersheds were selected for the Root River Field to Stream Partnership porject: the south branch of the Root River headwaters, located in glacial till in the western portion of the watershed; Crystal Creek, dominated by rolling hills and karst features; and Bridge Creek, consisting of bluff land in the east region of the watershed.

Rain in the region averaged 2% above the 30-year normal with 36.2 inches of rain. The study had a good mix of dry, normal and wet years. 2013 was the wettest year with 47.6 inches of rain. Field runoff was six times higher in 2013, too, mainly due to high rainfall hitting early in the growing season.

On average, 7% of the annual precipitation ran off the study fields. Most rain soaked into the ground, was used by the crop or evaporated into the air. The amount was highly variable by site and year. For instance, in 2013, 15% to 23% of the precipitation received ran off the fields, while in 2012, the amount was less than 2%. Nearly 50% of the annual water volume occurred when the soil was frozen, typically in March.

More than 90% of the annual runoff water and associated sediments/nutrient losses occurred during just four months: March, April, May and June.

Field sediment loss

Average sediment loss was 1,288 pounds per acre with loss ranging from 2 pounds to 8,457 pounds per acre per year. Eighty-seven percent of annual field loss occurred during certain storms in May and June. Losing soil at a rate of more than 1,000 pounds per acre is faster than can be naturally replenished, says Kevin Kuehner, Minnesota Department of Agriculture soil scientist. Keeping that number closer to 500 to 1,000 pounds per acre would be a good goal, especially if phosphorus is a concern for local receiving waters.

Field phosphorus loss

The average total P loss was 1.4 pounds per acre. Sixteen percent of P was lost in its dissolved form (not attached to sediment). Nearly 50% of the dissolved loss occurred when the ground was frozen.

Seventy percent P loss occurred in May and June and was strongly correlated with sediment loss. With good soil conservation practices, P losses could be held at or below one pounds per acre, Kuehner says.

Field nitrogen loss

The average total nitrogen loss was 10 pounds per acre in field surface runoff. When substantial soil loss occurred, total N exceed 34 pounds per acre at one site in 2013.

However, most nitrogen was not detected in surface runoff, but detected as nitrate-nitrogen in tile drainage, springs, streams, river and groundwater, averaging 29 pounds per acre from a tile drained field in Mower County and as high as 41 pounds per acre during a wet year.

Stream/watershed loss

Annual nitrate-N losses measured in the stream ranged from nearly 50 pounds in the headwaters to less than five pounds per acre in Bridge Creek. Geology and land use explain some of the large differences. Statewide watershed goals are to cut total nitrogen loading by 20% by 2025 and have no more than 10% of the groundwater wells test about 10 mg/L of nitrate-N.

Stream sediment and P loss averaged about 40% of that observed from field edges. This indicates existing downstream practices, such as grass waterways, sediment basins and other natural features, are likely trapping these materials, Kuehner adds.

During the six-year study, it was found that most soil loss occurred during a few days in May and June of each year with only one or two years accounting for the vast majority of the loss. For instance, in the headwaters and Bridge Creek watersheds, up to70% of the six-year sediment loss occurred during a very wet 39-day period in May and June 2013. By targeting critical source areas in these watersheds the study will determine how much lower losses can be cut, Kuehner says.

Crystal Creek watershed had the lowest soil loss over the six-year monitoring period. That could be attributed to the large amount of existing practices and one large sediment retention structure installed in the 1950s that treats more than 50% of the watershed land area, he adds.