This the third post in the Gorans Blog Series about the Gorans Discovery Farm Project. You can read the previous posts here.
How much water is running off the landscape?
A watershed is an area of land that drains to a given lake, river, stream, or wetland. The Lake Wakanda watershed includes the Kandiyohi, Willmar, and Peach Creek sub-watersheds.
We wanted to compare two of the drainage areas in the Lake Wakanda Watershed, the section that drains from around the City of Willmar and the section that drains from around our test field plots. To do so the precipitation and runoff/drainage had to be measured at the two sites. From the City of Willmar we were measuring surface runoff. From the fields, we were measuring drainage from the tile system.
Precipitation was measured daily using rain gauges at both the Fertilized Field plot and at the City Stormwater site. The average daily precipitation for each site was 0.12 inches. The two sites were not significantly different, so we knew any difference in the city runoff and agricultural drainage was not because of the amount of rainfall.
Precipitation (inches) from 1 April to 31 October for the City Stormwater and Fertilized Field.
|Site||2007||2008||2009||2010||2011||2012||2013||Average 2007-2013||30-Year Normal*|
|*Long-term normal is from 1981 to 2010 (National Oceanic and Atmospheric Administration).|
To measure the total amount of runoff and drainage we used several methods at each site. By using multiple methods, we could check the results against each other to be as accurate as possible. Multiple flow sensors (Doppler radar probes) measured the speed and depth of the water at both sites. We also used paddle wheels and bubblers/pressure transducers to verify the flow volumes. To compare the flow volumes, we took the total runoff or drainage from the site and divided that by the total acreage of the site, giving us the average daily flow depth. This allowed us to compare the two sites, despite the difference in size. The cumulative flow depth is the average daily flow depth multiplied by the total number of days.
Total precipitation and cumulative flow depth from April to October of 2007 to 2012 (excluding 2008) for the City Stormwater site and Fertilized Field.
|Description||Total Precipitation (inches)||Cumulative Flow Depth (inches)||Percent of Precipitation Resulting in Runoff/Drainage*|
|*This is a statistically significant difference|
In the City of Willmar, 94.9 inches of rain fell during 5 growing seasons, resulting in 43.7 inches of runoff (cumulative flow depth). That means 46% of the total rainfall in Willmar found its way to the City Stormwater monitoring site (County Ditch 23).
In comparison, 97.1 inches of rain fell on the Fertilized Field plot during 5 growing seasons, resulting in 26.6 inches of drainage water. For the field site, 30.5% of the total rainfall ended up in the tile drains and was pumped into a wetland bordering Lake Wakanda.
This showed us a higher, total percentage of the precipitation becomes runoff in the City of Willmar. This significantly higher amount can be attributed to the fact that more of the Willmar watershed is covered by impervious surfaces, such as streets, sidewalks, buildings, and parking lots (an estimated 54%). In contrast, the agricultural fields allow more water to soak into the soil and be used by the plants.
Comparing the Fertilized and Unfertilized Fields
Besides comparing the Fertilized Field to the City Stormwater site, we also wanted to compare the field sites to each other. We found the Fertilized and Unfertilized Fields had similar amounts of drainage (cumulative flow depth) due to the similar topography, drainage systems, and soil types.
In our next blog post, we will compare crop yields in the Fertilized and Unfertilized Fields and eventually how that impacts water quality.
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