Annual and Fall Precipitation are a Good Predictors of Tile Drainage from the Edge-of-Field

Discovery Farms collect water quality information from the edge-of-field monitoring sites.  These monitoring sites are situated at the edge of agricultural fields and collect water from areas ranging from 5 to 600 acres.  There are two types of runoff collected; surface runoff and tile drainage.  Surface runoff is overland flow originating from snowmelt or rainfall.  Tile drainage is water that infiltrates through the soil profile and moves into subsurface drains that remove excess water from agricultural fields.

The last two blogs (here & here) discussed the role of annual precipitation in surface runoff; documenting that the amount of annual precipitation is a poor predictor for surface runoff.  This was due mostly to the influence of precipitation timing and intensity.  This blog will discuss the role of annual precipitation in tile drainage.  Figure 1 displays tile drainage data from all Discovery Farms Minnesota sites from 2011-2015, including 8 different sites ranging from Southeastern to Northwestern Minnesota.  There is a significant and positive correlation between annual precipitation and tile drainage but, there is still some scatter in the dataset.  There are many more factors than just precipitation that determine the amount of tile drainage, including soils, slope, crop type, drainage design and precipitation intensity and timing, which may explain the scatter in the dataset.

AnnualTileDriangeVSAnnualPrecipitation
Figure 1: Annual tile drainage and precipitation at Discovery Farms Minnesota sites.

Looking as seasonal trends in precipitation gives more insight into the role of precipitation in the amount of tile drainage.  Discovery Farms water years run from October 1st through September 30th, meaning that data from water year 2015 includes data from October 2014 through September 2015.  Figure 2 displays tile drainage amounts from all Discovery Farms Minnesota sites and the corresponding fall precipitation from the year before (October through December).  There is a significant and positive correlation between October through December precipitation and tile drainage.  In general, more precipitation in the fall leads to higher tile drainage amounts the next spring and summer.  The correlation is higher for October through December precipitation than annual precipitation.

Figure 2: Annual tile drainage and corresponding fall precipitation at Discovery Farms Minnesota sites.
Figure 2: Annual tile drainage and corresponding fall precipitation at Discovery Farms Minnesota sites.

Unlike surface runoff, when assessing tile drainage from agricultural fields, annual precipitation and in particular, October through December precipitation are good indicators for the amount of tile drainage.  While there are many other factors that influence tile drainage, the amount of precipitation throughout the year and in the fall have a significant impact on the amount of tile drainage.  This further illustrates the differences between edge-of-field surface runoff and tile drainage and the need to understand the factors driving each.

 

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