Thursday, October 27, 2016

My last post discussed how Cedar River nitrate loading responded to flood of late September. This post builds on that with some excellent animations created by my colleague, Antonio Arenas. The text below refers to the animation that can be watched here:

  1. In watersheds with a lot of tile drainage, nitrate concentrations and loads follow a predictable pattern during and after storm events. When the rains come, we first get surface runoff directly to the stream. This is low nitrate water that dilutes higher nitrate water from other sources, namely tiles and shallow aquifers. We can see surface runoff hit the Cedar River at 0:12 of the animation.
  2. As rain starts to infiltrate downward, it dissolves nitrate in the soil profile before entering the tiles. This increased tile flow enters the stream network after peak surface runoff. We can see increased tile flows start to hit the river when nitrate concentrations rise (blue line). This begins at 0:16 of the video. Concentrations continue to rise even after the river discharge (black line) starts to decline. We describe this phenomenon as nitrate concentrations peak on the tail end of the hydrogaph (black line, discharge).
  3. Load is the mathematical product of discharge times concentration, and thus it is a little more difficult to describe what is happening. We can see load (red line) peak at 0:17 of the animation, just slightly after peak discharge. But loads begin to decline along with discharge, even while concentration continues to rise.

Also in the animation is a map showing accumulated precipitation in the Cedar Watershed. You need to watch it three or four times to soak it all in.