In the Raccoon River watershed, 50% of the annual precipitation falls from April through July. About 60% of the annual streamflow and 70% of the annual nitrate load (total mass) also occur from April-July. As we study these things, our challenge is to connect the facts into a coherent structure that tells a story. That process is much simpler than most people imagine. So, what’s the explanation behind these numbers? Half the rain produces 60% of flow which produces 70% of the nitrate. 50-60-70.
Precipitation (P) can evaporate (E) into the atmosphere; it can also get used by plants (transpiration-T), or enter the stream network (discharge signified by Q). Early in the season, the crops haven’t been planted or are still immature and T is very low. It’s still cool so E is low too. So spring rain produces a disproportionately large discharge (runoff).
There’s also a bit more nitrate available for transport in the spring. Spring applied N and that applied the previous fall; leftover N from last year’s crop; nitrate derived from the soil itself, not yet sucked up by plants. So the concentration of nitrate in the soil water is a little higher. Since load is the product of concentration x discharge, we get a multiplier effect. Thus, 50% of the rain produces 60% of the discharge produces 70% of the load. And if climate change projections are to be believed, this will only get enhanced because springtime rains are projected to increase the most here in Iowa.
The graph illustrates average nitrate concentration, average nitrate load, and average river discharge/flow (normalized to watershed area) progressing through the year. Look at July. During the first 13 days of July, nitrate concentration drops a modest 20%, but river discharge and load drop by almost 60%! What gives? Crop growth, that’s what. Corn and soybean growth kicks into overdrive and the plant roots start tapping into the deep subsurface soil moisture. Both water and nitrate are getting sucked up by the crops. Even though the rain keeps coming (on average), river flows drop off a cliff. A 40-acre field of corn uses about 800,000 gallons of water every day in the two weeks prior and two weeks after corn silking. That’s three times higher than water use when the corn is knee-high. No wonder streamflows decline so dramatically!