Muscatine

But of course we care if it came from Canada or Minnesota, since the whole point you're hopelessly trying to make is that we can differentiate Iowa Nitrogen from other nitrogen in the gulf. 

You are lost again.   We are trying to measure what portion of the nitrogen in the Gulf comes from Iowa.  And we're almost there.

Now.....start again.  From the top.  I sense your spirit dropping, and we all want you to just be happy. 

You can go back and review all the steps.   I don't have to start again.  My spirit has been depressed ever since you and the other boys couldn't understand this.   And the eleven year old girl did.

I fear she will be a teenager before you boys get it.

It's time for the next step.   When we know the area of the watershed and the total annual lbs. of nitrogen contributed from it we can calculate the lbs per square mile.   We don't care if some came from Canada or Minnesota.

I'll go to the next step and hope it prevents you from acting foolish again.
With the lbs per square mile we can multiply that times the square miles of the water shed in Iowa (not including that in Canada and/or Minnesota) and get the total nitrogen contributed by that river coming from Iowa.

Next step.  We can use the lbs. per square mile contributed by the one stream in Iowa and multiply that times the whole area of Iowa in square miles and get a crude value for the total contribution from Iowa.

But you boys want precision.   So we will measure the contribution from various streams which represent different population densities, ground cover, rain fall, and farming practices in the state.

Perhaps the Turkey River to represent the northeast corner of hills and timber cover.  The Des Moines and Skunk to represent the central area.   The Maquoteta to represent a transition from northeast and central.   The Nishnabotna to represent the western more dry area.   There is no limit on how many streams and partial drainage areas to measure to get a good approximation of what each land use area contributes.

When we have satisfied ourselves that we know the contribution of  Iowa we can then measure the total contribution of the Mississippi drainage area at New Orleans (including the portion from Canada) and add that from the several smaller rivers which drain directly into the Gulf from Texas, Alabama, Mississippi and Florida.   Maybe you would like to include some from Mexico too but the severely polluted area is concentrated close to the mouth of the Mississippi so that might not be necessary.

Now, having the total amount of nitrogen that is flowing into the Gulf, and the amount contributed by Iowa, it is simple math to determine the percentage contributed from Iowa.  I'm hoping you all can handle that calculation.   If not, let me know.

In the meantime, I'll just say it has been a pleasure to teach you how the percentage of nitrogen in the Gulf that comes from Iowa is measured.