We need more evidence on impacts of genetically modified crops

Here's the story at the New York Times.

One key sentence: 

"The improvement in water quality could prove to be the largest benefit of the [genetically modified] crops, the report said, though it added that efforts should be made to measure any such effect"

It amazes and disappoints me tremendously how little we know about the links between agriculture and water quality.  We know agriculture, broadly speaking, is perhaps a key source of all kinds of water pollution.  But which parts of agriculture are most important, and how much policies and structural changes ranging from conservation tillage, buffer strips, GM crops, shifts in cropping patters, and growth in confined livestock operations, really matter is, I think, a complete unknown.  There are some black box models that some have coaxed into spitting out numbers, but I really don't believe them.  I expect few do.

What I'd love to see is hard data linking changes in agriculture to changes in water quality outcomes. This hasn't been done because it is very hard to do.  But it is possible.  And if one could manage to do a good job for one particular policy or structural change, then there would be a litany of follow pieces to show impacts for other policies and structural changes.  Any dedicated number crunchers want to write a dissertation?