July is coming to a close, as is my time in Williamsburg for the summer. Over the past month, I have really been focusing on finding a quantitative pattern of agricultural field drainage. After going through the GIS steps on my chosen fields to find flow accumulation, I graphed the results using Kaleidagraph. After graphing the first few fields, a pattern began to emerge. Each field has one point of very high drainage (whether that be draining 10% of the field area or 50%), followed by subsequent points with lower drainage, and after a certain threshold, all the points drain only about 0.2% of the field area. This was a very interesting pattern to see on every field, and fits a logarithmic function with an R^2 value ranging from 78 to 99 (for all you non-math folks, that means the function is almost a perfect fit). This is just amazing to me! Every field naturally exhibits a logarithmic pattern of drainage, as you can see from these two graphs of fields in Stafford County and James City County. The reason some fields reach 100% of area drained (on the graph, 100% of cumulative flow accumulation divided by total flow accumulation on the y-axis) before others is simply because they are smaller fields and have less points along their field margin. These are pretty spectacular results to have at this point, I think.
In addition to the GIS work and graphing, I have also gone out in the field a few times. I have to get away from the computer sometimes! Once I pick fields adjacent to riparian buffers and go through all the GIS steps, I attempt to find a farm address and owner contact information. Finding information about who lives on a farm and who owns what land is surprisingly challenging for some counties, and I’ve had bad luck in getting in touch with people even if I do happen to find their phone number. All those troubles aside, I have managed to make it out to two fields this summer (thanks to my advisor Greg Hancock and fellow geology majors Max Cunningham and Shenandoah Raycroft for accompanying me out in the field at various points during the summer). When we go out in the field, we walk around the field margin (focusing on areas adjacent to the riparian buffer) and look for signs of concentrated flow off the field. When we find these, we first take a GPS point, then wander into the woods/buffer looking for a channel. Some of the off-field channels we have found are HUGE (as you can see from the picture below, taken in the forest next to a field in James City County). One of the largest was about 2 meters deep.
I’ll leave you with that image, and something to ponder. Riparian buffers are meant to trap nutrient and sediment pollution draining from nonpoint sources (such as agricultural fields), but do you think they are 100% effective if the runoff causes incised channels like the one pictured above? I think they are not, and hopefully by the end of the year, I will have data to back up the claim that riparian buffer policy as it is now is just falling short of what is needed to restore the Chesapeake Bay’s health.