Professor Ground Truthing Bay Diet Ag Runoff

 From the Bay Journal, Study aims for reality check on nutrient runoff from farms

At a farm in Prince George’s County, MD, Gurpal Toor and his students have been gathering water samples for more than three years. Their goal is to figure out how much nutrient pollution is running off a six-acre field — toward waterways and the Chesapeake Bay. It’s a question he still can’t answer with precision. There is too much year-to-year variation: changes in temperature, changes in rainfall, changes in what’s planted.

Providing an exact number for the amount of runoff — or advice about how to reduce it — would be “irresponsible” at this point, said Toor, a professor and agricultural extension specialist with the University of Maryland. “I don’t want to take a couple of years of data and tell everyone, ‘Here is the conclusion,’” he said.

 

Toor’s uncertainty starkly contrasts with figures used to guide Chesapeake Bay cleanup actions. While Toor struggles to understand what comes off a single field, figures from the state-federal Bay Program tell you with seeming precision the amount of nitrogen — a key nutrient — that comes off all 80,000 farms in the Bay watershed: 116,372,907.49 pounds in 2024.

That the Bay Program can determine what comes off all farmland to the hundredth of a pound seems a bit unlikely to Toor, who is in the fourth year of an effort that he views as something of a reality check.

Of course, as a professor, he knows the truth; the models used to estimate ag runoff spit out numbers to the limit of precision of the computer, probably several digits to the right of the decimal point, and whoever assembles the report simply rounds them off to the nearest even pound, without thinking very much about the how accurate the data that go into programming the model really are, or even what the error estimates of the results are.  And, if there are actually error bars on the estimates, they will be well hidden, because they are huge. We see this in the fish catch data too, where recreational fishing mortality is estimated from a chain of assumptions based on a pretty small sample size.

Toor is closely monitoring 15 small agricultural catchments in Maryland — essentially fields that drain to a specific point — ranging from 6 to 140 acres.

Funded by the Maryland Department of Agriculture and U.S. Department of Agriculture, it may be the largest detailed study ever attempted of how nutrients actually leave Bay-region farm fields in multiple settings. The goal is to better understand not only how many, but under what circumstances, nutrients are leaving actively managed fields and how that amount might be reduced.

Sounds like a worthwhile study, but unless it produces results that differ dramatically from model estimates, it will be difficult with a small number of sites to actually say the model is producing results at odds with reality.

The answer, as always, will be we need more money for a larger study.

The Wombat has Rule 5 Sunday: Doughnut Dolly up on time and under budget.