Every stream counts: NHDPlus HR in the Chesapeake Bay

To model a river, you first need a map of its water: every stream, and the land that drains into it. That map is called hydrography.

In the United States it is the National Hydrography Dataset, drawn from topographic maps and elevation data. Its two national editions for modeling differ in detail. NHDPlus Version 2 is mapped at 1:100,000, on one-arc-second elevation, about thirty meters, with about 2.6 million features. NHDPlus High Resolution is mapped mostly at 1:24,000 or finer, on one-third-arc-second elevation, about ten meters, with more than thirty million. That is the one we build on.

These are the Chesapeake Bay's four hydrologic regions, from the Susquehanna to the James.

Trace the mapped flow paths in them with NHDPlus Version 2, and you get about one hundred twenty-eight thousand kilometers.

Trace them with NHDPlus HR, the high-resolution version, and you get about two hundred thirty-four thousand: about one hundred five thousand kilometers more.

Lakes, ponds and reservoirs: six and a half thousand in Version 2, one hundred thirty-one thousand in HR.

The land is divided more finely, too: four hundred thirty-nine thousand catchments instead of eighty-four thousand. One for every 0.4 square kilometers, instead of one for every two.

Here is a typical basin, the Tioga River, at the top of the Susquehanna: nine hundred thirty catchments in Version 2, about four thousand four hundred in HR.

Each small catchment drains to its own short reach, so a model built on them can place every field, and every source, in the catchment it actually drains through.

SWATGenX builds its SWAT+ models from NHDPlus HR. In the Chesapeake, every stream counts.