Model the Reach You Study

You study one reach, one catchment, one site. The basin above it can be many times larger.

Nested modeling cuts the model at a boundary you can defend, and supplies what the upstream area would have contributed.

SWATGenX builds at four boundaries: a whole eight-digit basin, the watershed draining to a stream gauge, the watershed above any twelve-digit outlet you pick, and a single fourteen-digit catchment.

A gauge is the most useful cut. The boundary comes with an observed record, so the handoff between models is something you can check, not something you assume.

But a cut is a physical claim: that nothing you care about crosses it in both directions.

It fails with tidal backwater, with reservoir feedback, with long-residence constituents, and with groundwater. A gauge cuts the stream network. It does not cut the aquifer.

If you supply an upstream series, check that it arrives. Run with and without it, and difference the two runs just below the boundary. If the difference is exactly zero, the boundary is not being delivered.

Nested watershed modeling, at swatgenx.com/nested-watershed-modeling.