Setting the TFA based on sattelite data

NatCap Software Support
1

Hey all, I try to set the correct TFA value based on sattelite data and a shapefile I have on existing drainages in the area. The area of interest is the Pantanal, which is a very flat, floodplain-like landscape. One would say that this suits a higher TFA (>1000) because it regards floodplains rather than a landscape of narrow streams. However, using lower TFA values (<1000) provides streams_map outputs (Sediment model) that are better corresponding with aforementioned sattelite data and drainages shapefiles. It seems like generally, lower TFA values give a more detailed stream network.

Iā€™m also uncertain whether the drainage map provided includes year-round streams or gullies that are dry for a period every year. Do the stream maps provided by the model only count pixels as streams when it contains water year-round or also periodically?

Does anyone have experience in determining a TFA value, and would you recommend me to use high values (because of the nature of the landscape) or low values (because of the correspondance of the generated stream map with data)

2

Hi @Willem -

Choosing a TFA value is part art, part science. In general, we recommend using a TFA value that corresponds with a real-world stream network, so this would mean using the lower TFA value that corresponds with the drainages shapefile.

However, flat areas are tough to model. In particular, the Multiple Flow Direction (MFD) algorithm that InVEST uses tends to produce very wide streams in these areas (corresponding more to floodplains than streams), which may or may not reflect the usual bounds of the streams in reality.

The model generates the stream network based only on the DEM and TFA, it does not know whether those streams have water in them year-round or periodically. Since the SDR model works with annual average data, we try to use year-round streams, but those are not always possible to determine from a basic stream map. So if you know that the smaller tributaries are only periodic, you could choose a higher TFA value that creates a network without them.

~ Stacie

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