# How to create the longest flow path for multiple subwatersheds in ArcGIS Pro

Institute for Environmental and Spatial Analysis...University of North Georgia

## 1   What is the longest flow path?

Original (?) development: Smith, P. N. H., 1995. Hydrologic data development system. Transportation Research Record: Journal of the Transportation Research Board 1599, 118-127.

From the same research lab: Calculation of Hydrologic Parameters Using CRWR-PrePro

Longest flow path = Max cell value of (Upstream flow length raster + Downstream flow length raster)

## 2   Problems with the current Flow Length tool

The Flow Length tool in ArcGIS Pro does not support pour points for downstream flow length calculation.

How can we address this shortcoming if you want to calculate the longest flow path for multiple subwatersheds in the same study area?

## 3   Delineate subwatersheds

Watershed tool

• Flow direction raster
• Pour points raster (Outlets)
• Output: Watersheds raster

## 4   Repeat for each subwatershed

### 4.1   Clip the flow direction raster

Extract by Mask tool

• Flow direction raster
• Subwatershed
• Output: Flow direction raster for the subwatershed

### 4.2   Calculate the upstream flow length

Flow Length tool

• Flow direction raster for the subwatershed
• Upstream direction
• Output: Upstream flow length for the subwatershed

### 4.3   Calculate the downstream flow length

Flow Length tool

• Flow direction raster for the subwatershed
• Downstream direction
• Output: Downstream flow length for the subwatershed

### 4.4   Calculate the longest flow path

Raster Calculator tool

• `"up_length" + "down_length"`
• Output: updown_length

Find the maximum cell value

• `Con("updown_length" > YOUR_MAX_UPDOWN_LENGTH_INTEGER, 1, 0)`
• Output: lfp

### 4.5   Convert the longest flow path raster to feature

Raster to Polyline tool

• lfp
• Output: lfp