Flow Duration Index

Dimensionless index indicating level of flow control needed to match forest hydrology.

Description

Ecology Stormwater Guidance includes flow-related performance standards to protect receiving waters from degradation caused by changes in the hydrologic regime due to development. These performance standards rely on flow-duration matching, whereby flow durations from developed land are required to match pre-developed flow-durations for a range of discharge values. The flow duration standard is intended to prevent flashy flows in receiving stream channels.

We developed an index representing the magnitude of change to the flow-duration curve between flow thresholds. Thresholds were chosen based on Ecology’s LID and Flow Control Standards (Department of Ecology, 2014), which require flow-duration matching over the range between 8 percent of the 2-year peak discharge (lower threshold of the LID standard) up to the 50-year peak discharge (upper threshold of the flow-control standard).

The flow discharge index is calculated by summing the discharge over the simulation period between a high-flow and low-flow threshold. This index is then applied to hru/grid combinations in the stormwater heatmap to produce a spatially explicit mapping of flow alteration.

More information, including verification of results can be found in the technical reference.

Layer Access in Earth Engine

The javascript commands below can be used to access this layer within the Google Earth Engine Code Editor. A Google Earth Engine account is required.

// Import the layer data dictionary
var data = require('users/stormwaterheatmap/apps:data/public')

// To view data dictionary, print to the console:
print('Data:', data)

//Get this layer from the layer data dictionary: 
var layer_name = data.rasters["Flow Duration Index"]

Viewing

Individual objects contain all the info used in the stormwater heatmap. To add it to the map, add the layer object.

var display_image = layer_name.layer
Map.addLayer(display_image)

Analysis

To get the raw image data for analysis, access the eeObject key.

var raw_image = layer_name.layer.eeObject
Map.addLayer(raw_image,{},'Flow Duration Index')

Layer Full-Extent Download

Click the "download" link to initiate the download process. The data will download as a GeoTIFF file (the file size is listed beside the layer name) and a WGS 84 projection. The resolution is the finest available based on the source data.

Flow Duration Index (2.3 MB) download

Visualization

Palette

Colors
f0f9e8
a8ddb5
7bccc4
43a2ca
0868ac

Minimum: 1 index

Maximum: 10 index

Source

The Nature Conservancy

External Links