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Flow Duration Index

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


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"]


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


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')




Minimum: 1 index

Maximum: 10 index


The Nature Conservancy