Posted on January 23, 2023 (Updated on July 10, 2025)

GRASS GIS: Getting know about stream power index (SPI) computation by r.watershed function

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How does Arcgis calculate stream power index?

SPI is calculated using the following equation: SPIi = ln(DAi * tan(Gi)) where SPI is the stream power index at gridcell i, DA is the upstream drainage area (flow accumulation at gridcell i multiplied by gridcell area), and G is the slope at a grid cell i in radians.

What is r watershed?

r. watershed offers two methods to calculate surface flow: single flow direction (SFD, D8) and multiple flow direction (MFD). With MFD, water flow is distributed to all neighbouring cells with lower elevation, using slope towards neighbouring cells as a weighing factor for proportional distribution.

How do you calculate stream power?

Stream power is calculated as: ρgQs where ρ is density of water (1000 kg/m3), g is acceleration due to gravity (9.8 m/s2), Q is discharge (m3/s) and S is channel slope. Q is calculated as WdV where W is bankfull channel width (m), d is bankfull channel depth, and v is velocity (m/s).

What is SPI in GIS?

The Standardized Precipitation Index (SPI) is used to analyze meteorological droughts. SPI estimates the deviation of precipitation from the long-term probability function at different time scales (e.g. 1, 3, 6, 9, or 12 months).
 

What are the 3 main functions of a watershed?

WATERSHED FUNCTIONS



There are three processes within a watershed that can protect water quality if pre- served: water capture, water storage, and water release. A number of circumstances that can interrupt the capture, storage, and beneficial release of water are beyond human control.

What are the three types of watershed?

What are the types of watershed?

  • Macro watershed (> 50,000 Hect)
  • Sub-watershed (10,000 to 50,000 Hect)
  • Milli-watershed (1000 to10000 Hect)
  • Micro watershed (100 to 1000 Hect)
  • Mini watershed (1-100 Hect)


What are the 5 watershed features?

Watershed characteristics such as size, slope, shape, drainage density, land use/land cover, geology and soils, and vegetation are important factors affecting various aspects of runoff.
 

How is the power index calculated?

To calculate the power of a voter using the Banzhaf index, list all the winning coalitions, then count the critical voters. A critical voter is a voter who, if he changed his vote from yes to no, would cause the measure to fail. A voter’s power is measured as the fraction of all swing votes that he could cast.

How does Arcgis calculate stream order?

Open ArcTollbox > select Spatial Analyst Tool > Hydrology > Stream Order. Stream Order window appears, Input stream raster – Flow Accumulation Calculate Data (flow_accu_cal). Input flow direction raster – Flow Direction Data and then choose Output and click OK. Now see the result of the Stream order.
 

How does Arcgis calculate stream density?

Drainage Density is the total length of all the streams and Rivers in a Drainage basin divided by the total area of the drainage basin.