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Note: This document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade, and read the current manual page.

NAME - Calculate average natural discharge and minimum flow following regional law an


raster, hydropower, renewable energy

SYNOPSIS --help [-dfp] [q_spec=name] q_river=name [k_b=name] [k_n=name] [rain=name] [k_matrix=name] [m_matrix=name] [a_matrix=name] [mfd=name] [a_river=name] elevation=name [river=name] [lakes=name] streams=name [corr_fact=name] [env_area=name] threshold=float [--overwrite] [--help] [--verbose] [--quiet] [--ui]


Debug with intermediate maps
compute the discharge in the river with q_spec=flow map
compute the discharge in the river with q_spec computed as see manual
Allow output files to overwrite existing files
Print usage summary
Verbose module output
Quiet module output
Force launching GUI dialog


Name of the specific discharge [l/s/km2]
q_river=name [required]
Name of raster map with the discharge along river [m3/s]
Name of raster map with kb parameter
Name of raster map with kn parameter
Name of the specific with rain [mm]
Name of k area map with k parameter
Name of M area map with M parameter
Name of A area map with A parameter
Name of raster map with minimum flow along the river [m3/s]
Name of raster map with area of the basin along the river [m2]
elevation=name [required]
Name of input elevation raster map
Name of river network
Or data source for direct OGR access
Name of lakes network
Or data source for direct OGR access
streams=name [required]
Name of the new stream network
Name of corrective factors area map for environmental flow
Name of area with environmental restriction
threshold=float [required]
Minimum size of exterior watershed basin
Default: 100000

Table of contents

DESCRIPTION calculates the average natural discharge and the minimum flow discharge according to regional laws.


The natural discharge is the discharge of the streams which doesn't consider the existing power plants and the other structures exploiting the water of the river.
The Minimal Flow Discharge (MFD) is the amount of water which has to remain in the river to preserve the ecosystems. The legislation differs in each region. The MFD can be considered as a percentage of the current discharge, which is the discharge of the river considering the structures exploiting the water. The current discharge is often considered as the mean annual discharge.

However, a percentage of the current discharge cannot define precisely the MFD and each region has a different method to define it. For the moment, this module only considers the legislation applied on Piave basin in the Veneto region. New tabs with the legislation of other regions could be added.

The module computes two raster maps : the natural discharge and the MFD. On Piave basin, the natural discharge can be computed thanks to the input raster map with the values of specific discharge, and the MFD is calculated thanks to this formula :

QMFD = ( Kb + Kn ) * 177 * S0.85 * Qspec * 10-6
where Kb is the biological criticality index,
Kn is the naturalistic criticality index,
S is the catchment area, in km2,
Qspec is the specific flow-rate per unit area of the catchment, in l/(s.km2)
Kb is typically within the range of 1-1.6; higher values are chosen for a river whose aquatic ecosystem is considered to be of a particular environmental value.
Kn is typically within the range of 0-0.6; higher values of such index are used for basins having a particular naturalistic value, for instance national parks.
The values of Kb and Kn are imposed by the Piave River Catchment Authority (PRCA). They have different values depending on homogeneous segments which can be found in a table made by the PRCA. Also the values of Qspec depend on the area and are available in such a table.

Thanks to three raster maps respectively with the values of Kb, Kn and Qspec, and also the elevation raster map and the streams vector map, the module creates the two raster maps with the values of MFD and average natural discharge.


This example is based on the case-study of Mis valley in Belluno province, Veneto, Italy.

Here is the map of the Mis valley with colored areas to define the Kn, Kb and Qspec values.
Picture which gathers the input raster maps with Kn, Kb and Qspec values

According to the legislation for the Piave basin explained above, the legal values for the Mis valley are :
Kn = 0.4 in the whole region (yellow and red zones)
Kb = 1.4 in the yellow zone and 1.6 in the red zone
Qspec = 44 l/(s.km2) in the yellow zone and 43 l/(s.km2) elsewhere (red and white zones)

These values are put in three different raster maps : q_spec, k_b and k_n.
Here is the code used to create the raster maps with the MFD and the natural discharge. The basins are considered with a threshold of 10000 m. q_spec=q_spec output_q_river=discharge k_b=k_b k_n=k_n output_mfd=mfd elevation=elevation output_streams=streams threshold=100000

The following picture gathers the two output raster maps mfd and discharge which look like each other (yellow background with colored points following the river and containing the values of discharge). For a better understanding, the following picture also shows the border of the Mis valley and the streams.

Picture which gathers the output raster maps with valued of MFD and natural discharge, also showing the vector maps with the borders and streams of Mis valley

The white point is queried in GRASS to know the values of MFD and natural discharge. The following picture shows these values in m3/s.

Values of MFD and natural discharge (in m3/s) at the white point



Allegato alla delibera n. 4/2004 del Comitato Istituzionale del 3 marzo 2004
Piano stralcio per la gestione delle risorse idriche del bacino del Piave - Misure di Salvaguardia

from Autorità di bacino dei fiumi Isonzo, Tagliamento, Livenza, Piave, Brenta-Bacchiglione


Giulia Garegnani (Eurac Research, Bolzano, Italy), Sara Biscaini (University of Trento, Italy), manual written by Julie Gros.


Available at: source code (history)

Latest change: Monday Jan 30 19:52:26 2023 in commit: cac8d9d848299297977d1315b7e90cc3f7698730

Note: This document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade, and read the current manual page.

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