R.basin

Introduction

The module r.basin has been designed to perform the delineation and the morphometric characterization of a given basin, on the basis of an elevation raster map and the coordinates of the outlet. Here a tutorial based on NC sample dataset is presented.

Preparation

As a first step, we set the computational region to match the elevation raster map:

g.region rast=elevation@PERMANENT -ap
projection: 99 (Lambert Conformal Conic)
zone:       0
datum:      nad83
ellipsoid:  a=6378137 es=0.006694380022900787
north:      228500
south:      215000
west:       630000
east:       645000
nsres:      10
ewres:      10
rows:       1350
cols:       1500
cells:      2025000

For the basin's delineation, a pair of coordinates is required. Usually coordinates belonging to the natural river network don't exactly match with the calculated stream network. What we should do is calculating the stream network first, and then find the coordinates on the calculated stream network closest to the coordinates belonging to the natural stream network.

# Calculate drainage map
r.watershed -a elevation=elevation@PERMANENT accumulation=accum

# Extract the stream network
r.stream.extract elevation=elevation@PERMANENT accumulation=accum@testing threshold=20 stream_rast=stream_network

We no longer need the accumulation map:

g.remove rast=accum

Now that we have the calculated stream network, we should choose a pair of coordinates for the outlet belonging to it. Let's choose:

easting=636654.791181 northing=218824.126649

We no longer need the stream network map:

g.remove rast=stream_network

Usage of r.basin

We can run r.basin:

r.basin map=elevation@PERMANENT prefix=out easting=636654.791181 northing=218824.126649 threshold=20

Prefix parameter is a string given by the user in order to distinguish all the maps produced by every run of the program, i.e. every set of outlet's coordinates. Prefix must start with a letter. Threshold is the same parameter given in r.watershed and r.stream.extract. Physically, it means the number of cells that determine "where do the river begins". (This is an open issue for the hydrological science and a wide literature has been produced on the topic).

The output of r.basin consists of:

• Several morphometric parameters, which are printed in terminal and also stored in a csv file called out_elevation_parameters, in the working directory;
• Maps;
• Plots.

Morphometric parameters

Tot. cells 78560.0
===========================
Ipsometric | quantiles
===========================
145 | 0.025
143 | 0.05
141 | 0.1
137 | 0.25
129 | 0.5
119 | 0.75
121 | 0.7
110 | 0.9
100 | 0.975

##################################

Tot. cells 78560.0
Tot. area 7856000.0
Max distance 6809.546521
===========================
Whidth Function | quantiles
===========================
784 | 0.05
1477 | 0.15
2137 | 0.3
2543 | 0.4
2933 | 0.5
3606 | 0.6
4169 | 0.7
5144 | 0.85
6105 | 0.95

##################################
Output of r.stream.stats: 

Summary:
Max order | Tot.N.str. | Tot.str.len. | Tot.area. | Dr.dens. | Str.freq. 
  (num)   |    (num)   |     (km)     |   (km2)   | (km/km2) | (num/km2) 
        5 |        884 |      96.3436 |    7.8339 |  12.2983 | 112.8429 

Stream ratios with standard deviations:
 Bif.rt. | Len.rt. | Area.rt. | Slo.rt. | Grd.rt. 
  5.7374 |  2.6902 |   5.6815 |  1.5533 |  1.7962
  3.1961 |  1.9514 |   5.2294 |  0.1250 |  0.7482

Order | Avg.len |  Avg.ar  |  Avg.sl |  Avg.grad. | Avg.el.dif
 num  |   (km)  |  (km2)   |  (m/m)  |    (m/m)   |     (m)   
    1 |  0.0750 |   0.0069 |  0.0523 |     0.0384 |  3.1062
    2 |  0.1428 |   0.0311 |  0.0342 |     0.0277 |  4.1701
    3 |  0.4834 |   0.1732 |  0.0243 |     0.0205 |  9.0641
    4 |  2.4205 |   2.1885 |  0.0155 |     0.0134 | 24.1708
    5 |  1.1247 |   7.8339 |  0.0091 |     0.0046 |  5.1594

Order | Std.len |  Std.ar  |  Std.sl |  Std.grad. | Std.el.dif
 num  |   (km)  |  (km2)   |  (m/m)  |    (m/m)   |     (m)   
    1 |  0.0600 |   0.0052 |  0.0398 |     0.0287 |  3.2301
    2 |  0.1244 |   0.0230 |  0.0250 |     0.0196 |  4.4835
    3 |  0.4554 |   0.1353 |  0.0132 |     0.0122 |  7.1065
    4 |  2.2874 |   2.3843 |  0.0046 |     0.0061 | 12.9175
    5 |  0.0000 |   0.0000 |  0.0000 |     0.0000 |  0.0000

Order | N.streams | Tot.len (km) | Tot.area (km2)
    1 |       712 |      53.4067 |  4.8998
    2 |       137 |      19.5657 |  4.2562
    3 |        31 |      14.9849 |  5.3684
    4 |         3 |       7.2616 |  6.5655
    5 |         1 |       1.1247 |  7.8339

Order | Bif.rt. | Len.rt. | Area.rt. | Slo.rt. | Grd.rt. | d.dens. | str.freq.
    1 |  5.1971 |  1.9040 |   0.0000 |  1.5311 |  1.3892 | 10.8998 | 145.3120
    2 |  4.4194 |  3.3847 |   4.5144 |  1.4046 |  1.3472 |  4.5970 | 32.1883
    3 | 10.3333 |  5.0075 |   5.5742 |  1.5691 |  1.5365 |  2.7913 |  5.7745
    4 |  3.0000 |  0.4646 |  12.6376 |  1.7083 |  2.9120 |  1.1060 |  0.4569
    5 |  0.0000 |  0.0000 |   3.5796 |  0.0000 |  0.0000 |  0.1436 |  0.1277

##################################
Morphometric parameters of basin :
##################################
Easting Centroid of basin : 635195.00
Northing Centroid of Basin : 220715.00
Rectangle containing basin N-W : 632870 , 222890
Rectangle containing basin S-E : 637000 , 218720
Area of basin [km^2] : 7.8559625
Perimeter of basin [km] : 16.8287990515
Max Elevation [m s.l.m.] : 151.7396
Min Elevation [m s.l.m.]: 94.82206
Elevation Difference [m]: 56.91754
Mean Elevation [m s.l.m.]: 127.6772
Mean Slope : 2.93
Length of Directing Vector [km] : 2.38880562659
Prevalent Orientation [degree from north, counterclockwise] :
0.913351005341
Compactness Coefficient : 5.32106255399
Circularity Ratio : 0.348580442132
Topological Diameter : 127.0
Elongation Ratio : 0.470961456397
Shape Factor : 1.16984953656
Concentration Time (Giandotti, 1934) [hr] : 3.52654267443
Length of Mainchannel [km] : 6.715361467
Mean slope of mainchannel [percent] : 0.957339
Mean hillslope length [m] : 8145.381
Magnitudo : 621.0
Max order (Strahler) : 5
Number of streams : 884
Total Stream Length [km] : 96.3436
First order stream frequency : 79.0482388377
Drainage Density [km/km^2] : 12.2637550778
Bifurcation Ratio (Horton) : 5.7374
Length Ratio (Horton) : 2.6902
Area ratio (Horton) : 5.6815
Slope ratio (Horton): 1.5533
##################################

Maps

g.list rast

out_elevation_accumulation              out_elevation_hillslope_distance
out_elevation_aspect                    out_elevation_horton
out_elevation_dist2out                  out_elevation_shreve
out_elevation_drainage                  out_elevation_slope
out_elevation_hack                      out_elevation_strahler

g.list vect

out_elevation_basin       out_elevation_network
out_elevation_mainchannel out_elevation_outlet

Plots

The plots are stored in the working directory:

• out_elevation_ipsographic.png
• out_elevation_ipsometric.png
• out_elevation_width_function.png

References

• Rodriguez-Iturbe I., Rinaldo A.; Fractal River Basins, Chance and Self-Organization. Cambridge Press (2001)
• Di Leo M., Di Stefano M., Claps P., Sole A.; Caratterizzazione morfometrica del bacino idrografico in GRASS GIS (Morphometric characterization of the catchment in GRASS GIS environment), Geomatics Workbooks, n. 9 (2010) [1]