In SAGA GIS we have seen how hydrological analysis is done with SAGA GIS. However, the work we did there is not optimal:
We want to change all of this by working with SAGA GIS modules from inside a R Script.
Create a reproducible workflow with SAGA and RSAGA for flow accumulation from a digital elevation model.
Visualize the results in R.
SAGA has to be installed on your computer and R needs to know where.
library(RSAGA)
rsaga.env()Search for SAGA command line program and modules...
Done$workspace
[1] "."
$cmd
[1] "saga_cmd"
$path
[1] "/usr/bin"
$modules
[1] "/usr/lib64/saga"
$version
[1] "9.3.1"
$cores
[1] NA
$parallel
[1] FALSE
$lib.prefix
[1] "lib"If for some reason (e.g. working with a university computer) RSAGA throws an error, try the following:
library(link2GI)
linkSAGA() # wenn Pfad zu SAGA unbekannt
myenv <- rsaga.env(path="C:\\Program Files\\QGIS 3.8\\apps\\saga-ltr")rsaga.get.libraries()Search for SAGA command line program and modules...
Done [1] "climate_tools" "contrib_perego"
[3] "db_odbc" "db_pgsql"
[5] "docs_html" "garden_fractals"
[7] "garden_games" "garden_webservices"
[9] "grid_analysis" "grid_calculus_bsl"
[11] "grid_calculus" "grid_filter"
[13] "grid_gridding" "grid_spline"
[15] "grid_tools" "grid_visualisation"
[17] "grids_tools" "imagery_classification"
[19] "imagery_isocluster" "imagery_maxent"
[21] "imagery_opencv" "imagery_photogrammetry"
[23] "imagery_segmentation" "imagery_svm"
[25] "imagery_tools" "io_esri_e00"
[27] "io_gdal" "io_gps"
[29] "io_grid_image" "io_grid"
[31] "io_shapes" "io_table"
[33] "io_virtual" "io_webservices"
[35] "pj_georeference" "pj_geotrans"
[37] "pj_proj4" "pointcloud_tools"
[39] "shapes_grid" "shapes_lines"
[41] "shapes_points" "shapes_polygons"
[43] "shapes_tools" "shapes_transect"
[45] "sim_air_flow" "sim_cellular_automata"
[47] "sim_ecosystems_hugget" "sim_erosion"
[49] "sim_fire_spreading" "sim_geomorphology"
[51] "sim_hydrology" "sim_ihacres"
[53] "sim_landscape_evolution" "sim_qm_of_esp"
[55] "sim_rivflow" "statistics_grid"
[57] "statistics_kriging" "statistics_points"
[59] "statistics_regression" "ta_channels"
[61] "ta_cliffmetrics" "ta_compound"
[63] "ta_hydrology" "ta_lighting"
[65] "ta_morphometry" "ta_preprocessor"
[67] "ta_profiles" "ta_slope_stability"
[69] "table_calculus" "table_tools"
[71] "tin_tools" "vis_3d_viewer" rsaga.get.modules(libs = "ta_hydrology")Search for SAGA command line program and modules...
Done$ta_hydrology
code name interactive
1 0 Flow Accumulation (Top-Down) FALSE
2 1 Flow Accumulation (Recursive) FALSE
3 2 Flow Accumulation (Flow Tracing) FALSE
4 4 Upslope Area FALSE
5 6 Flow Path Length FALSE
6 7 Slope Length FALSE
7 10 Cell Balance FALSE
8 13 Edge Contamination FALSE
9 15 SAGA Wetness Index FALSE
10 18 Flow Accumulation (Mass-Flux Method) FALSE
11 19 Flow Width and Specific Catchment Area FALSE
12 20 Topographic Wetness Index FALSE
13 21 Stream Power Index FALSE
14 22 LS Factor FALSE
15 23 Melton Ruggedness Number FALSE
16 24 TCI Low FALSE
17 25 LS-Factor, Field Based FALSE
18 26 Slope Limited Flow Accumulation FALSE
19 27 Maximum Flow Path Length FALSE
20 28 Flow between fields FALSE
21 29 Flow Accumulation (Parallelizable) FALSE
22 30 Isochrones Variable Speed FALSE
23 31 CIT Index FALSE
24 32 Terrain Flooding FALSErsaga.get.usage(lib = "ta_hydrology", module = "SAGA Wetness Index")Search for SAGA command line program and modules...
Done
library path: /usr/lib/saga/
library path: /usr/lib/saga/
library name: libta_hydrology
library : ta_hydrology
Usage: saga_cmd ta_hydrology 15 [-DEM <str>] [-WEIGHT <str>] [-AREA <str>] [-SLOPE <str>] [-AREA_MOD <str>] [-TWI <str>] [-SUCTION <double>] [-AREA_TYPE <str>] [-SLOPE_TYPE <str>] [-SLOPE_MIN <double>] [-SLOPE_OFF <double>] [-SLOPE_WEIGHT <double>]
-DEM:<str> Elevation
grid, input
-WEIGHT:<str> Weights
grid, input, optional
-AREA:<str> Catchment Area
grid, output, optional
-SLOPE:<str> Catchment Slope
grid, output, optional
-AREA_MOD:<str> Modified Catchment Area
grid, output, optional
-TWI:<str> Topographic Wetness Index
grid, output
-SUCTION:<double> Suction
floating point number
Minimum: 0.000000
Default: 10.000000
-AREA_TYPE:<str> Type of Area
choice
Available Choices:
[0] total catchment area
[1] square root of catchment area
[2] specific catchment area
Default: 2
-SLOPE_TYPE:<str> Type of Slope
choice
Available Choices:
[0] local slope
[1] catchment slope
Default: 1
-SLOPE_MIN:<double> Minimum Slope
floating point number
Minimum: 0.000000
Default: 0.000000
-SLOPE_OFF:<double> Offset Slope
floating point number
Minimum: 0.000000
Default: 0.100000
-SLOPE_WEIGHT:<double> Slope Weighting
floating point number
Minimum: 0.000000
Default: 1.000000 params <- list(DEM = "dem.tif", TWI ="twi")
rsaga.geoprocessor(lib = "ta_hydrology",
module = "SAGA Wetness Index",
param = params)dem.tif is a file on your computer! NOT an R Object!
Some SAGA modules are easier to execute via wrapper functions (see https://cran.r-project.org/web/packages/RSAGA/index.html)
With such a wrapper, the Topographic Wetness Index can be computed like this:
rsaga.wetness.index(in.dem = "dem.tif", out.wetness.index = "twi")