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+---
+title: "Using GRASS, NumPy, and Landlab for Scientific Modeling"
+author: "Anna Petrasova"
+date: 2025-10-01
+date-modified: today
+categories: [NumPy, raster, terrain, intermediate, Landlab, Python, hydrology, matplotlib]
+description: >
+  Learn how to integrate NumPy arrays with GRASS tools
+  on an example using Landlab modeling framework.
+image: thumbnail.webp
+other-links:
+- text: "Intro to GRASS Python API"
+  href: "https://grass.osgeo.org/grass-devel/manuals/python_intro.html"
+  icon: link
+- text: "Check out new xarray-grass interface"
+  href: "https://github.com/lrntct/xarray-grass"
+  icon: link
+format:
+  ipynb: default
+  html:
+    toc: true
+    code-tools: true
+    code-copy: true
+    code-fold: false
+engine: jupyter
+execute:
+  eval: false
+jupyter: python3
+---
+
+# Introduction
+
+This short tutorial shows how to integrate NumPy arrays with GRASS tools to create a smooth workflow for scientific modeling and analysis in Python.
+
+Specifically, it demonstrates a complete workflow: generating a terrain surface in GRASS, importing it into [Landlab](https://landlab.csdms.io/) (an open-source Python library for running numerical models of Earth-surface dynamics), running an erosion model, and then returning the results to GRASS for further hydrologic analysis.
+
+With the [grass.tools API](https://grass.osgeo.org/grass-devel/manuals/python_intro.html) (introduced in GRASS v8.5), tools can now accept and return NumPy arrays directly, rather than working only with GRASS rasters. While native rasters remain the most efficient option for large-scale processing, NumPy arrays make it easy to connect GRASS with the broader Python scientific ecosystem, enabling advanced analysis and seamless use of familiar libraries.
+
+::: {.callout-note title="How to run this tutorial?"}
+
+This tutorial is prepared to be run in a Jupyter notebook locally
+or using services such as Google Colab. You can [download the notebook here](grass_numpy_integration.ipynb).
+
+If you are not sure how to get started with GRASS, checkout the tutorials [Get started with GRASS & Python in Jupyter Notebooks](../get_started/fast_track_grass_and_python.qmd)
+and [Get started with GRASS in Google Colab](../get_started/grass_gis_in_google_colab.qmd).
+
+:::
+
+![](logos.webp)
+
+# Generating a fractal surface in GRASS
+
+First we will import GRASS packages:
+
+```{python}
+# import standard Python packages
+import os
+import sys
+import subprocess
+
+# for some environments, this path setup can be skipped
+sys.path.append(
+    subprocess.check_output(["grass", "--config", "python_path"], text=True).strip()
+)
+
+# import GRASS Python packages
+import grass.script as gs
+import grass.jupyter as gj
+from grass.tools import Tools
+```
+
+Create a new GRASS project called "landlab". Since we will be working with artifical data, we don't need to
+provide any coordinate reference system, resulting in a generic cartesian system.
+
+```{python}
+gs.create_project("landlab")
+```
+
+Initialize a session in this project and create a `Tools` object we will use for calling GRASS tools:
+
+```{python}
+session = gj.init("landlab")
+tools = Tools()
+```
+
+Since we are generating artificial data, we need to specify the dimensions (number of rows and columns).
+We also need to let GRASS know the actual coordinates we want to use; we will do all that by setting
+the [computational region](https://grass.osgeo.org/grass-stable/manuals/g.region.html). Lower-left (south-west) corner of the data will be at the coordinates (0, 0),
+the coordinates of the upper-right (nort-east) corner are number of rows times cell resolution and number of columns times cell resolution.
+We chose a cell resolution of 10 horizontal units, which would impact certain terrain analyses, such as slope computation, and certain hydrology processes.
+
+```{python}
+rows = 200
+cols = 250
+resolution = 10
+tools.g_region(s=0, w=0, n=rows * resolution, e=cols * resolution, res=resolution)
+```
+
+## Creating a fractal surface as a NumPy array
+
+We will create a simple fractal surface with [r.surf.fractal](https://grass.osgeo.org/grass-stable/manuals/r.surf.fractal.html).
+
+{{< fa wand-magic-sparkles >}} The trick is to use the `output` parameter with the value `np.array` to request a NumPy array instead of a native GRASS raster. {{< fa wand-magic-sparkles >}}
+This way GRASS provides the array as the result of the call:
+
+```{python}
+import numpy as np
+
+fractal = tools.r_surf_fractal(output=np.array, seed=6)
+```
+
+::: {.callout-note title="NumPy arrays in GRASS version < 8.5"}
+
+Directly passing NumPy arrays to GRASS tools and receiving them back is a new feature in GRASS v8.5.
+If you work with older versions of GRASS, you can use
+[grass.script.array](https://grass.osgeo.org/grass-stable/manuals/libpython/grass.script.html#script.array.array):
+
+```{python}
+import grass.script as gs
+import grass.script.array as ga
+
+gs.run_command("r.surf.fractal", seed=6, output="fractal")
+fractal = ga.array("fractal")
+```
+
+:::
+
+Now we can display `fractal` array e.g., using matplotlib library:
+
+```{python}
+import matplotlib.pyplot as plt
+
+plt.figure()
+plt.imshow(fractal, cmap='terrain')
+plt.colorbar(label='Elevation')
+plt.title('Topography from r.surf.fractal')
+plt.xlabel('X-coordinate')
+plt.ylabel('Y-coordinate')
+plt.show()
+```
+
+![Fractal surface generated with r.surf.fractal displayed using matplotlib](fractal_numpy.webp)
+
+We can modify the array:
+
+```{python}
+fractal *= 0.1
+fractal = np.abs(fractal)
+
+plt.figure()
+plt.imshow(fractal, cmap='terrain')
+plt.colorbar(label='Elevation')
+plt.title('Modified topography from r.surf.fractal')
+plt.xlabel('X-coordinate')
+plt.ylabel('Y-coordinate')
+plt.show()
+```
+
+![Modified fractal surface](fractal_numpy_abs.webp)
+
+# From GRASS to Landlab
+
+Now, let's use Landlab's modeling capabilities to burn in an initial drainage network using the
+Landlab's [Fastscape Eroder](https://landlab.readthedocs.io/en/latest/generated/api/landlab.components.stream_power.fastscape_stream_power.html).
+
+Create the `RasterModelGrid` specifying the same dimensions we used for creating the fractal surface.
+Add the terrain elevations as a 1D array (flattened with `ravel`) to the grid's nodes under the standard field name "topographic__elevation".
+
+```{python}
+from landlab import RasterModelGrid
+
+grid = RasterModelGrid((rows, cols), xy_spacing=resolution)
+grid.add_field("topographic__elevation", fractal.ravel(), at="node")
+```
+
+Run the erosion of the landscape:
+
+```{python}
+from landlab.components import LinearDiffuser
+from landlab.components import FlowAccumulator, FastscapeEroder
+
+fa = FlowAccumulator(grid, flow_director="D8")
+fsp = FastscapeEroder(grid, K_sp=0.01, m_sp=0.5, n_sp=1.0)
+ld = LinearDiffuser(grid, linear_diffusivity=1)
+
+for i in range(100):
+    fa.run_one_step()
+    fsp.run_one_step(dt=1.0)
+    ld.run_one_step(dt=1.0)
+```
+
+Extract the resulting NumPy array and visualize it:
+
+```{python}
+elevation = grid.at_node['topographic__elevation'].reshape(grid.shape)
+
+plt.figure()
+plt.imshow(elevation, cmap='terrain', origin='upper')
+plt.colorbar(label='Elevation')
+plt.title('Topography after erosion')
+plt.xlabel('X-coordinate')
+plt.ylabel('Y-coordinate')
+plt.show()
+```
+
+![Eroded fractal surface with Landlab](fractal_landlab.webp)
+
+# From Landlab to GRASS
+
+Now we will bring the eroded topography back to GRASS for additional hydrology modeling.
+We will derive streams using the [r.watershed](https://grass.osgeo.org/grass-stable/manuals/r.watershed.html) and [r.stream.extract](https://grass.osgeo.org/grass-stable/manuals/r.stream.extract.html) tools.
+
+{{< fa wand-magic-sparkles >}} The `Tools` API allows us to directly plugin the NumPy `elevation` array into the tool call. {{< fa wand-magic-sparkles >}}
+
+```{python}
+tools.r_watershed(elevation=elevation, accumulation="accumulation")
+tools.r_stream_extract(
+    elevation=elevation,
+    accumulation="accumulation",
+    threshold=300,
+    stream_vector="streams",
+)
+
+```
+
+And visualize them using `gj.Map` on top of shaded relief:
+
+```{python}
+tools.r_relief(input=elevation, output="relief")
+
+m = gj.Map(width=400)
+m.d_rast(map="relief")
+m.d_vect(map="streams", type="line", color="blue", width=2)
+m.show()
+```
+
+![Streams derived from eroded topography in GRASS](streams.webp)
+
+Now if we want to store the eroded topography as a native GRASS raster, we can use
+[grass.script.array](https://grass.osgeo.org/grass-stable/manuals/libpython/grass.script.html#script.array.array)
+to create a GRASS array object with the dimensions given by the current computational region.
+Then we copy the NumPy array and write the raster map as GRASS native raster.
+
+```{python}
+import grass.script.array as ga
+
+# Create a new grasss.script.array object
+grass_elevation = ga.array()
+# Copy values from elevation array
+grass_elevation[...] = elevation
+# Write new GRASS raster map 'elevation'
+grass_elevation.write("elevation")
+```
+
+::: {.callout-note title="What about N-dimensional arrays?"}
+
+For 3D arrays, you can use [grass.script.array3d](https://grass.osgeo.org/grass-stable/manuals/libpython/grass.script.html#script.array.array3d) package.
+
+For N-dimensional arrays, check out the new [xarray-grass](https://github.com/lrntct/xarray-grass) bridge developed by [Laurent Courty](https://github.com/lrntct) {{< fa rocket >}}
+:::
+
+Let's visualize the new GRASS elevation map with streams by setting its color
+scheme to `srtm_percent` and rendering it in 3D using the
+[grass.jupyter.Map3D](https://grass.osgeo.org/grass-stable/manuals/libpython/grass.jupyter.html?highlight=map3d#module-grass.jupyter.map3d) class:
+
+```{python}
+tools.r_colors(map="elevation", color="srtm_percent")
+elevation_3dmap = gj.Map3D()
+elevation_3dmap.render(
+    elevation_map="elevation",
+    resolution_fine=1,
+    vline="streams",
+    vline_width=3,
+    vline_color="blue",
+    vline_height=3,
+    position=[0.50, 0.00],
+    height=5600,
+    perspective=10,
+)
+elevation_3dmap.show()
+```
+
+![Resulting elevation with streams in 3D](elevation_3d.webp)
+
+And now let's augment it with [Nano Banana AI](https://nanobanana.ai/):
+
+![AI edited landscape](thumbnail.webp)
+
+---
+
+The development of this tutorial and the `grass.tools` package (developed by Vaclav Petras) was supported by NSF Award #2322073, granted to Natrx, Inc.
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