Screen rivers for a shared-water annex

PythonexpertSpatial analysis

An illustrative river-basin commission is drafting the annex of transboundary rivers for a regional water agreement. The last draft listed any river whose line touched a second country, which put a river that spends three kilometres in a neighbour on the same page as the Danube. The commission wants a list built from measured lengths: how many countries hold a meaningful stretch of each river, and how dominated the river is by its largest holder, so that the annex can distinguish a river that is shared from one that merely crosses a line.

Write a script that produces the transboundary rivers index.

The datasets are mounted where the script runs:

/data/rivers.geojson — 461 river and lake centrelines, MultiLineStrings
/data/countries.geojson — 241 countries and dependencies

geopandas, shapely, pyproj, pandas and numpy are available.

Stage 1 — Cut and measure. For every river and every country it intersects, the geodesic length (pyproj.Geod, WGS 84) of the river inside that country, in km.

Stage 2 — Per river. total_km — the river's length inside all countries; n_countries — how many countries hold at least 5 km of it; dominant_share — the largest single country's length divided by total_km, 3 decimals.

Stage 3 — The index. Assign to a variable named `result` a GeoDataFrame of the rivers with n_countries ≥ 2, carrying the river geometry and river_id, name, n_countries, dominant_share, total_km, ordered by dominant_share ascending — the most evenly shared first. Do not write a file — the runner reads result.

The input files

/data/rivers.geojson — EPSG:4326; the geometry is in geometry (MultiLineString).

column	type	meaning
`river_id`	string	River identifier, `RV-` and four digits
`name`	string	Name as labelled
`name_en`	string	English name
`featurecla`	string	River \| Lake Centerline
`scalerank`	integer	Prominence, lower is more prominent

/data/countries.geojson — EPSG:4326; the geometry is in geometry (Polygon / MultiPolygon).

column	type	meaning
`country_id`	string	CO- plus the ISO 3166-1 alpha-3 code
`name`	string	Short name
`name_long`	string	Long name
`iso_a3`	string	ISO 3166-1 alpha-3; -99 where Natural Earth assigns none
`iso_a2`	string	ISO 3166-1 alpha-2; -99 where Natural Earth assigns none
`continent`	string	Continent
`subregion`	string	UN subregion
`pop_est`	number	Population estimate (persons)
`pop_year`	integer	Year of the estimate
`gdp_md`	number	GDP, millions of US dollars (USD m)
`gdp_year`	integer	Year of the GDP figure
`economy`	string	Natural Earth economy class
`income_group`	string	World Bank income group

Run executes the script in your browser and shows you its output and a preview. Submit executes it again on the server and grades what result holds.

What to submit

Submit: A Python script that assigns its output to result.
Input files: /data/rivers.geojson · /data/countries.geojson
Required properties: river_id, name, n_countries, dominant_share, total_km

Objectives

Intersect every river with every country it touches and measure each piece geodesically
Aggregate the pieces per river into a count of meaningful holders and a dominance share
Assign the shared rivers, ranked by how evenly they are shared, to result

Constraints

These are where the teaching is. Read them twice.

Length is geodesic on WGS 84 — never planar in a projection
A country counts toward n_countries only with at least 5 km of the river
dominant_share is over the river's length inside countries, not its full mapped length
result carries the river geometry with river_id, name, n_countries, dominant_share and total_km

Graded requirements

Cuts rivers at borders with an intersection
Measures length geodesically
How many rivers are shared by two or more countries
Carries river_id, name, n_countries, dominant_share and total_km
The most countries any one river runs through
Total river kilometres in the shared set
The smallest dominant share in the index

Modelled on UNECE, The Second Assessment of Transboundary Rivers, Lakes and Groundwaters (2011, https://unece.org/second-assessment-transboundary-rivers-lakes-and-groundwaters), which inventories shared waters. This exercise uses generalised Natural Earth centrelines and an illustrative five-kilometre screening convention; its holder shares are teaching measures, not UNECE figures.

Python

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Files: /data/rivers.geojson, /data/countries.geojson · assign result

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What will be checked

7 scored, 0 informational

Cuts rivers at borders with an intersection
13%
Measures length geodesically
13%
How many rivers are shared by two or more countries
27%
Carries river_id, name, n_countries, dominant_share and total_km
7%
The most countries any one river runs through
13%
Total river kilometres in the shared set
13%
The smallest dominant share in the index
13%