Add MFD flow direction and accumulation (#946)#956
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brendancol merged 5 commits intomasterfrom Mar 4, 2026
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Implements Qin et al. (2007) multiple flow direction algorithm that partitions flow from each cell to all downslope neighbors. The adaptive exponent adjusts based on the ratio of maximum to mean downslope gradient. Supports all four backends.
46 tests covering correctness, NaN handling, boundary modes, fixed vs adaptive exponent, cross-backend equality (numpy, dask, cupy, dask+cupy), dtype acceptance, and Dataset support.
Adds flow_direction_mfd to the hydrology reference docs, the README feature matrix, and the hydrology user guide notebook with adaptive vs fixed exponent comparison plots.
Adds side-by-side comparison of all three flow direction algorithms and MFD exponent mode comparison (p=1 vs adaptive vs p=8).
Accumulates upstream contributing area through all MFD flow paths, weighted by directional fractions from flow_direction_mfd. Supports numpy, cupy, dask+numpy, and dask+cupy backends.
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Summary
flow_direction_mfd-- partitions flow from each cell to all downslope neighbors using the MFD algorithm with an adaptive exponent (Qin et al. 2007). Output is a 3-D(8, H, W)array of flow fractions.flow_accumulation_mfd-- accumulates upstream contributing area through all MFD flow paths, weighted by directional fractions. Each cell starts at 1 and distributes its value downstream proportionally.Test plan
pytest xrspatial/tests/test_flow_direction_mfd.py-- 46 tests passpytest xrspatial/tests/test_flow_accumulation_mfd.py-- 23 tests passexamples/user_guide/11_Hydrology.ipynb)Closes #946