QGIS
The free, open-source GIS that has become the default spatial platform for water engineers worldwide.
Python + Google Earth Engine without the pain — interactive mapping and analysis of petabyte-scale water data.
Install
pip install geemapgeemap, by Qiusheng Wu, wraps the Google Earth Engine Python API in an interactive mapping package built for notebooks: browse and query EE's petabytes of satellite data, run analyses (surface-water dynamics, flood extents, ET, snow cover), and export results — with hundreds of tutorial notebooks that have become the de facto EE curriculum.
For water engineers, it is the lowest-friction path to questions like "how has this reservoir's surface area changed since 1990" using JRC Global Surface Water or Landsat directly, no downloads involved.
Other geospatial & hydro-gis tools covering similar workflow stages.
The free, open-source GIS that has become the default spatial platform for water engineers worldwide.

500+ fast geospatial analysis tools with the best DEM hydrology toolkit in open source.
JAXA's 30m global DSM from PRISM stereo imagery — a strong third opinion alongside SRTM and Copernicus DEM.
A 30m global DEM reaching to 83° latitude — the go-to where SRTM/NASADEM have no coverage at all.
Esri's free hydrology framework for ArcGIS Pro — terrain preprocessing and the data model behind many agency workflows.
ESA's TanDEM-X-derived global DEM — the most accurate free global surface model available today.