Introduction

Hydra is a water infrastructure simulation platform written in Rust. It is built as a suite of domain engines sharing one toolchain: a desktop GUI, a hydra CLI, and a Rust SDK.

See Engines for what each engine covers.

Water Distribution Engine

Extended-period simulation (EPS) of hydraulic behaviour and water quality dynamics across pressurised pipe networks, computing the full time history of flows, pressures, and constituent concentrations at every node and link.

Hydraulics

• Head-loss formulas: Hazen-Williams, Darcy-Weisbach, Chezy-Manning (with minor losses)
• Demand models: Demand-Driven Analysis (DDA) and Pressure-Dependent Analysis (PDA)
• Emitters: pressure-dependent outflow at junctions
• Leakage: FAVAD (Fixed and Variable Area Discharge) model
• Pumps: head-curve (1/3-point and custom), constant-power, variable-speed patterns
• Valves: PRV, PSV, FCV, TCV, GPV, PBV, PCV
• Tanks: cylindrical and volume-curve geometries, overflow mode
• Controls: simple time/level/pressure controls, rule-based controls with priorities
• Solver: Global Gradient Algorithm (GGA) with sparse Cholesky factorisation

Water Quality

• Modes: chemical constituent, water age, source trace
• Transport: Lagrangian segment-based advection
• Reactions: first-order and zero-order bulk and wall decay, limiting potential, roughness correlation
• Sources: concentration, mass booster, flow-paced booster, setpoint booster
• Tank mixing: complete (CSTR), two-compartment, FIFO (plug flow), LIFO

I/O

• Input: EPANET .inp format, any 2.x release (local files, HTTP URLs)
• Output: EPANET-compatible .out binary format, .rpt text report, .json report
• Unit systems: all 11 EPANET flow unit variants (CFS, GPM, MGD, IMGD, AFD, LPS, LPM, MLD, CMH, CMD, CMS)

Relationship to EPANET

The water distribution engine’s hydraulic and quality solvers were derived by studying EPANET’s mathematical foundations. Hydra is not an EPANET clone or compatibility layer; it is a distinct solver that models the same physics. Where the two diverge, Hydra’s result is authoritative.

The same principle will apply to each engine Hydra adds: it reads the established source-model format for its domain, and models the physics independently rather than reimplementing the reference tool.

For migration guidance, see Migrating from EPANET.

See INP Format Support for current EPANET input coverage.

Urban Drainage Engine

Continuous and event simulation of stormwater and wastewater collection systems on the SWMM data model. Available from the hydra CLI, the Rust SDK, and the desktop app, where a drainage model is imported, then edited, run and explored.

Hydrology

• Runoff: rainfall-runoff on subcatchments, driven by rain gages with inline or external records
• Infiltration: Horton and Modified Horton, Green-Ampt and Modified Green-Ampt, Curve Number
• LID controls: low-impact development units placed on subcatchments, layer by layer
• Snowmelt: snow packs with plowable, impervious, and pervious surfaces
• Groundwater: aquifers coupled to subcatchments
• RDII: rainfall-derived inflow and infiltration via unit hydrographs
• Climate: climate files and the Hargreaves evaporation relation

Hydraulics

• Routing: Preissmann-slot dynamic-wave routing
• Links: conduits with open and closed section geometry, pumps, orifices, weirs, outlets
• Nodes: junctions, outfalls, storage units, flow dividers
• Street drainage: street cross-sections, transects, and HEC-22 street inlets
• Controls: rule-based controls with priorities and PID modulation

Water Quality

• Buildup and washoff: pollutants accumulating on land uses and washing off in storms
• Treatment: node treatment expressions
• Transport: pollutant routing through the network

I/O

• Input: SWMM .inp format (local files, HTTP URLs)
• Auxiliary files: climate files and hotstart files (use and save)
• Output: SWMM-compatible .out binary format and text report

Relationship to SWMM

The engine models the same physics as SWMM with its own solver and its own convergence and conservation criteria. This is the same relationship the water distribution engine has to EPANET. Where the predecessor’s behaviour is reproduced, substituted, or corrected, the model import says so through named notices rather than silently rewriting.

For migration guidance, see Migrating from SWMM.

See INP Format Support for current SWMM input coverage.