This page explains the terminology used throughout Hydra’s documentation and in .inp files. If you are familiar with EPANET, most of these will be review.
A passive conduit between two nodes. Carries flow and produces headloss due to friction.
Pump
An active link that adds energy to the flow. Defined by a head-flow curve or a constant power rating.
Valve
A control device that regulates flow or pressure. Types include PRV (pressure-reducing), PSV (pressure-sustaining), FCV (flow-control), TCV (throttle-control), and GPV (general-purpose).
The total energy of water at a point, expressed as a height of water (metres or feet). Equal to elevation + pressure head + velocity head. Velocity head is typically negligible in distribution networks.
Pressure
Gauge pressure at a node — the difference between the total head and the node elevation. Positive pressure means the water is above atmospheric.
Headloss
The loss of energy as water flows through a pipe, due to friction and minor losses. Higher flow or smaller diameter means more headloss.
Demand
The rate at which water is withdrawn at a junction (litres/second, gallons/minute, etc.).
Emitter
A pressure-dependent outflow device at a junction, used to model sprinklers, leaks, or irrigation outlets. Flow is proportional to a power of the local pressure.
Demand-Driven Analysis (DDA)
Hydraulic mode where all demands are fully satisfied regardless of pressure. The default for most network models.
Pressure-Dependent Analysis (PDA)
Hydraulic mode where demand delivered at each junction depends on the local pressure. More realistic under low-pressure or deficit conditions.
FAVAD leakage
Background pipe leakage modelled using the Fixed and Variable Area Discharge method. Specified per pipe in the [LEAKAGE] section.
A simulation that runs for a period of time (hours or days) and tracks how the system state evolves — as opposed to a single steady-state snapshot.
Hydraulic timestep
The interval at which the solver recomputes the full network hydraulic state. Typically 1 hour.
Reporting step
The interval at which results are saved. Must be a multiple of the hydraulic timestep.
Pattern
A time series of multipliers applied to a base value (demand, pump speed, reservoir head, etc.) to simulate variation over the simulation period. A multiplier of 1.0 means the base value is used unchanged.
Curve
An XY dataset defining a relationship: pump head vs. flow, pump efficiency vs. flow, tank volume vs. level, or valve headloss vs. flow.
A dissolved substance (e.g. chlorine, fluoride) tracked through the network. Reactions consume or produce the constituent as it moves through pipes and tanks.
Water age
The time elapsed since water entered the network from a source. Longer age can indicate stale or degraded water.
Source trace
Tracks the fraction of water at each point in the network that originated from a specified source node. Useful for source blending analysis.
Bulk reaction
A chemical reaction occurring in the water volume (e.g. chlorine decay in the bulk flow).
Wall reaction
A chemical reaction at the pipe wall (e.g. chlorine consumption by biofilm or pipe material).
Quality source
An injection of a constituent into the network at a node. Types include concentration setpoint, mass injection, flow-paced booster, and setpoint booster.
EPANET network input file. Plain text, defines all network elements, options, and patterns. This is the file you load into Hydra.
.out
Binary output file. Contains time-series results for every node and link at every reporting step. EPANET-compatible — usable by existing post-processing tools.
.rpt
Plain-text report. Summary of simulation results in EPANET report style.
.json
JSON report. Summary-level results including warnings, energy usage, and flow/mass balance.