Security is more than storage
Water security depends on usable volume, replenishment, water quality, extraction limits, transfer capacity and the ability to manage demand. Define the service outcome first, including the acceptable frequency, duration and severity of restrictions or shortfalls.
Follow the water balance
At each time step, account for inflows, releases, abstraction, evaporation, spills and other losses. Apply consistent units and storage geometry. Water below a pump intake can remain in the model while being unavailable for supply.
Stress-test a portfolio
Compare source combinations under dry sequences, climate sensitivities, infrastructure outages and higher demand. Diversification is useful only if sources do not all fail under the same conditions. Include energy, treatment, environmental, cultural and affordability constraints in the assessment.
Connect thresholds to actions
A drought trigger should connect an observable indicator to an action, owner and delivery time. Test whether the intervention can start before the remaining buffer is consumed. Explore drought response planning and a worked regional assessment framework.
Define reliability, resilience and vulnerability
Reliability describes how often a defined service target is achieved. Resilience concerns recovery after a failure or disturbance. Vulnerability describes the severity or consequences of failures. Two options can have the same annual shortfall volume but very different maximum interruption durations and community consequences.
Specify the denominator and time step for every metric. Daily time reliability can hide within-day service failures, while annual volume reliability can hide a concentrated shortage in a critical month. Use a set of metrics that matches the decision rather than a single percentage.
Work a simple shortfall calculation
State whether the requested volume represents unrestricted demand or a restriction target.
Consider a ten-day example with demand of 2 ML/day. Supply is 2 ML/day on eight days and 1 ML/day on two consecutive days. Time reliability is 80%; total delivered volume is 18 ML against 20 ML requested, so volume reliability is 90%. Maximum consecutive shortfall duration is two days, and the maximum daily deficit is 1 ML.
Those numbers describe the chosen sequence. They are not probabilities for the future. To estimate future risk, use defensible inflow, demand and outage scenarios or probabilistic methods with clearly explained assumptions. Include the uncertainty associated with limited historical records and non-stationary conditions.
Model usable water and practical supply
Distinguish total storage from water above the intake or minimum operating limit. Represent evaporation using the appropriate changing area and climate inputs, and include other material losses. A minimum abstraction level should not necessarily prevent storage from declining further through evaporation.
Source quality can reduce treatable output before physical water runs out. Include events such as salinity, turbidity or algal problems where evidence supports them. Emergency supply may need mobilisation, permissions, connection works and quality management. An option that activates after storage is exhausted cannot provide the assumed protection.
Compare portfolios with correlated failures
Several sources can share climate, energy, transfer or treatment dependencies. Two river intakes may fail during the same regional drought; multiple pumps may depend on one electricity supply; alternative raw sources may use the same treatment plant. Count the actual diversification benefit, not just the number of sources.
Test combinations of demand reduction, operational changes, usable storage and new supply against common sequences and service targets. Include the energy and environmental consequences of maintaining readiness. A rarely used source may have minimum operating or restart requirements that affect both cost and response time.
Turn a trigger into an actionable pathway
A trigger links an observed state or forecast to a decision with enough time to act. For example, a storage threshold should be derived from projected demand, plausible inflow, accessible volume and implementation lead time. Copying a percentage-full trigger from another storage ignores its geometry and system role.
Define who monitors the trigger, who decides, how uncertainty is handled and what happens if delivery is delayed. Review the pathway after major events or demand changes. The Source tutorial demonstrates a closing daily balance and shortfall metrics; purified recycled water and desalination develop alternative-source considerations.
Sources & further reading
- Source: integrated water resource modelling ↗eWater Toolkit · Living technical overview
External source · Checked 24 September 2026 - Water resources planning guideline ↗Environment Agency, Natural Resources Wales & Ofwat · Living guidance
External source · Checked 24 September 2026 - Western Regional Water Strategy ↗NSW Government · 2022 strategy; living overview
External source · Checked 24 September 2026
Source findings are distinguished from editorial interpretation. Apply current local criteria and project evidence when making engineering decisions.