Water Resources Assessment and Water Balances

Introduction

Water-resources decisions depend not merely on how much water exists, but on where, when, in what quality, and with what reliability it is available. A basin may receive abundant annual rainfall yet experience seasonal scarcity because precipitation, demand, storage and environmental requirements are poorly aligned. Conversely, a relatively dry basin may support dependable supplies through storage, conjunctive use, demand management and carefully governed allocation.

A water balance is an application of conservation of mass to a defined control volume and period. It is the foundation for catchment assessment, reservoir planning, groundwater management, drought analysis, environmental-flow protection and allocation policy. Its usefulness depends on consistent boundaries and terminology. In particular, gross abstraction, return flow and net depletion must not be combined in ways that double-count water.

Learning Objectives

After studying this chapter, readers should be able to:

• formulate basin-, catchment- and project-level water balances;
• distinguish gross abstraction, return flow and net consumptive depletion;
• assess spatial and temporal water availability;
• naturalise observed streamflow without double-counting upstream uses;
• estimate dependable yield and reliability from hydrological records;
• apply reservoir continuity and sequent-peak analysis;
• incorporate environmental-flow requirements into allocation decisions;
• evaluate uncertainty, data limitations and governance implications.

1. The Water-Balance Principle

1.1 Control volume and accounting period

Every water balance must identify:

1. Spatial boundary: reservoir, aquifer, catchment, administrative basin or project system.

2. Temporal boundary: storm, month, season, hydrological year or multiyear planning horizon.

3. Included stores: surface water, soil moisture, groundwater, snow, reservoirs and distribution storage.

4. Included transfers: rainfall, streamflow, groundwater flow, abstractions, returns, imports and exports.

For any consistently defined control volume,

\[

\Delta S = \sum I-\sum O

\]

where \(\Delta S\) is the change in stored water, \(I\) denotes external inflows and \(O\) denotes external outflows. Positive \(\Delta S\) indicates storage accumulation.

1.2 Integrated basin balance

If the control volume includes both natural hydrological stores and water users, abstractions and returns occurring entirely inside the basin are internal transfers. A suitable balance is

\[

\Delta S=P+Q_{in}+G_{in}+W_{imp}-ET-Q_{out}-G_{out}-W_{exp},

\]

where:

• \(P\) = precipitation over the basin;
• \(Q_{in},Q_{out}\) = surface-water inflow and outflow;
• \(G_{in},G_{out}\) = groundwater inflow and outflow;
• \(W_{imp},W_{exp}\) = engineered imports and exports;
• \(ET\) = total evapotranspiration, including water consumed by irrigated crops where relevant;
• \(\Delta S\) = aggregate change in surface, soil-moisture, groundwater and artificial storage.

All terms must refer to the same period and normally be expressed as a volume, such as million cubic metres (Mm³).

1.3 Balance for a hydrological source subsystem

If users are outside the modelled source subsystem, gross abstraction is an outflow and recoverable return flow is an inflow:

\[

\Delta S_h=P+Q_{in}+G_{in}+W_{imp}+R-ET_n-Q_{out}-G_{out}-A_g,

\]

where \(A_g\) is gross abstraction, \(R\) is return flow entering the defined subsystem, and \(ET_n\) excludes use-related consumption already represented through \(A_g-R\).

Equivalently, if

\[

D=A_g-R,

\]

then

\[

\Delta S_h=P+Q_{in}+G_{in}+W_{imp}-ET_n-Q_{out}-G_{out}-D.

\]

Here \(D\) is net depletion for the specified boundary and period. One must use either gross abstraction with return flow, or net depletion alone—not net abstraction plus return flow. The simple relation \(D=A_g-R\) also requires that returns occur within the relevant boundary and accounting period and remain usable. Exports, evaporation, incorporation into products and polluted or delayed returns may constitute depletion.

1.4 Depth–volume conversion

A water depth over an area is converted to volume by

\[

V=dA.

\]

Because 1 mm over 1 km² equals 1,000 m³,

\[

V(\text{Mm}^3)=\frac{d(\text{mm})A(\text{km}^2)}{1000}.

\]

Worked Example 1: Annual catchment balance

A 250 km² catchment receives 900 mm of annual precipitation. During the same hydrological year, surface inflow is 12 Mm³ and an interbasin scheme imports 5 Mm³. Annual evapotranspiration is 540 mm, surface outflow is 82 Mm³, groundwater outflow is 8 Mm³, and exported municipal water is 4 Mm³. Estimate annual storage change using an integrated catchment boundary.

Precipitation volume:

\[

P=\frac{900(250)}{1000}=225\text{ Mm}^3.

\]

Evapotranspiration volume:

\[

ET=\frac{540(250)}{1000}=135\text{ Mm}^3.

\]

Therefore,

\[

\Delta S=225+12+5-135-82-8-4=13\text{ Mm}^3.

\]

The catchment gained 13 Mm³ of storage during the year. This is an annual climatic balance; it does not use storm effective rainfall, which is an event-scale quantity representing rainfall available for direct runoff after losses.

2. Levels of Water-Resources Assessment

2.1 Basin assessment

A basin assessment evaluates total renewable water, upstream–downstream interactions, groundwater exchanges, environmental needs and interbasin transfers. It supports strategic allocation and transboundary management. Large basins should be divided into hydrologically meaningful sub-basins because basin-wide annual averages conceal local scarcity.

2.2 Catchment assessment

Catchment studies commonly estimate rainfall, evapotranspiration, runoff, recharge and storage change. For an ungauged catchment, regionalisation, rainfall–runoff modelling and remotely sensed data may supplement sparse observations. Model parameters should be transferred only between catchments with defensibly similar climate, topography, soil, geology and land cover.

2.3 Project assessment

A project-level balance addresses whether a reservoir, irrigation scheme, industrial facility or water-supply system can meet defined services. It may include:

• source inflow;
• direct rainfall on storage;
• reservoir evaporation and seepage;
• conveyance losses;
• gross withdrawals and recoverable returns;
• environmental releases;
• spill and operational storage change.

For a reservoir,

\[

S_t=S_{t-1}+I_t+P_t^r-R_t-E_t-Se_t-Sp_t,

\]

where \(S_t\) is end-period storage, \(I_t\) is catchment inflow, \(P_t^r\) is direct rainfall volume on the reservoir, \(R_t\) is controlled release or withdrawal, \(E_t\) is evaporation, \(Se_t\) is seepage leaving the modelled storage, and \(Sp_t\) is spill. Reservoir area may vary with storage, so rainfall and evaporation sometimes require iterative calculation using an area–capacity curve.

3. Data Preparation and Naturalisation

3.1 Principal data sources

Assessment may draw upon:

• rain gauges, weather radar and satellite precipitation;
• stream gauges and rating curves;
• groundwater levels and pumping records;
• reservoir level–area–capacity surveys;
• meteorological estimates of evapotranspiration;
• abstraction licences and metered use;
• census, crop and industrial-production data;
• satellite observations of inundation, soil moisture, snow and actual evapotranspiration.

A long record is not automatically reliable. Gauge relocation, changing rating curves, land-use change, reservoir construction and missing observations may create non-homogeneity.

3.2 Natural and observed flow

Observed streamflow may already reflect upstream abstraction, return flows, reservoir regulation and land-use change. Naturalisation reconstructs the flow that would probably have occurred under a defined reference condition:

\[

Q_{nat}=Q_{obs}+D_{up}+\Delta S_{reg}-W_{imp,up},

\]

in a simplified period balance, where \(D_{up}\) is verified upstream net depletion and \(\Delta S_{reg}\) represents the effect of upstream regulation under a stated sign convention. Detailed naturalisation is normally performed at monthly or daily resolution.

Gross upstream withdrawals must not simply be added unless associated returns are treated consistently. Likewise, existing use must not be subtracted again from observed flow when the gauge already reflects that depletion. Analysts should state explicitly whether each flow series is observed, regulated, reconstructed or naturalised.

3.3 Closure error and uncertainty

A measured balance rarely closes exactly. The residual is

\[

\varepsilon=\sum I-\u001bsum O-Delta S_{obs}.

\]

The residual may result from measurement error, omitted groundwater exchange, unmetered abstraction, inconsistent timing or storage uncertainty. It should not automatically be assigned to a convenient component such as recharge.

Uncertainty should be reported through ranges, confidence intervals, sensitivity analysis or probabilistic simulation. Independent data sources—for example, streamflow, groundwater levels and remotely sensed evapotranspiration—can help diagnose implausible closure.

4. Spatial and Temporal Availability

4.1 Renewable, accessible and allocable water

Renewable water is replenished through the hydrological cycle over the selected period. Accessible water is physically and economically developable under available technology. Allocable water is the amount that may be assigned after accounting for environmental flows, existing rights, water quality, infrastructure capacity, operational reserves and legal constraints.

Thus, total runoff is not equivalent to available project yield.

4.2 Spatial variability

Precipitation, geology, land cover and demand are spatially heterogeneous. Useful tools include isohyetal mapping, Thiessen polygons, gridded climate products, digital elevation models, groundwater contouring and sub-basin water accounts. Spatial analysis should preserve upstream–downstream connectivity; water counted as available upstream cannot be independently counted again downstream.

4.3 Temporal variability

Annual totals can conceal seasonal deficits. Assessment should use a time step appropriate to the system response:

• daily or sub-daily for floods and run-of-river abstraction;
• monthly for seasonal reservoirs and irrigation planning;
• annual or multiyear for strategic drought and aquifer-storage analysis.

Aggregation can exaggerate yield because an annual surplus cannot satisfy a dry-season deficit unless adequate storage and conveyance exist.

4.4 Flow-duration curves

A flow-duration curve plots discharge against the percentage of time it is equalled or exceeded. A flow denoted \(Q_{90}\) is equalled or exceeded approximately 90% of the time and represents a low-flow statistic. It is not automatically an environmental-flow requirement or a safe abstraction limit; ecological seasonality, cumulative abstraction and water quality must also be considered.

5. Dependable Yield and Performance

5.1 Definition

Dependable yield is the water quantity that can be supplied at a specified reliability under defined infrastructure, operating rules, environmental constraints and hydrological conditions. It is not a fixed physical property of a river.

Common performance measures include:

Time reliability

\[

Rel_t=\frac{N_s}{N},

\]

where \(N_s\) is the number of periods in which demand is fully met and \(N\) is the total number of periods.

Volumetric reliability

\[

Rel_v=1-rac{\sum Def_t}{\sum Dem_t},

\]

where \(Def_t=\max(0,Dem_t-Sup_t)\).

Resilience measures how rapidly a system returns to satisfactory performance after failure. A common estimator is the number of failure-to-success transitions divided by the number of failure periods.

Vulnerability measures the magnitude of deficits, such as mean or maximum deficit during failed periods.

Reliability must always state the time step. A 95% annual reliability is not equivalent to 95% monthly reliability.

Worked Example 2: Reliability across a 20-year record

Annual water available after environmental requirements and unavoidable losses is:

92, 84, 76, 68, 61, 73, 88, 95, 79, 65, 58, 71, 83, 90, 74, 69, 62, 77, 86 and 72 Mm³.

Assume no carryover storage and a constant target supply of 70 Mm³/year.

Six years—68, 61, 65, 58, 69 and 62 Mm³—fail to meet the target. Hence,

\[

Rel_t=\frac{14}{20}=70\%.

\]

The deficits are 2, 9, 5, 12, 1 and 8 Mm³, totalling 37 Mm³. Therefore,

\[

Rel_v=1-rac{37}{20(70)}=0.9736\approx97.4\%.

\]

The high volumetric reliability but lower time reliability shows that failures are frequent enough to matter, although mostly moderate in volume.

There are three failure-to-success transitions and six failed years, giving an estimated resilience of

\[

Res=\frac{3}{6}=0.50.

\]

Mean deficit during failed years is \(37/6=6.17\) Mm³, and maximum deficit is 12 Mm³.

Sorting the record gives 58, 61, 62, 65, 68, 69, 71, 72, 73, 74, 76, 77, 79, 83, 84, 86, 88, 90, 92 and 95 Mm³. A yield of 62 Mm³ is met in 18 of 20 years, or 90% of the record. Under this empirical no-storage criterion, 62 Mm³/year is the largest observed-value yield meeting at least 90% annual reliability. The estimate remains uncertain because the record is finite and may not represent future climate.

5.2 Storage–yield analysis

For a required release \(R_t\) and inflow \(I_t\), the sequent-peak recursion is

\[

K_t=\max[0,K_{t-1}+R_t-I_t],

\]

where \(K_t\) is accumulated storage deficit. The maximum \(K_t\) over a complete representative sequence is the minimum active storage required in an idealised system, provided the sequence starts and ends under appropriate cyclic conditions. Evaporation, seepage, dead storage, operating rules and environmental releases must be included when material.

A design yield can be found iteratively: propose a target release, simulate continuity, test reliability and storage constraints, and adjust the target. Stochastic streamflow generation or climate ensembles may supplement historical simulation where records are short or non-stationarity is important.

6. Water Scarcity and Demand Assessment

Scarcity may be:

• physical, when hydrologically available water is insufficient;
• economic, when water exists but infrastructure or institutions cannot deliver it;
• seasonal, when supply and demand occur at different times;
• quality-induced, when pollution makes water unsuitable;
• institutional, when weak allocation or enforcement prevents equitable access.

A simple annual stress ratio is

\[

WSI=\frac{D}{A},

\]

where \(D\) is annual withdrawal or depletion and \(A\) is a consistently defined available resource. Interpretation depends strongly on whether environmental requirements, return flows and groundwater mining are included. The indicator should therefore not replace a monthly, spatially distributed balance.

Demand forecasts should distinguish withdrawals from consumption. Municipal withdrawal may largely return through wastewater, whereas irrigation consumption can be high. Efficiency improvements do not always create basin-scale savings: reducing canal seepage may also reduce aquifer recharge or downstream return flow. Savings should be evaluated as reductions in net depletion, not merely reductions in diversion.

7. Environmental-Flow Requirements

Environmental flows are the quantity, timing, duration, frequency and quality of flows needed to sustain freshwater ecosystems and the human livelihoods dependent on them. A single minimum discharge cannot reproduce an ecologically functional regime.

Major assessment approaches include:

1. Hydrological methods: percentages of natural flow, flow-duration indices or seasonal statistics.

2. Hydraulic-rating methods: relate discharge to habitat variables such as wetted perimeter or depth.

3. Habitat-simulation methods: model habitat suitability for selected species or life stages.

4. Holistic methods: construct a flow regime using multidisciplinary ecological and social evidence.

Environmental requirements should ideally be based on naturalised flow and specified by season. They may include dry-season baseflow, seasonal freshes, channel-maintaining floods and limits on rates of flow change. Environmental releases must be incorporated before estimating water available for new allocation, rather than treated as optional water remaining after abstraction.

8. Illustrative Case: Green Valley Supply Project

Background

Green Valley is a fictional catchment considering a municipal reservoir. The observed annual flow at the proposed dam site is 80 Mm³. Audited records show that existing upstream activities cause 12 Mm³/year of net depletion. Adding this depletion to observed flow produces a naturalised annual series of 92 Mm³. Gross upstream withdrawals are not used because their associated return flows are already reflected in the net-depletion estimate.

The monthly naturalised inflows are:

10, 8, 7, 6, 5, 4, 4, 5, 7, 10, 14 and 12 Mm³.

Existing upstream net depletion is 1 Mm³ each month, so current-condition project inflows are:

9, 7, 6, 5, 4, 3, 3, 4, 6, 9, 13 and 11 Mm³.

Problem

The project must provide:

• 26 Mm³/year of gross municipal withdrawal;
• 43 Mm³/year of environmental release immediately below the dam;
• 4 Mm³/year of net reservoir evaporation after direct rainfall.

Municipal return flow enters the river downstream of the project control boundary. It therefore cannot be deducted from the reservoir withdrawal when estimating source capacity.

Analysis and approach

For preliminary monthly analysis, municipal withdrawal plus net evaporation is represented as 2.5 Mm³/month. Environmental releases for the 12 months are:

3, 3, 3, 3, 3, 3, 3, 3, 4, 5, 6 and 4 Mm³.

Total monthly requirements are consequently:

5.5, 5.5, 5.5, 5.5, 5.5, 5.5, 5.5, 5.5, 6.5, 7.5, 8.5 and 6.5 Mm³.

Annual inflow is 80 Mm³ and annual requirement is

\[

26+43+4=73\text{ Mm}^3,

\]

leaving an annual surplus of 7 Mm³. However, annual totals do not demonstrate dry-season feasibility.

Applying sequent-peak analysis, accumulated deficits are:

0, 0, 0, 0.5, 2.0, 4.5, 7.0, 8.5, 9.0, 7.5, 3.0 and 0 Mm³.

The maximum idealised storage deficit is therefore 9 Mm³.

Outcome

The preliminary minimum active storage is 9 Mm³. Final design storage must be larger if allowances are required for dead storage, sedimentation, forecast error, operational reserve or more severe multiyear droughts. A full design would simulate the complete historical or stochastic monthly record rather than one representative year.

Learning points

• Observed flow already incorporates upstream depletion; existing use must not be subtracted twice.
• Naturalised flow is useful for ecological assessment and scenario comparison, but current-condition inflow is required for project operation.
• Gross project withdrawal is appropriate when returns occur outside the source boundary.
• An annual surplus can coexist with a substantial seasonal storage requirement.
• Environmental releases materially affect dependable project yield.

9. Governance, Ethics and Professional Practice

Water balances influence rights, livelihoods and ecosystems. Analysts have an ethical duty to disclose assumptions, uncertainty, data gaps and boundary choices. Optimistic treatment of return flows, groundwater recharge or future inflow can overallocate a basin and transfer risk to downstream communities.

Good governance requires transparent allocation registers, metering, compliance monitoring, drought rules and protection against cumulative abstraction. Transboundary basins additionally require shared data, agreed naturalisation methods and procedures for managing exceptional droughts.

Groundwater abstraction deserves particular caution. Pumping that exceeds recharge may initially be supplied by storage depletion and later by reduced river baseflow or induced recharge. A balance that labels all pumping as renewable because annual recharge is numerically similar may ignore location, timing and ecological impact.

10. Contemporary Methods and Best Practice

Remote sensing now supports precipitation estimation, reservoir-area mapping, snow monitoring, actual evapotranspiration and broad-scale groundwater assessment. Geographic information systems integrate these products with catchment boundaries and infrastructure. Machine-learning models can assist gap filling and streamflow forecasting, but they do not remove the need for physical consistency, independent validation and uncertainty analysis.

Climate change challenges the assumption that historical hydrology is stationary. Contemporary planning increasingly uses stress testing, multiple climate scenarios, adaptive operating rules and trigger-based drought plans. Robust decisions perform acceptably across several plausible futures rather than relying on one forecast.

Professional best practice includes:

• defining boundaries and time steps before calculating;
• maintaining consistent units and sign conventions;
• separating gross withdrawal, return flow and depletion;
• labelling flow records as observed, regulated or naturalised;
• evaluating seasonal and multiyear variability;
• reserving environmental flows before new allocation;
• testing yield against reliability, resilience and vulnerability;
• reporting uncertainty and avoiding false precision;
• independently reviewing high-consequence balances.

Chapter Summary

Water-resources assessment applies conservation of mass to a clearly specified spatial and temporal control volume. In an integrated basin balance, internal abstractions and returns are not separately counted. In a source-subsystem balance, gross abstraction may be paired with return flow, or both may be replaced by net depletion. Mixing these formulations causes double-counting.

Observed streamflow may reflect upstream use and regulation, whereas naturalised flow reconstructs a reference condition. Availability must be assessed spatially and temporally because annual basin totals can conceal local and seasonal deficits. Dependable yield is defined relative to reliability, infrastructure, operating rules and environmental constraints. Time reliability, volumetric reliability, resilience and vulnerability describe different aspects of system performance.

Environmental flows form a prior constraint on allocation and should represent ecologically important components of the flow regime. Reservoir continuity and sequent-peak analysis connect inflow timing to required storage. Reliable professional practice also requires uncertainty analysis, transparent governance, defensible naturalisation and adaptation to climatic non-stationarity.

Key Terms

Allocable water: Water that may be legally and operationally assigned after relevant constraints are satisfied.

Control volume: The spatial system across whose boundary water inputs and outputs are accounted.

Dependable yield: Supply that can be delivered at a specified reliability under defined conditions.

Environmental flow: Flow regime required to sustain aquatic ecosystems and dependent livelihoods.

Flow-duration curve: Relationship between discharge and its percentage exceedance time.

Gross abstraction: Total water withdrawn from a source before accounting for return flow.

Hydrological year: Twelve-month period selected to represent a complete annual hydrological cycle.

Naturalised flow: Reconstructed flow under a stated reference condition without specified human influences.

Net depletion: Gross abstraction minus usable return flow within the relevant boundary and period.

Non-stationarity: Change in the statistical characteristics of hydrological processes over time.

Reliability: Proportion of time or demand volume for which the required service is achieved.

Resilience: Ability or rate at which a system recovers from failure.

Return flow: Withdrawn water that re-enters the defined hydrological system.

Sequent-peak analysis: Recursive method for estimating storage needed to meet a specified release sequence.

Storage change: Difference between water stored at the end and beginning of an accounting period.

Vulnerability: Magnitude of adverse consequences or deficits when system failure occurs.

Water scarcity: Condition in which usable supply is insufficient to satisfy demands and environmental needs.

Water-balance residual: Unexplained difference between measured inputs, outputs and storage change.

Review Questions

Short Answer Questions

1. Why must a water balance specify both a spatial boundary and an accounting period?

2. Distinguish gross abstraction from net depletion.

3. What is the meaning of \(Q_{90}\) on a flow-duration curve?

4. Why can an annual water surplus coexist with seasonal scarcity?

5. Define volumetric reliability.

Long Answer Questions

1. Derive and explain integrated-basin and source-subsystem water balances, including the correct treatment of return flow.

2. Discuss the data and procedures required to naturalise an observed streamflow record.

3. Compare hydrological, hydraulic, habitat-simulation and holistic environmental-flow methods.

4. Explain how storage, operating rules and environmental releases influence dependable yield.

5. Critically examine the implications of climate non-stationarity for water-resources assessment.

Analytical / Application Questions

1. A 400 km² catchment receives 750 mm/year of precipitation and loses 500 mm/year through evapotranspiration. Surface and groundwater outflows are 70 and 12 Mm³/year, respectively, while imports equal 6 Mm³/year. Calculate annual storage change, assuming no other boundary fluxes.

2. Using monthly inflow and demand data for a proposed reservoir, apply the sequent-peak method and explain which additional allowances would be required for design storage.

3. An observed river flow is 120 Mm³/year. Upstream gross abstraction is 35 Mm³/year, verified return flow is 20 Mm³/year, and regulation causes no annual storage change. Estimate naturalised flow and explain why subtracting the gross abstraction again from observed flow would be incorrect.

Multiple Choice Questions with Answers

Question 1

In an integrated basin balance that includes water users, how should abstractions and returns occurring entirely within the basin be treated?

A. Both should be external outflows

B. They are internal transfers and should not be separately counted

C. Abstraction should be added as an inflow

D. Return flow should be ignored but abstraction deducted

Correct Answer: B. They are internal transfers and should not be separately counted

Question 2

A source subsystem has gross abstraction of 30 Mm³ and usable return flow of 18 Mm³ within the same period and boundary. What is net depletion?

A. 12 Mm³

B. 18 Mm³

C. 30 Mm³

D. 48 Mm³

Correct Answer: A. 12 Mm³

Question 3

What volume is represented by 800 mm of rainfall over 150 km²?

A. 12 Mm³

B. 53.3 Mm³

C. 120 Mm³

D. 1,200 Mm³

Correct Answer: C. 120 Mm³

Question 4

An observed flow is 80 Mm³/year and verified upstream net depletion is 12 Mm³/year. With no annual regulation effect or imports, what is the naturalised flow?

A. 68 Mm³/year

B. 80 Mm³/year

C. 92 Mm³/year

D. 104 Mm³/year

Correct Answer: C. 92 Mm³/year

Question 5

A flow denoted Q90 on a flow-duration curve is the flow that is:

A. Equalled or exceeded about 90% of the time

B. Exceeded only during the wettest 10% of years

C. Exactly 90% of mean annual flow

D. Automatically available for abstraction without ecological effects

Correct Answer: A. Equalled or exceeded about 90% of the time

Question 6

In a 20-year record, a target is fully achieved in 17 years. What is time reliability?

A. 15%

B. 17%

C. 80%

D. 85%

Correct Answer: D. 85%

Question 7

Total demand over a simulation is 1,000 Mm³ and cumulative deficit is 40 Mm³. What is volumetric reliability?

A. 4%

B. 40%

C. 96%

D. 104%

Correct Answer: C. 96%

Question 8

In sequent-peak analysis, what does the maximum accumulated deficit represent in an idealised system?

A. Minimum active storage required

B. Mean annual inflow

C. Reservoir dead storage only

D. Maximum flood spill

Correct Answer: A. Minimum active storage required

Question 9

Why should existing upstream abstraction not automatically be subtracted from gauged downstream flow?

A. All abstractions increase downstream flow

B. Gauged flow may already reflect upstream depletion

C. Stream gauges measure only groundwater

D. Upstream use never affects project yield

Correct Answer: B. Gauged flow may already reflect upstream depletion

Question 10

Which statement best describes environmental-flow assessment?

A. It requires only a single constant minimum flow

B. It should be undertaken only after all economic demands are allocated

C. It considers ecologically relevant flow magnitude, timing, duration and frequency

D. It is equivalent to reservoir evaporation loss

Correct Answer: C. It considers ecologically relevant flow magnitude, timing, duration and frequency