Andrew%20D.%20Dehoff,%20P.E%20and

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Position Analysis and Forecast-Based Water Supply
OperationsReducing Risk and Saving Money by
Operating Smarter

• Andrew D. Dehoff, P.E and
• Daniel P. Sheer, Ph.D., P.E.
• October 22, 2004

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Conowingo Pool and surrounding area
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Conflicting Objectives

• FERC minimum pool elevation
• FERC minimum flow requirements
• FERC recreation requirements
• Municipal withdrawals
• Habitat/fishery needs
• Salinity intrusion

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Limited Range of Operations
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No reservoir system is fail safe

• There is always some risk of running out of
  water.
• Water supply planning and operations can reduce,
  but not eliminate that risk.
• Position Analysis and forecast-based operations
  can allow you to manage risk explicitly and
  effectively

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Reducing Risk

• Capacity expansion reduces the risk of water
  shortage
• Smart operating policies can also reduce the risk
  of water shortage
• What are you willing to do or pay to achieve an
  acceptable level of reliability?

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Traditional Approach

• Safe Yield
  
• The amount of water that can be safely
  withdrawn from a reservoir (system) during some
  specified drought.
• Implied Reliability
• Safe yield of record
• 50-year safe yield
• 20-year safe yield
• Etc.
  

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Traditional Approach, contd.
Demand
Supply/Demand
Supply
Safe Yield
Time
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Traditional Approach, contd.
Demand
Supply/Demand
Supply
Safe Yield
Time
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Safe Yield Isnt a measure of reliability

• Safe yield in all but the simplest systems is
  more than the sum of the safe yields of
  individual facilities
• How the system is operated is very important
• Safe Yield isnt as Safe as It Sounds
• A system operated at historical safe yield will
  run out of water in any worse drought
• A system operated at 20 year safe yield will run
  out of water an average of 1 in 21 years

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Operations are Crucial to Supply

• Increasing water available during droughts
  increases reliability
• Conjoint operations of all facilities can
  substantially increase the water available during
  droughts
• The objective of such operations is to minimize
  water loss through spill or seepage

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Conjoint Use Simple Example

• Reservoir A - minimum spring inflow 25 of
  storage
• Reservoir B - minimum spring inflow 125 of
  storage
• Rule - empty reservoir B before drawing Reservoir
  A down more than 25
• If the drought is 2 years long, the system will
  benefit from a complete refill of Reservoir B in
  the spring of the second year.

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Examples of Conjoint Operations

• Upper and Lower Delaware Basin reservoirs
• Conowingo Pool and Baltimore City reservoirs
• Kansas River Basin
• Potomac River Basin

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Measuring Reliability

• In actual operations, emergency measures will
  prevent a system from running out of water
• Short-term conservation
• Alternative higher cost or lower quality sources
• Emergency measures will be undertaken in many
  droughts, even if they are less severe than the
  drought used to calculate safe yield
• Expected frequency, duration, and severity of
  drought emergencies are good measures

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Water Shortage Phases
Percent of Years in Each Phase
Phase 1
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Frequency, Severity and Duration

• Evaluated with a simulation model like OASIS
• Model includes facilities, demands, facility
  operating policies and drought management
  policies
• Model runs for a long (50 year or longer)
  record, usually of historical flows
• Model outputs include when emergency measures are
  imposed
• Model also produces output that can be used for
  environmental and economic evaluations

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Operations to Increase Reliability

• Low cost conservation measures implemented early
  can avert high cost draconian measures later.
• Water saved days demand_reduction/day
• Costs are more than monetary costs
• Costs of reducing 50 may be orders of magnitude
  larger than the cost of reducing 10

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Operating Rules

• Rules will also include provisions that minimize
  the negative effects of implementing emergency
  measures
• Rules will include operations to increase water
  available
• Rules should be adequate to handle any historical
  drought with a reasonable margin of safety.
• Rules will NOT be traditional safe yield rules

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Operating Rules Can Impact

• Demands
• Environmental requirements
• Balancing of supplies

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Operating Rules Can Consider

• Present state of the resource
• Storage
• Groundwater levels
• Inflow forecasts
• Time of year
• Demand forecasts
• Other factors

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We Cant Forecast Weather, But

• Streamflow depends on both weather and the
  dryness or wetness of the drainage basin
• The snow, soil and aquifers in the basin are
  reservoirs and we can get some useful information
  about how much water they will or can contribute
  to a stream or to wells over the next several
  months
• That information can be used to help make better
  operating decisions

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NOT Particularly Useful Forecasts

• Expected value of flow over the next x months
  -on average, there will always be enough water
• Flows are likely to be higher than normal or
  lower than normal - OK, worry a little less or a
  little more
• Worst case scenario - wonderful if youre
  paranoid, otherwise useless unless you know its
  probability (most are arbitrary)

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Useful Forecasts

• Flow time series and their probabilities
• Flow totals for various periods into the future
  and their probabilities

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Historical Time Series
Streamflow
time
t0
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Conditional Time Series Forecasts

• All start from todays flows
• Can be produced using statistical methods or
  rainfall runoff models that represent surficial
  and deep aquifer interactions (and snow, if
  appropriate
• Usually produce equally likely traces based on
  the assumption that any years historical weather
  is equally likely to repeat

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Time series conditioned upon todays basin
conditions
Streamflow
time
t0
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Forecasts to Useful Information

• Run the simulation model for each of the equally
  likely forecasts
• If there are 50 equally likely traces, and the
  reservoir falls below a given level in only 5 of
  the simulations, then the probability of falling
  below that level is 10 given the rules used in
  the simulation
• Probabilities of other consequences, including
  environmental consequences and utility revenue
  can also be evaluated

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How to respond

• If the probabilities of untoward events are too
  high, change the rule and test again. If it
  works and youre happy with the rule, do it.
• Find a rule that works for the long-term so you
  dont have to change rules on the fly

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Creating a Rule That Uses Forecasts

• Many forms are possible
• The Rocky Mount rule
• If the probability of the reservoir falling below
  25 in the next 8 weeks is gt 20 institute water
  use restrictions and reduce instream flow
• A possible (and untested) Conowingo Rule
• If the probability of Marietta flow falling below
  2500 cfs in the next summer is gt 20 begin
  Baltimore pumping

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Effective Rules

• Can NOT miss any droughts
• Must provide enough advance notice for remedial
  action to be effective
• Minimize the number of false alarms

Developed by trial and error through the use of
simulation models.
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Evaluating Rules
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Checking that the Rule is Sufficient
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Rocky Mount Results

• Change in operating rule replaced 70M pipeline
• Required emergency measures infrequent and not
  severe
• NCDNR agreed to tie reductions in instream flow
  requirements to imposition of demand restrictions
• Environmental impacts of infrequent reductions in
  low flow (still well above natural flows)
  judged less than impacts of building and
  operating the pipeline

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Thank You