17.2 Source Supply, Yield, and Demand
Key Takeaways
- A usable supply is limited by the weakest applicable constraint among source availability, withdrawal authority, intake or well delivery, treatment capacity, storage, and transmission.
- Average-day, maximum-day, and shorter peak demand describe different time periods and must be compared with capacity expressed over the same period.
- Volume equals flow rate multiplied by time; the WPI table gives 1 MGD as 694 gpm and 1 L/s as 0.0864 MLD for rapid conversions.
- No universal reserve margin proves a source is adequate; drought, water quality, outage, permit, and operating conditions define the scenario.
Supply is a chain, not one nameplate number
The WPI outline lists supply under Source Water Characteristics. For an operator, source adequacy means more than water appearing in a river, reservoir, spring, or aquifer. The amount that can actually serve customers depends on a chain: water available under defined hydrologic conditions, legal withdrawal or allocation, intake or well performance, raw-water conveyance, treatability, plant capacity, finished-water pumping, storage, and transmission. The usable rate for a scenario cannot exceed its limiting step.
Keep these terms separate:
| Term | Operational meaning | Why it can change |
|---|---|---|
| Source availability | Water physically present and accessible under stated conditions | Drought, recharge, streamflow, reservoir level, competing withdrawals |
| Tested well or intake rate | Rate demonstrated under stated test and operating conditions | Water level, screen or intake condition, interference, fouling |
| Authorized withdrawal | Amount and conditions allowed by the applicable authority | Permit, allocation, season, minimum-flow or operating conditions |
| Treatment capacity | Rate the approved process can reliably treat to requirements | Raw-water quality, unit outage, backwash, residuals handling |
| Demand | Water needed over a defined interval | Population, weather, fire flow, irrigation, leakage, industrial use |
Do not add unlike limits and call the sum yield. A reservoir allocation expressed as annual volume does not by itself establish the instantaneous intake rate. A pump nameplate does not prove aquifer supply. A plant rated above its source rate cannot create raw water. Storage shifts when water is delivered, but it does not create new long-term source yield.
Put demand and capacity on the same clock
Average-day demand averages use across a selected period. Maximum-day demand is the highest daily total in the period. Peak-hour or another short peak describes a still shorter interval. Their values and ratios are system-specific. Compare a daily source volume with daily demand, or convert a continuous rate to daily volume before comparing. Also distinguish metered production from billed customer use: plant process water, authorized uses, meter error, leakage, and the timing of storage changes can make them differ.
The basic relationship is Volume = Flow × Time. Useful WPI conversion-table values are 1 MGD = 694 gpm and 1 L/s = 0.0864 MLD. When converting directly, keep enough digits for the work and round only the final answer as instructed.
Worked U.S. customary example
Two wells can deliver 480 and 390 gallons per minute (gpm) under the stated test condition. Combined source rate is 480 + 390 = 870 gpm. Daily source volume is:
870 gpm × 1,440 min/day ÷ 1,000,000 gal/MG = 1.2528 MGD
The treatment train is limited to 0.95 million gallons per day (MGD), while forecast maximum-day demand is 0.82 MGD. Under this stated all-units-available condition, the treatment step is the bottleneck and the arithmetic margin is 0.95 − 0.82 = 0.13 MGD. That is not a universal adequate reserve. If the 390-gpm well is unavailable, remaining supply is 480 × 1,440 ÷ 1,000,000 = 0.6912 MGD, below the forecast maximum day even though the plant itself can treat more. Operators then follow the approved shortage, storage, interconnection, or demand-management plan.
Worked metric example
A source can deliver 60 liters per second (L/s), and the plant limit is 4.5 megaliters per day (MLD). Convert source rate: 60 L/s × 0.0864 MLD per L/s = 5.184 MLD. With a 3.9-MLD maximum-day demand, the plant is again the limiting step and its arithmetic margin is 4.5 − 3.9 = 0.6 MLD. If a 26-L/s source unit fails, 34 L/s × 0.0864 = 2.9376 MLD, which does not meet 3.9 MLD. The unit conversion supports the decision; it does not decide which contingency is authorized.
The WPI table also defines per-capita water use as produced volume divided by population. If production is 720,000 gallons per day for 4,800 people, 720,000 ÷ 4,800 = 150 gpcd. The metric relationship is identical: 3,600,000 liters per day divided by 24,000 people equals 150 liters per capita per day (Lpcd). This production indicator supports trend and planning comparisons, but it is not automatically indoor customer consumption. Use the same production boundary and time period when comparing years.
Judge yield under stated conditions
Yield must be qualified. A pump test may establish a well rate for a test duration and water-level response. A water-right document may set a different authorized maximum. A planning study may estimate a dependable supply for selected drought, environmental-flow, or infrastructure assumptions. None is a universal sustainable number. WPI publishes no fixed percentage reserve or source-to-demand ratio for every jurisdiction.
For an operating review, record source level or flow, pumping rate and duration, raw-water quality, unit availability, treatment constraints, storage trend, demand period, permit conditions, and weather. Compare like conditions over time. A falling reservoir, increasing well drawdown, deteriorating raw-water quality, or repeated inability to recover storage can reduce practical supply before a pump reaches its nameplate limit.
Scenario: high capacity, wrong conclusion
A plant can treat 2.0 MGD, its intake pumps total 2.4 MGD, and current demand is 1.6 MGD. During low flow, the authority permits only 1.4 MGD withdrawal. Calling the system adequate because treatment exceeds demand ignores the controlling source condition. The correct comparison identifies 1.4 MGD as the present limit, confirms storage and other approved supplies, notifies the responsible role, and applies the system's shortage plan.
Official source trail
A source can provide 1.3 MGD, treatment is limited to 1.0 MGD, and authorized withdrawal is 0.9 MGD under the current low-flow condition. Which value limits production from this chain now?
A source delivers 25 L/s continuously. Using the WPI conversion 1 L/s = 0.0864 MLD, what daily volume is available?
Which statement is most defensible when source capacity exceeds forecast maximum-day demand by 15 percent?