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Tampa Bay Water provides water to six member governments, three cities and three counties, with a regional demand of 200 to 250 MGD (million gallons per day). The utility uses an integrated drought-resistant supply system composed of:
- 13 wellfields
- 8 groundwater treatment facilities
- Surface water treatment plant
- Desalination treatment plant
- 9 pump stations
- 270 miles of transmission mains.
The overarching question is how much water may the Tampa Bay region need by 2040?
It would be easy to answer this question if water supply and demand were known, fixed amounts. The truth is supply and demand aren’t fixed; they change due to weather, population, socioeconomics, and more. Planning for the future involves looking at historical records to anticipate supply and demand.
Let's begin with a look at annual rainfall and evapotranspiration rates here in the Tampa Bay area (see figure below).
Percent of annual rainfall that ends up as evapotranspiration as a function of annual rainfall.
Note that between 55% and 90% of the rain returns to the atmosphere as evapotranspiration rather than running off as surface water, with evapotranspiration declining as rainfall increases. This rate of evapotranspiration affects surface water amounts. How will a changing climate affect rainfall rate, evapotranspiration and resulting supplies of surface water?
Now, let's look at the annual surface water availability in the Tampa Bay Water area (see figure below).
The annual surface water availability in the Tampa Bay Water region.
Rainfall, and consequently surface water availability, varies seasonally. Rainfall and river runoff are greatest in the four months around summer and early fall. Tampa Bay Water stores surface water from the Hillsborough and Alafia Rivers to meet demand year-round.
Rainfall also varies annually as exemplified in 2017 when a drier-than-average winter and spring led into a winter with greater-than-average rainfall plus a hurricane. The overall annual actual rainfall, however, was below the average.
Demand is driven by socioeconomic parameters, including population growth, income, and price. Climate impacts the demand, and there are strategies for both passive and active demand management. Key uncertainties to consider when planning for water supply in the future include how climate will impact supply sources, level of service, regulatory requirements, finance, and sustainability.
"Forecasting is the art of saying what will happen, and then explaining it why it didn’t.” – Anonymous
Before evaluating supply, we predict demand. As the graph below reinforces, forecasting demand is challenging and rarely accurate.
Forecasts made between 1967 and 2012 and the actual water demand (black solid line).
The graph below demonstrates the challenges in predicting how many housing units to provide for. Note that each year forecasts a different number, often on the order of tens of thousands of units. For every 50,000 additional housing units, roughly 11 MGD would be required and 10 MGD requires $150M to $250M in capital investment. (A Hot-Dry climate scenario would shift demand upward by almost 20 MGD.) The ability to forecast accurately becomes a matter of supply as well as economy.
Range of variability in housing units based on the baseline year for each projection.
Once future demand requirements are established, Tampa Bay Water considers supply sources.
- Identify options such as expanding surface water treatment plants or expanding our desalination capacity.
- Stress test to see if a given project concept is worth further investigation. Conduct a full Monte Carlo simulation (334 demand-supply pairs.)
Each full run took about 5 to 6 hours on a cluster of computers. Over a hundred project concepts were evaluated using the level of service criteria to meet demand. We grouped the potential scenarios and narrowed it to three surface water project concepts:
- Surface water treatment plant expansion with existing source water
- Desalination facility expansion with existing source water
- New groundwater treatment plant via net benefit from SHARP program.
Consider the graph below:
The year the new Master Water Plan would need to be initiated.
Based on projected water delivery requirements, our water plan would need to shift by 2028.
It is important to look at a wide range of supply, demand, and operations possibilities before planning or building infrastructure. Also, define system-level metrics. For example, how frequently might there be a water shortage? Depending on the supply and demand, there should be a time frame associated with any given scenario.
Once expected future conditions are agreed upon, policy makers evaluate how much risk they can tolerate before a new supply will be required.
Now consider the graph below showing the range of projected demand on Tampa Bay Water:
Range of demand projections Tampa Bay Water can expect between 2014 and 2045.
The question to be asked, and answered from this data is this: How much risk can be tolerated before new supply is required?
Residual Risk Management was evaluated based on future demand using a water shortage mitigation plan, which includes supply and demand management, forecasting, using climate information to inform the public, and different scenarios in different phases, etc. While each of these was explicitly modeled, there is a lot of uncertainty inherent to understanding hydrology, hydrological cycles, and socioeconomic factors.
Tampa Bay Water Mitigation Plan.
