2025 Orange County Water Demand Projection Model
6. UWMP Scenarios
The 2025 Urban Water Management Plan requires tabulation of the long-term projected water use over the next 20 years, in 5-year increments, for three types of hydrology: Normal Year: The demand a Supplier believes will occur during normal conditions. It could be a single year or an average range of years that most closely represent the average water demand. This is provided by the Baseline Forecast in Section 5. Single Dry Year: A year that represents higher water demand on the Supplier. Provided by finding the historical year most sensitive to all weather parameters (2010 – present) Five Consecutive Year Drought: The demand during the driest five-year historical sequence for the Supplier. Provided through a separate regression on total Orange County demand and historical weather (2004 – present)
6.1.1
Single Dry Year Scenario
The single dry year is the historical year in which weather factors have the largest combined impact as measured by a hot-dry index (HDI). The HDI was developed in four steps: 1) Calculate the HDI for each month in the historical weather series. This monthly hot-dry index (MHDI) is calculated for each member agency (a) and monthly observation (m) as the sum of the estimated weather effects for each sector model (j). The weather effects for each sectoral model are derived from the historical weather values (W) and weather model parameters (β) assigned to the weather variables in each model (i), which vary by member agency ( Equation 6-1 ).
𝑀𝐻𝐷𝐼 ,, = 𝑒𝑥𝑝൫∑ 𝛽 ,, 𝑊 ,,, ൯
Equation 6-1
2) Calculate the HDI for each calendar year in the historical weather series. The monthly index values are summed across the month of any given annual period to derive a set of annual index values (AHDI) for each member agency and sector ( Equation 6-2 ).
𝐴𝐻𝐷𝐼 , = ∑ 𝑀𝐻𝐷𝐼 ,, ଵଶ ୀଵ
Equation 6-2
3) Weight the annual sector indices by the proportion of total annual water sales attributed to each sector (w) to define the final hot-dry index (HDI) values used to evaluate and select the hot-dry periods for scenario development ( Equation 6-3 ). The weights are based on the estimated proportion of sales by sector for as many full annual records were available for each agency from 2010 to 2024.
𝐻𝐷𝐼 =∑ 𝑤 ×𝐴𝐻𝐷𝐼 , ସ ୀଵ
Equation 6-3
4) Determine the maximum value of HDI for each member agency across all years in the historical weather series. Retain weather data for the selected calendar year for each member agency.
6-1
Appendix G - 70
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