# Phoenix Water-Use Seasonality

Load this only when estimating a customer's next bill. It explains why
Phoenix-area water usage swings so hard across the year and gives a simple
multiplier you can apply.

## The pattern

Phoenix is a desert. Indoor water use (kitchen, bath, laundry) is roughly
flat all year. The swing comes almost entirely from **outdoor** use:
landscape irrigation and evaporative cooling. That load tracks heat. It is
near zero in winter and peaks in mid-summer, when many households run
irrigation daily and evaporative coolers run for weeks.

As a rule of thumb, summer usage runs **3 to 4 times** winter usage for a
typical single-family home with a yard. The shoulder months (spring and
fall) sit in between and are the steepest *changes* month to month — that is
where a "rising" or "falling" trend label is most reliable.

## Seasonal index (multiply the annual monthly average by this)

| Month | Index | Note |
|------:|:-----:|------|
| Jan   | 0.55  | winter low |
| Feb   | 0.55  | winter low |
| Mar   | 0.70  | irrigation begins |
| Apr   | 0.90  | ramping |
| May   | 1.20  | ramping fast |
| Jun   | 1.55  | near peak |
| Jul   | 1.75  | peak |
| Aug   | 1.70  | peak |
| Sep   | 1.35  | cooling off |
| Oct   | 1.00  | shoulder |
| Nov   | 0.70  | winding down |
| Dec   | 0.55  | winter low |

The indices average to ~1.0 across the year. So if a home averages 6,000
gallons/month annually, expect ~3,300 in January and ~10,500 in July.

## Estimating next month's bill

1. Take the customer's **trailing 12-month average** gallons (or the best
   average you have).
2. Multiply by next month's index from the table to get expected gallons.
3. Convert gallons to dollars using the tier schedule on the statement
   (base charge + per-1,000-gallon tiers). Summer usage usually pushes a
   household into a higher tier, so the dollar jump is steeper than the
   gallon jump.
4. Give a range, not a point estimate: **±15%** is reasonable, because
   weather, a single irrigation-timer change, or a leak can move a month
   meaningfully.

## Watch-outs

- A summer bill that is *lower* than the index predicts can mean a broken
  irrigation timer or a vacant home — worth flagging, not just celebrating.
- A winter bill that is *higher* than the index predicts is a classic
  **leak** signature (indoor use should be flat in winter). Flag it.
- New sod, a new pool, or a new evaporative cooler resets the household's
  baseline; one season of history will mislead you until it settles.