How Rain, Splash-Out & Evaporation Affect Salt Levels

One of the most common questions from new saltwater pool owners is, "Where does the salt go?" After adding hundreds of pounds of salt to their pool, many are surprised when they need to add more later in the season. Understanding the physical processes that affect your pool's salt concentration is key to effective and efficient pool maintenance. The short answer is: **salt only leaves your pool when water leaves your pool.** This guide will break down the primary factors—rain, splash-out, backwashing, and evaporation—and explain how each one impacts your salt level.

The Core Principle: Salt Does Not Evaporate

This is the most important concept to grasp. When water evaporates from your pool surface on a hot, sunny day, only the pure water (H₂O) turns into vapor and leaves. The salt, minerals, and all other dissolved solids are left behind. This means that evaporation does not lower your salt parts per million (ppm) reading. In fact, it does the opposite.

How Evaporation Increases Salt Concentration

Imagine you have a glass of saltwater. If you leave it in the sun, the water level will go down, but the amount of salt in the glass remains the same. Since you have the same amount of salt in less water, the concentration (ppm) has actually increased. The same happens in your pool. During long, dry periods with lots of evaporation and no fresh water being added, your salt ppm can creep up. While usually not a dramatic increase, it's a key reason why your salt level doesn't drop from evaporation.

Factors That Lower Salt Concentration

Since salt only leaves when water leaves, any process that removes water from the pool and requires you to add fresh, unsalted water will dilute the salt concentration and lower your ppm. These are the real culprits for salt loss.

1. Rainwater

Rain is essentially distilled water with no salt content. A heavy downpour adds a significant amount of fresh water to your pool, diluting the existing salt. If the rain is heavy enough to overflow the pool, it's physically removing salty water and replacing it with fresh water. After a major storm, it's crucial to test your salt level, as it will almost certainly be lower. You'll then need to use a Pool Salt Calculator to determine how much salt to add back.

2. Splash-Out and Drag-Out

This is a major, often underestimated, source of salt loss, especially in pools that see a lot of activity. Every time someone cannonballs, jumps, or splashes, they are physically ejecting salty water from the pool. Similarly, when swimmers get out of the pool, water drips off their bodies and swimsuits onto the deck—this is called "drag-out." Over the course of a summer with lots of swimming, this can add up to a significant volume of water loss, which you then replace with fresh water, lowering your salt ppm.

3. Backwashing Your Filter

If you have a sand or D.E. (diatomaceous earth) filter, you need to backwash it periodically to clean it. The backwashing process involves reversing the flow of water through the filter and sending hundreds of gallons of dirty, salty pool water to waste. When you refill the pool with fresh water to bring the level back up, you have significantly diluted the salt concentration. Backwashing is one of an owner's most direct and impactful ways of lowering the salt level.

4. Leaks in the Pool or Plumbing

A leak, even a small one, is a constant drain of salty water from your pool. If you find yourself having to add fresh water to your pool more than once a week to maintain the water level (and it's not due to heavy splash-out), you likely have a leak. This constant cycle of losing salty water and adding fresh water will steadily decrease your salt ppm and require you to add more salt than usual. Our guide on how to find a pool leak can help diagnose this issue.

Practical Summary: When to Test and Add Salt

Based on these factors, you can develop a smart testing routine:

• Monthly Check: A regular monthly salt test is a good baseline during the swim season to catch any gradual changes.
• After Heavy Rain: Always test your salt level after a significant rainfall event.
• After a Lot of Use: If you've had a big pool party with lots of splashing, it's wise to check your levels a day or two later.
• After Backwashing: Always test your salt after backwashing your filter.
• If You Suspect a Leak: If you're adding water more than usual, test your salt level. A consistently dropping salt level is a strong indicator of a leak.

By understanding that salt loss is tied to water loss, you can move from guessing to knowing. This saves you from adding salt unnecessarily (which can damage equipment) and ensures your salt chlorine generator always has the fuel it needs to keep your pool sanitized and sparkling.