Outlet pressure and flow: the physics of pouring

The fundamental mechanism: gravity, not pressure

Haws watering cans use gravity to move water. When you tip the can toward the outlet, the weight of the water creates pressure at that point. This pressure — called outlet pressure — pushes the water out through the spout and any attached rose or downspout.

Haws cans do not use pumps, sealed systems, or stored pressure. The outlet pressure exists only while the can is tipped and water is moving. It disappears when you stop pouring.

Understanding outlet pressure

Outlet pressure is determined by two factors:

The height of water inside the can. The vertical distance between the water surface and the outlet creates what's known as head pressure. When the can is full, this distance is greatest. As you pour and the water level drops, the distance decreases, so outlet pressure decreases. This is why a pour becomes weaker as the can empties.

Your tipping angle. The angle at which you tip the can affects how much water weight works to push the outlet forward. A steeper angle produces higher outlet pressure. A shallow angle produces lower outlet pressure. You control this continuously during pouring.

Between these two variables, outlet pressure is never constant. It's highest at the start of a pour when the can is full and tipped steeply. It's lowest toward the end of a pour when the can is nearly empty and tipped less steeply.

What outlet pressure does

Outlet pressure determines how fast water leaves the spout and how it behaves after leaving.

Without a rose. Water exits as a single continuous stream. Higher pressure produces a faster, more forceful stream that travels further. Lower pressure produces a slower stream.

With a rose fitted. A rose divides the stream into multiple jets that pass through small holes in the face. These jets then break into droplets and form a spray pattern. Outlet pressure is essential to keep this spray stable. If pressure drops too low, the spray collapses into dribbles and runs across the face rather than projecting outward. Each rose has a minimum operating pressure below which the spray breaks down.

What "flow" means

Can flow is a classification describing the relative amount of water a watering can delivers per unit time, compared to other cans. It's governed by the size of the spout opening (the ferrule) and how the spout is shaped.

Haws classifies cans into flow categories: Low, Medium, High, and Very High.

A higher flow classification means water leaves the spout more quickly for a given outlet pressure. This suits tasks where you want to cover ground steadily. A lower flow classification delivers water more gradually, allowing you to control the amount of water more accurately. This suits tasks where precise placement matters.

Flow is not the same as outlet pressure. Two cans with different flow classifications can have the same outlet pressure, but one will deliver water faster than the other. Flow is determined by the can's geometry; outlet pressure is created during pouring.

The three variables that govern watering

Your experience of watering — how fast the water comes out, how steady the stream or spray is, how much control you have — results from three interdependent variables working together.

Head pressure: the amount of water still in the can, and how steeply the can is tipped.

Can flow: how much water the spout is designed to let through.

Outlet restriction: if a rose is fitted, it restricts flow. Finer spray weights (smaller holes) restrict more. Coarser spray weights (larger holes) restrict less.

At full pour with a full can and a gentle rose, you might have a steady, controlled flow. At the end of that same pour with the can nearly empty, you have the same flow classification but much lower outlet pressure, so the water comes more slowly. If you've fitted a finer rose for delicate plants, you have more restriction, so flow is lighter. If you've fitted a coarser rose, you have less restriction, so flow is heavier.

All three variables interact. Changing any one changes how the can behaves in your hand.

Why this matters

Understanding these principles helps you choose the right can for your tasks and get the behaviour you want in use.

If you're watering seedlings, you need low outlet pressure and fine restriction to keep the spray gentle. You might use a smaller can so you refill frequently and avoid pouring from a full can, which would increase pressure. You might select a fine spray rose and tip the can slowly and gently.

If you're watering established borders and shrubs, you can use higher outlet pressure and less restriction. A larger can reduces refill trips. A coarser spray or no rose at all moves water quickly.

Outlet pressure varies during every pour, and you control it. This is why pouring technique — the angle and speed at which you tip — affects what the water does. Haws cans are designed to give you this control consistently.