Monsoon Drainage for Arizona Horse Arenas

Arizona monsoon season runs roughly June 15 through September 30, and during peak storms a single cell can drop 1 to 3 inches of rain in under an hour. Onto bone-dry desert ground that cannot absorb it. Onto an arena pad that — if not properly engineered — turns into a pond that takes days to dry out.

Why standard arena drainage fails in Arizona

Most outdoor arena drainage assumes 0.25 to 0.75 inches of rain per hour, with subsoil that absorbs at least some of the runoff. Arizona violates both assumptions:

• Monsoon rates are 4 to 10x higher.
• Caliche subsoil is impermeable. Water has to be moved off the pad — it will not soak in.

Engineering monsoon-grade drainage

A properly built Arizona arena combines four systems:

• Crowned grading. 2 to 3 percent slope from centerline to edges for outdoor arenas, sized for the worst expected rainfall event.
• Perimeter French drains. Tied to discharge points outside the arena footprint, sized for monsoon flow.
• Free-draining sub-base. Compacted aggregate that moves water laterally to the drains.
• Geotextile separation. Prevents footing migration into the sub-base over time.

Covered arenas change the drainage problem

A covered arena removes the rainfall load, but you still need perimeter drainage to handle splash, blown rain, and runoff from the roof. Roof drainage on a 100x200 covered arena moves serious water — it has to go somewhere that is not into your footing.

The caliche connection

None of this drainage works if caliche is left in place under the arena. Read our Phoenix caliche guide for the excavation side.

Building or rebuilding in Arizona?

See our Arizona service area or call 888-995-0622 for a free site assessment.