The excerpt explains how O'ahu's water cycle is driven by prevailing northeast trade winds and southwest Kona winds, producing orographic rainfall primarily on the windward Koʻolau side. Rain infiltrates porous volcanic dike rock, where it is stored in chambers and later accessed via horizontal dike wells. Excess runoff feeds streams, waterfalls, and forested watersheds, eventually percolating to form a basal freshwater lens that floats atop denser salt water beneath a non‑porous cap rock. On the leeward Kona side, artesian wells pump water from the basal lens, but over‑pumping can draw salt water upward, creating brackish water. The text includes a concise vocabulary list of key hydrological and geological terms.
Topics
Trade Winds, Kona Winds, and Orographic Rainfall on O'ahu
How prevailing north‑east trade winds and south‑west Kona winds generate moisture, and how the Koʻolau mountains force air upward to produce daily orographic rainfall on the windward side.
Porous Dike Rock and Dike Wells: Natural Aquifer Capture
The role of volcanic dike rock as a sponge‑like reservoir, the formation of underground chambers, and the use of horizontal dike wells to extract stored rainwater.
Basal Freshwater Lens and Cap Rock: Fresh‑Salt Water Interface
The formation of a freshwater lens that floats on denser salt water beneath an impermeable cap rock, and how this lens supplies the island’s potable water.
Artesian Wells on the Kona Side and Saltwater Intrusion Risk
How artesian wells pump water from the basal lens on the leeward side, and why over‑pumping can draw brackish water into the supply, making it unsuitable for drinking.
Trade Winds and Kona Winds: Drivers of O'ahu's Rainfall Patterns
Explanation of the northeast trade winds and southwest Kona winds and how they influence moisture transport around the island.
Orographic Rainfall on the Windward (Koʻolau) Side
How moist air is forced upward by the high Koʻolau mountains, cooling and condensing into heavy, daily rain on the windward side.
Porous Dike Rock and Dike Wells: Natural Aquifer Extraction
Characteristics of volcanic dike rock as a sponge‑like reservoir and the use of horizontal dike wells to tap stored groundwater.
Watersheds and Runoff: Surface Water Flow to the Basal Lens
The role of saturated land, streams, waterfalls, and forested watersheds in channeling rainwater toward subsurface storage.