In the last 40 years, Hawaii has lost 20 percent of its annual rainfall. Forecasts produced by the University of Hawaii predict that if this decrease continues at its current rate, by the last quarter of this century, the leeward sides of most Hawaiian islands will have 70 percent to 90 percent less rainfall than today.

Why is all this happening? The lee sides of the islands seldom get tradewind rains, and depend on the great storms of winter — cold fronts dipping down from the north, and Kona Storms coming from the west — for their rainfall. Warming seas have caused these storms to migrate north, and storm after storm is passing the islands by.

But the windward side is also in trouble. As the seas have warmed, the path of the tradewinds coming across the Pacific has moved to the south. Historically, the trades have reached the islands from the east northeast, hitting the Koolaus head-on, with the mountains forcing their warm, moisture-laden air to rise, where it cooled and condensed, creating 65 percent of our rainfall. Arriving now from the east, the tradewinds are glancing by the Koolaus, producing considerably less rising action and considerably less rainfall.

But the story gets worse. The warming seas have killed the winds altogether on many days. Forty years ago, we had tradewinds 291 days a year; now it’s 210 days. And with temperatures continuing to rise, we can expect to lose more tradewind days. No trades, no rain on the Koolau side.

What will this ever-decreasing rainfall do to our aquifers? As the decades progress, they will become further shrunken.

On Oahu, the Central Aquifer is a key source in meeting the daily requirements of much of the island. From it, the Honolulu Board of Water Supply pipes 14 million gallons daily to Waianae, Ewa and the primary urban center. Additionally, that aquifer sits at a higher elevation, and much of its water seeps downhill to fill the great Pearl Harbor Aquifer.

By the end of the century, Central Oahu is expected to have 30 percent to 50 percent decrease in rainfall, which means less water to recharge the crucial Central Aquifer.

The streets and houses of Mililani and Waipio already create hard surfaces, which keep rain from penetrating to that aquifer. The planned Koa Ridge development will cover over another major area. That land is currently farmed, which provides open space for rainfall as well as crop irrigation water for aquifer recharge. Ideally it must be kept in farmland if future generations are to have fresh water.

There is a somewhat similar problem in the Kapolei-Ewa area. Historically, when it rained or the land was irrigated, water seeping down through the ground mixed with seawater that had moved into the aquifer through the volcanic rock substructure, and formed brackish water. This water was too saline to drink, but worked fine for watering farm crops and golf courses, and other nondrinking uses. From sugar times, it has been the irrigation source for the Ewa plain.

Today, most of the Ewa plain is covered with houses and paved streets. Rainwater hits hard surfaces and is carried to the ocean. The Ho‘opili and University of Hawaii-West Oahu farmland are the only remaining open spaces where rain can fall and lands can be irrigated, replenishing the aquifer. But they, too, are scheduled for development. When this land is covered, and the freshwater recharge is lost, the aquifer water will become too saline for use.

Warming seas and the great changes in wind and rainfall they would cause were not known when the Koa Ridge, Ho‘opili and UHWO developments were approved. But they are real, going to happen, and disastrous. For the survival of future generations, we must keep all of these current farmlands open with rain and crop irrigation replenishing our aquifers.

Kioni Dudley, a Kapolei resident, is a retired educator. For more, see www.2050Hawaii.com.