As drought-plagued western states watch their water sources literally dry up, California is digging deeper to tap the most basic source of all: groundwater.

Amid a worsening drought scorching much of the western U.S., California’s numerous and varied potential drinking water sources are becoming unusable or, when purchased from Colorado or other districts, too costly. As reservoirs, rivers and washes dry out, state water resource administrators are boosting use of alternative supplies to ensure water keeps flowing.