Consider a seasonally dry landscape in a hilly or mountainous region where little or no rain or snow falls for months at a time. How is it that months into the dry period, forests can remain green and productive and streams can keep flowing? The answer must be that earlier precipitation remains available, as subsurface moisture to trees and as groundwater that slowly drains to rivers. This subsurface water supply maintains not only trees and streams but also entire water-dependent terrestrial and river ecosystems. These streams also become the rivers that serve as the main water supplies for downstream hydropower and for agricultural and urban users.

All of this water storage occurs in the critical zone, the near-surface layer of Earth where coevolving geomorphic, hydrologic, geochemical, and ecological processes create dynamic, deep water-storing systems out of solid, nearly impermeable bedrock. But under what conditions, and by what mechanisms, is subsurface moisture maintained or, in multiyear drought conditions, say, insufficient? Understanding controls on critical zone properties and processes like water storage, as well as how the critical zone will respond to land use and climate change, and how this knowledge can enhance ecosystem resilience and prediction of drought effects will be, well, critical for many years to come.

In California, with its wet, cool winters and dry, hot summers, programs at two Critical Zone Observatories (CZOs) have intensively monitored and quantified water use by the forest (evapotranspiration, or ET) and potential sources of moisture in the subsurface for more than a decade.  At the Southern Sierra CZO, along a transect from the oak savannahs near the San Joaquin Valley city of Fresno to the conifer forests of the high Sierra Nevada, researchers have investigated effects of elevation and a climate gradient on moisture. At the Eel River CZO, west of Laytonville, amid the conifer–hardwood forests and savannah of the Northern Coast Ranges, they have looked at effects of sharply contrasting bedrock properties in a common climate.

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