A housing has at least one port for allowing entry of subterranean water into an interior of the housing when the housing is buried in soil. The housing has an inlet fitting configured to be coupled to drip tubing and an outlet for discharging water conveyed from a pressurized water source conveyed through the tubing. A valve includes a valve member that is movable to open and close a fluid communication path between the inlet fitting and the outlet. A hygroscopic member is positioned in the interior of the housing and is capable of absorbing subterranean water and expanding to move the valve member to close the fluid communication path. A spring element is positioned to bias the valve member to open the fluid communication path.