An integrated system for monitoring sap flow and simultaneously performing data-logging and automatically scheduling irrigation in a field; comprising a plurality of in situ sap flow gauges that perform its comprehensive computer-controlled tasks in the field being irrigated. This plurality of sap flow gauge can be detached from and reattached to a like plurality of plant stems as appropriate. A scaling mechanism is used to calculate crop water usage in variously-sized fields. An integrated portable computer apparatus is programmed to perform a plurality of actions including sap flow monitoring via a controller and data-logger; data-logging and automatic irrigation scheduling; a plurality of sap flow gauges; an automatic rain gage; a water meter; an irrigation valve actuator; and any auxiliary and independent weather or soil monitoring sensors. The system is connected to a PC, mounted in a weatherproof enclosure, and powered by rechargeable battery, solar panel, or AC mains power with battery backup. Computerized monitoring and control procedures are initialized via a custom-developed graphical user interface. At any predetermined interval set by a user, the depth or volume of water required to balance transpiration losses is calculated and irrigation is then automatically triggered. The system affords closed loop control with water flow from soil through plant to atmosphere and from irrigation--and having the benefit of a feedback loop.