During pervaporative irrigation, water is transported across a polymer membrane by the process of pervaporation due to a chemical potential gradient. The membrane retards the transport of contaminants, thus reducing the risk of soil degradation when irrigating with low quality water (e.g. brackish or saline water). To irrigate the pervaporative membrane is formed into a pipe, buried in the soil and filled with water.  As well as providing in-situ water treatment of low quality water, the irrigation flux from this membrane irrigation system inherently responds to soil moisture content, providing more water when the soil is dry or when crops take it up (water ‘on demand’). Such a system has significant possibilities to provide water-efficient irrigation. Laboratory experiments presented here show that the flux across the membrane is related to the humidity in the soil thus the flow and condensation of water vapour in the soil have a significant effect on the system. These experiments also demonstrate the importance of osmotic effects on the flux from the system. A model has been developed to simulate the flux from the irrigation system in different soil types and different atmospheric conditions. This model has been used to investigate the interaction of the system with moisture sinks within the soil, showing that the flux from the pervaporative irrigation system responds to the demand from surrounding moisture sinks within the soil. This demonstrates how plants could interact with the pervaporative membrane to increase the flux and provide sufficient water to sustain growth, as has been observed in field trials.