Mechanisms of water transport through soil from a pervaporative irrigation system

Abstract

This thesis presents the results of an investigation into the transport of water through soil from a pervaporative irrigation tube. This irrigation system uses a selective polymer membrane that allows water to cross by the process of pervaporation, which releases water into the soil in vapour phase. The polymer membrane is formed into a tube, buried in the soil and filled with saline water. The selectivity of the membrane retains salt within the tube whilst water is released, so the system treats the water and distributes it for irrigation simultaneously. The distribution of water from the system, rather than the treatment efficiency, was the focus of this project. Although in the field the system will interact with the plants it irrigates, in this work, for simplicity, plants were neglected. Instead a series of laboratory experiments were conducted to quantify the flux from the pervaporative tube in different soil types and in different humidity conditions. The results of these experiments highlighted, for the first time, the significance of vapour flow and condensation in the soil during the pervaporative irrigation process. Soil types with high water sorption at low relative humidity (e.g. saline sand) had an increased amount of condensation in the soil which resulted in an increased flux from the pipe. A moisture sorption isotherm was a useful predictor of this behavior. A numerical model was then developed to simulate the experimental findings. Experimental results had demonstrated that diffusion and condensation of water vapour through the soil were significant processes hence these were the main focus of the modelling work. Liquid flow was also simulated when the water content in the soil was sufficient. To simplify the representation of the pervaporative tube it was assumed that the membrane acted as a 100% humidity boundary in the soil thus it was assumed that it was the transport of water through the soil that limited the flux of water from the pervaporative tube rather than the membrane structure itself. The similarity between the observed and simulated results supported the basis of this conceptual understanding of pervaporative irrigation. This research raises a number of further questions regarding the interaction between plants and pervaporative membranes. These include how plants might interact with vapour flows through the soil and the influence of soil salinity on the interaction between the pervaporative membrane and the plant roots.