Detecting water stress and irrigation time in viticulture from stem's water content and electrical conductivity measurements

About 75% of accessible fresh water resources is used for irrigation with about 40% water use efficiency. It is possible to save considerable amounts of water by an efficient irrigation scheduling. Otherwise, excess water use may lead to environmental and economical problems. Therefore, a more accurate, sensitive, simple (farmer-friendly) and automatable method for monitoring of crops’ water stress and irrigation time is vital. Methods based on monitoring plant water status are preferred to those measuring soil water status or climatic conditions. Because of the limitations in monitoring plant water status (particularly in orchard trees) in roots and leaves, stem water content measurement (θs) is suggested. The objective of this research was to detect water stress and irrigation time in vines from stem water content (θs) and electrical conductivity measurements (ECs) using TDR (Time-Domain Reflectometer) method. With this aim, four vines were selected and planted in the research area. A 70 mm long probe made from three rods of 1.7-mm diameter stainless steel at 50-mm spacing was installed in each stem. Probes were connected to a multiplexer by 8.0 m coaxial cables and to TDR by 1.0 m coaxial cables. ECs, θs and tree stem temperatures were measured every 30 minutes parallel to soil water content and climatic parameters. Irrigation was performed when 70 % of the available water was consumed within 0-90 cm soil profile by replenishing to the field capacity. The relationship between soil water content and ECs and θs were monitored between April-2011 and September-2011. Stem water content of vines increased significantly in early spring. Later, it fluctuated as a function of plant water stress or soil water status. While the soil water content varied between 18 and 28% between two irrigation's, stem water contents changed between 14 and 18%. It gradually decreased towards the end of growing season and eventually gave a minimal response to soil water content variations. ECs and θs produced the same trend. A statistically significant positive linear relationship was found between soil water content and ECs. This implies that ECs can be replaced with the stem as well as soil water content in detecting water stress and irrigation time.