Moistube ™ Irrigation (MTI) Discharge Under Variable Evaporative Demand

We investigated the conceptual capability of Moistube irrigation to discharge under zero applied positive pressure under varied climatic conditions by inducing an artificial evaporative demand (Ed) or negative pressure around moistube tubing. This was premised on the null hypothesis that an artificially induced Ed or negative pressure does not impact moistube discharge. Moistube tubing was enclosed in a 1 m long PVC conduit. A 20 l water reservoir placed on an electronic balance provided a continuous supply of water whilst a three-speed hot air blower facilitated the radiative factor and advection process. The procedure was conducted under varied climatic conditions of air velocities (ua) 1.2 m.s-1, 2.5 m.s-1, and 3.0 m.s-1 and the experiment run times were 159 h, 134 h and 10 h, respectively. The average temperature (Tave) and relative humidity (RH) data for ua = 1.2 m.s-1 were 53{degrees}C and 7.31%, whilst for ua = 2.5 m.s-1, Tave was 56{degrees}C and RH = 7.19%, and for ua = 3.0 m.s-1, Tave was 63{degrees}C and RH = 6.16%. The experimental data was input into the four variable Penman-Monteith method to compute the evaporative demand (Ed). For each of the air velocities, the respective Ed values obtained were 0.16, 0.31 and 0.36 mm.d-1. The Bowen ratios (r) were well below 1 (r < 1), which suggested a sufficient supply of moisture to evaporate. For Ed = 0.16 mm.d-1 the vapour pressure deficit (VPD) was 113.08 mbars, whilst for Ed = 0.31 mm.d-1 and for Ed = 0.36 mm.d-1 the VPD were 129.93 mbars and 150.14 mbars, respectively. The recorded discharges (q) at t = 10 hrs for Ed= 0.16 mm.d-1 was 7.67*10-3 l.hr-1.m-1, whilst for Ed = 0.31 mm.d-1 q = 14.5*10-3 l.hr-1.m-1, and for Ed = 0.36 mm.d-1 q = 20.8*10-3 l.hr-1.m-1. The imposed negative pressure causes an exponential increase in moistube discharge, thus disproving the null hypothesis. The higher the evaporative demand the higher the discharge. This phenomenon allows moistube irrigation to be used for deficit irrigation purposes and allows irrigators to capitalize on realistic soil matric potential irrigation scheduling approach.