Two-Dimensional Modelling of Nitrate Transport in Canola Field Under Moistube Irrigation Using HYDRUS 2D/3D

Tinashe Lindel Dirwai^1*Aidan Senzanje²Tafadzwanashe Mabhaudhi³

• ¹International Water Management Institute (South Africa), Pretoria, South Africa
• ²School of Life Sciences, University of KwaZulu-Natal, Scottsville, KwaZulu-Natal, South Africa
• ³London School of Hygiene and Tropical Medicine, University of London, London, London, United Kingdom

We demonstrate nitrate distribution and leaching in the soil profile under Moistube irrigation (MTI). Whilst other studies describe wetting patterns and qualitative nitrate retention potential, they do not simulate nitrate transport in variably saturated zones with detailed temporal-spatial fluxes. The experiment was conducted in a 20 m by 8 m naturally ventilated Greenhouse under three water application regimes, namely, (i) full irrigation at 100% ETc, (ii) deficit irrigation (DI) at 75% ETc, and (iii) 55% ETc thereby strengthening external validity of MTI beyond laboratorybased tests. Each irrigation regime had four plots measuring 2 m × 1 m which were physically and hydrologically separated by 1 m buffer the plots. Fertilizer was applied at a rate of 210 ppm over two split applications. Soil samples for fertility analysis were collected directly next to MTI lateral and 15 cm away from the MTI lateral at varied depths before and after 2 h, 4 h, 24 h, 48 h, and 72 h of each fertigation exercise. HYDRUS 2D/3D was used to simulate solute transport under the irrigation regimes. The partial factor productivity of applied N (PFPN) was used as a proxy for nitrogen use efficiency. The extreme deficit irrigation regime (55% ETc) had a pronounced leaching effect compared to the other two regimes. The full and optimal DI strategy yields were ≥ 1.15 ton.ha -1 and PFPN of 1.72 kg.kg -1 and 1.29 kg.kg -1 , respectively. The nitrogen budget revealed that the extreme deficit irrigation regime (55% ETc) had pronounced leaching, followed by the optimal deficit irrigation regimes. HYDRUS 2D/3D successfully simulated solute movement under full and optimal DI [nRMSE ≤ 0.24, EF ≥ 0.54, PBIAS ≤ -7.41%] and performed poorly under the extreme DI regime. It was concluded that fertigation events under optimal deficit MTI could be employed without the farmer incurring yield penalties. Moreover, this study positions MTI as a precision fertigation tool in climate-smart agriculture, especially for nitrate-sensitive zones.