بررسی توزیع رطوبت- نیترات و عمق نصب لاترال‌ها در آبیاری قطره‌ای سطحی و زیرسطحی

Subsurface and surface drip irrigation systems have been preferred by most experts, due to their numerous advantages over other irrigation methods. High frequency irrigation systems are difficult to plan and have complex design procedures to achieve sustainable irrigation conditions. At the same time, these production systems are becoming harder, in an effort to maximize the return on valuable and scarce resources such as water and nitrate. Advanced fertigation systems combine drip irrigation and fertilizer application to deliver water and nutrients directly to the roots, with the aim of synchronizing the applications with crop requirement, and keeping up the required concentration and distribution of fertilizer and water in the soil. Hence a clear comprehension of water movement in the soil is important for the design, operation and management of irrigation and fertigation under the surface and subsurface drip irrigation systems. At the same time, it is necessary to assess the performance of these irrigation systems, because considerable leaching can occur near drip lines, even under deficit irrigation conditions. The loss of nutrients, particularly nitrate, from drip irrigation systems can be costly and pose a serious threat for receiving water bodies. The installation depth of laterals in Subsurface Drip Irrigation (SDI) systems is one of the most important design parameters that if disregarded, deep percolation and nitrate losses could be considerable and lead to low system performance and adverse environmental consequences. In this research, in order to achieve the optimal management of these systems, the distribution patterns of moisture and nitrate were analyzed simultaneously. The main objectives were (1) to determine the optimal installation depths of drippers according to distribution uniformity of moisture and nitrate, and (2) to minimize the deep losses in subsurface and surface drip irrigation system.
All the equipment was installed in a drip irrigation system in accordance with the actual situation of a farm in the central laboratory of university of Tehran. Water was pumped through the polyethylene pipes (diameters of main, sub main, lateral pipe were 50, 20 16 mm, respectively) and transferred from a tank to emitters. The system consisted of the float (to maintain the water level in the tank) and valves (to direct the flow), filters (to avoid emitter clogging and improve distribution uniformity), a pressure gauge (to keep the pressure constant through all the experiments) and an on-off control valve (to control water flow in container). All experiments were carried out at the constant pressure of 2 bars. In order to regulate discharge rate, a bypass system was also designed to reduce the working pressure of the system. The experiments were carried out in a transparent plexy-glass tank (0.5m×1.22m×3m) for three different soil textures (fine, medium and heavy) with the irrigation intervals of 6hr. The drippers were installed at four different soil depths (0, 15, 30 and 45cm) and were operated with two different outflow rates (2.4 and 4 lit/s). Urea fertilizer was utilized and injected into the final third of irrigation intervals. Nitrate and moisture distribution patterns were monitored in different directions (horizontal, downward and upward) under the surface and subsurface drip irrigations.
Considering the distribution uniformity of moisture and nitrate and with the goal of minimizing the deep losses, the optimal installation depths of drippers were determined 30cm for clay soil texture, loamy and sandy. Moreover, in surface drip irrigation systems, the distribution pattern of nitrate extended to the depth of 20, 40 and 40cm, respectively, and did not go beyond the effective soil depth. Considering more gradual distribution of nitrate in clay soil texture (compared to the coarser textures), the injection time was considered longer for these soil samples. But, it is recommended to set the injection time as short as possible in course soil textures to lower nitrate losses. Finally, by considering the losses of water and nitrate in both surface and subsurface drip irrigation systems, the main design variable (Q: discharge rate of emitters (lit/hr), V: volume of applied water (lit), and F: nitrate concentration in fertigation (mg/lit)) were optimized for different treatments. The results showed that considering the optimized design variables could result in minimal moisture and nitrate distribution losses under the surface and subsurface drip irrigation systems. This could be used as a general guideline for all drip irrigation systems.