Evaluation of combined open ditch and subsurface drainage: Experimental data and optimization of specifications in arid Northwest China

Open ditches and subsurface drainage are effective measures for improving saline soils. Installations of subsurface drainage are now complementing surface drainage in Northwest China, but their optimisation has not been attempted. Therefore, the drainage and desalination performance of a combined subsurface drainage-open ditch system was analysed using two years of field experiments. The data were then utilised to calibrate and validate a water and salt transport model. The drainage volume in surface drains were 9-fold those in subsurface pipes. Additionally, 25 sets of orthogonal numerical experiments were designed with the subsurface pipe length, depth, and open ditch depth as variables. The results revealed that these three factors significantly affected the desalination efficiency of salinealkaline farmland (P < 0.05). The ditch depth, pipe length, and pipe depth F values were 9.954, 50.286, and 6.557, respectively, and no interactions were observed among these factors. When a single open ditch was used for drainage, the desalination rate initially increased and then decreased as the distance from the open ditch increased. The inflection point varied with the open ditch depth and occurred within a range of 32–43 m when the ditch depth was 180–300 cm. The combination of an open ditch and a subsurface pipe produced a larger desalination area, and its efficiency was 170 % that of a single open ditch. Within the inflection point range, the desalination rate increased with increasing ditch depth. Beyond the inflection point, subsurface drainage played a primary role, and the desalination rate increased as the subsurface drainage depth increased but remained relatively stable along the drainage direction. The optimal installation depth for subsurface pipes was estimated to be 90–110 cm, and the depth of ditches was 180–210 cm in a combined system. The maximum length for full flow in long-distance subsurface drainage was 750–850 m. This study provides references for the optimal application of combined subsurface drainage–open ditch systems in arid Northwest China.