Evaluating Moisture Distribution and Salinity Dynamics in Sugarcane Subsurface Drip Irrigation

The average sugarcane cultivation area in Iran's southwestern farms is 12,000 hectares wherein irrigation is by the furrow method and the average water consumption is about 30,000 m(3) per hectare. Therefore, the feasibility of subsurface irrigation method was examined to reduce the water consumption in this area. As records of earlier researches have shown that the subsurface drip irrigation (SDI) can be useful for the sustainable development of agriculture and conservation of the available water resources. This study applied it to cultivate two rows of sugarcane with supply pipes to report the moisture and salinity distribution in heavy-texture (clay-silt) soils. Finding the optimal conditions, as regards the drippers' distance, discharge and frequency, irrigation time and duration and wet bulb development, is important for the subsurface drip irrigation. Besides, due to the limitations of field studies, numerical models can be used to simulate the prevailing conditions. Therefore, this study has evaluated the accuracy of the 2D HYDROS model for simulating the wetting pattern. The results were then used to determine the pipes' best installation depth and the drippers' distance on them. Tests were done for a dripper discharge of 1.2 lit/hr; supply pipe installation depths of 15, 20 and 30 cm; and dripper distances of 50 and 60 cm in three iterations. Results showed that the moisture content in all treatments varied between the field capacity limit and the permanent wilting point in horizontal and vertical distances of up to 60 and 90 cm, respectively, from the supply pipes. During the study period, the soil salinity increased in the 0-30 cm layer and decreased in the 30-60 and 60-90 cm depths. The highest salinity occurred at the furrow bottom and the lowest was on the ridge. Finally, considering the extension of moisture distribution and salinity in soil, the best supply-pipe installation depth and dripper distance were found to be 20 and 50 cm, respectively. Using HYDRUS 2D, the moisture distribution around supply pipes was estimated at three installation depths for an inter-dripper distance of 60 cm. To evaluate the software accuracy, a TDR (time-domain reflectometer) was used to measure the moisture content at 15 points around the supply pipes; according to the normalized root-mean-square error (NRMSE) criterion, the accuracy was, respectively, fair, good and fair for a 60-cm inter-dripper distance and 15, 20 and 30 cm pipe installation depths.