Effect of surface and subsurface drip irrigation on yield and water use efficiency of tomato and salt distribution in soil profile

According to the arid and semi-arid climate conditions and insufficient rainfall in Bushehr province, southern Iran, water deficit is the most important limiting factor for agricultural development in this region. Therefore, the use of modern irrigation methods to prevent water loss in this sector is essential. Drip irrigation is of particular importance due to some specific technical characteristics and proper conditions that it provides in the root zone in terms of nutrition, ventilation, moisture and salinity. In recent years, use of drip-tapes to irrigate row crops has extended, due to some comparative advantages. As well, drip-tape irrigated tomato fields have developed in Bushehr province and many efforts have been made by the government to support the farmers. Therefore, the present study was designed to evaluate the effects of different modes of drip-tape installation (surface and subsurface) on yield and irrigation water use efficiency of tomato in the region. This study was carried out during three consecutive cropping seasons (2008-2011) at the Agricultural and Natural Resources Research Center of Bushehr (51° 32′ E and 35° 51′ E; elevation of 100 meters above sea level). The experiment was conducted as split plot in a randomized complete block design (RCBD), with four mods of drip-tape installation in three replications. The treatments included different installation of drip-tape: on soil surface (T1); within the small surface furrows (T2); in the depth of 10 cm from soil surface (T3); and in the depth of 20 cm from soil surface (T4). Accordingly, a drip-tape irrigation system was designed and installed based on the objectives of the experiment and arrangement of the treatments. The amount of irrigation water was calculated based on FAO-Penman-Monteith equation. The area of each experimental plot was 28 m2 and consisted of two laterals of 10 m long and 140 cm apart. A volumetric flow meter was used to control the irrigation of plots. In order to investigate the effect of considered treatments on salt distribution in root zone, soil was sampled using a manual auger in two stages: once in mid-growing season (after the end of winter rains) and again after harvesting. Sampling was done at horizontally distances of 0, 30, 60 and 90 cm from drip-tape and at depths of 0-30, 30-60 and 60-90. Also, for investigating the effect of the treatments on the uniformity of water distribution through the drip-tape, 8 apertures at the beginning, one third, two third and the end of the drip-tapes were selected. Then, sampling cans were placed at the selected points. By measuring the water collected in the cans, Christiansen coefficient of Uniformity (UC) was calculated using the obtained data. Generally, with subsurface installation of drip-tape, fruit yield and IWUE increased, compared with surface treatments. However, with increasing depth of drip tape placement from 10 cm to 20 cm, fruit yield and IWUE decreased. The highest fruit yield (48.0 t/ha) and IWUE (9.4 kg/M3) belonged to T3. However, this treatment had no significant difference with T2. The lowest fruit yield and IWUE belonged to the T1 treatment, which had the lower soil moisture content due to surface runoff and increased evaporation losses. In the T2 treatment, which is a common practice of farmers, the wetted area was limited to the floor width of the furrow. So, lateral expansion of wetted area in T2 was less than that of T1 treatment. Also, in T2 treatment, irrigation/rainfall water was stored in furrows, instead of producing runoff, and then gradually infiltrated into the soil. This feature helped to store moisture in the root zone and salt leaching from the soil. Therefore, the treatments of T3 and T2 had not significant difference from each other. Christiansen coefficient of Uniformity (UC) ranged from 86% (in T2 and T3 treatments) to 89% (in T4 treatment), but these changes were not significant. This was due to the use of appropriate water, proper filtration system and the use of new drip-tapes at each year. Salt distribution in soil profile was different under the evaluated treatments. Generally, in the middle decades of the growth season, due to the coincidence with winter rainfall and salt leaching with rainwater, salt accumulation in the surface layer was lower, but in the late decades, due to the end of the rainy season, premature heat and increased evaporation need of atmosphere, the salt movement toward the surface layer increased. For this reason, at the end of the growing season, in almost all evaluated treatments, the highest soil salinity was measured in surface layer of the soil (0-30 cm), at a distance of 30 cm from the location of the drip-tape. According to the results discussed, because of easy installation and management of drip-tape, T2 treatment preferred to subsurface treatments for the experiment location and similar regions.