Impacts of irrigation regimes with saline water on carrot productivity and soil salinity

Abstract

A three-year study was conducted to evaluate the effects of different irrigation regimes with saline water on soil salinity, yield and water productivity of carrot as a fall-winter crop under actual commercial-farming conditions in the arid region of Tunisia. Carrot was grown on a sandy soil and surface-irrigated with a water having an ECi of 3.6 dS/m. For the three years, a complete randomized block design with four replicates was used to evaluate five irrigation regimes. Four irrigation methods were based on the use of soil water balance (SWB) to estimate irrigation amounts and timing while the fifth consisted of using traditional farmers practices. SWB methods consisted in replacement of cumulated ETc when readily available water is depleted with levels of 100% (FI-100), 80% (DI-80) and 60% (DI-60). FI-100 was considered as full irrigation while DI-80 and DI-60 were considered as deficit irrigation regimes. Regulated deficit irrigation regime where 40% reduction is applied only during ripening stage (FI-DI60) was also used. Farmer method (Farmer) consisted in giving fixed amounts of water (25 mm) every 7 days from planting till harvest. Results on carrot production and soil salinization are globally consistent between the three-year experiments and shows significant difference between irrigation regimes. Higher soil salinity in the root zone is observed at harvest under DI-60 (3.1, 3.4, 3.9 dS/m, respectively, for the three years) and farmer irrigation (3.3, 3.6, 3.9 dS/m) treatments compared to FI-100 treatment (2.3, 2.6 and 3.1 dS/m). Relatively low ECe values were also observed under FI-DI60 and DI-80 treatments with respectively (2.7, 3, 3.5 dS/m) and (2.5, 2.9, 3.3 dS/m). ECe values under the different irrigation treatments were generally lower than or equal to the EC of irrigation water used. Rainfall received during fall and/or winter periods (57, 26 and 29 mm, respectively, during the three years) contributed probably to leaching soluble salts from the root zone. Highest carrot yields for the three years were obtained with SWB scheduling technique FI-100, (29.5, 28.7 and 26.8 t/ha) although we didn’t find significant differences with the regulated deficit irrigation regime (FI-DI60). Compared to FI-100, significant reductions in carrot yields were observed under DI-80 and DI-60 deficit irrigation treatments resulting from a reduction in roots number/m 2 and average root weight. The farmer’s method not only caused significant reductions in yield but also resulted in using 43–57% more water and increased soil salinity. For all irrigation treatments, carrot yields were higher in the first year compared to the two following years. Water productivity (WP) values reflected this difference and varied between 3.2 and 9.7 kg/m 3. The lowest WP values were observed for the farmer’s method, while the highest values were obtained under DI-60 deficit irrigation treatment. The scheduling technique using SWB with variable doses is more efficient than the traditional technique used by farmers in carrot production. The FI-100 irrigation scheduling seems to optimize the use of saline water in carrot production and to control soil salinity. Under situations of water shortage, adopting deficit irrigation strategies (FI-DI60 and DI-80) could be an alternative for irrigation scheduling of carrot crop under the conditions of Mediterranean arid in southern Tunisia.