Soil salinity and water productivity of carrot-millet system as influenced by irrigation regimes with saline water in arid regions of Tunisia

Abstract

Field studies were conducted for three years to determine the effects of irrigation regimes with saline water (3.6 dS/m) on soil salinity, yield and water productivity of carrot and millet under actual commercial-farming conditions in the arid region of Tunisia. Carrot and millet were grown during fall-winter and summer seasons on a sandy soil and surface and drip-irrigated with well water having an ECi of 3.6 dS/m. For three years, a complete randomized block design with four replicates was used to evaluate five irrigation regimes. Irrigation regimes consisted in water replacements of cumulated ETc at levels of 100% (SWB100, full irrigation), 80% (DI-80), 60% (DI-60), when the readily available water in SWB100 treatment is depleted, deficit irrigation during ripening stage (SWB100-DI60) and farmer method corresponding to irrigation practices implemented by the local farmers. The results showed that soil salinity was significantly affected by irrigation treatments. Higher soil salinity was maintained in the root zone with DI-60 and farmer irrigation treatments than full irrigation (SWB100). SWB100-DI60 and DI-80 treatments resulted also in low ECe values. Soil salinity was kept within acceptable limits for the growth of the crops grown in the rotation when SWB100, SWB100-DI60 and DI-80 strategies were employed. The rainfalls received during fall-winter and spring periods were effective in leaching salts from the soil profile. During the three year period, carrot and millet yield was highest for the SWB100 full treatment, (29.5, 28.7 and 26.8 t/ha for carrot and 27.2, 28.3 and 26.9 q/ha for millet) although no significant differences were observed with the regulated deficit irrigation treatment (SWB100-DI60). However, the DI-80 and DI-60 deficit irrigation treatments caused significant reductions in carrot and millet yields through a reduction in roots number and weight, panicle number, kernel number and weight in comparison with SWB100. The farmer’s method caused significant reductions in yield and resulted in using 43 to 57% and 27 to 47% more water, respectively, in the carrot and millet growing seasons and increased soil salinity. For all irrigation treatments, carrot yields were higher in first than the two following years and millet yield was higher in second than the first and third year. Water productivity (WP) values reflected this difference and varied, respectively, between 3.29 and 9.7 kg/m 3 and 0.35 and 0.95 kg/m 3 for carrot and millet crops. The lowest WP values occurred under the farmer’s method, while the highest values were obtained under DI-60 deficit irrigation treatment. SWB100 irrigation treatment provides significant advantage on yield and WP compared to farmer’s method in carrot and millet production under experimental conditions. Thus, for water-saving purposes, the SWB100 irrigation scheduling is recommended to optimize the use of saline water in carrot and millet production and to control soil salinity. Under situations of water shortage, the deficit irrigation strategies (SWB100-DI60 and DI-80) are recommended as a tool to schedule irrigation of carrot and millet crops under the arid conditions of southern Tunisia.