Time and amount of supplemental irrigation at different distances from tree trunks influence on soil water distribution, evaporation and evapotranspiration in rainfed fig orchards

This study was carried out in Estahban, Iran, during 2013 and 2014 to identify the optimal time and amount of supplemental irrigation at different distances from tree trunks on a rainfed fig orchard. This region is distinguished as the largest producer of dried figs in Iran and the world and its production is highly dependent on precipitation. A split-split plot statistical design with four replicates was used to conduct the experiment. Irrigation treatments based on the position of application around trees were: in a micro-catchment close to tree trunks; in three holes inside of tree canopies with a mean canopy of 3.2 m diameter (placed 1-1.1 m from tree trunks); and in four holes outside of tree canopies placed 2.1-2.2 m from tree trunks were used in this experiment. Irrigation time treatments consisted of: (a) in early spring; (b) in mid-summer and the three different volumes of irrigation water were used: no supplemental irrigation (control), and either 1000 or 2000 l of irrigation water per tree. Results showed a high evaporation rate in the area which used nearly half of the rainfall during autumn and winter. There was a significant positive correlation between soil water content (SWC) and temporal rainfall distribution as r** = 0.63 (p < 0.001) over two years. Under drought conditions, irrigated trees showed higher ETa that was close to the ETa in years with normal rainfall. Compared to irrigation in summer, irrigation in early spring kept higher SWC for a longer time period in the soil profile and irrigation far from trees increased soil surface evaporation especially in summer. This study suggests that using supplemental irrigation with 2000 l per tree of water in early spring near tree trunk could provide higher SWC and lower surface evaporation for rainfed fig orchards during drought conditions.