Investigation the Citrus Yield Reduction Affected by Groundwater Salinity in Mazandaran Province

One of the main factors of the reducing agricultural production is the irrigation water salinity. Mazandaran province is one of the most important citrus-producing area in Iran, where the groundwater used for supplying the irrigation water in citrus orchards of the province. The main objective of this paper was to determine map spatial and temporal variations of groundwater salinity (EC) in the coastal strip of Mazandaran province with an area of 8252 km2. The collected data of 300 wells of the study area were measured by Mazandaran Regional Water Authority for 9 consecutive years and defined for ArcGIS 10.7.1 software as the average ECs for each 6 months.

EC maps were obtained by ordinary kriging (OK) for each 6-month periods from and the beginning of 2012 to the end of 2020, by using ArcGIS 10.7.1 software. According to the derived empirical function of the effect of EC on the citrus yield (Y=100-13(EC-1.4)). According to this function, ECs of 1.40, 2.17, 2.94 and 3.71 dS/m, corresponding to relative yields of 100, 90, 80 and 70% were selected, respectively. Thereby, areas belonging to the classes of 100%, 90-100%, 80-90% and <70% were outlined. In addition, the Mann–Kendall test and Sen’s slope were used to project future changes.

The result of EC maps indicated that the salinity of groundwater increases from west to east of the province. The Mann-Kendall test result showed a significant temporal trend of the size of the area below the <70% yield (EC >3.71 dS/m) and 100% (EC <1.40 dS/m), presenting that the salinity of groundwater can be a concern for citrus orchards in the coastal strip of Mazandaran province. The equation given by Sen’s slope estimator denoted that if this trend is continuing, the area under 100% yield will reach to zero in 2021 and the yield of citrus in the coastal strip will reach below 70% at the latest by 2047.

This study has shown that the salinity of groundwater was increasing in the eastern part over the time and it covered a wider area around the Miankaleh lagoon, which may be due to the leakage of the lagoon to the groundwater aquifer in that area. The reduction of groundwater quality over the time can be attributed to two factors: reduced rainfall in recent years and digging illegal wells and subsequently consumed extraction of wells in the region. Taken together, it is suggested that the wells in the area will be exploited completely controlled to prevent the reduction trend of its quality.