Mapping pH of Groundwater and Determination of Vulnerability Areas for Citrus and Rice Growth in Mazandaran

Mazandaran province is one of the most important rice and citrus-producing areas in Iran, where most of the citrus orchards and agricultural fields are irrigated with groundwater. On the other hand, irrigation water pH is one of the basic qualitative factors that determine the solubility and biological availability of chemical components in the soil such as nutrients and heavy metals, and it can affect agricultural production.

The coastal strip of Mazandaran Province toward the southwest of the Caspian Sea is situated in the north of Iran with an area of 8,252 km2 between 35.77 to 36.99 N latitudes and 50.36 to 57.13 E longitudes. In this study, the temporal and spatial variations of groundwater salinity were studied in the coastal strip using data from 300 wells, collected by Mazandaran Regional Water Company. Data included mean pH for each 6-month period of 9 consecutive years, from 2012 until the end of 2020. pH maps and maps of the risk probability area for rice and citrus growth were obtained by using Ordinary Kriging (OK) and Indicator Kriging (IK) in ArcGIS 10.7.1 software, respectively. Classifications were selected according to the properties pH range for the growth of citrus (5.8, 8) and the optimum pH for rice (6.8) in OK method. The indicator amount of pH was considered equal to 6.8 in IK method. Thereby, areas belonging to different pH classes were outlined and places with the risk probability for growing the rice and citrus were identified.

The 11 different models for semivariograms were drawn, and the best one was chosen according to the lowest nugget-to-sill ratio, and thus Stable and Exponential were obtained as the highest frequency for first and second half-years. The indices of cross validation for each selected semivariogram were estimated within acceptable ranges. In Ik method, the pH of studying area was classified into 4 ranges of <5.8, 5.8–6.8, 6.8–8.0, >8, and the percentage area of each classification derived from the ArcGIS software, the average area of each classification during the studying period was calculated zero, 0.6, 83.5 and 15.9 percent, respectively. It showed that most part of the study area located in the range of 6.8-8. It means most rice fields and citrus orchards were irrigated by the groundwater with the pH close to neutral. The obtained maps in the OK method indicated that the pH of the groundwater was not acidic in any points and alkaline conditions were observed in the western and eastern parts of the province. Therefore, The IK method was used to further investigate and determine the vulnerable areas. The probability of pH risk in rice and citrus growth was classified into 4 ranges (0-20%, 20-40%, 40-60% and 60-100%), and the average percentage area of each classification along the period was estimated 94.9, 4.8, 0.3 and zero percent, respectively. Using the IK method, higher probability of groundwater pH reducing the yield in citrus orchards and rice fields was found in eastern parts of Mazandaran province, which was about 5% of total studying area. Also, the results of the study in these 9 consecutive years did not show any decreasing or increasing trend in pH changes and consequently the area under each classification.

Generally, the results indicated that the pH of groundwater for irrigating the citrus orchards and rice fields was appropriate in the most parts of the province and merely in the eastern part of the province, low water alkalinity may make a risk probability for rice and citrus growth in both western and eastern parts of the province. Due to the fact that alkaline water causes soil alkalinity and consequently reduces the solubility of phosphorus and some other plant nutrients in soil, it is suggested to supply the optimum required fertilization amounts of the nutrients in soil. However, the amount of fertilization should be on the basis of field research results. It is also proposed to study the condition of rice and citrus growth and the irrigated water in more details through the farms of western parts of the province. Due to the fact that most citrus orchards in this province are irrigated under the pressurized irrigation systems and using groundwater for irrigation, it is suggested that the Langelier Saturation Index (LSI) be examined in future research.