Study on relationship between soil salinization and groundwater table depth based on indicator Kriging

Abstract: Soil salinization is the most important land degradation processes in many arid and semi-arid areas with shallow groundwater table depth. Preventing and controlling soil salinization is an important guarantee of the sustainable development of agricultural and ecological environment. Due to an arid and semi-arid continental climate with low rainfall, high evaporation and shallow groundwater table, the Hetao irrigation district (HID), located along the Yellow River in Inner Mongolia, is a typical secondary salinized area in the north of China. In order to understand the relationship between soil salinization and groundwater table at district scale, the Jiefangzha sub-district, the second largest sub-district in HID, was selected as the study area. A total 53 groundwater observation wells and 41 soil salinity monitoring sites were installed in the sub-district. The depth of groundwater level was measured at the ends of March and April in 2012, and the soil salt content was measured at the depths of 0-0.1, 0.1-0.2, 0.2-0.4, 0.4-0.6 m of soil salinity monitoring sites at the end of April in 2012. The probability maps of the critical groundwater table depth and the soil salinity were generated by the indicator Kriging technique with low salinity threshold (2 g/kg), moderate salinity threshold (3 g/kg), and 4 groundwater table depth thresholds of 1.5, 2.0, 2.5 and 3.0 m. Spatial distributions of the soil salinity probability for each of the thresholds were compared with each of groundwater table depth thresholds, and the relationship between soil salinity and groundwater table depth were evaluated on the basis at the district scale. The results showed as following: 1) Surface soil salinity and groundwater table depth exhibited moderate spatial variability and moderate spatial dependence. The spherical model was the best-fitted model for the indicator semivariograms of soil salt and groundwater depth with different thresholds. 2) The spatial distribution of moderate salinization risk was greatly similar to that of the probability of groundwater depth less than 2.0 m, and the spatial distribution of low salinization risk was greatly similar to that of the probability of groundwater depth less than 2.5 m. Especially, the high salinization risk areas were usually corresponding to that of the high probability areas with groundwater depth less than the critical depth of 2.0 or 2.5 m. Meanwhile, the spatial distribution of the probability of groundwater depth less than 2.0 m was greatly similar to the spatial distributions of soil salinization types based on remote sensing inversion at the end of April in 2001, 2006 and 2012. These indicated that the critical depths of groundwater at the district scale were 2.0 and 2.5 m when the low and moderate salinization occurred on the surface soil in the Jiefangzha sub-district at the end of April, respectively. In the sub-district, the high probability area that the groundwater depth did not exceed the critical values gradually decreased from 2001 to 2012. These soils were mainly located in the area of east central and south west of the sub-district, and were corresponding to the soil of the high salinization risk area. 3) Compared with the probability map of groundwater table depth at the end of April, that at the end of March had higher degree of similarity with the spatial distribution of salinization risk of surface soil, which indicated that there was a certain lag effect of the groundwater table depth on soil salinization, and the low or moderate salinization risk would increase when the groundwater depth was less than the critical depth and maintained for a period of time. These results could be useful for the administrator to control groundwater table depth and prevent soil salinization at the district scale. The article also provides a reference for studying causes and regulation of secondary soil salinization at the district scale.