Objective of this study is to evaluate soil salinity using image analysis of remote sensing data and field data in order to develop, later on, the automatic interpretation of remote sensing data in the salt-affected areas.
The study area is located in the northwestern part of the Caspian Lowland, in the semidesert zone. The investigated area includes the territory of the Dzhanybek research station and its surroundings and amounts to about 7000 ha. Salt-affected soils are widespread throughout this area. This territory is characterized by the arid continental climate with a significant excess of evaporation over precipitation; the annual precipitation varies from 150 to 450 mm.
The investigated area is located on an exceptionally flat plain, with a well-pronounced meso- and microtopography with an amplitude from 10–30 cm (shallow depressions, or microlows) to 0.5–1 m (deep depressions, or mesolows). The soil parent material is represented by the homogeneous slightly salinized loesslike loam. The groundwater table is found at a depth of 3–5 m. In this study, virgin lands are under consideration.
Characterization of the Soil Cover
The soil cover in the investigated region is composed of the soils of mesolows and the soils of solonetzic complexes between these mesolows.
Mesolows have a length of up to several hundred meters and an area from 0.1 to 36 ha. They are occupied by dark-colored chernozem-like nonsaline soils, which are easily interpreted on the images.
Soil complex (soil association) is composed of three components: (1) meadow-chestnut (nonsaline and slightly saline) soils are located in the microlows; (2) light-chestnut both nonsolonetzic and solonetzic (from nonsaline to strongly saline) soils occupy different positions in the microtopography; and (3) solonchakous (strongly saline) solonetzes are located on the microhighs (flat elevated areas between the microlows). According to the World Reference Base for Soil Resources (1998), meadow-chestnut and light-chestnut soils are referred to as Haplic and Gypsic Kastanozems; and solonchakous solonetzes – as Salic (Gypsic, Calcic) Solonetzes. The soils are mainly characterized by the sodium sulfate or chloride-sulfate composition of salts.
Materials and Methods
Image analysis was conducted using ILWIS software, the supervised classification procedure. More than 100 pits and boreholes to a depth of 1 to 5 meters were made, from which about 1000 samples were taken and analyzed. Soil reaction and chemical composition of water extracts (1:5) were measured. Then, the total content of toxic salts was calculated as the sum of all ions in water extracts excluding those forming calcium sulfate and bicarbonate.
Results and Discussion
In the process of image analysis, all solonetzic complexes occupying the inter-padina areas were grouped with respect to the portion occupied by meadow-chestnut soils, as follows: (1) Soil complexes characterized by sparse distribution of meadow-chestnut soils (<5%); (2) Soil complexes, in which meadow-chestnut soils occupy 10-30% of the soil complex; (3) Soil complexes, in which more than 50% (50-70%) of the soil complex is occupied by meadow-chestnut soils. The portion of other soils (except for meadow-chestnut soils) and degree of soil salinity change regularly from one group to another. Approximate estimates show that salt reserves in the upper meter of the distinguished soil complexes differ significantly. The largest reserves of salts are in the complexes of the second group (70-120 t/ha), which are characterized by the maximum portion of solonetzes and medium portion of meadow-chestnut soils. The minimum salt reserves are in the complexes of the third group (10 t/ha), where the largest area is occupied by meadow-chestnut soils, and solonetzes are absent. The salt reserves in the complexes of the first group predominantly occupied by light-chestnut soils are 40 t/ha.
(1) Assessment of salt-affected soils in the Northern Caspian region, which is characterized by the small-area complex soil cover, is possible on the basis of interpretation of 5-m resolution satellite images. With the use of image processing, the types and salinity status of soils can be distinguished due to the specific reflectance of each component of the soil complex.
(2) Within the studied area, three groups of soil complexes are distinguished with respect to the portion of meadow-chestnut soils.
(3) Salt reserves in the upper meter differ considerably in the distinguished groups of complexes. The largest reserves are found in the areas occupied by complexes of the second group, which is characterized by the maximum portion of solonetzes and solonetzic soils.
This study was supported by the Russian Foundation for Basic Research, project nos. 04-04-48197 and 05-04-49098.