Part II.Tutorial examples, guidelines and exercises
D.1. Innovative methods of electromagnetic induction and remote sensing for mapping
overview)
M.V. Konyushkova (Russia)
There are two approaches used in the mapping and monitoring of saline and alkaline soils. The first approach is based on measuring soil properties with the use of proximal soil sensing technologies including electromagnetic induction (EMI). The second approach involves the identification of soil-landscape relationships in different environmental-agricultural situations followed by the mapping of such relationships with the use of traditional and digital mapping and remote sensing techniques.
Soil mapping with the use of electromagnetic induction
The EMI method is based on field measurements of the apparent electrical conductivity (ECa) with the use of a portable device that can be carried by hand or attached to a tractor (Fig. D.1).
The data are transferred from the device to a portable computer along with GPS coordinates that are recorded in real time. The Geonics EM-38 ground conductivity meter (Canada) is the most common device for measuring electrical conductivity through EMI, which allows the assessment of soil salinity within a layer 1-1.5 m thick. A more detailed description of this method can be found in published papers [9, 11]. Similar devices include the DUALEM-421 sensor that has been recently introduced in practice. This sensor is capable of measuring the ECa at six different depths, which allows creation of multidimensional (3D and even 4D) models of salt distribution in soils [21] (Fig. D.2).
108 Soil salinity manаgement manual | Part II.Tutorial examples, guidelines and exercises
Fig. D.1. A tractor equipped with an EM-38 ground conductivity meter and a satellite navigation system, with direct data transfer to a portable computer (USDA Salinity Lab, Riverside, USA).
Fig. D.2. The vertical distribution pattern of the apparent electrical conductivity ( σa, mSm/m) measured using a DUALEM-421 sensor in its horizontal ( Hcon) and perpendicular ( Pcon) positions, with different distances between its transmitter and receiver: a – 1 m, b – 2 m, c – 4 m.
The assessment of distribution of electrical conductivity values ( σ, mSm/m) (d) [21].
110 Soil salinity manаgement manual | Part II.Tutorial examples, guidelines and exercises
Measured values of soil electrical conductivity depend on the salinity of soil solution, the soil porosity and the type and quantity of clay particles in the soil [16]. The EMI method also has significant limitations that prevent the possibility of taking accurate measurements in dry conditions (i.e., on drylands or when moisture content of irrigated soils is low) or in soils and parent materials with irregular particle-size composition. In the absence of such limitations, the EMI method allows for a quick and reliable assessment of soil salinity over a large area. The comparison of EMI data with laboratory measurements of electrical conductivity of saturated soil-paste extracts shows correlation coefficients of 0.74–0.78 and very similar distribution patterns of data obtained by the two methods [10]. More precise measurements can be obtained by simultaneous testing of ECa and soil moisture, which enebles the possibility to separate the effects of salinity and moisture on the electrical conductivity of soil.
The EMI is a basic method applied in the soil salinity assessment system that was developed in the USA and now used worldwide [19].
Soil mapping by the use of remote sensing Soil salinity
Soil salinity assessments on the basis of remote sensing data involve the use of a series of spectral indices such as colour saturation and brightness indices, soil salinity index and vegetation index (for more detail see [5], pp. 26-28). Such indices illustrate relationships between different aspects of the study materials and their spectral characteristics.
Relationships between soil salinity and spectral indices differ for bare and vegetated soil surfaces. In cases of bare surfaces, soil salinity correlates with indices calculated by the addition or multiplication of brightness values of visible and near-infrared light, but has no connection with vegetation index.
An increase in soil salinity is reflected by an increase in brightness within the visible (especially, blue) and near-infrared spectra [12, 18]. However, the degree of association between these parameters depends on the state of the soil surface (e.g., whether it is moistened, cracked or trampled) as well as the mineralogical composition of salt crusts [16, 17] (Fig. D.3).
Fig. D.3. The influence of the salt crust surface on the reflectance capacity (ground survey using a Crop Scan radiometer) [16].
In cases of vegetated surfaces, soil salinity closely correlates with the vegetation index and indices calculated from the brightness of colour within the red and near-infrared spectra. The latter also reflects the condition of vegetation, which can be either poor (due to high salinity) or good (due to low salinity of soils) (Fig. D.4). Approaches to the assessment of soil salinity by use of the normalized difference vegetation index (NDVI) and the condition of vegetation (primarily, agricultural crops) have been described in numerous publications (e.g., [2, 7, 13, 14] etc.).
Fig. D.4. The relationship between albedo and the normalized difference vegetation index (NDVI) in different surface categories: 1 – water; 2 – 5 – soils overmoistened to different degrees; 6 – soils in good condition; 7 – 11 - soils having different degrees of salinity [14].
112 Soil salinity manаgement manual | Part II.Tutorial examples, guidelines and exercises
Salt-affected soils under halophytic plant communities (Salicornia, Spartina, Suaeda, Salsola, etc.) are characterized by a weak correlation between the vegetation index and soil salinity, because halophytes (in contrast to salt-tolerant crops such as cotton, maize and sugar cane) are only weakly dependent on differences in soil salinity levels (Fig. D.5).
Fig. D.5. The distribution of the NDVI values (measured using a hyperspectral portable radiometer) and the electrical conductivity of saturated soil-paste extract (ECe) under different plant communities (halophytes and salt-tolerant crops) [22].
Solonetz complexes
Approaches to mapping and monitoring Solonetz complexes (combinations of alkaline soils) on the basis of remote sensing data are still insufficiently developed, as compared to the above- described cases of saline soils. Existing approaches are based on the fact that different soils within Solonetz complex support different plant communities and that the condition of plants is closely associated with the degrees of soil salinity or alkalinity [5, 20] (Fig. D.6).
Figure D.6. The distribution of the TM4/TM3 values (760-900 nm/630-690 nm) and the cover (5) of different plant communities within Solonetz complex in Hungary (with spectral measurements conducted using an Exotech-100 radiometer) [20].
Solonetz complexes in the north of the Caspian Sea region have been mapped with the use of satellite imagery (Quickbird, GeoEye, Pleiades) that provided the basis for calculating the differences in vegetation indices (with higher values for Meadow Chestnut soils due to a dense cover of meadow vegetation) and brightness indices within the near-red spectrum (with lower values for Solonetz under moss-lichen communities). Guidelines for digital mapping of Solonetz complexes in the north of the Caspian Lowland on the basis of processing high-resolution satellite images can be found in the handbook on digital soil mapping (Fig. D.7) [8].
Quickbird Image (synthesized 4-3-1) Digital map of Solonetz complex
Fig. D.7. An example of a digital soil map of Solonetz complex compiled using the Quickbird Imagery [8].
114 Soil salinity manаgement manual | Part II.Tutorial examples, guidelines and exercises