USING ELECTROMAGNETIC INDUCTION TECHNIQUE TO DETERMINE SOIL SALINITY WITHIN LAYERS OF VARYING DEPTHS: A CASE STUDY IN THE MEDITERRANEAN FARMLANDS, TURKEY

摘要

Diagnosis of soil salinity and characterizing its spatial variability both vertically and horizontally are needed to establish control measures in irrigated agriculture. In this regard, it is essential that salinity development in varying soil depths be known temporally and spatially. The main objectives of this paper were to investigate potential use of electromagnetic induction technique, i.e. EM38 device, to determine soil salinity, ECe, at varying soil depths through utilizing apparent electrical conductivity, ECa, measurements at horizontal- and vertical-dipole configurations of the device, and to develop soil salinity maps to delineate severity of salinity in large-scale irrigation schemes. The salinity of a large area, irrigated with irrigation return flows in Yemisli irrigation district (7 110 ha), Adana, Turkey was assessed using the EM38 device at 112 randomly selected sites distributed homogeneously over the area in horizontal- (ECa-_H) and vertical-dipole (ECa-_V) configurations. The EM38 data was calibrated against standard soil extract conductivity measurements (ECe) of gravimetric samples collected from 20 randomly selected sites, representing the whole area. The values of ECa-H and ECa-_V had asymmetrical hypergeometric and pois-son distributions, respectively, with most readings in the non-saline range (ECe<4 dS m~(-1)) and a sharp transition to relatively high readings. Most salinity profiles were in regular configuration (i.e. ECa-_V > ECa-_H), except in areas with similar salinity values where the profiles were uniform (i.e. ECa-_H = ECa-_V). Soil salinity contour maps were produced from the 112 ECe values estimated from the EM38 data and the 20 measured values of ECe. Even though it was claimed that [e.g., 1, 2] horizontal and vertical dipole positions is suitable for diagnostic and agronomic purposes for the soil layers of 0-100 and 0-200 cm, respectively, this study showed that ECa-_H values should be preferred to ECa-_V to predict ECe values at all soil depths. This is especially true for assessing and characterizing salinity in 0-30 and 0-60 cm depths in the areas irrigated with low quality irrigation return flows. Theoretical frequency curves developed exclusively for varying depths in this work can be used for assessing the probability of observing a given soil salinity, or vice versa.