Abstract:
Soil salinity is an enormous problem for agriculture under irrigation as it affects the growth and development of plants in Ethiopia, the problem is widespread in the rift valley areas where middle Awash and Amibara irrigated farms are present. Therefore, this study was conducted to investigate the impact of depth of water table fluctuation on the occurrence of salt affected soil in Amibara irrigation scheme. Based on the existing piezometers points, additional ground water monitoring holes were repaired and installed at the previous points. Totally about thirty monitoring piezometers were used in this study, which are found in both Fluvisols and Vertisols areas. A Total of 180 soil samples with two sampling depths (0-30 cm and 30-60 cm) were collected at the months of August, October and December. Similarly ground water samples from thirty sampling points and one sample from Awash River at Melkasedi diversion weir, were taken for five consecutive months (August 2017 to December 2017 ) to evaluate the seasonal fluctuation of water table and water quality. Then, for all soil and water samples, all salinity and sodicity parameters were determined in the laboratory. To determine the relationship between depth of water table and soil salinity and sodicity, simple correlation coefficients were determined with SAS software. Based on the field measurement of water table level, shallow water level were recorded in most sampling points. About 29 and 71% of the piezometers in Fluvisols and 89% and 11% in Vertisols, revealed water table below 2 m and greater than 2 m, respectively from the ground surface. Based on the laboratory analysis result, the Awash River quality is in the medium range for irrigation with mean values of 1 ds/m ECw and 6.65 SAR. Together with shallow water table, higher mean ECe values of 5.10 and 2.98 ds/m and SAR values of 17.57 and 6.22, were recorded in Fluvisols and Vertisols areas, respectively. The analytical result of soil samples indicates, about 71.43, 19.05 and 9.52% in Fluvisols and about 77.78, 11.11 and 11.11% of the soil samples in Vertisols of sampling points grouped under normal, saline and saline sodic class, respectively. Correlation analysis result between irrigation water ECw and soil ECe revealed significant correlation (r= 0.99) in Vertisols at 30-60 cm depth at (P ≤ 0.05) level of significance. Correlation analysis result between depth of water table and soil salinity revealed significant correlation in Fluvisols, but not in Vertisols. The correlation result between ground water ECw and soil ECe showed a significant correlation (r= 0.73 and 0.71) and (r= 0.89 and 0.74) at 0-30 and 30-60 cm, respectively, in Fluvisols, while it showed a significant correlation (r= 0.86, 0.96 and 0.95) at 0-30 cm along the sampling months in Vertisols. The correlation result between ground water SAR and soil SAR showed a significant correlation (r= 0.75, 0.55 and 0.66) and (r= 0.62, 0.58 and 0.69) at 0-30 and 30-60 cm soil depths, in Fluvisols area but it showed a significant correlation (r= 0.93) at 0-30 cm at October in Vertisols area. Generally, in addition to quality reduction of Awash River water, poor management of irrigation, absence of adequate surface and subsurface drainage structures are aggravating soil salinity in the study area. Therefore, efficient irrigation practice and maintenance and construction of surface and subsurface drainage structures should be implemented as soon as possible to reduce the expansions of soil salinity and sodicity problem in Amibara irrigation scheme.