The influence of increased sodicity on soil hyadaulic conductivity and erosion by rain and runoff
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Increased soil salinity through irrigation, ground-water rise or surface runoff alters soil's physical and chemical characteristics and its ability to withstand erosional forces applied by rain and runoff. Experiments were conducted under a rainfall simulator to study how changes in sodium and salt concentration alter the hydraulic conductivity and erodibility of soils. Changes in saturated hydraulic conductivity (Ksat) were studied in the laboratory, using solution with different SAR and C (ionic concentration) values. For the Toohey soil, Ksat was severely reduced at intermediate SAR and low C values but remained high in the Redlands clay soil for all SAR values tested as long as C remained high. Erosion measurements were carried out in the flume of Griffith University's large rainfall simulator, on soils with and without sodium and salt treatments. Sediment loss significantly increased for the sodium-treated soils. Mean aggregate/particle size of the eroded sediment decreased with increased sodicity. High sodium concentration contributed to the weakening of soil aggregates and their dispersion under the raindrop impact. Electrical conductivity of runoff decreased exponentially with time during the erosion event. SAR and EC values of the sodium-treated soils also decreased with applied rainfall. The resultant SAR values for soils were similar to those which produced the lowest Ksat values in the lab experiment. These results indicate that increased sodicity contributes to increased soil erosion not only through its dispersive action and the reduction in aggregate stability, but also through reduction in the hydraulic conductivity of soils which in turn increases surface runoff.