Influence of drip irrigation layout on salt distribution and sap flow in effluent irrigated cotton

The influence of drip irrigation layout on soil water and salt distributions and associated water stress of cotton irrigated with treated sewage effluent was evaluated by comparing alternate-row irrigation, the standard practice in Israel for cotton, and every-row irrigation. It was expected that every-row irrigation would prevent water stress caused by drying roots, and also salt stress as it occurs under alternate-row irrigation at the edges of the wetted front, by pushing the salts below the root zone. The objective was to quantify the distribution and composition of salts and distribution of water in the root zone and their effect on cotton transpiration for the two treatments. Salt and soil water content were measured with Time Domain Reflectometry. Soil samples to 100 cm depth were analyzed to determine composition and amount of salts, and cotton stem sap flow was measured using the heat pulse method. Results showed higher soil water contents at the alternaterow treatment compared to the every-row treatment and higher soil water contents at 40 cm depth compared to 20 cm depth. The every-row treatment showed higher electrical conductivity at the cotton row compared to the alternate-row treatment, whereas the alternate-row treatment had a higher salinity at 40 cm overall. Analysis of soil samples confirmed larger accumulation of salts at the cotton row for the every-row treatment. Electrical conductivity of the soil water was below the salinity threshold value for cotton. No significant differences in salt composition were observed. Two weeks of continuous stem sap flow measurements of fully grown plants showed no differences in transpiration resulting from the treatments. It is concluded that differences in salinity resulting from the two irrigation treatments are not expected to influence yield and that contrary to the expectation higher salt accumulation occurs in the root zone in case of the every-row treatment as a result of lower water contents in the root zone and thus less percolation.