Impacts of Prolonged Drought on Salt Accumulation in the Root Zone Due to Recycled Water Irrigation

Abstract

Continuous use of recycled water (treated sewage effluent) over a long period of time may lead to the accumulation of salt in the root zone soil. This is due to the relatively higher levels of salt content in the recycled water compared to surface water. In this study, a laboratory column study was carried out to validate the HYDRUS 1D model under no rain condition. During the validation, the relative error and the % bias between observed and simulated soil water electrical conductivity (ECSW) were found to be low and varied in a range of 5–10 and 5–6 %, respectively. The validated model was then used to predict long-term (5 years) salt accumulation under drought conditions. The analysis of model predicted salt values showed a cyclical pattern of salt accumulation in the root zone, and this related to the variation in rainfall and evapotranspiration. The mean root zone ECSW in the 5th year was found to be within the highest salinity tolerance threshold for pasture (11.2 dS/m); however, the maximum root zone ECSW was found to be about 63 % more than the threshold. Irrespective of seasons, in 5 years time, ECSW at the depth of 1.0 m increased from 3.0 to 7.0 dS/m, which may pose a salinity risk to the groundwater table if there is a perched water table at a depth <1 m below the field surface. One of the management options to minimise long-term salt accumulation was also examined. By reducing the salt in recycled water by 50 %, it was possible to keep the ECsw within the recommended threshold values. Overall, the methodology developed in this study can be used to identify appropriate management options for sustainable recycled water irrigation.