Impact of long-term recycled water irrigation on crop yield and soil chemical properties

The variably-saturated flow and multi-component transport module UNSATCHEM of HYDRUS-1D was used to evaluate the impact of the long-term (2018-2050) application of recycled water (RCW) for irrigating perennial horticulture (almonds, pistachios), viticulture (grapevines), annual horticulture (carrot, onion, and potato), and pasture crops in representative soils from the Northern Adelaide Plains (NAP), South Australia. The input parameters for soil hydraulic, soil solution, and cation exchange data were determined for 14 soil profiles from the NAP region. For a warm-up period from 1970 to 2017, the model used historical climate data and low-salinity irrigation water. In the subsequent period (2018-2050), irrigation continued with RCW and projected meteorological conditions were obtained by considering expected future climate change. The average soil water salinity (EC_sw) at the end of the simulation period ranged from 2.9-10.5 dS/m across all soils and crops. Potential yields of salt-sensitive crops such as annual horticulture and almonds were reduced by 4-32% due to increased salinity in the soil. Similarly, the model predicted that the sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP) would increase above threshold values, typically considered as indicative of poor growing conditions for most crops. Relationships between SAR and ESP were developed for four representative soils, providing the threshold soil SAR that corresponds to a critical ESP (>6), which would lead to adverse soil health and crop growth impacts. Threshold SARs were derived for calcareous (SAR = 4), hard red-brown (SAR = 3.5), sand over clay (SAR = 6), and deep uniform to gradational (SAR = 3) soils. An increase in SAR and ESP in soils adversely affects soil structural stability and soil water movement, which can severely impact the sustainable crop production in the NAP region. Relationships such as those between SAR and ESP help in identifying critical soil constraints and assist in devising better guidelines for the sustainable use of recycled water for irrigated agriculture.