Three study decades on irrigation performance and salt concentrations and loads in the irrigation return flows of La Violada irrigation district (Spain)

Irrigation district salt balances identify the main sources and sinks of salts and quantify salt loads in irrigation return flows. Salt balances were performed in La Violada Irrigation District during the 80s (1982–1984), 90s (1995–1998) and 00s (2006–2008) decades. Total Dissolved Solids (TDS) and loads in irrigation return flows were related with changes in irrigation performance and infrastructures during these decades. TDS increased linearly to increases in Irrigation Consumptive Use Coefficient (ICUC) (P < 0.01), and decreased with increases in Drainage Fraction (DRF) (P < 0.001). The salt balances revealed that 82% of the exported salts originated from soil gypsum dissolution. Annual salt yields in return flows were high and similar in the 80s and 90s (about 19.3 Mg/ha year), and halved in the 00s (9.9 Mg/ha year) due to comparable reductions in irrigation and drainage volumes derived from a better irrigation performance. Irrigation season salt yields were high and steady around 14.1 Mg/ha for ICUC < 51% and DRF > 66%, and decreased exponentially for values above and below these thresholds, respectively. Therefore, the key management strategy to reduce salt discharge to downstream areas is to decrease drainage volumes by improving irrigation management.

► Soil gypsum dissolution accounts for 82% of total salt loads.
► Salt loads decrease exponentially above a threshold irrigation efficiency.
► Salt loads decrease exponentially below a threshold drainage fraction.
► Salt loads are affected by changes in irrigation infrastructures and management.
► Increased irrigation performance is the key strategy to minimize salt pollution.