Environmental impact of irrigation in la violada district (Spain): I. Salt export patterns

Salt loading in irrigation return flows contributes to the salinization of the receiving water bodies, particularly when originated in salt-affected areas as frequently found in the middle Ebro River basin (Spain). We determined the salt loading in La Violada Gully from the total dissolved solids (TDS) and flows (Q) during the 1995 to 1998 hydrological years. Since this gully collects flows from various sources, an end-member mixing analysis (EMMA) was performed to quantify the drainage flow from La Violada Irrigation District (VID). Three flow components were identified in La Violada Gully: drainage waters from VID (Qd); tail-waters from irrigation ditches, spill-over, and seepage from the Monegros Canal (Qo); and ground water inflows (Qg) originating in the dryland watershed. Gypsum in the soils of VID was the main source for salts in La Violada Gully (flow-weighted mean TDS=1720 mg L-1, dominated by sulfate and calcium). The contribution of Qg to the total gully flow during the 1996 irrigation season was low (6.5% of the total flow). The 1995 to 1998 annual salt load average in La Violada Gully was 78 628 Mg, 71% of which was exported during the irrigation season. The 1995 to 1998 irrigation season salt load average in Qd was 43 015 Mg (77% of the total load). Thus, irrigated agriculture in VID was the main source of salt loading in this gully, with a yield of 11.1 Mg of salts per hectare of irrigated land for the irrigation season. Efficient irrigation systems and irrigation management practices that reduce Qd are key factors for controlling off-site salt pollution of these gypsum-rich irrigated areas.