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The analysis of irrigation and drainage management and their effects on the loading of salts is important for the control of on-site and off-site salinity effects of irrigated agriculture in semi-arid areas. We evaluated the irrigation management and performed the hydrosalinity balance in the D-XI hydrological basin of the Monegros II system (Aragon, Spain) by measuring or estimating the volume, salt concentration and salt mass in the water inputs (irrigation, precipitation and Canal seepage) and outputs (evapotranspiration and drainage) during the period June 1997-September 1998. This area is irrigated by solid-set sprinklers and center pivots, and corn and alfalfa account for 90% of the 470 ha irrigated land. The soils are low in salts (only 10% of the irrigated land is salt-affected), but shallow (<2 m) and impervious lutites high in salts (average ECe = 10.8 dS m(-1)) and sodium (average SAR(e) = 20 (meq l(-1))(0.5)) are present in about 30% of the study area.

The global irrigation efficiency was high (Seasonal Irrigation Performance Index = 92%), although the precipitation events were not sufficiently incorporated in the scheduling of irrigation and the low irrigation efficiencies (60%) obtained at the: beginning of the: irrigated season could be improved by minimising the large post-planting irrigation depths given to corn to promote its emergence. The salinity of the irrigation water was low (EC = 0.36 dS m(-1)), but the drainage waters were saline (EC = 7.5 dS m(-1)) and sodic (SAR = 10.3 (meq l(-1))(0.5)) (average values for the 1998 hydrological year) due to the dissolution and transport of the salts present in the lutites. The discharge salt loading was linearly correlated (P < 0.001) with the volume of drainage. The slope of the daily mass of salts in the drainage waters versus the daily volume of drainage increased at a rate 25% higher in 1997 (7.6 kg m(-3)) than in 1998 (6.1 kg m(-3)) due to the higher precipitation in 1997 and the subsequent rising of the saline watertables in equilibrium with the saline lutites. Drainage volumes depended (P < 0.001) on irrigation volumes and were very low (194 mm for the 1998 hydrological year), whereas the salt loading was moderate (13.5 Mg ha(-1) for the 1998 hydrological year) taking into account the vast amount of salts stored within the lutites. We concluded that the efficient irrigation and the low salinity of the irrigation water in the study area allowed for a reasonable control of the salt loading conveyed by the irrigation return flows without compromising the salinization of the soil’s root-zone. (C) 2001 Elsevier Science B.V. All rights reserved.