Irrigation management for optimizing crop production and nitrate leaching on grassland

Six supplementary irrigation management options for grazed grassland were defined and their effects on both agricultural production and nitrate leaching to the groundwater were studied. Data were available from the De Marke experimental farm for sustainable dairy farming. The calibrated and validated simulation models SWACROP and ANIMO were used to calculate the effects of the different irrigation management options on crop transpiration, and on water fluxes and nitrate concentrations for three fields of the farm. Comparisons with the common practice at the farm were made. A change in application volume from 25 to 15 mm per irrigation resulted in higher irrigation efficiencies and lower annual water use for supplementary irrigation with only small changes in the ratio of actual and potential transpiration (T-a/T-p). The advisory system 'irrigation planner' generally also resulted in high irrigation efficiencies combined with a reduction of water use and a small effect on the transpiration ratio. The different irrigation strategies had no significant effect on nitrate concentrations in the leachate from two dry fields studied. For the relatively wet field in this study an increase of irrigation water use would improve agricultural production conditions and reduce nitrate concentrations at 1 m depth. For the evaluation of environmental effects of irrigation management options it is advised to assess the actual nitrate concentrations and not only the water fluxes, which potentially cause solute leaching. (C) 2001 Elsevier Science B.V. All rights reserved.