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Local infiltration tests on 1.5 m long blocked furrows were carried out on a loam soil to assess N fertiliser leaching under furrow irrigation where ridging operations entails placing nitrogen on the upper part of the ridge. This article focuses on the impact of flow depths, or water application depth (WAD), on nitrogen movement in seven 1.5-m long blocked furrows. For a first irrigation event, a WAD greater than or equal to 240 mm, significantly reduced the heterogeneity of the N concentration profiles measured at the top of the ridge and beneath the furrow. The virtually homogeneous N soil distribution with depth permitted the determination of the nitrogen balance throughout the season using soil samples obtained at the beginning and end of the season as well as the determination of nitrogen present in the crop tissue. This is not possible when there is a heterogeneous N soil profile at the end of the irrigation season, as observed under moderate WAD conditions. In addition, a substantial WAD delivered during the first irrigation event, and at a period where the plant N requirements are high, does not affect crop yield potential.
A modelling approach, adapted to 2D water and solute transfer, allowed us to validate the estimated N leaching resulting from N balance performed under high WAD conditions. About 22 kg/ha of N was leached behind the root zone (around 10% of the nitrogen application) during the irrigation season under high WAD (240 mm during first irrigation and around 200 mm for the following irrigation events). Modelling should be used to estimate N leaching under low or moderate WAD conditions because of the heterogeneous N distribution within the root zone.
