+ 3 1-50-3636 140, J. J. M .de. Jongabiol. rug ABSTRACT A ground-based X-band radar was directed almost horizontally into the top of a mature ash during 5 months. The aim was to relate radar backscatter to hydrologic properties of leaves. The results show that: (1) Due to the retention of rain on the surface of leaves, the backscatter increased temporarily during and after showers. (2) The backscatter decreased the first days after a shower. This decrease is attributed to depletion of internal water-storage tissues. (3) The backscatter increases towards the end of the season. This trend is related to a decreased transpiration rate and increased water content of leaves.