Rainfall interception (RI) over a comprehensible outdoor urban scale model (COSMO) was investigated from the perspective of water and energy balance. COSMO was consisted of an impervious concrete cubes with 1.5 m height and 0.1 m thickness. A total of 512 blocks were regularly aligned on an impervious flat concrete base with a surface area of 50 m x 100 m and thickness of 0.15 m. A portion of the COSMO site (6 m x 6 m) was used as a catchment for the RI experiment. This catchment was enclosed by water-proof fencing (5 cm high). Thus, horizontal water exchange between the catchment and its surrounding area and infiltration into the concrete were assumed to be zero. Such unique setup allowed the direct estimation of the RI as the residual of measured rainfall and runoff.
RI in COSMO was largely different from that in forests. RI was 6% of total rainfall on average and smaller than typical values in forests, 10-50%. The correlations between RI and any rainfall quantities such as total rainfall and rainfall duration were not observed unlike the correlations found in forests. Instead, most RI occurred in the first several hours of rainfall and then RI rapidly decreased with time during rainfall. RI strongly depended on the saturation deficit at the beginning of the rainfall event. The latent heat flux equivalent to RI approximately balanced by heat conduction from the concrete surfaces.
As the possible reasons to cause the different behaviors of RI between COSMO and forests, differences in the canopy structures are considered: 1) complete surface area index (SAI), 2) efficiency for scalar transfer in relation to roughness size, and 3) heat balance in relation to volumetric heat capacity on the roughness elements.
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