Numerical Determination of Planting Layer Thickness in Rooftop Garden from the Aspect of Soil Water Consumption

Urban Farm DB summary

Artificial planting soil layers used in settings such as rooftop gardens typically have a two-layer structure: a coarse drainage layer and a planting layer. Assuming no irrigation, this study examined the required layer thickness based on soil physical properties and weather conditions. Using 30 years of rainfall records for Tokyo, the logarithmic cumulative distribution of the number of days between rainfalls was well fitted by a normal distribution, with the number of days between rainfalls corresponding to 95% and 99% occurrence probabilities being 29 and 64 days, respectively. Numerical calculations for a two-layer system with a 100 mm lower layer examined the relationship between layer thickness and the time until the planting layer wilts on average from initial moisture; at 99% probability, the required planting layer thickness was 236 mm, roughly half the 424 mm required for a single-layer system.