Modelling reference evapotranspiration for vertical green (in urban areas)

Vertical green is promoted as climate change mitigation and adaptation measure, and it provides green space for the urban population. However, it could be used in urban water management as well if its evapotranspiration, thus its water demand would be predictable.

For optimal performance, plants need to be provided with water, nutrients, and rooting space. But irregular precipitation, drought periods, and lack of natural water storage necessitate additional irrigation preferably by local water sources (such as rainwater runoff and greywater).

The amount of water needed for irrigation can be calculated using the Penman-Monteith approach which quantifies evapotranspiration of vegetated horizontal surfaces. For Vertical Green, the Penman-Monteith equation has already been tested. In that way, water demand of VGS can be calculated for hourly time steps based on radiation, wind speed, and vapor pressure deficit expressed by air temperature and relative humidity data.

The needed meteorological data can be measured on-site or derived, thus adapted – verticalized - from remote climate stations, depending on data availability, and needed accuracy of the results. This study models water demand using (1) on-site measured meteorological data, (2) ‘verticalized’ remote station data, and (3) remote station data. We then compare simulated evapotranspiration with measured lysimetry data for a ground-based Vertical Greenery system of Fallopia baldschuanica monitored in Berlin, Germany.

This study finds radiation and vapor pressure deficit to have the highest impacts on the variance of the results while wind speed has the lowest impact. In this contribution, we present the developed model, verticalization methods for the input parameters and validate the performance of the model based on measured water demands.