Soil Moisture Measurements and Evapotranspiration in Extensive Green Roofs

Abstract

In his master's thesis, a concept within stormwater management, the green roof, will be examined. The first part of this thesis is a literature study on the hydrology of extensive green roofs and the factors influencing the hydrological performance of the roofs. This has been done in order to map existing literature and to gain knowledge for the residual work. There is an emphasis on evapotranspiration, which is important for regeneration of the green roofs in between storm events. By measuring soil moisture for the roof, one will gain knowledge on how well the roof release water through evapotranspiration. However, there are uncertainties regarding the sensors measuring soil moisture when used on green roofs. With this in mind, a huge part of this thesis has been to calibrate these sensors, in a laboratory environment. Existing models for evapotranspiration are used to calculate this based on local field data. The factors affecting the green roofs that are being discussed are substrate characteristics, roof geometry, vegetation and the age of the green roofs, as well as the dry weather period prior to a rainfall, seasonal variations and event characteristics. Various models for estimating so-called potential evapotranspiration are presented, and a select few models will be used to calculate this for the green roof field at Risvollan, Trondheim. Along with correct data for soil moisture, the actual evapotranspiration can be calculated. The calibration of the sensors are carried out under stable laboratory conditions. The sensors measure the dielectric permittivity of the material, and by the use of an equation providing the relationship between this and the volumetric water content, the water content is found. At first, a standard calibration were carried out for the two types of substrates that are to be found at Risvollan. It is established that the difference between the substrates are to such a degree that there s a need for equations specifically for each substrate. The substrates at Risvollan has a depth of 5 cm, and the field roof has additionally two plots without substrate. One plot has a thin felt mat, while the other one has a type of rockwool. Therefore, it s important to do a calibration specifically for the green roof build-up. This is done in the laboratory as well, in the scale of small boxes of 21x23 cm. It turns out there s a deviation compared to the standard calibration, when considering the roof plots with substrates. For the roof plot with the felt mat, there was no success in establishing a new equation.