# Hydraulic Characterization of Green Roof Substrates by Evaporation Experiments
## 1 Introduction
Then, the unsaturated hydraulic conductivity function is determined from the pressure head profile and the changes in water content distribution. A simplified version of [PERSON]'s method was proposed by [22] in which tensiometers are installed at only two depths within a short soil column. However, the linearizing assumptions of the simplified method with respect to time, space and the water content-pressure head relationship could result in marked deviations from the true hydraulic properties for the coarse-textured pore media that are commonly used for green roof design [23]. Apart from this, other limitations may affect the evaporation method. Water cavitation in the tensiometers, typically occurring around \(-70\) to \(-90\) kPa, limits the measurement range on the dry end [19; 20]. On the wet end, the major limitations arise from the inability to obtain accurate estimates of the hydraulic conductivity because the hydraulic gradients are too small and subject to uncertainties in tensiometric readings [19; 24]. However, many hydrologic and agronomic studies require soil hydraulic property measurements at both lower and higher tensions. Therefore, the integration of evaporation data with independent measurements conducted for both the wet and/or the dry ends seems a valuable solution to improve the soil hydraulic functions' reliability. Water retention data at low pressure head values can be readily obtained by the pressure plate apparatus [25] whereas measurements of near-saturated hydraulic conductivity may be obtained from steady-state head-controlled infiltration experiments, like the unit hydraulic gradient (UHG) [26]. Although the combination of these two techniques is attractive, to our knowledge, measurements of near-saturated hydraulic conductivity on the same sample used for evaporation experiments were conducted only by [19; 27]. Furthermore, provided that \(\theta\)(\(h\)) and \(K\)(\(h\)) data collected from the evaporation method are generally fitted by closed-form empirical functions like the van Genuchten-Mualem(VGM) model [18], the consistency between modelled functions and additional steady-state measurements (pressure plate and UGH data) may be problematic and need to be specifically assessed. This is particularly true close to saturation where, due to the influence of the macropore domain, unimodal functions may be inappropriate to describe the hydraulic properties of green roof substrates [3; 28; 29; 30]. Parameter optimization based on an inverse solution of the Richards equation has been largely used for soil hydraulic characterization (e.g., [31; 32]). One of the advantages of the inverse method is the flexibility in modelling the hydraulic properties of the porous media. Though numerically more expensive, inverse modelling was considered preferable to simplified evaporation methods for coarse media with narrow pore-size distribution [33]. The optimization module of the Hydrus-1D model [9] was used to estimate the water retention curve and the hydraulic conductivity function of green roof substrates from simulated rainfall experiments [34; 35; 36; 37] but the feasibility of estimating the parameters of the bimodal VGM [28] from an inverse approach was not explored. The present study was performed with the main objective of developing a laboratory procedure for the hydraulic characterization of green roof artificial substrates based on evaporation and steady-state UHG experiments. The hydraulic properties of the substrates obtained with the direct Wind method were compared with those obtained by numerical inversion of the evaporation transient experiments performed by Hydrus-1D software. The agreement with independent near-saturated \(K\) measurements was assessed with the aim of establishing the reliability of unimodal and bimodal VGM models to describe the hydraulic properties of the considered artificial substrates.