## 3 Benefits of Biochar Amendment in Green Roofs
### Runoff Retention
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Approach** & **Substrate** & **Biochar Setup** & \begin{tabular}{c} **Retention Rate** \\ **without Biochar** \\ **(\%)** \\ \end{tabular} & \begin{tabular}{c} **Retention Rate** \\ **with Biochar (\%)** \\ \end{tabular} \\ \hline \multirow{6}{*}{[65]} & \multirow{6}{*}{Pilot-scale} & \multirow{6}{*}{\begin{tabular}{c} Light expanded clay \\ aggregate, coir chips, and mulch \\ \end{tabular} } & 7.5\% mixed—medium & \multirow{6}{*}{\begin{tabular}{c} 70 (summer), \\ 50 (autumn) \\ \end{tabular} } & 21.95–53.05 \\ \cline{5-5}  & & & 15\% mixed—medium & \multirow{6}{*}{\begin{tabular}{c} 27.44–58.54 \\ 50 (autumn) \\ \end{tabular} } & 27.44–58.54 \\ \cline{5-5}  & & & 7.5\% buried—medium & \multirow{6}{*}{\
\begin{tabular}{c} 70 (summer), \\ 50 (autumn) \\ \end{tabular} } & 35.98–59.76 \\ \cline{5-5}  & & & 7.5\% mixed—fine & \multirow{6}{*}{
\begin{tabular}{c} 24.39–68.29 \\ 36.59–67.48 \\ \end{tabular} } \\ \cline{5-5}  & & & & & \\ \hline
[70] & Pilot-scale & Topsoil & NA & 32.1 & 46.6 \\ \hline
[71] & Pilot-scale & \begin{tabular}{c} Gravel, sand, silt, clay, \\ pumice, Fiber Life \\ compost, and paper fiber \\ \end{tabular} & NA & \
\begin{tabular}{c} Not saturated: \\ 19.3–32.9 \\ \end{tabular} &
\begin{tabular}{c} Not saturated: \\ 29.6–32.5 \\ \end{tabular} \\ \hline
[72] & Pilot-scale &
\begin{tabular}{c} Peat, vermiculite, perlite, \\ and sawdust \\ \end{tabular} & NA & 72.54 & 72.08 \\ \hline
[73] & Soil column & Natural clay soil & NA & 45.5 & 69.3 \\ \hline
[74] & Pilot-scale & Peat, vermiculite, perlite, \\ and sawdust & NA & NA & \
\begin{tabular}{c} Substrate (5, 10, 15, \\ and 20 mm): 47.1, \\ 52.71, 56.99, and \\ 62.64 \\ \end{tabular} &
\begin{tabular}{c} Substrate (5, 10, 15, \\ and 20 mm): 47.7, \\ 53.99, 60.42, and 66.5 \\ \end{tabular} \\ \hline
[75] & Software modeling &
\begin{tabular}{c} Scoria (\(\leq\)8 mm), scoria \\ (7 mm aggregate), and \\ coir \\ \end{tabular} & 40\% \(\
icefrac{{p}}{{v}}\) & Maximum 12\% improvement annually \\ \hline
[76] & Pilot-scale &
\begin{tabular}{c} Crushed, recycled brick, \\ compost, peat, and \\ crushed bark \\ \end{tabular} & NA & Maximum 10\% improvement \\ \hline
[77] & Field-scale & NA & 2.5, 5, and 10\% \(\
icefrac{{w}}{{w}}\) & 36.6 & 41.5, 39, and 37.8 \\ \hline
[62] & Software modeling & \
\begin{tabular}{c} Loam and expanded \\ clay \\ \end{tabular} &
\begin{tabular}{c} 3 cm and 5 cm depths \\ of biochar \\ \end{tabular} & 55.05–65.29 & 66.9 and 69.15 \\ \hline
[78] & Pilot-scale & Heat-expanded clay & \
\begin{tabular}{c} 15\% \(\
icefrac{{p}}{{v}}\) \\ \end{tabular} &
\begin{tabular}{c} 36\% improvement \\ \end{tabular} \\ \hline
[79] & Pilot-scale & \begin{tabular}{c} Crushed clay, peat, and \\ green compost \\ \end{tabular} & \begin{tabular}{c} Pelleted wood (10 and \\ 15 cm deep substrates) \\ \end{tabular} & 25 and 30 & 33 and 45.5 \\ \cline{5-5}  & \
\begin{tabular}{c} Pumice, lapillus, peat, \\ and green compost \\ \end{tabular} & Flake wood (10 and 15 \\ \cline{5-5}  &
\begin{tabular}{c} and green compost \\ \end{tabular} & 38 and 45.5 & 41.5 and 47 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._used a soil column to evaluate the hydrological performance of GRs modified by biochar. The results showed a considerable increase in runoff retention from 45.5% to 69.3%. In the study of [PERSON], [PERSON], [PERSON] and [PERSON] (2009), both types of biochar made from pelleted wood and flake wood in both substrate depths of 10 cm and 15 cm enhanced runoff retention. Similarly, biochar of [PERSON], [PERSON], [PERSON] and [PERSON] (2010) improved runoff retention in different cases of substrate depths (5, 10, 15, and 20 cm). [PERSON], [PERSON], [PERSON] and [PERSON] (2010) concluded that the amendment of 40% (\(v/v\)) biochar to scoria-based GRs could result in an increase in the annual runoff retention, of 12% when modeled with Melbourne's climatic conditions.