## 4 Discussion  
### Assumptions  
This study explored the effects of land cover change among impervious surfaces, turflawns, and forests in Lehigh Valley and within the city of Bethlehem. The results were based on model runs with and without LULCC using the MACA-downscaled and bias-corrected NCAR CESM model RCP8.5 scenario from 2019 to 2099. The results were all presented as differences from a control run without any LULCC change. Lehigh Valley is the equivalent of one grid of the CESM climate model, which means that for future data, MACA only captures the trend of a single grid. The bias correction is important to ensure that the range of climate variation throughout the time period being modeled is reasonable and therefore responsible for the correct range of ecosystem responses. The bias correction in MACA is performed with respect to the GridMET data, which provided the historical climate data [23]. We examined the historical and future climate input variables to ensure they were properly bias-corrected. MACA has been shown to perform extremely well in temperature, humidity, wind, and precipitation through the methods of calculation, allowing for the joint downscaling of temperature and dew point temperature [24]. This study uses the same climate conditions for all the experiments; thus the climate serves as a control, and the only thing changing between the experiments is the LULC datasets. Implementing the change to LULC in a single year may seem unrealistic, but the total area change, if spread out over 80 years, is consistent with historical rates of change. The area change generally falls within Pennsylvania's average annual land cover change of 115,981 km\({}^{2}\)yr\({}^{-1}\)[25]. While the 50% conversions of forests to lawn and impervious cohorts in experiments 3 and 4 fall within that range when accumulated over 80 years, the 10% experiments are less than that average rate. When breaking down the land use change provided by NRCS into land use types, the average rate of land use change in developed areas of 15,302 km\({}^{2}\)yr\({}^{-1}\) is less than the amount of area converted in this study in a single year, with even the 10% conversion of impervious to forest changing 18,823 km\({}^{2}\) in a single year (Table S2). However, the rates of change we imposed are consistent with historical rates but just implemented in a single year rather than spread out over 80 years. To explore the effects of applying the LULCC in a single year, we repeated experiments 6 and 12 with the same total change applied on a decadal basis. When converting from impervious to forest (Figure S8a), there is virtually no difference in carbon or water variables (Figure S8b,c) because the new forest is still very young and therefore does not substantially increase the vegetation carbon. While young, growing forests are stronger carbon sinks, the relative area each decade is still fairly small. When converting from forest to impervious (Figure S9a), however, the change in vegetation carbon and cumulative NCE (Figure S9b) takes a different path, with a more gradual change than the original results, but ends up in the same place by the end of the century, although cumulative NEP is virtually the same. The runoff (Figure S9c) is less at first since not as much impervious surface is being created immediately but ends up at the same values by the end of the century. Studies have shown that at least 10% LULCC is required to see a significant change in carbon storage [1]. In this study, the only experiments in which over a 10% change in the total region occurred were in the 25 and 50% conversion of forest to either lawn or impervious (Table S3). The 25% and 50% experiments for F2L and F2I were outside \begin{table}
\begin{tabular}{c c c} \hline \multicolumn{3}{c}{**Biomass of Bethlehem**} \\ \hline  & **gC/m\({}^{2}\)** & **TgC** \\ \hline
**West** & 16,134 & 43 \\ \hline
**South** & 16,083 & 4 \\ \hline
**North** & 21,893 & 161 \\ \hline
**East** & 15,778 & 32 \\ \hline \end{tabular}
\end{table}