# Evaluating the Drainage Density Characteristics on Climate and Drainage Area Using LiDAR Data
## 4 Summary and Conclusions  
The aim of this study was to re-examine the dependence of drainage density on climate and the size of the drainage area. A total of 121 watersheds with pristine natural conditions were selected based on LiDAR data availability. The climate aridity index was used as an indicator of long-term climate. The range of climate aridity index for the study watersheds was from 0.3 to 10.4. Valley networks were extracted from the 1-m LiDAR-based DEMs using the topographic curvature threshold derived from connected component analysis, and the drainage density of each watershed was calculated. When the climate aridity index is smaller than 1, the negative correlation between drainage density and climate aridity index presented in Madduma Bandara [39] was not observed in the data from this study; there was no strong positive or negative correlation. The decreasing trend in the data from Madduma Bandara [38] is thought to be caused by the elevated human interference over a 100 year period that affected the drainage characteristics of the watersheds. When the climate aridity index is within the range 1-6, both the dataset from Melton [33] and the current study showed a positive correlation between drainage density and climate aridity index. Overall, drainage density was greater than that of the data from Melton. The reason for the positive shift was caused by the use of high-resolution DEM in this study; coarse DEM produced smaller drainage density as small valleys were not detectable. When the climate aridity index is greater than 7, the drainage density of the dataset from Melton was greater than that of the current study. The difference might have been  
Figure 10: The relationship between drainage density (\(D_{d}\)) and drainage area from the watersheds in this study: (**a**) 42 watersheds from the humid region (\(E_{P}/P<1\)) and (**b**) 79 watersheds from the arid region (\(E_{P}/P>1\)). caused by the difference in watershed size; Melton's watersheds (0.001-0.006 km\({}^{2}\)) were smaller than the watersheds in the current study (0.40 km\({}^{2}\)). A negative correlation between drainage density and drainage area was presented in the datasets from Melton and reconfirmed in the current study. The decreasing trend of drainage density as drainage area increases was more prominent when the climate aridity index was greater than 1, which means a stronger decreasing trend was observed between drainage density and drainage area in arid regions than in humid regions. This study revisited previous studies on the drainage density characteristics related to climate aridity index and drainage area. The results of this study added a new perspective to understanding drainage density data. Land use change, climate, and input topography data resolution have significant impact on the relationship between drainage density and climate aridity index. The valley networks used in this study were extracted from LiDAR data, which represent the hydrological conditions of the LiDAR acquisition periods only. Thus, the results may contain an unexpected seasonality effect. It can be improved by introducing other contemporaneous climate data, i.e., soil moisture conditions, in future research. The application of the methodology could be expanded to broader climate conditions as well as urban watersheds to understand the general characteristics of a drainage network. Additionally, more study watersheds with a broader range of drainage areas can be selected and included in future analysis. Conceptualization, S.K. ; Formal analysis, S.K. ; Methodology, S.K. and S.-K.Y. ; Resources, S.-K.Y; Writing--original draft preparation, S.K., S.-K.Y, and N.C.; Writing--review and editing, S.K., S.-K.Y., and N.C. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. The data presented in this study are available on request from the corresponding author.