**4. Discussion**

The study area has undergone significant land use change from 1980 to 2015. A significant loss of farmland was observed in the period between 1980 and 2015. During this period, most of the farmlands are replaced by built-up lands in the south-eastern part of the study area. After the national survey was carried out in 1985, the land area of Xiamen City expanded outward through reclamation [42]. The major land use change in Xiamen is attributable to land reclamation and urban development in the past years. The process of urban construction in reclaimed land and building new residential areas started to rise after 1985, which is reflected in the runoff increase after 1990 [45,46]. In 1980, after the city was declared as specific economic zone, most of the farmlands and forestlands were converted into urban areas between 1985 and 2005. After 2005, there was major sea reclamation which increased the urban area. New initiatives for industrial and economic development occurred in the period of 1990 to 2005, and an urban renewal program took place between 2003 and 2012 [31]. The change in land use significantly affected the runoff hydrology of the city as a result of urban development. The high values of runoff gradually expanded outside Xiamen Island and were mainly distributed in the areas with increased urban construction [40]. Results show that particular increase in constructed land with higher CN value contributes to higher runoff, whereas farmland, forestland, and grassland with lower CN contribute to a lower percent of runoff [30]. Therefore, change in land use, particularly an increase in urban areas, corresponds to an increase in runoff. As a result of rapid urbanisation, significant increase in surface runoff is observed in the island and outer areas of mainland, which poses higher risks for urban as well as coastal floods [47]. Similar results depicting the strong impact of urbanisation on surface runoff has been obtained by previous studies [2,7,17,48]. Relevant studies have shown that the extent to which urbanisation affects hydrological response depends on spatial and temporal scale, physical geography, landscape composition, and physical and climatic characteristics [49]. The study area, being a lowland coastal area, is relatively flat, and a major part of the area has a slope of less than 15%. Therefore, the slope of the area had a less significant effect on the surface runoff. Additionally, the main objective of the study is to analyse the impact of land use change on the surface runoff, especially due to rapid urbanisation. The impact of slope on the runoff characteristics (peak flow and runoff velocity) are not studied in detail. However, future studies could help better understand the effect of slope on peak flow and runoff velocity.

This study demonstrated the use of a GIS-based SCS–CN method to assess the effects due to land use change on surface runoff by integrating spatial data and hydrological parameters. A GIS-based approach proved to be a reliable tool for quantifying the impact of land use change on runoff with respect to change in CN, which is a function of soil, land use, and moisture conditions [50]. The input data of the soil map and rainfall were based on actual field data, and the CN values of each land use type were obtained from the USDA standard table. The simulation of runoff was validated by comparing the observed and simulated annual flow in the study area, which shows that the design runoff simulated by the model is well accepted. Therefore, the model produced accurate and reliable results of runoff incorporating different spatial aspects. The results were consistent with the results obtained by previous studies of runoff simulations in Xiamen [40,42,51].

To fully comprehend the impacts of spatiotemporal land use change on abrupt or gradual flood peaks and runoff flow, various geographical parameters that influence the runoff should be considered in future studies. In the study, other human and environmental factors affecting runoff, such as the construction of dams, reservoirs and underground extraction, drainage systems, temperature, canopy cover, and soil loss, etc. are given little attention. Therefore, it is necessary to conduct extensive studies including these factors in the future.
