4.2. Urban Spatiotemporal Change Detection
A change detection analysis was applied for the five time periods (1985–1990, 1990–2000, 2000–2007, 2007–2014 and 2014–2020) for Riyadh, Jeddah and Makkah.
Figure 3 shows the spatiotemporal change of urbanization between 1985 and 2020, and
Table 4 lists the percentage of urban growth during the six time periods for each ring buffer from the urban core centre in Riyadh. The urban areas were clustered around the centre between 1985 and 1990 within 20 km only from the urban core (
Figure 3A). During the 1990–2000 period, new patches were created, particularly in the north-eastern section (
Figure 3B). Mainly, this development occurred alongside the road networks that contributed significantly to the urban growth in the city. However, most of the expansion in Riyadh occurred between 2000 and 2007, when urban cover increased by about 40%. Growth occurred in nearly all directions after 2000. New developments in the north-east and north-west sectors indicated that urban areas were expanding toward the outskirts of the city. Like the 2000–2007 period, the expansion continued during 2007 to 2014 in the same direction, but to a lesser extent (about 18%). However, the growth rate decreased sharply after 2014, with the rate of urbanization increasing by almost 9% between 2014 and 2020.
Figure 4 shows the spatiotemporal change detection of urban cover in Jeddah between 1985 and 2020, and
Table 5 represents the increased rate of urbanization for each period in percentage terms. The urban area in Jeddah increased significantly between 1985 and 2020, with most of the growth, about 41%, occurring between 1990 and 2000. Unlike Riyadh, the 2000–2007 period witnessed a relatively low rate of urbanization, while the higher rate of urbanization was during 2007 to 2014, when growth nearly doubled. While the rate of urbanization during the 2014–2020 period was greater than the growth rate of Riyadh for the same period, the percentage increase in urbanization was low compared with other periods. The spatial distribution was controlled by the coastline in the western part of the city. Most of the expansion between 1985 and 2000 was within 25 km of the urban core centre and mostly occurred in the northern sector. After 2000, urban cover increased significantly in the north and exceeded the distance of 35 km from the urban core by 2014. There was also a remarkable development in the south-east sector during that time.
Change detection maps during the five time periods between 1985 and 2020 in Makkah are shown in
Figure 5, and the percentage increase of the urban area for each period is shown in
Table 6. In common with the other cities, Makkah has experienced rapid development during the last 35 years. Like Jeddah, most of its development, about 43%, occurred between 2007 and 2014 (
Table 6). The city also experienced a high rate of urbanization between 1990–2000 and 1985–1990, by 30% and 29%, respectively. While Makkah is highly urbanized within its administrative boundary, urban growth was distributed mostly in the north-eastern sectors between 2007 and 2014 (
Figure 5D). The development decreased significantly after 2014 and the rate of urbanization was very low in contrast with other periods.
4.3. Urban Patterns and Processes
Figure 6 shows a comparison analysis of the spatial metrics between Riyadh and Jeddah for each distance (5 to 40 km) from the city centre, and
Figure 7 presents a similar comparison for the entire landscape of both cities (>40 km). The percentage of urban cover increased between 1985 and 2020 in the entire landscape for both cities (
Figure 7A). The PLAND values were greater than 85% in the central area of Riyadh (five kilometres distance from the urban centre), while the PLAND values in Jeddah were between 60% and 85% between 1985 and 2020 (
Figure 6A). The urban central area in Riyadh showed an increase in urbanization between 1985 and 2007, but it decreased after that. In Jeddah, however, the expansion in the central area continued to increase between 1985 and 2020. This indicates that the central part of Jeddah is more active than that of Riyadh, which had already been developed by 2007. It is also clear that the PLAND values decreased with increasing distance from the urban centre in both cities. However, it was observed that the curve direction changed slightly after 2000 in Riyadh, with the change observed within the 15-km buffer zone. This indicates that the vacant lands within the buffer zone are likely to be occupied by urban areas. Therefore, the urban system in Riyadh is becoming more clustered.
The patch density (PD) in
Figure 6B shows different results for the two cities. During the 1985 period, a significant number of patches was associated with Jeddah compared with other periods. PD peaked near the city core at 10 to 15 km of the buffer zone and declined thereafter. However, during this period the PD value was higher for the entire landscape. Similarly, the PD values increased near the city centre during 1990 and peaked at 10 to 15 km, but the patch numbers declined for each distance at 20 km. This suggested that the urban patches during 1985 were more fragmented than in other periods. The landscape was comprised of small and unconnected patches of urban land. The landscape of the urban area in Riyadh was less fragmented and relatively less patchy than in Jeddah. This indicates that urban patches in Jeddah, especially during 1985, were likely mixed with other land cover areas, while urban areas dominated the landscape in Riyadh during the same period. The results also showed an increase of the PD value in the entire landscape of Jeddah during 2020 (
Figure 7B), indicating that there were new and small urban patches that were not connected to the previous patches. It is suggested, therefore, that the urban land in Jeddah is likely to be more fragmented in the future.
Edge density (ED) is another metric that can explain the process of urban expansion. Like PD, there is a pronounced differentiation between both cities. ED reached its peak during 1985 within a radius of five kilometres in Jeddah and then declined as the distance increased from the city centre. Within the same radius, the ED values decreased throughout the period, indicating that urban areas around the city centre fused together between 1985 and 2020. Riyadh, on the other hand, showed a different ED curve. While the ED declined between 1985 and 2020 within the five-kilometre buffer zone, the ED value in 1985 was low compared with Jeddah (
Figure 6C). The comparison analysis for the entire boundaries of both cities showed that ED increased in Riyadh between 1985–2014 and declined between 2014–2020, while in Jeddah, the ED declined between 1985–2000 and increased substantially between 2000–2020. These results indicate that the new urban developments after 2014 occurred near existing developments in Riyadh. Thus, the urban patches tend to be more clustered and the landscape is increasingly dominated by urban areas. In contrast, urban areas in Jeddah tend to be dispersed and the urban landscape is mixed with the other land cover types.
The FRAC_AM metric explains the shape of the urban area by which the distribution of urbanization is either a normal or complex process. The FRAC_AM values varied between Jeddah and Riyadh (
Figure 6D). Riyadh showed a relatively simple shaped urban form, in which the FRAC_AM values were less than 1.1 for all buffer zones between 1985 and 2020 (
Figure 6D). These results indicate that the urban form tended to be more compact and clustered. However, the situation was different in Jeddah. The FRAC_AM was higher for all time periods and the highest value was during 1990 (
Figure 7D). Within the five-kilometre buffer zone, the FRAC_AM values decreased throughout the period (
Figure 6D), indicating that the urban pattern in the central area moved from the complex form to compactness between 1985 and 2020. The FRAC_AM values are expected to decrease in the central area due to the lack of change in urban cover near the city centre. Nevertheless, the FRAC_AM is likely to increase within the city boundary, resulting in a more complex-shaped and fragmented urban form.
The Euclidean nearest neighbour distance was computed by the mean (ENN_MN) and standard deviation (ENN_SD), and it is presented in
Figure 6E,F for each buffer zone and
Figure 7E,F for the entire landscape during each time interval. Urban patches in Jeddah in 1985 were relatively aggregated, although the distance increased to its maximum in 2007 by about 225 metres. The distance decreased during the 2014 and 2020 periods. In Riyadh, the ENN_MN curve shows a relatively sine wave shape between 1985 and 2020 (
Figure 7E). The highest value was during 1990, by around 200 metres, while the lowest was during 2014 by approximately 175 metres. On the other hand, the ENN_SD diagram shows different results (
Figure 7F). Riyadh experienced a high value of ENN_SD during 1985 and 1990, while the values decreased during 2000, 2007 and 2014, and increased by 2020. The different shape between ENN_MN and ENN_SD between the cities indicates that the urban landscape in Jeddah has a dispersed pattern of patches, but it is more regular than Riyadh, which has an irregular landscape but a concentrated pattern of patches.
The contagion index (CONTAG) is shown in
Figure 6H for each buffer zone and
Figure 7H for the entire landscape. The CONTAG values around the central area (5 to 15 km) were higher in Riyadh than Jeddah for all time periods, indicating that the area around the urban core was most dispersed and fragmented in Jeddah, while in Riyadh, the central area was more aggregated, especially during 2007, 2014 and 2020. This was found in Jeddah during 1985 and 1990, where the landscape of the city was mostly heterogenous within the central area. In general, the CONTAG values decreased for both cities with a distance from the urban centre to the 20-kilometre buffer in Riyadh and 15-km buffer in Jeddah, and increased as the distance increased, showing a wave-like shape. For the entire landscape, Jeddah showed relatively higher CONTAG values than Riyadh, and the CONTAG decreased throughout time in both cities (
Figure 7H).
Figure 8A–H shows a comparison of spatial metrics of urban growth in Makkah for multiple buffer zones (5 to 20 km), and
Figure 9 shows the same metrics for the entire landscape during 1985, 1990, 2000, 2007, 2014 and 2020. The percentage of urban landscape increased in Makkah between 1985 and 2020 (
Figure 9A). The landscape of the central area (five-kilometre buffer) was covered by urban land use by about ~80% during the 2014 and 2020 periods (
Figure 8A). However, the percentage of urban area decreased with the increase of the distance from the urban core centre. In general, Makkah experienced a high rate of urbanization between 2007 and 2014.
The patch density (PD) also increased between 1985 and 2020 in the entire landscape of Makkah (
Figure 9B), as the number of patches (NP) increased during the same time (
Figure 9G). Analysis of the buffer zones shows that within the five-kilometre buffer, the PD values were relatively high during 1985–1990 and decreased after that (
Figure 8B), indicating that in the early stage of development, the central area was more fragmented and most patchy. During this time the landscape became urbanized around the city centre and the PD values decreased. Additionally, the PD values decreased as the distance increased from the urban core during 1985–1990 and increased between 2000–2020. The 2020 period showed the highest value of PD compared with other periods when the distance increased from the urban centre.
The edge density (ED) values also show a variation in results over space and time (
Figure 8C and
Figure 9C). Since the urban centre showed higher ED values during 1985, 1990, 2000 and 2007, the ED values decreased during these periods when the distance from the urban core increased. The ED values decreased around the central area and increased for the entire landscape during the 2014 and 2020 periods. These results suggest that the central area was dispersed and mixed with other land cover types during 1985. The landscape then became more clustered and dominated mostly by urban areas by 2014–2020 as the ED values decreased throughout time.
The area weighted mean patch fractal dimension (FRAC_AM) values are presented for each buffer zone in
Figure 8D and
Figure 9D for the entire study area. The FRAC_AM values increased between 1990–2000 and continued at nearly the same level between 2000–2007 and decreased between 2007–2014 (
Figure 9D). The results of FRAC_AM in
Figure 8D show an increase as the distance increases from the urban core after 2000, indicating that the landscape of the city (>10 km) tended to be a more complex and fragmented urban form. The shape of the central area, however, tended to be at a regular form in recent years (2014 and 2020) as the values become closer to one, which indicates a relatively simple shape.
ENN_MN and ENN_SD are presented in
Figure 8E,F for each buffer zone and
Figure 9E,F for the entire study area. The 2000 period showed the highest value of the average nearest neighbour and dropped after that time (
Figure 9E). This indicates that the 2000 period was the initial phase of diffusion in Makkah as large distances separated new development. For each buffer zone, the ENN_MN values showed an increase as the distance increased from the city centre (
Figure 8E), indicating that the urban patches near the core centre are clustered while the patches become isolated as the distance increases from the urban centre. On the other hand, the standard deviation nearest neighbour peaked in 1985 (
Figure 9F), which indicates that the distribution of urban patches was irregular across the landscape. However, the SD values decreased after 1985, indicating that the urban patches became uniform across the landscape.
The contagion metric is presented in
Figure 8H for each buffer zone and in
Figure 9H for the entire landscape. The contagion was lowest within the central area (five kilometres), especially during the 1985 and 1990 periods. During this time, the urban configuration was most fragmented around the city centre. Moreover, in 1985–1990, the contagion increased as the distance increased from the urban core, indicating that during 1985–1990, the landscape was homogenised within 15 to 20 km from the urban centre. In general, contagion decreased between 1985 and 2020 for the entire landscape, suggesting that urban patches of Makkah where dispersion occurred in the early years had become more interspersed in recent years.