As far as urban centers are economic hubs, urbanization is an inevitable phenomenon throughout the world. Thus, the environmental damages due to urbanization in Ethiopia could be alleviated by letting the urban development has to be based on the master plan and structural plan. The central arguments explicitly explored in this paper are the positive contributions of urbanization on the urban tree cover and built structures over the expenses of the low ground cover (grass, bare soil, and other herbs). Subsequently, expanding cities based on such proper urban planning would help to lower the heat island of the human-dominated landscapes.
Studies on the urban land surface cover and temporal changes based on sample points using i-Tree software have been commonly conducted in many US cities (e.g., Nowak & Greenfield, 2012; Nowak & Greenfield, 2020), some European urban centers (e.g., Mills et al., 2015; Doick et al., 2017; Doick et al., 2020), and few Asian cities (e.g., Atasoy, 2020). To our knowledge, no study has been conducted so far following similar approaches to explore the trends of urban land surface cover changes in cities and towns in Africa. Thus, this study has reported pioneer findings on the land surface cover dynamics and environmental implications of the cities and towns in Ethiopia. As far as urban centers are similarly characterized in terms of ground cover type dynamics, the results of this study were compared to the findings of similar studies conducted in Europe, the US, Asia, and other regions of the world.
Attempts were made to analyze the current and change of the urban fabrics from which the urban land is covered. This helps to understand the impacts of urbanization on land surface cover compositions and change dynamics. Subsequently, it can assist to rethink the proper balancing of the grey, green, and blue cover classes of the built environment. Thus, understanding the current green-grey cover proportion of the built environment can enhance the sustainability of cities and towns (Banzhaf et al., 2018; Corbane et al., 2020).
Materials that cover every land parcel of the study area were estimated based on the 500 random sample points of the i-Tree software that were distributed across the study area. Indeed, studies on urban land use and cover change analysis have been increasingly using spatial scale satellite images that could affect the quality data presentation to study the ground cover types of urban areas with fine scale. A study by Parmehr et al. (2016) compared land cover quantified using i-Tree software based on 1000 random points and remotely sensed data shows that the variation between the two results was 1%. Thus, approaching this research using a different procedure would provide a similar output.
4.2. Land surface cover and Temperature changes
Although urban tree coverage can be increased in line with urbanization, the expansion of built-up areas exuberates the destruction of natural elements, including vegetation cover and soil. The results of this study show that a large area of pervious land coverage of the study area, such as herbaceous/grass and bare land/soil declined, which could be due to the expansion of the grey coverage (e.g., building, road, and other impervious types). The gaining of grey surface cover (+ 24%) is over the expenses of the coverage loss of grass (-2.8%) and bare soil (-34.6%) covers. This implies that plantable spaces of the city have been converted into grey cover to expand other infrastructures. This result is in line with the study by Nowak & Green (2018), who reported the impervious surface cover was gained from the loss of trees, grass, and herbaceous coverage. Thus, total canopy cover from large trees and shrubs could temporally be increased in cities and towns, whereas the losses of lower layer ground covers (herbs, soil) decreased as the rate of urbanization intensified.
Understanding the land use history of areas around cities and towns is critically important to comprehend the positive or negative impacts of urbanization on land surface cover. The expansion of built areas over the previously forested area can play a great role in declining the green coverage of an urban area (Nowak & Green, 2018). In this study, however, tree canopy coverage increased as the rate of urbanization increased. Likewise, the percentage of areas with high and low/very low NDVI values increased and decreased respectively within the stated timeframe. This could be because an important part of Hawassa city has been expanded into the previously undeveloped agricultural area (barren land) which had no tree cover until 2011.
There are driving factors that affect the decline or gaining of green coverage in the urban center around the world. For instance, tree coverage could be raised due to the increment in tree planting initiatives and activities around the human-dominated landscapes, such as institutional areas, commercial and residential areas (Nowak & Greenfield, 2020). Moreover, town planning, changes in vegetation preference of community, and management activities (Doick et al., 2020) can contribute to the gaining of TCC in the face of rapid urbanization. The first structural plan of Hawassa city was prepared in 1951 with a total land of 120 ha of which most of the land was allocated for housing purposes (Federal Urban Planning Institute, 2006). The updated five-year structural plans (2007–2021) revealed that neighborhood green spaces, closed natural parks, and public recreational green infrastructures are important elements of the land use types of the city. Thus, the increment in the green area coverage per capita from 0.52m² in 1998 to 2.0m² by the year 2011 is one of the indicators that the structural plan is an important tool to raise the green coverage of the city. Subsequently, vacant spaces with few patches of vegetation and dominated by bare land in 2011 have been turned into closed parks and other recreational areas. For instance, the presently closed public park with enacted natural forest called Millennium Park was a vacant space with scatter native tree species in 2011; and significant green spaces established along the lakeside as a buffer of the Hawassa Lake could significantly contribute to increasing the TCC. It was clearly stated in the structural plan that the strategies employed to attain the required green coverage were encouraging environmental advocators, private sectors, and mobilizing the community to participate in tree planting and green area developments.
These positive roles of urbanization towards TCC could contribute to lowering the land surface temperature of the study area. This study shows that the LST of the study area decreased by -1.84°C between 2011 and 2021, while the green coverage in general and tree canopy cover in particular raised by + 7.2% and + 9.8%, respectively. The increment of tree canopy cover following the settlement expansion towards the natural area has contributed to lowering the LST. Gill et al., (2007) also reported that raising the urban green spaces by 10% in the most built part of a city contributes to decreasing the maximum surface temperature by 2.2°C. The authors also pointed out that this amount of green spaces addition in a high and low emission scenario decreases LST by 2.4°C and 2.5°C, respectively. The same authors also reported that the removal of 10% green space coverage in high and low emission scenarios could raise the LST by 7°C and 8.2°C, respectively. Similarly, the current study confirmed that increasing trees, shrubs, grass, and herb, collectively green coverage by 9.8% or raising the TCC alone by 7.2% contributes to decreasing the maximum and average LST by -2.17°C and − 1.67°C, respectively.
It was not possible to carry out a correlation analysis between the LST derived from raster images and TCC estimated using the i-Tree canopy software due to the nature of the data from this software. Thus, the TCC was replaced by the NDVI to test the effects of vegetation cover on LST. The NDVI values are usually between − 1 and + 1; however, areas with different types of ground cover type could have various levels of NDVI values. In the case of the current study, the average NDVI value in 2021 was higher (0.23) than in 2011 with 0.17. About 17% (880 ha) of the total area of the city in 2021 had the maximum NDVI value of 0.5, while small land size (490 ha) of the city had the highest vegetation coverage with the same NDVI value in 2011. The urban area is dynamic and the ground cover type is subjected to frequent temporal and spatial changes. Thus, the NDVI values vary across the study area. According to Ya’Acob et al. (2014), water bodies, rocky/barren land, greenery, and dense forest have less than zero (negative), -0.1 to 0.1 (value close to zero), 0.1–0.4 zero (low positive), and up to + 1 NDVI values, respectively. According to the NDVI values of the current study, more than 20% of the city in 2011 had sites with no vegetation coverage (NDVI = < 0.1), whereas this proportion decreased to 18% in 2021. There are green spaces and some patches of the urban forest before 2011 and still exist in Hawassa city where the highest NDVI values in both periods could be recorded. According to Warkaye et al. (2018), the highest NDVI with 0.5–0.7 values were reported in the green belt, urban green spaces, and river banks, while the lowest values with 0.31–0.5 in the closed urban park of Addis Ababa city. However, this study was focused on selected parks only, while the current study was conducted on the entire city of Hawassa. Areas with 0.2 to 0.4 NDVI were classified as medium vegetation and more than 0.4 value was considered as areas with high vegetation cover (Nandargi & Kamble, 2017). Regarding the NDVI spatial pattern shown in Fig. 8, the highest land surface area of the city with 0.3–0.5 NDVI values in 2021, whereas the major part of the city (66.7%) in 2011was with less than or equal to 0.20 NDVI value due to the less coverage of green spaces.
According to the result of the statistical analysis with R2 = 0.472 for 2011 and 0.592 for 2011, about 47% and 59% of the LST in 2011 and 2021 respectively decreased due to the vegetation coverage in the study. In addition to the vegetation components, the water bodies could also play important roles to reduce the LST. Adulkongkaew et al. (2020) suggested the conversion of the built area into a landscape with 20% tree canopy, 30% of blue components (water bodies), and 40% of shrubs and other green elements is effective to reduce the LST of urban areas. Thus, it is critically important to take into consideration the proportion of land cover composition at the urban planning and implementation stages to reduce the impacts of heat island effects.
The Normalized Difference Vegetation Index and tree canopy cover can be used as indicators of LST trends. The highest temperature at the center of the city and other built areas where the lowest NDVI values records indicates that the heat island effect is due to the low vegetation cover and thermal properties of the ground cover materials. This study confirmed that LST decreased as both TCC and NDVI increased within the 10 year time. Thus, the two variables can be used interchangeably to examine the effects of vegetation cover on LST trends. The statistical analysis result of the study revealed that vegetation cover and the heat island effect have a reverse correlation. This study is in line with the research by Warkaye et al. (2018) conducted in Addis Ababa city who concludes that the NDVI value of different urban parks increased as the LST of the study area decreased from 1985 to 2015. Thus, increasing annual urban tree coverage in urban centers is the effective strategy of urban heat island effective adaptation strategy.