1. Introduction
Temporally and spatially, urbanization is an uneven process supporting residential expansion, including growth in population size of individuals living in urban areas and expansion of physical structures in an urban setup in addition to the previously existing structures [
1,
2]. Urbanization is directly changing and affecting the environment, as it is made distinct by the increasing built-up and impervious areas at the expense of wetland areas and agricultural landscapes. Such actions result in the transformation of natural landscapes into agricultural landscapes [
3]. Consequently, this leads to environmental degradation through deterioration of vegetation and sealing, often resulting in increasing surface runoff, soil erosion, surface water contamination, and exploitation of natural habitats [
4,
5,
6]. Muller et al. [
7] highlights that urbanization is one of the greatest factors contributing to biodiversity loss due to the expansion of industrial, residential and commercial business areas.
The world’s population is projected to increase from 7.0 to 9.3 billion by 2050 [
8]. Therefore, during that stipulated period urban areas worldwide are anticipated to absorb large numbers of the growing population. Urbanization is a continuous process, and megacities such as Delhi, India had a total increase in population of 47.02% within a decade between 1991 and 2001 [
9]. For China, the urbanization growth rate tripled from 17.9% to 57.4% between 1978 and 2016, and it is projected to reach 70% by 2035 [
10]. Although countries like Ethiopia are among the least urbanized countries in the world, high rates of in-migration to urban cities have been investigated and projected to reach 42.1% of the total population by 2050 [
11]. Up-to-date data and information regarding the trends and status of urban ecosystems are required to enable the development of sustainable strategies on improving livelihoods in urban settings. In particular, in sub-Saharan Africa, up-to-date data and information on population are fundamental for developing ways to curb urban demographic transitions [
12]. It is estimated that sub-Saharan Africa’s urban population will rise by 60–70% by 2050, with most people occupying small cities due to the fact of cultural, socio-economic and political influences [
12,
13]. Although previous studies have indicated urban growth in terms of population size, studies on land use and land cover (LULC) changes and the key drivers of such changes remain scarce yet important.
To further understand the relationship between urbanization and environmental alteration, LULC changes need to be assessed and evaluated to determine the extent and the rate at which human activities are contributing to shifts in the environment. The integration of remote sensing data to monitor the state and dynamics of the Earth’s surface provides reasonable results in a short space of time [
6], compared to on-site surveying techniques [
14,
15]. Moderate resolution Landsat Thematic Mapper images (TM) are a standard tool used for urban mapping and change detection analysis and were used, for example in Minnesota, USA between 1986 and 2002 for LULC changes [
16]. Landsat images in combination with socio-economic data have been used to determine the effects associated with development and land use shifts. For example, spatial dynamics of LULC changes were analysed for the Nairobi urban area and showed that the built-up area quadrupled from 1.9% in 1976 to 8.6% of the total area in 2000 [
17]. For Zimbabwe, Hove and Tirimboi [
18] indicated that on a national scale, vast numbers of people migrated to Harare from rural homes soon after independence in 1980. Wania et al. [
19] reported on the expansion of built-up areas of Harare using high-resolution SPOT images.
Investigation of LULC change dynamics and classification in heterogeneous landscapes using moderate-resolution satellite imagery potentially has challenges due to the fact of spectral confusion resulting in misleading information [
20,
21]. Accurate observation of LULC changes by remote sensing is a vital component of promoting sustainability. Enhancement of land cover class delineation using remote sensing indices and machine learning algorithms such as random forest (RF) and support vector machines (SVMs) have the relatively desirable characteristic of improving multispectral classification [
20,
21,
22,
23,
24]. The effectiveness of mapping land-cover types using spectral indices is primarily the result of their ability to characterize relative features of interest over a wide range of the spectrum [
25]. The Harare Metropolitan Province was chosen as a case study because of its vastly reported pressures due to the presence of high population and urbanization rates for this metropolitan area in the northern High Veldt of Southern Africa [
19,
26]. Harare Metropolitan Province, being the capital city of Zimbabwe, faces increasing population growth as do other metropolitan cities largely because they are associated with better livelihoods and as centres for economic activities, public services, and amenities [
24,
27]. Among others, Chirisa and Muhomba [
28] revealed that Epworth, a Harare Metropolitan Province district, has approximately 70% of its inhabitants living on unauthorized, non-serviced land and thereby compounding the increased settlement and spread of the metropolitan area. Thus far, monitoring urban growth trends is an important tool for understanding previous trends and present growth patterns and potentially unravelling possible coming developments and their likely impacts [
29]. Identifying empirical drivers of urban structure change is based on past and current state of LULC changes for the Harare Metropolitan Province. The current study aimed to investigate the axis of change and expansion of the Harare Metropolitan Province. In view of the resource constraints in a developing country, freely available remote sensing data were applied to assess the direction of the urban expansion. Furthermore, the study sought to determine the explanatory drivers of the changes in LULC for the Harare Metropolitan Province.
4. Discussion
Urbanization has modified the Harare Metropolitan Province through the expansion of built-up areas at the expense of vegetation, cropland and water bodies (
Figure 3,
Figure 4). Similarly, urbanization processes investigated in Daqahlia, a city in Egypt, depicted that the built-up area expanded from 4.2% of the area under investigation in 1985 to 36.3% in 2010, while areas used as cropland shrank by 30.7% and areas covered by water decreased by 0.45% [
27]. An increase of 219.5% in LULC change, mainly attributed to land development at the expense of cropland, fallow land, water, shrub and bare land, was revealed in the Shanghai metropolis between 1997 and 2008 showing the impact of land use change and population growth in urban areas [
51]. Akure city in the southwest of Nigeria experienced a similar loss of areas covered by vegetation and water bodies as the Harare Metropolitan Province due to the fact of built-up area expansion, from 5.1% in 1986 to 53.41% in 2014 for the total area under investigation [
6].
In the current study, a SVM classification method was applied because it potentially produces better accuracy in a confusion matrix compared to other neural networks, maximum likelihood and decision trees when mapping LULC [
50,
51]. However, possible sources of error in the calculations may have emerged from geometric rectification, accuracy in digitizing topographic maps and combining different data sources. The spectral differences and characteristics between Landsat 5 TM and Landsat 8 OLI sensors may have affected the accuracy of the thematic maps [
23]. Despite these potential discrepancies, the classification and results obtained in the current study have relatively high accuracy considering urban area spectral heterogeneity characteristics and spectral confusion from land cover classes, and the results agree with other published scientific studies carried out at the national and regional level (
Supplementary Materials Table S1) [
6,
24,
26,
27]. The use of hyper-spectral data and aggregation of urban built-up areas have been observed to improve and enhance the analysis of remote sensing data in urban areas [
50,
52]. The utilization of additional built-up, water and vegetation remote sensing indices bands on the Landsat imagery scenes provide a substantial improvement in the mapping of an urban area using moderate-resolution imagery [
21,
44,
45]. However, misclassifications and reduction of areas covered by water bodies (water class) might have resulted from the increasing density of water hyacinths along the streams due to the fact of contaminated sewage effluents deposited in the water ecosystem [
52,
53]. This is directly linked to the inflow of effluents from industries, sewage disposal (punctual sources) and urban agriculture (diffuse sources). Consequently, alteration of water bodies as aquatic weeds scattering on the surfaces potentially influences classification (
Table 4).
The current study looked at the axis of urban development and the drivers posing LULC changes. This study revealed that the distance to the nearest major and secondary roads have a large impact on urban expansion and development. The binary logistic regressions highlight that built-up area development occurred predominantly along the major roads and in dense road network areas (i.e., secondary roads), due to the high connectivity and easy access to transport facilities. Hegazy and Kaloop [
27] reiterate that urban growth follows development along highways or already established cities as a result of population growth and socioeconomic factors. Nevertheless, high-density residential areas are expanding towards the periphery of the south, southeast, southwest and northwest of Harare Metropolitan Province. For comparison, the results of modelling the distance characters in Bucharest were in line with the findings of the current study—major and secondary roads impact positively on urban growth and expansion of built-up areas [
53]. For Bucharest, however, independent variables such as distance to lakes and rivers were not significant, while for the Harare Metropolitan Province, these variables have significant influence, as revealed by the binomial regression analysis between 2008 and 2018. Henceforth, the exclusion of the core city of Bucharest from the Bucharest Metropolitan area could have reduced the ability of the model to detect some independent characters. Still the geographic location of the study areas, socioeconomic structure and population sizes are highly different.
The current study reveals that zones of urban area or built-up area expansion were associated with relatively gentle undulating slopes (
Figure S1) which can be attributed to low housing costs, cheap land acquired through housing schemes and informal urban settlements [
19]. The southwest part of the Harare Metropolitan Province was the main direction of urban spread between 1984 and 1990 (
Figure A1). This was associated with the expansion from first old Harare high-density suburbs that were designated during the colonial period such as Highfields, Mufakose and Rugare, among others [
54]. These areas are associated with high-density road networks, low costs and economic residential units compared to the low and mediumdensity residential areas. Between 1990 and 2000, expansion of the Harare Metropolitan Province dominated much in the southwest, west and southeast (
Figure 4,
Figure A1). This expansion can be attributed to urban sprawling and rampant informal settling due to the presence of socio-economic factors in the eastern direction—that is, the Epworth suburbs and Dzivarasekwa extension introduced in 1991 [
55,
56]. Between 2000 and 2008, significant expansion of built-up areas was directed towards the south and northwest direction (
Figure 4,
Figure A1) regardless of “Operation Murambatsvina” (Restore Order), a clean-up campaign that was carried out in June 2005 and left Harare with dismal identifiable illegal, urban built-up structures [
31].
The expansion of Harare Metropolitan Province between 2008 and 2018 was dominant in the southern direction, and the city was expanding towards its peripheries (
Figure 4,
Figure A1). The far continued expansion towards the southeast resulted from unplanned urban development because of population pressure [
28]. Bureaucracy and rigid and stringent procedures relating to construction plans approval by local authorities posed a hindrance towards sustainable urban growth [
57,
58,
59]. Fast urban built-up area growth was observed on the southeast parts of Harare [
19] resulting in marginalized urban residents. These marginalized urban residents are residing in poorly serviced areas which are of relatively low cost due to the shunning absence of proper water and sewer reticulation systems. This unplanned development accompanied with poor sanitary conditions resulted in increased chances of severe health issues [
60,
61].
The northeast of Harare Metropolitan Province is composed of low-density residential suburbs and is characterized by medium gradient hills covered by high-vegetation density compared to high-density residential suburbs (
Figure 4) [
34,
54]. Due to the stratification of Harare, high income earners were pronounced to occupy these vegetation-enriched suburbs [
55]. Construction on these landscapes is costly resulting in the variable “slope” as a significant driver for urban-built up area growth since the larger proportion of Harare residents occupied flat to gentle undulating landscapes in the south and other parts of Harare (
Figure 4,
Figure S1). Urban built-up area expansion towards the high-density residential area in the northwest direction follows the establishment of housing schemes such as the University of Zimbabwe’s Association of University project and Hatcliffe Consortium development [
31,
59]. The Hatcliffe Consortium development was a government initiative on Operation “Garikai/Hlalani kuhle” projects meant to provide housing units towards the “Operation Murambatsvina” victims [
31]. On the other hand, high population density on the northwest suburbs of Harare coincides with increasing urban built-up area expansion [
19,
30].
The geometry of road network reveals an influence on the spatial distribution and spread of urban built-up area as characterized by the regression models capturing the changes between 1984 and 2018 (
Figure 5). This indicates a linear pattern on the built-up areas and the road network systems, contributing to the rapid spreading of informal settlements within the road spheres. Easy access to transportation network systems paves the way for a linear and grid distribution pattern of built-up areas in urban cities such as the Harare Metropolitan Province. The urban population increased faster than anticipated, resulting in accelerated rates of informal settlements and the erratic provision of decent housing by the Zimbabwean Government [
28,
58,
62]. Increased unrestrained built-up area expansion and spread in the south and east of the Harare Metropolitan Province reveals largely urban sprawl [
26,
28]. Thus, these increasing rates of informal settlements within the metropolitan area have negatively impacted water resources. Thereby, driving the “distance to the nearest streams and open water” variables as influential characters of the urban built-up area expansion. This has also been attributed to the invasion of the Harare Metropolitan Province’s ecosystem with urban construction activities [
26,
37,
59]. The outward spreading of built-up areas was not evenly distributed. It concentrated largely in the south and southeast parts of the Harare Metropolitan Province, where fairly flat landscapes occur. This contributes to cheap residential construction costs [
19] compared to the strongly rolling, sloppy landscape in the northeastern suburbs of the Harare Metropolitan Province (
Figure 4,
Figure A1) [
34].
Without negating population growth rate as a driving force for urban growth, there is a correlation between urban expansion and population growth rate as substantiated by previous Harare Metropolitan Province population statistics [
29]. The Harare Metropolitan Province population is estimated to have increased from approximately 830,000 in 1982 [
28] to 2,098,199 in 2012 [
29]. This huge growth of urban population posed large threats to water bodies (open water and streams) thereby making water resources vulnerable. Consequently, urban sprawl and unplanned rampant settling in the Harare Metropolitan Province concurred with the deterioration of water bodies since 2000. Overall, this study indicates that the popularity of water resources as amenities for land estates declined due to urban growth which has led to the degradation of water resources through various human activities.