Remote sensing-based detection and assessment of vegetation biomass and water content of urban green spaces in Mariental, Namibia

Abstract

Urban green spaces (UGS) improve air quality and the hydrological cycle, which, in turn, results in a cooling effect on the microclimate. However, the maintenance of UGS in terms of water demand may be costly, particularly in arid and semi-arid environments, thus, achieving a balance is imperative. Mariental is situated in an arid zone and has the highest average temperature variations among the urban areas in Namibia. This study was therefore aimed at assessing biomass and vegetation water content (VWC) of green spaces in Mariental as proxies for gauging its effort towards ameliorating the town’s microclimate in a water-deficient environment. The detection and assessment were based on two indices, the Modified Soil Adjusted Vegetation Index 2 (MSAVI2) and Normalised Differenced Water Index (NDWI), derived from a dry season Sentinel-2 image, acquired in August 2018. Field validation, focusing on the types of UGS, was carried out during the same week as the image acquisition. MSAVI2 depicted UGS and other non-biomass classes better than NDWI. Approximately 1% (6.6 ha) of Mariental is occupied by UGS as estimated using MSAVI2. The NDWI and MSAVI2 recorded mean values of 0.14 (very low) and 0.61 (low), respectively. The maximum values, derived for the central business district (CBD), were categorised as very high for the moisture (0.38)
and biomass (0.79) indices. The estimated green space area translates to a per capita of 5m2, with the minimum of the global average ranging between 5m2 and 50m2 per capita. Assessing the water demand for existing vegetation types will complement these results
for a knowledge-based decision to expand Mariental’s UGS while maintaining effective water management and improving its microclimate.