Use of multi-source remotely sensed data in monitoring the spatial distribution of pools and pool dynamics along non-perennial rivers in semi-arid environments, South Africa

Abstract

This study explored the use of multi-source remotely sensed data in monitoring the spatial distribution of pools and pool dynamics in two distinct semi-arid sites in South Africa. The factors that control the pool dynamics were also examined. Three water extraction indices were used, these included Normalised Difference Water Index (NDWI), Modified NDWI and Normalised Difference Vegetation Index. In addition, random forest classifier and Sentinel-1 SAR data were used in mapping pools and pools dynamics for both sites. Overall, the remotely-sensed methods detected and mapped pools with acceptable accuracy, except for small pools (<400 m 2 ). The results suggest that flow occurrences and rainfall are key in controlling temporal changes in pools sizes, and there was no interaction between pools and groundwater. The study showed that remote sensing methods are essential for filling ground monitoring gaps in non-perennial rivers and determining hydrological processes and water availability from pools in semi-arid environments.

Using the water balance approach to understand pool dynamics along non-perennial rivers in the semi-arid areas of South Africa

“…However, the study site received rainfall amounting to 112 mm/year in 2018, 91 mm/year in 2019, and 182 mm/year in 2020. There is no distinct wet/dry seasons (Maswanganye et al, 2022). According to Petersen et al (2017), the catchment has a Mean Annual Runoff (MAR) of 16.32 Mm 3 /year or 2.5 mm/year.…”

“…For some of the NPRs, when flows cease water occurs in pools along these rivers. These pools are of importance for aquatic life as refugia and surrounding communities as a source of water for livestock, garden watering and domestic use (Maswanganye et al, 2022). Pools occurring along NPRs have been recognised for their ecological importance (Ilhéu et al, 2020;Marshall et al, 2016;Sheldon et al, 2010).…”

“…Routine monitoring of water storage in pools along NPRs has not been included in most national hydrological monitoring systems, partly because these systems are often considered to have low value (Rodríguez-Lozano et al, 2020), and due to the absence of adequate financial resources. There are also physical limitations, as some of these pools are not easily accessible, and some may disappear after flow events, depending on river-bed material (Hattingh, 2020;Maswanganye et al, 2022). However, very few studies have shown that remote sensing can provide useful information about these pools, including their spatial distribution and size (Maswanganye et al, 2021;Seaton et al, 2020).…”

“…However, very few studies have shown that remote sensing can provide useful information about these pools, including their spatial distribution and size (Maswanganye et al, 2021;Seaton et al, 2020). Maswanganye et al (2022) found that river flows are the major controlling factor of pool dynamics and suggested that rainfall is important for delaying the drying out of pools in the semi-arid and arid environments of South Africa. However, the study also expressed that there is a need to assess pools in detail, in order to gain a better understanding of their hydrodynamics.…”

“…Although the water balance has been applied to understand the dynamics of water bodies across the globe, it has not been used to understand pools along NPRs. Maswanganye et al (2022) and Bourke et al (2020) suggest that the water balance approach can assist in improving the understanding of pool dynamics, which could be useful in the management of NPRs and their contributing catchments. Therefore, this study aims to improve the understanding of pool dynamics or water storage changes in pools along non-perennial rivers (NPRs) in the semi-arid environments of the Karoo region of South Africa.…”