Seasonal and Spatial Dynamics of Surface Water Resources in the Tropical Semi-Arid Area of the Letaba Catchment: Insights from Google Earth Engine, Landscape Metrics, and Sentinel-2 Imagery
Abstract
1. Introduction
2. Materials and Methods
2.1. Study Area
2.2. Data Acquisition and Analysis
2.3. Spectral Derived Indices
2.3.1. Extraction of Surface Water Bodies
2.3.2. Validation
- True positive (TP): number of accurate extracted water pixel points.
- False negative (FN): number of undetected water pixel points.
- False positive (FP): number of erroneously extracted water pixel points.
- True Negative (TN): number of non-water pixel points.
- Overall accuracy (%) = (TP + TN) × 100/Total number of pixel (T)
2.4. Spatial Configuration
- (i)
- Number of Water Bodies (NW): This metric represents the total number of distinct water bodies within a defined area. Monitoring NW helps to understand the proliferation or reduction in aquatic habitats, influenced by factors such as climate change, land use changes, or conservation efforts. This metric was calculated by converting the raster (water image) into vector data using the reduceToVectors function in Google Earth Engine (GEE), applying the parameter ee.reducer.countEvery to count the number of water bodies within the study area.
- (ii)
- Mean Area of Water Bodies (MAW): This metric provides the average size of water bodies, offering insights into habitat availability and the potential to support biodiversity. Changes in MAW may indicate ecological shifts, such as the drying up of smaller ponds or the expansion of larger lakes. To calculate MAW, the area of each water body was determined using the GEE function area. The area in hectares was then calculated by dividing by 10,000 using the divide function. The average water body area was computed using the following formula:
- (iii)
- Total Area of Water Bodies (TAW): This metric quantifies the total area covered by water bodies within a landscape. It reflects the overall extent of aquatic ecosystems, which is crucial for water resource management, flood control, and maintaining ecological balance. Fluctuations in TAW can indicate environmental changes or the effectiveness of water conservation efforts.
- (iv)
- Coefficient of Variation (CV): This metric is derived from the standard deviation of MAW, divided by the mean area of water bodies (MAW), and multiplied by 100. It measures the relative variability in the sizes of water bodies, highlighting the landscape’s heterogeneity. A high CV suggests a diverse range of water body sizes, which can be beneficial for supporting different species and ecological functions. Conversely, a low CV may indicate uniformity, potentially affecting habitat diversity.
3. Results
3.1. Water Indices and Validation
3.2. Intra and Inter-Seasonal Dynamics of Surface Water Bodies
4. Discussion
5. Conclusions
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
Abbreviations
Google Earth Engine | GEE |
Modified normalized difference water index | MNDWI |
Short wavelength infrared | SWIR |
False negative | FN |
False positive | FP |
Total Area of Water Bodies | TAW |
Mean Area of Water Bodies | MWA |
Number of Water Bodies | NWB |
Normalized difference water index | NDWI |
Coefficient of Variation | CV |
True positive | TP |
True Negative | TN |
Sentinel Water Index | SWI |
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Band Name | Band Number in GEE | Band Resolution |
---|---|---|
Coastal Aerosol | SR_B1 | 60 |
Green | SR_B2 | 10 |
Red | SR_B3 | 10 |
Blue | SR_B4 | 10 |
Vegetation Red Edge | SR_B5 | 20 |
Vegetation Red Edge | SR_B6 | 20 |
Vegetation Red Edge | SR_B7 | 20 |
Near-Infrared (NIR) | SR_B8 | 10 |
Narrow NIR | SR_B8A | 20 |
Water Vapor | SR_B9 | 60 |
Shortwave Infrared 1 (SWIR-1) | SR_B11 | 20 |
Shortwave Infrared 2 (SWIR-2) | SR_B12 | 20 |
Shortwave Infrared 3 (SWIR-3) | SR_B13 | 20 |
Cloud Mask | QA60 | 60 |
Water Indices | Formulae | Threshold | Reference |
---|---|---|---|
Normalized difference water index (NDWI) | NDWI = (Green − NIR)/(Green + NIR) | NDWI > 0 | [34] |
Modified normalized difference water index (MNDWI) | MNDWI = (Green − SWIR2)/(Green + SWIR2) | MNDWI > 0 | [35] |
Sentinel 2 Water Index (SWI) | SWI = (Red edge − SWIR2)/(Red edge + SWIR2) | SWI > 0 | [18] |
(a) | Dry Season | ||||||||
---|---|---|---|---|---|---|---|---|---|
NDWI | MNDWI | SWI | |||||||
OA% | UA% | PA% | OA% | UA% | PA% | OA% | UA% | PA% | |
2017 | 99 | 99 | 100 | 94 | 92 | 99 | 99 | 99 | 100 |
2018 | 98 | 98 | 100 | 96 | 95 | 99 | 98 | 98 | 100 |
2019 | 99 | 98 | 100 | 96 | 96 | 99 | 99 | 98 | 100 |
2020 | 96 | 94 | 99 | 92 | 92 | 99 | 98 | 97 | 99 |
2021 | 99 | 96 | 100 | 98 | 97 | 99 | 99 | 99 | 100 |
2022 | 100 | 100 | 100 | 99 | 99 | 100 | 98 | 97 | 100 |
(b) | Wet season | ||||||||
NDWI | MNDWI | SWI | |||||||
OA% | UA% | PA% | OA% | UA% | PA% | OA% | UA% | PA% | |
2017 | 99 | 99 | 100 | 99 | 99 | 100 | 99 | 98 | 100 |
2018 | 98 | 98 | 100 | 98 | 98 | 99 | 98 | 98 | 100 |
2019 | 99 | 98 | 100 | 96 | 95 | 100 | 99 | 98 | 100 |
2020 | 96 | 95 | 99 | 94 | 92 | 99 | 95 | 94 | 99 |
2021 | 99 | 98 | 100 | 99 | 98 | 99 | 99 | 99 | 100 |
2022 | 100 | 100 | 100 | 98 | 80 | 100 | 98 | 97 | 100 |
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Mashala, M.J.; Dube, T.; Ayisi, K.K. Seasonal and Spatial Dynamics of Surface Water Resources in the Tropical Semi-Arid Area of the Letaba Catchment: Insights from Google Earth Engine, Landscape Metrics, and Sentinel-2 Imagery. Hydrology 2025, 12, 68. https://doi.org/10.3390/hydrology12040068
Mashala MJ, Dube T, Ayisi KK. Seasonal and Spatial Dynamics of Surface Water Resources in the Tropical Semi-Arid Area of the Letaba Catchment: Insights from Google Earth Engine, Landscape Metrics, and Sentinel-2 Imagery. Hydrology. 2025; 12(4):68. https://doi.org/10.3390/hydrology12040068
Chicago/Turabian StyleMashala, Makgabo Johanna, Timothy Dube, and Kingsley Kwabena Ayisi. 2025. "Seasonal and Spatial Dynamics of Surface Water Resources in the Tropical Semi-Arid Area of the Letaba Catchment: Insights from Google Earth Engine, Landscape Metrics, and Sentinel-2 Imagery" Hydrology 12, no. 4: 68. https://doi.org/10.3390/hydrology12040068
APA StyleMashala, M. J., Dube, T., & Ayisi, K. K. (2025). Seasonal and Spatial Dynamics of Surface Water Resources in the Tropical Semi-Arid Area of the Letaba Catchment: Insights from Google Earth Engine, Landscape Metrics, and Sentinel-2 Imagery. Hydrology, 12(4), 68. https://doi.org/10.3390/hydrology12040068