**3. Results**

#### *Temporal and Spatial Accuracy Assessment*

Classification of surface reflectance, using a CART classifier ran within GEE and trained separately for both Landsat-7, and -8 on collected field data, produces 95.6% and 94.1% accuracy confiners for LS-7 and LS-8, respectively, for the reference year 2015 (Table 3). The gap-filling algorithm's performance to predict land use type based on the seven-year pattern surrounding missing pixels hovers between 87 to 92% depending on the year (Table 4).


**Table 3.** Validation results of land use classification for Landsat-7 and Landsat-8.


**Table 4.** Validation results of land use classification for Landsat-7 and Landsat-8. Years with SLC malfunction are shaded in grey considering the higher uncertainties.

An overview of all land use classification maps after gap-filling and post-processing for logical land use transitions is presented in Figure 3. The maps depict all years with full data-availability and therefore the highest reliability ranging from 2001–2002, and 2013–2019. A visual inspection of the temporal dynamics indicates that the central regions in Ngoc Hien have been subject to a high frequency of land use changes whereas towards the coast, specifically the Western Cape, stable havens of dense mangrove forest have to large extent been minimally subjected to forest clearance and land use change. Over the years, three core mangrove forests areas can be identified (See Figure 1, for commune locations); (1) the cluster in the south-east of Ngoc Hien, including the eastern part of Tan An commune and adjacent southern Tam Giang Tay commune, (2) the mangrove forest strip along the southern coastline, and (3) the largest core mangrove area is located in the western tips of Ngoc Hien district; Dat Mui commune which encloses the Ca Mau Cape National Park and the north-western part of Vien An commune.

The total land use changes over time are highlighted in Figure 4. These results show that the total shares of land use cover between four classes have remained relatively stable with yearly fluctuations. In the span of two decades, we find sparse mangrove cover fluctuating between 33,000 ha and 40,000 ha and dense mangrove forests hovering between 18,000 ha and 24,000 ha. The average net change of dense mangroves over the 2001–2019 equals −0.01% annually. In recent years since 2013, we observe upward trends in dense mangrove forest cover as well as built environment and mudflats. Sparse mangrove cover has been decreasing in that same period. Waterbodies/ponds have remained more or less equal.

Despite these overall trends, the figure conceals the spatial dynamics behind the distribution of mangrove forests and land use changes that characterize the region. To gain insight into the spatial distribution of land use changes, an aggregated trend map was created (Figure 5) to identify hotspots of mangrove forest change while leaving changes of non-forest classes out of consideration. Generally, the dense mangrove patches in the Eastern regions have shrunk in size and extent, whereas the core mangrove forests on the Western Cape have seen an expansion through sedimentation and seaward colonization. Ngoc Hien's central communes have seen an increased integration of dense mangroves patches in the mosaic of waterbodies/ponds and sparse mangrove cover. Areas most prone to mangrove forest loss and degradation are the eastern central regions, possibly due to its vicinity to districts with higher urban development, infrastructure, and aquaculture production [51].

The trend map presented in Figure 5 gives insight into the spatial distribution of temporal dynamics of mangrove forest loss and gains. The map highlights how coastal erosion along the southward coastline and sedimentation along the westward frontier have resulted in losses and gains of mangrove forests. The strip of dense mangrove forests serving as a protective buffer along Ngoc Hien's southern coastline has seen shifting land use changes; on the one hand, inland regrowth and increased connectivity are observed, whereas on the other hand coastal erosion is becoming increasingly severe

leading to seaward losses of dense mangrove forest. In Ngoc Hien's Western regions, Dat Mui and Vien An communes (see Figure 1, for commune locations), the remaining core mangrove forests have seen relatively few land use changes except for seaward mangrove expansion and colonization caused by coastal sedimentation (Figure 5).

**Figure 3.** Time series of land use maps of Ngoc Hien District from 2001–2002 and 2013–2019, displayed are all years with full data availability (non-SLC malfunction).

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**Figure 4.** Temporal changes in total land cover surface across four land use classes in Ngoc Hien district. Years with SLC malfunction are shaded in grey considering the higher uncertainties.

**Figure 5.** Mangrove forest trend map (2001–2019) of 150 × 150 m box averages.
