Evaluating water level changes at intertidal zones is complicated because of dynamic tidal inundation. However, water level changes during different tidal phases could be evaluated using a digital surface model (DSM) captured by unmanned aerial vehicle (UAV) with higher vertical accuracy provided by a Global Navigation Satellite System (GNSS). Image acquisition using a multirotor UAV and vertical data collection from GNSS survey were conducted at Kilim River, Langkawi Island, Kedah, Malaysia during two different tidal phases, at high and low tides. Using the Structure from Motion (SFM) algorithm, a DSM and orthomosaics were produced as the main sources of data analysis. GNSS provided horizontal and vertical geo-referencing for both the DSM and orthomosaics during post-processing after field observation at the study area. The DSM vertical accuracy against the tidal data from a tide gauge was about 12.6 cm (0.126 m) for high tide and 34.5 cm (0.345 m) for low tide. Hence, the vertical accuracy of the DSM height is still within a tolerance of ±0.5 m (with GNSS positioning data). These results open new opportunities to explore more validation methods for water level changes using various aerial platforms besides Light Detection and Ranging (LiDAR) and tidal data in the future.

Estimating surface elevation changes in mangrove forests requires a technique to filter the mangrove canopy and quantify the changes underneath. Hence, this study estimates surface elevation changes underneath the mangrove canopy through vegetation filtering and Difference of DEM (DoD) techniques using two epochs of unmanned aerial vehicle (UAV) data carried out during 2016 and 2017. A novel filtering algorithm named Surface estimation from Nearest Elevation and Repetitive Lowering (SNERL) is used to estimate the elevation height underneath the mangrove canopy. Consequently, DoD technique is used to quantify the elevation change rates at the ground surface, which comprise erosion, accretion, and sedimentation. The significant findings showed that region of interest (ROI) 5 experienced the highest volumetric accretion (surface raising) at 0.566 cm³. The most increased erosion (surface lowering) was identified at ROI 8 at −2.469 cm³. In contrast, for vertical change average rates, ROI 6 experienced the highest vertical accretion (surface raising) at 1.281 m. In comparison, the most increased vertical erosion (surface lowering) was spotted at ROI 3 at −0.568 m. The change detection map and the rates of surface elevation changes at Kilim River enabled authorities to understand the situation thoroughly and indicate the future situation, including its interaction with sea-level rise impacts.

Groundwater plays a vital role in the global water cycle as a source of water for human use in daily life. The problem of groundwater depletion attracts researchers to understand the phenomenon of terrestrial water storage (TWS) and the primary technique used to monitor changes in groundwater mass in the subsurface. Accurate quantification is subtle due to the weakness of gravity measurement methods, which cover a wide range with high precision. A global evaluation of improvements in groundwater storage used a calculation tool that may involve temporal differences in TWS. The Gravity Recovery and Climate Experiment (GRACE) and the GRACE Follow-On (GRACE-FO) satellite missions were able to monitor changes in water mass in the basin and calculate changes in water levels by measuring gravity variations and quantifying groundwater tables at 1-micron precision. This paper aims to discuss GRACE, GRACE-FO and hydrological variables in the monitoring of groundwater depletion during the drought season. This paper presents an estimation technique using satellite gravimetry and hydrological methods, as well as a study of several case studies in central Amazon (Brazil), Murray-Darling (Australia) and Mongolia Basin. Previous observations, including TWS-hydrological variables, trends in groundwater depletion and drought intensity, have been discussed as a vital outcome of the paper as a whole.

Estimating surface elevation changes at bare earth riverbank is challenging because of the non-visibility effect of this phenomenon, especially for short observation periods. Hence, the spatio-temporal detection at a short-term period determines the geomorphological changes by comparing the same area at different observation epochs. This study attempts to assess surface elevation changes components which include erosion and accretion, using the geomorphological changes detection (GCD) technique based on multi-temporal unmanned aerial vehicle (UAV) imagery. Using a multi-rotor UAV and high accuracy Global Navigation Satellite System instrument, the data acquisition process was conducted in Kilim River in the December 2016 and December 2017 to complete a one-year interval. To generate a very high accuracy of orthomosaics and digital elevation model (DEM), the structured from motion and multi-view stereo techniques were used. Then, GCD method, which involves a difference of DEM (DoD) was performed to evaluate the surface elevation changes that comprising erosion and accretion using GCD analysis tool. This study discovered that surface raising was more dominant than surface lowering, at 31.70% (raw) and 31.67% (thresholded), the per cent elevation raising at 68.30% (raw) and 68.33% (thresholded), the per cent imbalance (departure from equilibrium) at 18.30% (raw) and 18.33% (thresholded) and the net to total volume ratio at 36.60% (raw) and 36.67% (thresholded). The results showed how the UAV platform provides a way to evaluate surface elevation changes in bare earth areas with decent accuracy and enables further study on river geomorphological-related issues in the future. Keywords: DEM, GCD Analysis, GNSS, SfM-MVS, Surface Elevation Changes, UAV Photogrammetry