# Groundwater and Urban Planning Perspective
## 2 Exploring the Connection between Urban Development and Urban Subsurface Hydrology  
The direction of the first analysis includes two steps outlined in Figure 1: (i) an analysis of the urban planning documents and support studies for 17 large cities in Romania, exploring the reliance of the General Urban Plan (GUP) on groundwater documentation; (ii) the analysis of strategic documentation [23] for the city of Bucharest, namely the Integrated Urban Development Strategy for Bucharest (strategy for absorbing EU funds, elaborated in 2021) and of the strategic concept for Bucharest 2030 (elaborated in 2011); in addition, the analysis of urban planning documentation, namely the current general urban plan (drafted in 1998, with extended validity until the new GUP is completed).  
Figure 1: Research methodology for exploring the connection between urban development and urban subsurface hydrology. Source: elaborated by the authors.  
Based on the land instability map of Bucharest, the second direction presented in Figure 1 focuses on discerning the role of groundwater within the main subsidence triggering factors and analyzing its relation to the urban planning framework. By monitoring vertical ground displacement it is possible to indirectly estimate the generated subsidence phenomena, with one of the most important of these phenomena being related to the dynamics of urban groundwater. The recent method for monitoring urban subsidence is the satellite remote sensing technique that uses Persistent Scatterer Interferometry (PSI), which allows for vertical ground displacements to be determined to the millimeter order [24] by measurements made along the Line of Sight (LoS) of the satellite [25]. PSI is a radar-based technique from the Interferometric Synthetic Aperture Radar (InSAR) family [26]. The InSAR operates by generating an interferometric pair using two radar scenes, acquired approximately from the same look angle, and over the same area, at different times [27]. If more interferograms are generated, their phase difference is used for estimating the displacements in the monitored area along the Line of Sight (LoS) of the satellite [28]. Considering the available radar data and the particularities of the studied area, different methods are used for estimating the vertical ground displacements from LoS displacements. For the PSI technique, the ground displacements are determined in temporally stable highly reflective ground features known as persistent scatterers (PS) [25] using multiple SAR interferograms generated from a time series of satellite radar data. The main advantages of the SAR techniques consist of its large area monitoring capability and high temporal resolution [27]. As an example, Sentinel-1, the current European SAR missions, provides a large amount of satellite data and has a revisit time of 6 days when both satellites of the constellation are considered. The Sentinel-1 Interferometric Wide Swath (IW) products, which are used for PSI, have a swath of 250 km, [29] facilitating an analysis at city level, or even at regional level. One disadvantage of the SAR technique is represented by the temporal and spatial decorrelation, which leads to a lack of ground displacement information for land surfaces covered with vegetation or for areas where land changes occur during the studied time interval [30].