# Inland Water Level Monitoring from Satellite Observations:
## 1 Introduction  
Inland water bodies, wetlands, and their dynamics have a key role in a variety of scientific, economic, and social applications. As rivers, lakes, and wetlands form the main freshwater recourses, are crucial to water resource management [1], biodiversity [2; 3], climate change impacts [4; 5; 6; 7], agricultural productivity [8], and modeling between land and atmosphere interactions [9; 10]. Thus, inland water bodies are an integral part of the global water cycle and there is a need for a globally homogeneous, continuing monitoring system for an effective management of terrestrial water resources. However, there is limited understanding of the distribution of the global hydrological cycle. Both the water surface elevation of inland water bodies and the spatial and temporal dynamics of water storage, are not sufficiently known. Historically, observations for understanding the global hydrological cycle and the Earth system in general rely on in situ gauge measurements. In situ sensors help to retrieve water level heights, but the network is sparse, particularly in areas with difficult access. TheGlobal Runoff Database Centre has established a network with gauge stations providing river discharge data in 159 countries [11]. The spatial distribution of rain gauge measurements is not dense enough to represent the input with high accuracy to the hydrological cycle. Also, the water storage estimates from traditional in situ observation show great uncertainty [12; 13]. Moreover, the prohibitive cost of state-of-the-art sensors is a significant factor that hinders their global widespread installation, and as a result, it contributes to the inadequate monitoring of the hydrological cycle, while in some cases the scientific community faces restricted access to in situ data, often due to political circumstances or agreements pertaining to transboundary water sharing [14]. In this context, space-based remote sensing could be considered an innovative cost-effective technique that provides up-to-date measurements from various sensors with global coverage and homogeneous accuracy. Spaceborne sensors designed for monitoring terrestrial water levels can provide measurements with high temporal coverage. Space-based water cycle monitoring is entering a new era in view of the wealth of present and future spaceborne missions. This scoping review paper examines the performance of satellite altimetry missions used to retrieve inland water surface heights. Satellite altimetry constitutes the dominant technique for inland water level monitoring. The principle of altimetry is to emit wave pulses and to receive the reflected echoes recording the round-trip time of the pulse. The measured backscatter energy as a function of time is called the waveform. Altimetry, originally aimed in the 1960s at oceanography and geodesy using a radar altimeter to extract sea water level [15], has demonstrated its potential for monitoring other components of the hydrosphere such as rivers, lakes, and wetlands. The concept of using satellites to observe water surface heights was initially proven successful through various missions launched by NASA in the 1970s, such as the Skylab-3 in 1973, the GEOS-3 in 1975, and the Seasat in 1978 [16; 17; 18]. A total of 20 altimetry satellites have been launched in the last three decades. In the 1980s, the altimetry Geosat mission was launched [19], while the real upsurge of altimetry came after the launch of the very successful NASA-CNES Topex-Poseidon (T/P) mission in the 1990s [20]. Especially, the significant upsurge of altimetry for inland water came after the launch of the ERS-1 mission, which had three tracking modes which was very efficient over non-ocean surfaces. However, the use of altimetry for monitoring inland water bodies was facilitated by the advent of two different developments: The Open-Loop Tracking Command (OLTC), first implemented by the Jason-2 satellite mission [21], and the Synthetic Aperture Radar (SAR) mode, first used in the CryoSat-2 mission [22; 23].