# A Multisensor Approach to Satellite Monitoring of Trends in Lake Area, Water Level, and Volume
## 1 Introduction  
### Background: Remote Sensing of Lake Hydrology  
The world's \(\sim\)117 million lakes and reservoirs [1] have an important role in the water and carbon cycles at regional scales, and provide a wide range of ecosystem services and economic benefits [2; 3; 4]. In some regions, lakes are known to be declining or disappearing due to disparate causes, ranging from climate change to human water withdrawals for agriculture, industry, and domestic consumption [5; 6]. Lakes in arid regions are particularly sensitive to changes in local to regional hydrology [6; 7; 8], which is a troubling fact, given their disproportionate importance in the ecology and economic systems of these water-limited areas. Other anthropogenic impacts on lakes may also have severe consequences, such as the introduction of invasive species [9; 10], overfishing [10; 11], and pollution [12; 13]. Remote sensing is widely used in limnology and lake management for identifying, measuring, and characterizing the properties of lakes [14; 15; 16; 1]. Lake surface area has been successfully mapped using optical and microwave sensors in environments from the tropics to the poles [14; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44;  
I'observation de la Terre\") series of satellites. Historically, issues of sensor calibration and radiometric quality, atmospheric correction, and data distribution hindered the development of fully automated, high-temporal resolution systems for lake monitoring using these optical remote sensing platforms. Since 2000, the launch of operational, global monitoring systems, such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Sentinel satellite constellation, including products and/or tools for deriving surface reflectance from the raw top-of-atmosphere spectral radiance measurements, has enabled the development of automated algorithms for frequent monitoring of lake surface areas from space. The vertical dimension of lake volume (water level or bathymetry) can be measured using laser altimetry [7; 24; 25; 26; 27; 28] or radar altimetry [29; 30; 31; 32; 28]. Although it was only operational from 2003-2009, and experienced technical problems that limited its data collection to a few operational periods per year, the Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud, and Land Elevation Satellite-1 (ICESat-1; [24]) provided spot measurements of water level for many lakes worldwide, and comparison of measurements between operational periods enabled the estimation of changes in water level. The use of satellite laser altimetry from space is likely to become far more widespread with the recent successful launch of ICESat-2. Due in part to the influence of the oceanographic community, the science of water-level measurement from radar altimetry has matured rapidly, and been supported by a temporally overlapping series of platforms, including ERS-1 and -2, the Ocean Topography Experiment (TOPEX/Poseidon), Envisat, Jason-1 and -2 Ocean Surface Topography Mission (OSTM), and Cryosat-2. For the lake research and management community, the Global Reservoir and Lake Monitor (G-REALM; [33]) is a widely-used automated system for assimilating radar altimetry across multiple platforms for the development of long-term records of water-level change in large lakes worldwide. Compared to laser altimetry, satellite radar altimetry typically provides a coarser spatial resolution, making it suitable for only the largest lakes worldwide, but with the advantage of a long and continuous historical record and independence from the constraints of cloud cover. Recently, a number of studies have used simultaneous analysis of lake area (via image classification) and water level (via altimetry) to derive changes in lake volume. In [34], the changing area and water level of Lake Victoria were measured using MODIS and ERS/Envisat, respectively, for a 22-year period. A similar approach was used for Lake Urmia in Iran [35], using MODIS with Envisat/Cryosat-2.