# A Multisensor Approach to Satellite Monitoring of Trends in Lake Area, Water Level, and Volume
## 1 Introduction
### Study Region  
The Toshka Lakes basin is located in the Western Desert of southern Egypt, west of the Nile's Lake Nasser reservoir (Figure 1a). Although there is evidence for river channels and lakes in the region during the middle Pleistocene [48], the region is currently a hyper-arid desert (Figure 2) and was devoid of surface water prior to the development of an artificial spillway and channel (22 km length) from Lake Nasser in 1978 to protect the dam during times of high flow on the Nile [49]. Beginning in September 1998, increasing water levels in the Nile brought Lake Nasser to the elevation of the spillway, leading to the formation of the first Toshka lake (Lake 1). As waters continued to rise during the late-1998/1999 flood season, and subsequent flood seasons up to 2001/2002, a series of additional lakes were formed by waters spilling out of Lake 1 into downstream basins to the west (Figure 1b). After 2002, water levels in the Toshka Lakes began dropping, and by mid-2017, only Lake 4 remained extant (Figure 3). During the early 2000s, a short-lived fishery developed on the lakes, with over 7000 tonnes produced in 2004 [50], but its growth was quickly halted and reversed by the salinization and eventual disappearance of the lakes. The formation and subsequent desiccation of the lakes also had a significant impact on local climate [51]. Due to the isolation of the region and the broad spatial scale of the lake basins, satellite remote sensing has been a primary source of information on the dynamic hydrology of the Toshka Lakes. The specific approaches have been varied, and are summarized in Table 1. What sets the present study apart from these earlier works is its combination of high-temporal frequency observation of lake surface area, water level, and volume, for the entire life cycle of the lakes, with comparisons among different methodological choices and data sources. The first of these four prior studies, one by [PERSON] and [PERSON] [7], used relatively coarse-resolution optical imagery from the Advanced Very High-Resolution Radiometer (AVHRR) and MODIS, in an adaptive, sub-pixel analysis algorithm to estimate the surface area of each lake on 145 dates, from lake formation in 1998 to mid-2006. The MODIS imagery used in this study was Level-1B (geometrically corrected top-of-atmosphere spectral radiance). Surface area measurements were validated by comparison to higher-resolution Landsat images. Water levels for four of the six lakes were measured using ICESat-1 GLAS, although Lake 3 had only a single date of GLAS measurements and thus could not be used to estimate rates of change in water level. For Lake 5, a pre-inundation digital elevation model (DEM) was obtained from the Shuttle Radar Topography Mission (SRTM) and was used to infer changes in water level as its basin filled and then began drying. Lake surface area reached a maximum of 1740 km\({}^{2}\) in or around December 2001. The volume of Lake 5 (from the Shuttle Radar Topography Mission [SRTM] DEM) reached a maximum of 3.13 \(\times\) 10\({}^{9}\) m\({}^{3}\) in \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Time** & **Data Sources** & **Outcomes** & **Reference** \\ \hline
1998–2006 & 145 MODIS Level-1B/AVHRR images; Landsat; ICESat GLAS; SRTM DEM & Lake area (all lakes); water level and volume (Lake 5 only) & [PERSON] \& Lillesand 2007 [7] \\ \hline
2002 and 2006 & 7 ASTER/SPOT-4 images; DEM digitized from topographic maps & Lake area; water level \& volume; evaporation & [PERSON] et al. 2008 [52] \\ \hline
1998–2007 & 27 Landsat/ASTER images; SRTM DEM & Lake area; geologic/ geomorphological controls & [PERSON] et al. 2008 [53] \\ \hline
2000–2013 & 14 MODIS MOD13 images & Lake area; vegetation/soils in riparian zone & [PERSON] 2015 [23] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Prior remote sensing studies of the hydrology of the Toshka Lakes. Figure 3: Formation, expansion, and desiccation of the Toshka Lakes.