# A Multisensor Approach to Satellite Monitoring of Trends in Lake Area, Water Level, and Volume
## 4 Discussion  
Both the dual objectives of this study were achieved: (1) Various methodological improvements in the remote sensing of water storage in lakes were described and implemented in an automated system for measurement of lake surface area, water level, and volume; and (2) this system was used to develop new, comprehensive, highly detailed datasets on the life cycle of the Toshka lakes since their formation 20 years ago. All three of the informally-stated hypotheses from Section 1.3 were borne out. For Hypothesis 1, the adaptive sub-pixel analysis algorithm for estimation of lake surface area worked well across a wide range of spatial scales; the output from the 1000 m resolution AVHRR imagery merged smoothly with the 250 m MODIS output (Figure 5), and both were generally consistent with the 30 m Landsat (Figures 8 and 9) and 10 m SPOT imagery. As noted in [22], the widely used process of setting a \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Lake 1** & **Lake 2** & **Lake 3** & **Lake 4** & **Lake 5** & **Lake 6** & **Total** \\ \hline Volume (\(10^{9}\) m\({}^{3}\)) & 13.18 & 0.43 & 6.61 & 12.72 & 3.05 & 0.08 & 26.54 \\ Date of maximum & 1999-02-05 & 1999-02-26 & 1999-12-13 & 2002-03-14 & 2002-03-14 & 2002-03-14 & 2001-12-02 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Maximum volumes and dates of maximum for each lake. Figure 12: Hypsgraphic curves for each lake, relating lake area to depth (from AST14 DEMs). \"hard\" land/water threshold in a near-infrared band or spectral index can cause problems due to slight variations in scene illumination, sensor calibration, or imperfectly corrected atmospheric conditions. The use of an adaptive algorithm for this threshold is conceptually preferable, as is the modeling of sub-pixel water fraction for shoreline pixels, rather than the use of a simple binary classification. For Hypothesis 2, the version of the lake area algorithm using ELI had a lower noise level than the version with NDLI (Figure 6). However, like its equivalent vegetation index EVI, ELI does require the presence of a blue spectral band, in addition to the red and near-infrared bands used in NDLI. For Hypothesis 3, the two methods for estimating water levels by combining lake area maps and DEMs gave generally similar results. The reported trends in water level (Table 5) were virtually identical for Lakes 1 and 4; for Lakes 3 and 5, Method 2 gave rates of decline that were 14% and 10% faster than Method 1. The observed rates of decline in ICESat-1 GLAS were 3% and 11% faster than the DEM-based methods for Lakes 1 and 4 respectively; for Lake 5, ICESat's rate fell between those from the two DEM-based methods. The recent launch of ICESat-2 should provide an excellent source for lake water measurements at a finer scale than those obtainable by satellite radar altimetry. The data produced in this study can be used to address other questions about the past and future of the Toshka lakes, such as the partitioning of water losses between evaporation and recharge, and the anticipated expiration of the last remaining lake. With no outlet for surface water discharge from this closed depression, the decline in water levels is attributable to two factors: evaporation and groundwater recharge. [PERSON] et al. (2015) used data from floating rafts on Lake Nasser, just east of the Toshka Lakes, to estimate evaporation rates from the reservoir. The mean daily evaporation rates at the two stations closest to the Toshka Lakes spillway were 5.78 mm/day (Allaqi) and 5.90 mm/day (Abusembel). Comparing these rates and the mean of Methods 1 and 2 for water level measurement in this study suggests that approximately 86% of the water loss observed here was via evaporation, with 14% via groundwater recharge. The groundwater recharge fraction was highest for Lake 3 and lowest for Lake 4. As noted in Section 1.2, [PERSON] et al. (2015) reported a rate of decline in water level during from 2002-2006 of (approximately) 7 mm/day.