# A Multisensor Approach to Satellite Monitoring of Trends in Lake Area, Water Level, and Volume
## 3 Results
### Water Level, Mean Depth, and Volume  
As described in Section 2.3, two methods were used to estimate water level by combining lake surface area maps with the ASTER DEM mosaic. Method 1 involved measuring the average elevation \begin{table}
\begin{tabular}{c c c c c c c} \hline  & **Lake 1** & **Lake 2** & **Lake 3** & **Lake 4** & **Lake 5** & **Lake 6** & **Total** \\ \hline Area (km\({}^{2}\)) & 617 & 63 & 515 & 634 & 255 & 19 & 1722 \\ Date of max. & 1999-02-05 & 1999-02-26 & 1999-12-13 & 2002-03-14 & 2002-04-06 & 2002-03-14 & 2001-12-01 \\ \hline \end{tabular}
\end{table} 
Table 4: Maximum surface area and date of maximum for each lake. Figure 8: Lake surface area (km\({}^{2}\)) for each lake, 1998–2017. Colored lines are merged AVHRR/MODIS; black dots are individual lake measurements from Landsat, for validation. Figure 9: Scatterplot of lake surface area (km\({}^{2}\)) with coarse-resolution sensor area (MODIS or AVHRR) on the X axis and fine-resolution area (Landsat) on the Y axis. of the observed shoreline of the lake, while Method 2 involved constructing a histogram of elevations within each lake basin from the DEM (i.e., the count of pixels at each increment of elevation) and finding the elevation at which the cumulative histogram matched the observed lake area in km\({}^{2}\). Figure 10 shows a comparison of the results of these two methods for all six lakes (for Lake 5, whose basin was not filled until after the date of the SRTM DEM; Method 1 was also used with the SRTM data as an additional comparison). ICESat-1 GLAS laser altimetry points are also shown for each lake crossed by one of the ICESat orbit tracks (see map in Figure 7). Because each GLAS orbit track across a given lake may include many individual laser measurements, each lake's GLAS water level estimate can be described in terms of both a mean and standard deviation. The mean values (for 76 combinations of lake and orbit track) are used in Figure 11 and Table 5; the standard deviations ranged from 0.016 to 0.091 m, with a mean standard deviation of 0.041 m.  
Figure 10: Comparison of methods for estimating water level (in colored lines) for all six lakes, with validation data from ICESat-1 GLAS laser altimetry (black diamonds). In general, water levels increase and decrease prior to 2003, due to seasonal inflows from the Nile, and then decrease continuously and approximately linearly from 2003 onward, indicating a consistent net water loss due to evaporation and groundwater recharge. Table 6 shows the date on which each lake reached its maximum average depth. In principle, the date of maximum _average_ depth could differ from the date of maximum surface area, but in this case, the dates are the same for all lakes except Lake 5 are the same (cf. Table 4). The rates of change in water level during the 2003-2009 operational period of the ICESat-1 GLAS mission are given in Table 5; these results are less reliable for Lakes 2 and 6 due to their small size and short duration of existence during this time period. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Lake 1** & **Lake 2** & **Lake 3** & **Lake 4** & **Lake 5** & **Lake 6** & **Total** \\ \hline Average & & & & & & & \\ depth (m) & 21.4 & 6.9 & 12.8 & 20.1 & 12.0 & 4.0 & 21.3 \\ \hline Date of max & 1999-02-05 & 1999-02-26 & 1999-12-13 & 2002-03-14 & 2002-02-10 & 2002-03-14 & 1999-02-05 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Maximum average depth and date of maximum for each lake. Figure 11: Lake volume (10\({}^{9}\) m\({}^{3}\) = 1 km\({}^{3}\)), 1998–2017. Total of Lakes 1–6 shown in black. Hypsgraphic curves for all six lakes are shown in Figure 12. The lowermost range of the curve for Lake 4 is uncertain, because it is based on the basin's bathymetric ASTER DEM with spline interpolation to estimate elevations within its late-2017 water extent (Section 2.3 above). In each case, zero on the depth axis is set based on the maximum areal extent of the lake during the entire 1998-2017 (AVHRR and MODIS) time series.