(a) Model optimization for the soiling type observed at the Homa Bay station, where a significant amount of persistent soiling is present. The blue and green lines show the time series of soiling losses produced by the calibrated original and modified HSU models, respectively. The background in panel (a) indicates the calibration period, while the validation period is denoted by the shaded background, along with the dashed lines. The observed soiling losses at Homa Bay are depicted by the solid orange line across the entire campaign period. Panel (b) shows the observed daily rain sum (orange line) at the station, as well as the corresponding ERA5 values (blue line).