Two-decade of monitoring spatio-temporal variation of surface albedo over a large saline lake

Saline lakes play a crucial role in regulating the regional climate, supporting unique biodiversity, and providing a diverse range of economic benefits. However, as a result of growing water withdrawals for human use, most of the large saline lakes worldwide are desiccating at a substantial rate. Water level decline and salinity rise affect physico-chemical characteristics of saline lakes including surface albedo. Water surface albedo impacts lake color and evaporation. Here, we investigate spatio-temporal variation of surface albedo over Lake Urmia, in northwest Iran, using the MODerate Resolution Imaging Spectroradiometer (MODIS) albedo product (MCD43D) from 2000 to 2019. Satellite-derived shortwave albedos were validated against in-situ surface albedo data measured at an online net-radiometer station on the lake. We identified two spatial patterns through Lake Urmia: 1) a decreasing trend from the outer shallow zones toward the deep inner parts, and 2) a higher mean albedo of the south arm compared to the north arm in summer. Moreover, the lake albedo varies seasonally with lake level and reaches its peak between September and October. This is mainly due to an increased concentration of total suspended solids (TSS) and phytoplankton (Duanalliea spp.) growth, which accounts for the lake red color between mid-spring and early autumn. Results also revealed that concurrent with the lake level drop since 2000, both lake-averaged surface albedo and its seasonal variation have constantly increased. The increased lake albedo affects net absorbed radiation by the lake and limits lake evaporation. Consequently, we emphasize that for large saline lakes which experience significant areal fluctuations seasonally, the use of a constant albedo to estimate lake evaporation and heat budget is inadequate. Instead, satellite-derived albedo maps encompassing the effect of lake depth, TSS, and phytoplankton growth can be used with confidence. Our findings can contribute to enhanced water, energy, and salt balance models for saline lakes by better estimation of their surface albedo. 

Keywords: Surface albedo, Lake Urmia, MODIS, Water level, Phytoplankton