SMAP Satellite Microwave Radiometry to Monitor River Flow and Lake Level in the Lower Mekong Basin

Abstract The NASA Soil Moisture Active Passive Mission (SMAP) satellite passive microwave radiometry (PMR) capability is demonstrated for measurements of river stage, river discharge, and lake level with in situ gauging data in the Lower Mekong Basin (LMB). Five river gauging locations with distinct characteristics in the Mekong River system and a location for the Tonle Sap Lake were selected. The SMAP PMR method was validated with forward‐split, backward‐split, and full‐record approaches. Results from the three different validations were consistent and well compared with in situ gauging data at all the locations. Both the narrowest (42‐m width, Songkhram River) and the widest river (1,735‐m width, Mekong River) achieved high correlation values ≥0.9 and Nash‐Sutcliffe Efficiencies >0.8. The SMAP PMR observations of rivers and lake captured seasonal and interannual patterns of river change corresponding to flood and drought conditions. The synergy of SMAP with satellite Ka‐band PMR and Soil Moisture and Ocean Salinity (SMOS) data over multiple decades identified flood and drought events, and abrupt changes in river flows in the LMB corresponding to the operations of the two largest dams, Xiaowan and Nuozhadu, on the Lancang (upper Mekong) River. After these two dams went into operation, wet‐season flow stage in the lower Mekong River did not again reach the 2.33‐year flood stage, and dry‐season water level dropped below the lowest stage recorded in the 2015 exceptional drought year. The PMR method enables river and lake monitoring with global coverage on a daily to nearly daily basis over decades.