SMAP satellite microwave radiometry to monitor river flow and lake level in the Lower Mekong Basin

The NASA Soil Moisture Active Passive Mission (SMAP) passive microwave radiometry (PMR) capability is demonstrated for measurements of river stage, river discharge, and lake level using in situ gauging data in the Lower Mekong Basin (LMB). Five river gauging locations with different characteristics in the Mekong River system and another location for the Tonle Sap Lake were chosen. The SMAP PMR method was validated with forward-split, backward-split, and full-record approaches. Results from the three different validations were all compared with in situ gauging data at all the locations. Both the narrowest river (42-m width, Songkhram River) and the widest (1735-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 Ka-band PMR and Soil Moisture and Ocean Salinity (SMOS) data over multiple decades identified flood and drought events, and abrupt changes of 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 operations, wet-season flow stage in the lower Mekong River did not reach the 2.33-year flood stage and dry-season water level dropped to below the lowest stage in the 2015 severe drought year. The PMR method enables a breakthrough for river and lake monitoring with global coverage on a nearly-daily to daily basis.