EVAPOTRANSPIRATION (ET) IS THE SECOND LARGEST FLUX IN THE TERRESTRIAL WATER BALANCE AFTER PRECIPITATION AND HENCE IT IS FUNDAMENTAL COMPONENT OF THE GLOBAL WATER ENERGY AND CARBON CYCLES. THE FLUX OF WATER VAPOR FROM THE LAND SURFACE TO THE ATMOSPHERE HAS THREE COMPONENTS: SOIL EVAPORATION TRANSPIRATION AND EVAPORATION OF INTERCEPTED WATER FROM VEGETATION CANOPIES. EXISTING OBSERVATIONS REPRESENT THE TOTAL ET FLUX NOT THE INDIVIDUAL COMPONENTS. WE PROPOSE TO USE SMAP L2_SM_P DATA TO GENERATE A GLOBAL OBSERVATIONALLY-BASED SOIL EVAPORATION DATASET CALLED E-SMAP. WE WILL USE THIS PRODUCT TO ESTIMATE SOIL EVAPORATION BASED ON THE TIME RATE-OF-CHANGE OF SOIL MOISTURE AND INCLUDE ADJUSTMENTS FOR OTHER FLUXES THAT AFFECT THE SURFACE SOIL LAYER WATER BALANCE. THE E-SMAP APPROACH IS ANALOGOUS TO USING SMAP AS A GLOBAL-SCALE LYSIMETER THAT IS CONTINUOUS THROUGH TIME AND PROVIDES INFORMATION AT SCALESRELEVANT TO VARIOUS HYDROLOGIC APPLICATIONS. LIKE ALL METHODS E-SMAP HAS BOTH STRENGTHS AND LIMITATIONS; THEREFORE WE WILL EVALUATE WHERE AND WHEN E-SMAP PROVIDES USEFUL INFORMATION ABOUT SOIL EVAPORATION. THE PROPOSED RESEARCH HAS FOUR OBJECTIVES:1. DEVELOP A METHOD TO MONITOR SOIL EVAPORATION USING SMAP_L2_P SURFACE SOIL MOISTURE DATA SUPPLEMENTED BY SMAP L4 AND RELATED DATASETS. L4 PRODUCTS AND OTHER DATASETS WILL BE USED TO ESTIMATE THE FLUX TO UNDERLYING SOIL AND TRANSPIRATION FROM THE SURFACE SOIL LAYER. THESE DATA WILL ALSO BE USED TO IDENTIFY AND EXCLUDE INTERVALS WITH PRECIPITATION WHEN SIGNIFICANT INFILTRATION PRECLUDES THEPROPOSED WATER BALANCE APPROACH. GAPS WILL BE FILLED USING COMPLEMENTARY DATASETS. UNCERTAINTY WILL BE QUANTIFIED BY COMBINING ALL CONTRIBUTING ERRORS.2. EVALUATE THE E-SMAP SOIL EVAPORATION ALGORITHM VIA COMPARISON TO RELATED IN SITU AND REMOTELY SENSED OBSERVATIONS AND MODELED PRODUCTS. THE ESOIL ESTIMATED FROM E-SMAP WILL BE DIRECTLY COMPARED TO ET DATA (E.G. MOD16 ET) FOR TIMES WHEN TRANSPIRATION IS KNOWN TO BE MINIMAL (ESOIL~ET). IN SITU SOIL MOISTURE OBSERVATIONS WILL BE USED TO ASSESS THE DIRECTION AND MAGNITUDE OF VERTICALREDISTRIBUTION OF SOIL WATER IN THE E-SMAP WATER BALANCE.3. GENERATE A DAILY SOIL EVAPORATION PRODUCT USING THE E-SMAP APPROACH. THE SPATIAL DOMAIN OF THE E-SMAP PRODUCT WILL MATCH THAT OF THE SMAP L2 DATA. E-SMAP DATA WILL INCLUDE ESTIMATES OF UNCERTAINTY AND A PRODUCT-GENERATION FLAG INDICATING GAP-FILLING BOTH WHICH ARE CRITICAL FOR GUIDING APPROPRIATE USE OF THE EVAPORATION PRODUCT IN MODEL EVALUATION STUDIES.4. EXAMINE THE PARTITIONING OF ET INTO SOIL EVAPORATION AND TRANSPIRATION AT THE GLOBAL SCALE ACROSS LAND COVER AND LAND USE. THE ESMAP PRODUCT IN CONJUNCTION WITH REMOTELY-SENSED AND MODELED ET WILL BE USED TO ANALYZE THE PARTITIONING OF ET ON A MONTHLY TIMESCALE. THIS APPLICATION OF E-SMAP WILL BE FOCUSED ON DIFFERENCES IN ET PARTITIONING RELATED TO LAND USE AND LAND COVER. THIS PROPOSAL WILL ADDRESS 2.1 UTILIZATION OF SMAP PRODUCTS FOR PROCESS STUDIES: SMAP PRODUCTS WILL BE USED IN A UNIQUE WAY TO EXAMINE ONE OF THE THREE PRIMARY FLUXES OF THE TERRESTRIAL WATER CYCLE. THE PROPOSED APPROACH IS DIFFERENT FROM AND COMPLEMENTARY TO EXISTING OBSERVATIONAL ET DATASETS. THUS SMAP PRODUCTS WILL PERMIT A MORE COMPLETE UNDERSTANDING OF SOIL EVAPORATION INCLUDING: AN IMPROVED MECHANISTIC UNDERSTANDING OF THE SOIL EVAPORATION PROCESS ITS SPATIAL AND TEMPORAL FLUCTUATIONS AND ITS RELATIONSHIP WITH TRANSPIRATION AT THE GLOBAL SCALE. THIS KNOWLEDGE WILL GUIDE IMPROVED FORMULATIONS OF SOIL EVAPORATION AND TRANSPIRATION IN HYDROLOGIC AND BIOGEOCHEMICAL MODELS. THE RESULTS REPRESENT AN IMPORTANT STEP TOWARDS MORE COMPLETE CHARACTERIZATION OF THE WATER ENERGY AND CARBON CYCLES. THE PRODUCTS WILL ALSO SHOW HOW LAND USE CHANGE AND AGRICULTURE INFLUENCE THE PATHWAYS BY WHICH WATER IS RETURNED FROM THE LAND SURFACE TO THE ATMOSPHERE.