MOTIVATION: THE STABILITY OF THE GREENLAND ICE SHEET IN A WARMING CLIMATE IS A CRITICAL SOCIETAL CONCERN. PREDICTING GREENLAND'S CONTRIBUTION TO SEA LEVEL REMAINS A CHALLENGE IN PART DUE TO OUR LIMITED PRACTICAL UNDERSTANDING OF ICE SHEET BASAL MOTION PARTICULARLY THE WAY IN WHICH THAT BASAL MOTION CHANGES THROUGH TIME. THIS IS CRITICAL AT THE TRANSITION BETWEEN INLAND AND TIDEWATER GLACIER ENVIRONMENTS WHERE RATE LIMITING PROCESSES MAY DETERMINE FUTURE DISCHARGE. THE BASAL PHYSICS OF THIS TRANSITIONAL ZONE AND OUTLET GLACIERS IN GENERAL IS CURRENTLY POORLY REPRESENTED IN ICE SHEET MODELS. APPROACH: WE PROPOSE TO INVERT SATELLITE-DERIVED ESTIMATES OF SURFACE VELOCITY AND SURFACE ELEVATION HISTORIES TO IMPROVE CHARACTERIZATION OF HYDROLOGY-DETERMINED PATTERNS OF BASAL MOTION. TO DO THIS WE WILL DERIVE A SIMPLIFIED BUT REPRESENTATIVE MODEL OF SUBGLACIAL HYDROLOGY WHICH CAN BE SOLVED EFFICIENTLY. IN CONJUNCTION WITH A BASAL MOTION PARAMETERIZATION (I.E. A SLIDING LAW) WE WILL FIND THE SPATIALLY AND TEMPORALLY CONSTANT PARAMETER VALUES THAT MINIMIZE THE MISFIT BETWEEN MODELED AND OBSERVED SURFACE VELOCITIES SUBJECT TO SURFACE GEOMETRY EVOLUTION FROM OPERATION ICEBRIDGE AND ICESAT. WE WILL VALIDATE THE SUITABILITY OF THE DERIVED BASAL STATE MODEL BY HINDCASTING THE PERIOD 1979-2015 WITH AN ICE SHEET MODEL USING CLIMATE FORCING FROM A REGIONAL CLIMATE MODEL. DELIVERABLES: (1) A NEW APPROACH TO INTEGRATING SURFACE ELEVATION HISTORIES AND ICE VELOCITY CHANGES TO CONSTRAIN BASAL PROCESSES. (2) A BASAL STATE MODEL DERIVED FROM THIS APPROACH THAT IS COMPATIBLE WITH OBSERVATIONS READY TO BE IMPLEMENTED INTO ICE SHEET MODELS TO IMPROVE THEIR USEFULNESS FOR SEA LEVEL PREDICTIONS. (3) FLUX DIVERGENCE MAPS FROM MEASURED AND MODELED VELOCITY FIELDS THAT CAN BE COMPARED WITH LASER ALTIMETRY TO AID IN DH/DT PARTITIONING. UNIQUENESS: THIS WILL BE THE FIRST TIME THAT INVERSE METHODS HAVE BEEN APPLIED TO A SPATIALLY-VARIABLE COUPLED ICE FLOW/BASAL HYDROLOGY MODEL TO CONSTRAIN BASAL MOTION IN A GENERALIZABLE AND TIME EVOLVABLE WAY. RELEVANCE TO NASA CRYOSPHERIC SCIENCE: THIS PROPOSAL DIRECTLY RESPONDS TO THIS CALL'S REQUEST TO USE OBSERVATIONS FROM ICESAT OIB AND OTHER SATELLITE RESOURCES TO BETTER "CHARACTERIZE THE DYNAMIC PROCESSES CONTROLLING ICE FLOW AND RELATED CHANGES IN ICE SHEET ELEVATION AND MASS BALANCE ESPECIALLY TO IMPROVE ICE SHEET MODELS USEFUL FOR PREDICTION OF SEA LEVEL RISE". THIS WORK WILL ALSO DIRECTLY ENHANCE THE VALUE OF NASA'S LASER ALTIMETRY MISSIONS BY PROVIDING BETTER ESTIMATES OF FLUX DIVERGENCE NEEDED TO PARTITION THE SURFACE ELEVATION CHANGE SIGNAL BETWEEN SURFACE MASS BALANCE AND CHANGES IN ICE DYNAMICS.