SOIL MOISTURE IS IDENTIFIED BY THE WORLD METEOROLOGICAL ORGANIZATION AS AN ESSENTIAL CLIMATE VARIABLE (ECV). ROOT ZONE SOIL MOISTURE (RZSM) IN PARTICULAR IS CRITICAL FOR UNDERSTANDING HYDROLOGIC FLUXES LINKING SURFACE AND SUBSURFACE PROCESSES AND THE INTERPLAY BETWEEN THE WATER AND CARBON CYCLES. ACCURATE SOIL MOISTURE DATA IN THE ROOT ZONE ARE CRITICAL TO AGRICULTURE (ESPECIALLY FOOD PRODUCTION) AND ARE OF NATIONAL IMPORTANCE. MOREOVER SOIL MOISTURE INFORMATION IS ESSENTIAL TO PROVIDING INFORMATION ON DROUGHT SEVERITY AND TO PREDICT THE LIKELY EFFECTS OF FOOD AVAILABILITY. SIMULTANEOUS KNOWLEDGE OF DEEP (10 TO>100 CM) AND SHALLOW (0-10 CM) SOIL MOISTURE WILL ENABLE ESTIMATION OF EVAPOTRANSPIRATION AND AQUIFER RECHARGE. THE CROP SIMULATION MODELS (CSM) NEEDED FOR CROP YIELD FORECASTS USE PROFILE SOIL MOISTURE INFORMATION. PRESENTLY ONLY NEAR-SURFACE (0-10 CM) SOIL MOISTURE IS GLOBALLY OBSERVED BY SATELLITES AT THE PRESENT TIME WITH RZSM PRODUCED THROUGH DATA ASSIMILATION. P-BAND OR LOWER (<500 MHZ) MICROWAVE OBSERVATIONS ARE REQUIRED TO ACHIEVE A SENSING DEPTH SUFFICIENT FOR THE MEASUREMENT OF RZSM. FROM SPACE THIS NECESSITATES A VERY LARGE (>10 M) ANTENNA MAKING SUCH A MISSION INFEASIBLE. THIS PROPOSAL AIMS TO RETRIEVE PROFILE SOIL MOISTURE AT DIFFERENT DEPTH (FROM THE SURFACE DOWN TO THE ROOT-ZONE) FROM REFLECTIVITY MEASUREMENTS OBTAINED USING SIGNALS OF OPPORTUNITY (SOOP) REFLECTOMETRY. THIS IS A NEW REMOTE SENSING TECHNIQUE THAT UTILIZES FORWARD SCATTERED SIGNALS FROM COMMUNICATION SATELLITE TRANSMITTERS IN A BISTATIC RADAR CONFIGURATION. SOOP FEATURES GOOD ASPECTS OF BOTH RADIOMETRY AND RADAR. SOOP ESSENTIALLY BENEFITS FROM THE SAME SOURCES THAT COULD CAUSE RFI SOURCES TO OTHER INSTRUMENTS! FOR THESE REASONS I/P-BAND SOOP PROVIDES A VIABLE PATH TOWARDS SPACEBORNE RZSM MEASUREMENTS. THE SOOP INSTRUMENT WAS SUCCESSFULLY BUILT AND DEMONSTRATED UNDER THE NASA IIP-13 PROJECT. THE SOOP REFLECTIVITY MEASUREMENTS AT DIFFERENT FREQUENCIES RANGING FROM I-BAND (137 MHZ) TO S-BAND (2.3 GHZ) PROVIDE SOIL MOISTURE INFORMATION AT MULTIPLE DEPTHS. SOOP MEASUREMENTS IN BARE SOIL AND UNDER VARYING BIOMASS (THROUGH THE CORN GROWING SEASON) WILL BE USED TO STUDY THE RELATIONSHIP TO CHANGING SOIL MOISTURE CONDITIONS. THIS RESEARCH WILL COMPARE EXPERIMENTAL ESTIMATES OF REFLECTIVITY OBTAINED FROM THE TOWER AND AIRBORNE CAMPAIGNS TO IN SITU SOIL MOISTURE OBSERVATIONS. EXISTING ELECTROMAGNETIC THEORY WILL BE ADAPTED TO THE FORWARDSCATTER PROBLEM IN WHICH THE SCATTERING CAN BE ASSUMED TO BE PREDOMINANTLY SPECULAR AT LOW FREQUENCIES. INITIALLY THE SENSITIVITY WILL BE EVALUATED EMPIRICALLY USING STATISTICAL METHODS BUT AS THE FORWARD MODEL IS DEVELOPED A PHYSICAL BASIS FOR THE SENSITIVITY WILL BE PROPOSED. VARIOUS APPROACHES TO THE INVERSION EITHER FITTING A MODEL FUNCTION TO THE MEASUREMENTS OR INCORPORATING SOME HYDROLOGICAL MODELING WILL BE EVALUATED. THE GOAL OF THIS PROPOSAL IS TO APPLY ELECTROMAGNETIC SCATTERING THEORY TO DEVELOP NEW FORWARD MODELS TO ESTIMATE SOIL MOISTURE USING AIRBORNE AND FIELD EXPERIMENTAL MEASUREMENTS. WE PROPOSE TO DEVELOP FIRST GENERATION INVERSION ALGORITHMS INCORPORATING THESE FORWARD MODELS FOR THE ESTIMATION OF THE SOIL MOISTURE PROFILE AND TO DEFINE THEIR LIMITATIONS (FOR EXAMPLE ABOVE-GROUND BIOMASS DENSITY).