CURRENT TECHNIQUES TO MAP THE EXTENT AND DURATION OF SURFACE FLOODING OR INUNDATION (INUNDATION DYNAMICS) AT APPROPRIATE SPATIAL AND TEMPORAL SCALES FALL SHORT IN KEY AREAS. UNDERSTANDING HOW TEMPORAL CHANGES IN LAKES RIVERS AND WETLANDS AFFECT OTHER COMPONENTS OF THE HYDROLOGIC AND CARBON CYCLES REQUIRES INNOVATIVE ADVANCES IN HOW THEY ARE MONITORED. MICROWAVE REMOTE SENSING IS GENERALLY ACCEPTED AS THE BEST WAY TO CHARACTERIZE INUNDATION DYNAMICS BECAUSE UNLIKE OTHER METHODS THE SIGNAL CAN PENETRATE CLOUDS AND VEGETATION CANOPIES. HOWEVER TRADITIONAL METHODS OF MICROWAVE REMOTE SENSING USING EITHER RADIOMETERS OR SYNTHETIC APERTURE RADARS (SARS) CANNOT PROVIDE DATA AT BOTH A HIGH SPATIAL RESOLUTION AND FREQUENTLY ENOUGH TO PROPERLY CHARACTERIZE INUNDATION DYNAMICS. THE OUTCOME OF THE PROPOSED RESEARCH WILL BE A NEW MICROWAVE REMOTE SENSING TECHNIQUE TO MAP INUNDATION THAT IS NOT SUBJECT TO MANY OF THE LIMITATIONS OF RADIOMETER OR SAR DATA. THIS RESEARCH WILL ENHANCE THE UNDERSTANDING OF THE EARTH'S SURFACE TERRESTRIAL WATER BUDGET BY PROVIDING HIGH RESOLUTION RAPID REPEAT SURFACE INUNDATION MAPS FOR THE TROPICAL REGIONS. L-BAND SIGNALS TRANSMITTED FROM GLOBAL NAVIGATION SATELLITE SYSTEM (GNSS) SATELLITES AND RECEIVED BY THE SATELLITES THAT COMPRISE THE CYCLONE GNSS (CYGNSS) CONSTELLATION WILL BE USED TO CREATE DYNAMIC INUNDATION MAPS. BECAUSE CYGNSS IS COMPRISED OF EIGHT SATELLITES THE TEMPORAL REPEAT CYCLE IS MUCH SHORTER THAN FROM ONE SATELLITE ALONE AND THE CYGNSS DATA OVER LAND SHOW SENSITIVITY TO EXTREMELY SMALL SURFACE WATER FEATURE ON THE ORDER OF TENS OF METERS. THIS INVESTIGATION WILL NOT ONLY YIELD MAPS OF DYNAMIC INUNDATION FOR THE TROPICS. IN ADDITION KNOWLEDGE AND EXPERIENCE WILL BE GAINED THAT IS APPLICABLE TO FUTURE GNSS RECEIVER CONSTELLATIONS. GNSS TRANSMITTING SATELLITES WILL BE IN ORBIT FOR DECADES TO COME SO THERE IS THE POTENTIAL TO MAP INUNDATION WITH GREATER DATA CONTINUITY THAN IS POSSIBLE WITH TRADITIONAL REMOTE SENSING TECHNIQUES. THE PRIMARY OBJECTIVE OF THE PROPOSED RESEARCH IS TO DEVELOP A RETRIEVAL ALGORITHM TO MAP SURFACE INUNDATION EXTENT WITH FORWARDSCATTERED GNSS SIGNALS RECEIVED BY CYGNSS AND OTHER SPACEBORNE RECEIVERS EVEN IN THE PRESENCE OF A SIGNIFICANT VEGETATION CANOPY. THE RETRIEVAL ALGORITHM WILL BE VALIDATED IN AREAS OF KNOWN INUNDATION AND THOSE WITH IN SITU OBSERVATIONS. DYNAMIC SURFACE INUNDATION MAPS USING SPACEBORNE GNSS REFLECTION DATA WILL BE GENERATED FOR THE TROPICS. THESE SURFACE INUNDATION MAPS WILL PROVIDE DATA THAT ARE UPDATED MORE FREQUENTLY AND AT A FINER SPATIAL RESOLUTION THAN IS CURRENTLY POSSIBLE WITH EXISTING SENSORS. THESE MAPS WILL BE COMPARED TO EXISTING REMOTE SENSING DATASETS OF INUNDATION. THE PROPOSED WORK IS DIRECTLY RELEVANT TO THE A.25 THP SOLICITATION UNDER SECTION 2.2 "OTHER HYDROLOGICAL VARIABLES" AND IS RESPONSIVE TO THE REQUEST TO "IMPROVE REMOTE SENSING ALGORITHMS AND THEIR PERFORMANCE THAT PRODUCE THESE LAND-SURFACE HYDROLOGY DATA PRODUCTS: GROUNDWATER ROOT-ZONE SOIL MOISTURE EVAPOTRANSPIRATION (OR TRANSPIRATION) SURFACE WATER AND RIVER DISCHARGE." MAPS OF SURFACE WATER EXTENT BASED ON FORWARD-SCATTERED L-BAND GNSS SIGNALS PROVIDE AN INDEPENDENT MEASUREMENT OF INUNDATION THAT COMPLEMENTS PRODUCTS BASED ON RADAR BACKSCATTER. THE CYGNSS-BASED MAPS WILL DOCUMENT INUNDATION EXTENT AND VARIATIONS THAT EXIST TODAY AND THUS HELP CONTEXTUALIZE POTENTIAL FUTURE CHANGES WHICH ADDRESSES THE THP OBJECTIVE OF USING "SATELLITE-BASED REMOTELY SENSED DATA FOR CHARACTERIZING UNDERSTANDING AND PREDICTING THE TERRESTRIALLY LINKED COMPONENTS OF THE HYDROLOGIC CYCLE."