The Water Security Indicator Model - Global Land Data Assimilation System (WSIM-GLDAS) Monthly Grids, Version 1 data set identifies and characterizes surpluses and deficits of freshwater, and the parameters determining these anomalies, at monthly intervals over the period January 1948 to December 2014. The data set uses the land surface model outputs from NASA's Global Land Data Assimilation System, covering the global extent, to generate anomaly values for the following parameters at a gridded resolution of 0.25 degrees: temperature, precipitation, soil moisture, potential minus actual evapotranspiration, runoff, total blue water (flow-accumulated runoff), composite index of water surplus, and composite index of water deficits. These data are provided in terms of return periods, scientific units, and standardized (normalized) anomalies, and are computed over 1-month, 3-month, 6-month, and 12-month temporal periods of accumulation, referred to as integration periods. Anomaly values are present in terms of return periods with respect to a fitted Generalized Extreme Value (GEV) probability distribution function over a historical baseline period of January 1950 to December 2009, at a global spatial resolution of 0.25 degrees over the monthly, 3-month, 6-month, and 12-month periods of integration. Parameter values (location, scale, shape) of the fitted GEV probability distribution, which are fit separately for each calendar month, are distributed per parameter for each integration period.