This report documents the computer program INFIL3.0, which is a grid-based, distributed-parameter, deterministic water-balance watershed model that calculates the temporal and spatial distribution of daily net infiltration of water across the lower boundary of the root zone. The bottom of the root zone is the estimated maximum depth below ground surface affected by evapotranspiration. In many field applications, net infiltration below the bottom of the root zone can be assumed to equal net recharge to an underlying water-table aquifer. The daily water balance simulated by INFIL3.0 includes precipitation as either rain or snow; snowfall accumulation, sublimation, and snowmelt; infiltration into the root zone; evapotranspiration from the root zone; drainage and water-content redistribution within the root-zone profile; surface-water runoff from, and run-on to, adjacent grid cells; and net infiltration across the bottom of the root zone. The water-balance model uses daily climate records of precipitation and air temperature and a spatially distributed representation of drainage-basin characteristics defined by topography, geology, soils, and vegetation to simulate daily net infiltration at all locations, including stream channels with intermittent streamflow in response to runoff from rain and snowmelt. The model does not simulate streamflow originating as ground-water discharge. Drainage-basin characteristics are represented in the model by a set of spatially distributed input variables uniquely assigned to each grid cell of a model grid. The report provides a description of the conceptual model of net infiltration on which the INFIL3.0 computer code is based and a detailed discussion of the methods by which INFIL3.0 simulates the net-infiltration process. The report also includes instructions for preparing input files necessary for an INFIL3.0 simulation, a description of the output files that are created as part of an INFIL3.0 simulation, and a sample problem that illustrates application of the code to a field setting. Brief descriptions of the main program routine and of each of the modules and subroutines of the INFIL3.0 code, as well as definitions of the variables used in each subroutine, are provided in an appendix.