Mapping monthly distribution of daily light integrals across the contiguous United States

The daily light integral (DLI) is a measurement of the total amount of photosynthetically active radiation delivered over a 24-hour period and is an important factor influencing plant growth over weeks and months. Contour maps were developed to demonstrate the mean DLI for each month of the year across the contiguous United States. The maps are based on 30 years of solar radiation data for 216 sites compiled and reported by the National Renewable Energy Lab in radiometric units (watt-hours per m(-2).d(-1), from 300 to 3,000 nm) that we converted to quantum units (mol.m(-2).d(-1), 400 to 700 nm). The mean DLI ranges from 5 to 10 mol.m(-2).d(-1) across the northern U.S. in December to 55 to 60 mol.m(-2).d(-1) in the southwestern U.S. in May through July. From October through February, the differences in DLI primarily occur between the northern and southern U.S., while from May through August the differences in DLI primarily occur between the eastern and western U.S. The DLI changes rapidly during the months before and after the vernal and autumnal equinoxes, e.g., increasing by more than 60% from February to April in many locations. The contour maps provide a means of estimating the typical DLI received across the U.S. throughout the year.