Mapping Monthly Distribution of Daily Light Integrals Across China

Daily light integral (DLI, mol·m^-2·d^-1) is the total amount of photosynthetically active radiation received over a 24-hour period. It is a useful light measurement related to plant growth. To provide an estimate of the light quantities delivered to different locations across the country throughout the year, contour maps were developed using solar global-horizontal radiation data recorded from 1973 to 2002 at 45 meteorological observation stations in China. The maps demonstrated the mean daily light integral for each month of the year across the contiguous China. Northern China received the lowest DLI of 5-10 mol·m^-2·d^-1 in December, while northwestern China had the highest DLI of 45-50 mol·m^-2·d^-1 in May, June, and July. From October through March, DLI primarily differed between northern and southern China, while from May to August DLI varied between eastern and western China. The DLI changed quickly during the months before and after the vernal and autumnal equinoxes. These contour maps can be used to estimate the necessity and/or amount of supplemental lighting inside greenhouses in various geographical locations.