Mapping Daily Light Integrals for Greenhouse Production

The daily light integral (DLI) is a measurement of the total amount of photosynthetically active radiation (PAR) received on a flat surface over a 24-hour period and is an important factor influencing plant growth over weeks and months. A 10 km gridded map of DLI for each month was created using the 1998-2009 National Solar Radiation Database (NRSDB) developed by the National Renewable Energy Lab (NREL) and the State University of New York/Albany (SUNY). We converted Global Horizontal Irradiance (GHI in Kwh m-2 d-1) to quantum units (mol m-2 d-1) using a conversion of 7.058. Many aspects of plant production in a greenhouse are impacted by DLI such as the root and shoot growth of seedlings and cuttings, quality of the finished plants, and timing of critical phenological events such as flowering. The resultant tables and maps can assist commercial growers who routinely monitor and record the DLI received by their crops to help them easily determine when they need supplemental lighting or retractable shade curtains.