Light and Water Use Efficiency as Influenced by Clouds and/or Aerosols in a Rainfed Spring Maize Cropland on the Loess Plateau

Using the flux data in a rainfed spring maize (Zea mays L.) field on the Loess Plateau during the growth seasons in 2013 and 2014, we analyzed variation in light use efficiency (LUE) and water use efficiency (WUE) in relation to clouds and/or aerosols. The peak values of monthly average diurnal LUE and WUE were found at sunset and at similar to 7: 00 AM, respectively. During the period of rapid growth, LUE and WUE decreased linearly with increasing clearness index (K-t). Total photosynthetically active radiation (PAR(t)) and evapotranspiration (ET) increased linearly with increasing K-t. Gross primary productivity (GPP) increased rapidly with increasing K-t when K-t < 0.4 but was less responsive to K-t when K-t > 0.4. The slope of the linear equation between ET and available energy was 0.33 kg H2O m(-2) d(-1) in both years, and the slope of the linear equation between GPP and diffuse PAR (PAR(f)) was 0.86 and 0.83 g C m(-2) d(-1) in 2013 and 2014, respectively. However, GPP only increased sharply with increasing direct PAR (PAR(r)) when PAR(r) < 5 mol PAR(r) m(-2) d(-1). When K-t decreased, the ratio of PAR f to PAR(t) increased, producing a more uniform irradiance of the canopy and resulting in increases in LUE and WUE. These findings highlight the fact that clouds and/or aerosols have important effects on the carbon and water cycle in rainfed spring maize fields on the Loess Plateau.