Prominent Impact of Diurnal Rainfall Variations on Evapotranspiration and Gross Primary Productivity in Forests over Low Latitudes

Rainfall amount and its variability on various time scales can significantly influence ecosystem evapotranspiration (ET) and gross primary productivity (GPP). However, little attention has been paid to how ET and GPP respond to diurnal rainfall variations. Here we investigate the effect of diurnal rainfall variations on ET and GPP, based on data at 14 forest sites between 35°S-35°N from FLUXNET2015. Diurnal rainfall variations are represented by four indices. The timing of precipitation within one day is represented by the fraction of daytime precipitation (Frc) and the rainfall peaking time h r p e a k . The frequency of precipitation (Freq) is expressed in hours with rainfall out of 24 hours on the day. An Unranked Gini index (UGi) is newly introduced to represent the unevenness of diurnal rainfall distribution in both timing and frequency. Our results illustrate prominent influences of diurnal rainfall variations on ET and GPP at the studied forest sites in lower latitudes. Given an amount of daily total precipitation, more daytime rainfall either in intensity or frequency (larger Frc or Freq), especially occurring around noon (shorter period from h r p e a k ), could lead to less daily total ET and GPP. More unevenly distributed rainfall being more away from noon (larger UGi) could result in more daily total ET and GPP. These influences become stronger with more daily total rainfall except for Freq. The regression coefficients of ET and GPP to UGi could reach 0.6, which is twice of Frc and h r p e a k . During a rainfall, solar radiation, air temperature and vapor pressure deficit would decrease, leading to decreases in ET and GPP. The concurrent diurnal variations in rainfall and other meteorological variables drive changes in ET and GPP. Considering diurnal variations of rainfall in observations and models would be helpful to improve our understanding of atmosphere-biosphere interactions in the future.