Time-frequency insights: Uncovering the drivers of reference evapotranspiration across China

Reference evapotranspiration (ETo) is an important variable required in many disciplines and is influenced by many factors. However, the bivariate and multivariate relationships between ETo and affecting factors across multiple time-frequency domains remain unknown. Here, we identified the primary factors affecting ETo across time-frequency domain in 653 meteorological stations of mainland China based on the combination of wavelet transform coherence (WTC) and multiwavelet coherence (MWC) methods. The results indicated that ETo and all affecting factors (solar radiation, Rs; vapor pressure deficit, VPD; air temperature, Ta; wind speed, u2) during 1967–2016 exhibited a frequency ranging from 2 days to 211 months, and had a continuous annual (374 d) periodicity (except u2) for almost all sites. Results of percentage area of significant coherence (PASC) of WTC indicated that VPD or Rs is the dominant single factor driving variations of ETo across time-frequency space in majority sites (66.3 % and 32.0 %, respectively), while u2 is only dominant in limited (11) sites. This quite differs from the daily scale, where daily ETo was primarily influenced by daily Rs at 361 sites, daily VPD at 286 sites, and daily Ta at 6 sites. Results of MWC showed that the explanation for the time-frequency variations of ETo can be further improved using two-factors in 40.7 % of all sites as indicated by absolute increased PASC of MWC by 5 %. Overall, we found that the variation of ETo across time-frequency domain can be well explained by using only one variable (VPD or Rs) in 59.3 % of all sites, while by combinations of VPD-Rs and VPD-u2 in remaining sites. This study provides novel insights into understanding the variations of ETo across multiple time-frequency spaces.