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

Reference evapotranspiration (ET o ) is an important variable required in many disciplines and is influenced by many factors. However, the bivariate and multivariate relationships between ET o and affecting factors across multiple time-frequency domains remain unknown. Here, we identified the primary factors affecting ET o 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 ET o and all affecting factors (solar radiation, R s ; vapor pressure deficit, VPD; air temperature, T a ; wind speed, u 2 ) during 1967–2016 exhibited a frequency ranging from 2 days to 211 months, and had a continuous annual (374 d) periodicity (except u 2 ) for almost all sites. Results of percentage area of significant coherence (PASC) of WTC indicated that VPD or R s is the dominant single factor driving variations of ET o across time-frequency space in majority sites (66.3 % and 32.0 %, respectively), while u 2 is only dominant in limited (11) sites. This quite differs from the daily scale, where daily ET o was primarily influenced by daily R s at 361 sites, daily VPD at 286 sites, and daily T a at 6 sites. Results of MWC showed that the explanation for the time-frequency variations of ET o 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 ET o across time-frequency domain can be well explained by using only one variable (VPD or R s ) in 59.3 % of all sites, while by combinations of VPD-R s and VPD- u 2 in remaining sites. This study provides novel insights into understanding the variations of ET o across multiple time-frequency spaces. • We revealed the drivers of reference evapotranspiration across China from time-frequency insights. • Wavelet transform and multiwavelet coherence methods were employed to assess the driving forces of ET o . • Variability of ET o and meteorological factors (except for u 2 ) shows annual periodicity for almost all sites. • VPD is the primary single factor driving variations in ET o across time-frequency domain in 66.3 % of all sites. • Two factors cannot further improve explanation changes of ET o across time-frequency domain in 59.3 % of all sites.