Long-term monthly pan evaporation time series registered at 40 meteorological stations located within Mexico’s state of Zacatecas were analyzed in order to identify their trends. In addition, we analyzed the power spectrum signals of evaporation anomaly series in order to identify their important frequencies and its possible connection with periodic phenomena. Results suggest that negative trends are prevailing over positive trends. We found negative linear trend for 27 out of 40 pan evaporation time series; 18 of the 27 decreasing trends were significant at p<0.05. On the other hand, 13 out of 40 pan evaporation trends were positive but only 3 at significant level (p<0.05). Moreover, noise in all of these monthly pan evaporation series tends to be a persistent behavior. Additionally, we found that important frequencies could be related with the yearly cycle, quasi–biannual cycle, ‘El Niño Southern Oscillation’ phenomena and sunspot cycle.
Key words: Linear trends, fractal dimension, power spectrum density, yearly cycle, quasi–biannual cycle, El Niño Southern Oscillation, sunspot cycle.
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