## 3 Results and Discussion  
### Temperature Trends  
In this section, two different approaches to studying temperature trends are considered. Firstly, the results of the trend analysis of temperature for the entirety of California (Table 1) show that a positive trend exists in the state as a whole. The highest value is found in November (+0.04 \({}^{\circ}\)C year\({}^{-1}\)) and it is statistically significant. The same arises with July, August, summer and autumn, where the trend reaches +0.03 \({}^{\circ}\)C per year. It is noteworthy that January has shown the same trend (+0.04 \({}^{\circ}\)C year\({}^{-1}\)) over the period studied as November but is not statistically significant. It is remarkable that February has shown no trend. Secondly, the results of analysing temperature trends (positive and negative) for each one of the meteorological stations on monthly, seasonal and annual levels from 1980 to 2019 in the State of California as well as their statistical significance at a confidence level of 95% are shown in Figures 2 and 3. Figure 2 shows that at least 60% of the meteorological stations showed positive trends for each month, in the different seasons and on an annual scale with the highest fraction of positive trend in November, reaching 92% of the stations. Alternatively, it was revealed that January, June, July, August and November are the months that have shown the highest percentages of stations with statistical significance. In November, significant trends are observed in almost half of the stations studied (40%). In addition, autumn is the season that showed the highest percentage of meteorological stations with positive trends (90%) and 35% of them statistically significant. In spring, however, only 72% of the trends are positive and only are 4% statistically significant. In contrast to other months, 57% of the trends in February are negative but none of them are statistically significant. According to the monthly results (Figure 3), in January the spatial distribution of positive trends was found all over the state, close to 80% of the stations as can be seen in Figure 2. The highest increases (+0.056 \({}^{\circ}\)C year\({}^{-1}\)) were found in the Sacramento and San Joaquin Valley, where 30% of the stations were statistically significant. Although the \begin{table}
\begin{tabular}{c|c c c c c c c c c} \hline \hline  & January & February & March & April & May & June & July & August & September \\ \cline{2-10} Slope & 0.04 & 0 & 0.02 & 0.02 & 0.01 & 0.03 & 0.03 & 0.03 & 0.01 \\ \(p\) value & 0.06 & 0.87 & 0.37 & 0.53 & 0.44 & 0.07 & 0.03 \({}^{\star}\) & 0.01 \({}^{\star}\) & 0.23 \\ \cline{2-10}  & October & November & December & Winter & Spring & Summer & Autumn & Annual \\ \cline{2-10} Slope & 0.01 & 0.04 & 0.02 & 0.02 & 0.01 & 0.03 & 0.03 & 0.01 \\ \(p\) value & 0.58 & 0.02 \({}^{\star}\) & 0.25 & 0.25 & 0.38 & 0.02 \({}^{\star}\) & 0.02 \({}^{\star}\) & 0.13 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mean temperature trends (\({}^{\circ}\)C year\({}^{-1}\)) for the entire State of California. \({}^{\star}\) Statistical significance at a 95% confidence level. observational period and time scale are different (1910-2003), our result agrees with the research [69] that revealed a rise in annual mean temperatures in this area. This rapid warming in the Valley seems to be caused by the development of irrigated agriculture [70] in contrast to the slight increase they had shown in Sierra Nevada. Irrigated agriculture increases the minimum temperature (T\({}_{\text{min}}\)) values in these areas and seems not to have an effect on maximum temperatures (T\({}_{\text{max}}\)), which explains the increase in mean temperatures. The San Joaquin and Sacramento rivers that originate in the Sierra Nevada and the mountainous regions in the north run through this region. The confluence of the rivers occurs in the Sacramento-San Joaquin Delta.