The response of air temperatures to widespread irrigation may represent an important component of past and/or future regional climate changes. The quantitative impact of irrigation on daily minimum and maximum temperatures (T_min and T_max) in California was estimated using historical time series of county irrigated areas from agricultural censuses and daily climate observations from the U.S. Historical Climatology Network. Regression analysis of temperature and irrigation changes for stations within irrigated areas revealed a highly significant (p < 0.01) effect of irrigation on June–August average T_max, with no significant effects on T_min (p > 0.3). The mean estimate for T_max was a substantial 5.0°C cooling for 100% irrigation cover, with a 95% confidence interval of 2.0°–7.9°C. As a result of small changes in T_min compared to T_max, the diurnal temperature range (DTR) decreased significantly in both spring and summer months. Effects on percentiles of T_max within summer months were not statistically distinguishable, suggesting that irrigation’s impact is similar on warm and cool days in California. Finally, average trends for stations within irrigated areas were compared to those from nonirrigated stations to evaluate the robustness of conclusions from previous studies based on pairwise comparisons of irrigated and nonirrigated sites. Stronger negative T_max trends in irrigated sites were consistent with the inferred effects of irrigation on T_max. However, T_min trends were significantly more positive for nonirrigated sites despite the apparent lack of effects of irrigation on T_min from the analysis within irrigated sites.

Together with evidence of increases in urban areas near nonirrigated sites, this finding indicates an important effect of urbanization on T_min in California that had previously been attributed to irrigation. The results therefore demonstrate that simple pairwise comparisons between stations in a complex region such as California can lead to misinterpretation of historical climate trends and the effects of land use changes.