Temporal Trend Analysis of Meteorological Variables and Reference Evapotranspiration in the Inter-Mountain Region of Wyoming

Long-term trends in reference evapotranspiration ( ET ref ) and its controlling factors are critical pieces of information in understanding how agricultural water requirements and water resources respond to a variable and changing climate. In this study, ET ref , along with climate variables that directly and indirectly impact it, such as air temperature ( T ), incoming solar radiation ( R s ), wind speed ( u ), relative humidity ( RH ), and precipitation ( P ), are discussed. All variables are analyzed for four weather stations located in irrigated agricultural regions of inter-mountain Wyoming: Pinedale, Torrington, Powell, and Worland. Non-parametric Mann−Kendall (MK) trend test and Theil–Sen's slope estimator were used to determine the statistical significance of positive or negative trends in climate variables and ET ref . Three non-parametric methods—(i) Pettitt Test (PT), (ii) Alexandersson’s Standard Normal Homogeneity Test (SNHT), and (iii) Buishand’s Range Test (BRT)—were used to check the data homogeneity and to detect any significant Trend Change Point (TCP) in the measured data time-series. For the data influenced by serial correlation, a modified version of the MK test (pre-whitening) were applied. Over the study duration, a statistically significant positive trend in maximum, minimum, and average annual temperature ( T max , T min , and T avg , respectively) was observed at all stations, except for Torrington in the southeast part of Wyoming, where these temperature measures had negative trends. The study indicated that the recent warming trends are much more pronounced than during the 1930s Dust Bowl Era. For all the stations, no TCPs were observed for P ; however, significant changes in trends were observed for T max and T min on both annual and seasonal timescales. Both grass and alfalfa reference evapotranspiration ( ET o and ET r ) had statistically significant positive trends in at least one season (in particular, the spring months of March, April, and May (MAM) or summer months of June, ...