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

• Main Authors: Vivek Sharma, Christopher Nicholson, Antony Bergantino, Suat Irmak, Dannele Peck
• Format: Article
• Language: English
• Published: MDPI AG 2020-07-01
• Series: Water
• Subjects: evapotranspiration; climate variables; trend analysis; Wyoming
• Online Access: https://www.mdpi.com/2073-4441/12/8/2159

Long-term trends in reference evapotranspiration (<i>ET_ref</i>) 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, <i>ET_ref</i>, along with climate variables that directly and indirectly impact it, such as air temperature (<i>T</i>), incoming solar radiation (<i>R_s</i>), wind speed (<i>u</i>), relative humidity (<i>RH</i>), and precipitation (<i>P</i>), 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 <i>ET_ref</i>. 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 (<i>T_max, T_min, </i>and<i> T_avg</i>, 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 <i>P</i>; however, significant changes in trends were observed for <i>T_max</i> and <i>T_min</i> on both annual and seasonal timescales. Both grass and alfalfa reference evapotranspiration (<i>ET_o</i> and <i>ET_r</i>) 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, July, and August (JJA) at all stations, except the station located in southeast Wyoming (Torrington) where no statistically significant positive trends were observed. Torrington instead experienced statistically significant negative trends in <i>ET_o</i> and <i>ET_r</i>, particularly in the fall months of SON and winter months of DJF. Over the period-of-record, an overall change of +26, +31, −48, and +34 mm in <i>ET_o</i> and +28, +40, −80, and +39 mm in <i>ET_r</i> was observed at Pinedale, Powell, Torrington, and Worland, respectively. Our analysis indicated that both <i>ET_o</i> (−3.4 mm year⁻¹) and <i>ET_r</i> (−5.3 mm year⁻¹) are decreasing at a much faster rate in recent years at Torrington compared to other stations. Relationships between climate variables and <i>ET_o</i> and <i>ET_r</i> on an annual time-step reveal that <i>ET_o</i> and <i>ET_r</i> were significantly and positively correlated to <i>T_avg, T_max, R_s, R_n, </i>and<i> VPD</i>, as well as significantly and negatively correlated to <i>RH</i>.