Temporal trend analysis of meteorological variables and reference evapotranspiration in the inter-mountain region of Wyoming

Authors: SHARMA, VIVEK - University Of Wyoming; NICHOLSON, CHRISTOPHER - University Of Wyoming; BERGANTINO, ANTONY - University Of Wyoming; IRMAK, SUAT - University Of Nebraska; Peck, Dannele

Submitted to: Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/28/2020
Publication Date: 7/30/2020
Citation: Sharma, V., Nicholson, C., Bergantino, A., Irmak, S., Peck, D.E. 2020. Temporal trend analysis of meteorological variables and reference evapotranspiration in the inter-mountain region of Wyoming. Water. 12(8). Article e2159. https://doi.org/10.3390/w12082159.
DOI: https://doi.org/10.3390/w12082159

Interpretive Summary: The measurement of evapotranspiration is important because it helps agricultural researchers and producers understand how much water crops require, depending on weather conditions. Crop water demand, in turn, affects the amount of water used from streams, rivers, and groundwater aquifers. Crop water demand might be changing in response to changing climate conditions, such as precipitation, temperature, relative humidity, wind speed, and more. To help agricultural producers adjust their cropping and irrigation practices in a changing climate, we need to understand how crop evapotranspiration is changing through time. Therefore, we analyzed long-term climate and evapotranspiration data at four locations in the state of Wyoming: near Pinedale, Powell, Torrington, and Worland. Results show that temperatures have risen steadily at all stations, except for Torrington in southeastern Wyoming, where temperatures are trending downward. At the three stations with increasing temperatures, evapotranspiration for grass and alfalfa has increased in summer months (June-August). These results indicate that rising temperatures have increased the amount of water required by an acre of grass or alfalfa. This knowledge could help inform agricultural producers' cropping and irrigation decisions, especially in areas where water is becoming more scarce.

Technical Abstract: Long-term trends in reference evapotranspiration (ETref) 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, ETref, along with climate variables that directly and indirectly impact it, such as air temperature (T), incoming solar radiation (Rs), 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 (1963–2017), Torrington (1922–2017), Powell (1908–2017), and Worland (1921–2017). The Mann-Kendall trend test and Theil-Sen's slope estimator were used to determine the statistical significance of positive or negative trends in climate variables and ETref. A statistically significant positive trend in maximum, minimum, and average annual temperature (Tmax, Tmin, and Tavg, 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 (from 1970–2017) are much more pronounced than during the 1930s Dust Bowl Era. Both grass and alfalfa reference evapotranspiration (ETo and ETr) had statistically significant positive trends in at least one season (in particular the spring months of MAM or summer months of 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 ETo and ETr, 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 ETo and +28, +40, -80, and +39 mm in ETr was observed at Pinedale, Powell, Torrington, and Worland, respectively. Relationships between climate variables and ETo and ETr on an annual time-step reveal that ETo and ETr were significantly and positively correlated to Tavg, Tmax, Rs, Rn, and VPD, as well as significantly and negatively correlated to RH.