Analysis of Evapotranspiration Variability and Trends in the Arabian Peninsula

Problem statement: Climate variability is receiving much attention recently because it has significant effects on water resources and therefore on the livelihood of society especially in water scarce countries such the Kingdom of Saudi Arabia (KSA). Approach: The aim of this study was to explore changes in the reference evapotranspiration (ET_o) using data from 27 weather stations through years 1980-2008 and to identify trend directions in the ET_o as an indicator to climate variability in the region. ET_o was calculated using FAO-56 Penman Monteith method and trends analyses were performed with non-parametric statistics proposed by Mann-Kendall and Sen slope estimator. Results: Results indicated that the average ET_o varied from 5 mm day⁻¹ in January to more than 15 mm day⁻¹ in July with extreme average values ranged from 4 mm day⁻¹ in January to 18 mm day⁻¹ in July. The trend analyses indicated that the average annual maximum and minimum daily ET_o steadily increased over the study period. The ET_o annual daily average had increased from about 9.6 mm day⁻¹ in 1980 to 10.5 mm day⁻¹ in 2008. Trend analysis with Mann-Kendall and Sen Slope gave consistent results and were able to detect trend direction and its significant level for each month and for each station. In general, most of stations showed an increasing trend especially in the Northern parts of KSA and along the longitudinal line of 45°. ‘Yenbo’ station showed a significant increasing trend in ET_o for the entire year. The increasing trends prevailed during most of the year except in the winter months from October to January. Even during these winter months, significant decreasing trends were observed for only four stations. Conclusion: It is recommended to perform similar studies on other parts of the Arabian Peninsula using wider databases, and to consider the increasing trend of ET_o in the planning for agricultural and water resources projects.