Quantitative Trend, Sensitivity and Contribution Analyses of Reference Evapotranspiration in some Arid Environments under Climate Change

The temporal trend of reference crop evapotranspiration (ET0) and contribution of associated meteorological factors to the ET0 trend were assessed for 17 arid areas. Sensitivity of ET0 to changes in key meteorological variables was also analyzed. To study temporal trend of ET0, Mann-Kendall trend test was employed. Quantitative contribution and sensitivity analyses were carried out, respectively, using a dimensionless relative sensitivity coefficient and detrending method. Results indicated that ET0 has an increasing trend in 70.6, 64.7, 70.6, 76.5 and 70.0%, of sites respectively, in winter, spring, summer, autumn and entire year. This positive trend was significant (p <= 0.05) in 47.0, 35.3, 35.3, 29.4 and 35.3% of sites, respectively, for the same seasons. There was a significant change-point in winter, spring, summer, autumn and annual ET0 series at 64.7, 52.9, 64.7, 64.7 and 82.3% of stations, respectively. In 35.3 and 35.3% of sites, solar radiation and wind speed were the most sensitive climatic factors on ET0, respectively. ET0 exhibited the highest sensitivity to the relative humidity changes in coastal sites. Changes of wind speed contributed much more than other factors to the annual ET0 trend in 58.8% of investigated sites. The negative trend in wind speed nearly nullified the positive effects of increased air temperature on ET0 over 1966-2012 in 23.5% of stations. Changes in ET0 were attributed to wind speed changes in most locations. Given the upward trend of ET0 in the majority of locations, proper water management is required to avoid negative impacts of climate change in arid regions.