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Impacts of climate variability on reference evapotranspiration over 58 years in the Haihe river basin of north China

Tang, Bo, Tong, Ling, Kang, Shaozhong, Zhang, Lu
Agricultural water management 2011 v.98 no.10 pp. 1660-1670
Food and Agriculture Organization, air temperature, climate, climate change, evapotranspiration, vapor pressure, watersheds, wind speed, China
Physically, evaporative demand is driven by net radiation (Rₙ), vapour pressure (eₐ), wind speed (u₂), and air temperature (Tₐ), each of which changes over time. By analyzing temporal variations in reference evapotranspiration (ET₀), improved understanding of the impacts of climate change on hydrological processes can be obtained. In this study, variations in ET₀ over 58 years (1950–2007) at 34 stations in the Haihe river basin of China were analyzed. ET₀ was calculated by the FAO Penman–Monteith formula. Calculation of Kendall rank coefficient was done by analyzing the annual and seasonal trends in ET₀ derived from its dependent climate variables. Inverse distance weighting (IDW) was used to analyze the spatial variation in annual and seasonal ET₀, and in each climate variable. An attribution analysis was performed to quantify the contribution of each input variable to ET₀ variation. The results showed that ET₀ gradually decreased in the whole basin over the 58 years at a rate of −1.0mmyr⁻², at the same time, Rₙ, u₂ and precipitation also decreased. Changes in ET₀ were attributed to the variations in net radiation (−0.9mmyr⁻²), vapour pressure (−0.5mmyr⁻²), wind speed (−1.3mmyr⁻²) and air temperature (1.7mmyr⁻²). Looking at all data on a month by month basis, we found that Tₐ had a positive effect on dET₀/dt (the derivative of reference evapotranspiration to time) and Rₙ and u₂ had negative effects on dET₀/dt. While changes in air temperature were found to produce a large increase in dET₀/dt, changes in other key variables each reduced rates, resulting in an overall negative trend in dET₀/dt.