تغییرات زمانی مکانی تبخیر و تعرق مرجع و عوامل اقلیمی موثر بر آن در استان یانان، جنوب غرب چین ۱۹۶۱-۲۰۰۴ Spatiotemporal variability of reference evapotranspiration and its contributing climatic factors in Yunnan Province, SW China, 1961–۲۰۰۴

۱ Introduction Evapotranspiration (ET), the sum of evaporation and plant transpiration, is a central element of the hydrological cycle, governing the moisture transfer to the atmosphere and thereby influencing fundamental properties of terrestrial ecosystems such as runoff, soil moisture and plant growth (Fisher et al. 2011). ET is of particular concern in Asia where the convection above the Tibetan Plateau and hence the transfer of latent energy to the atmosphere directly influences the intensity of the Asian monsoon system (Thomas 2008). Thus, understanding the spatiotemporal variations of ET is a vital component in regional hydrological studies in Asia. In a warming climate, the hydrological cycle is expected to intensify (IPCC 2007). The main argument is that according to the Clausius–Clapeyron equation a warmer atmosphere will be able to hold more water and hence allow for higher evaporation. However, despite globally increasing temperatures, most studies have shown that measured pan evaporation and calculated potential evapotranspiration (PET) are declining at both global (Roderick et al. 2009; McVicar et al. 2012) and regional scales (Chattopadhyay and Hulme 1997; Thomas 2000; Liu et al. 2004). A decrease of PET clearly points to changes in atmospheric water demand and therefore to changes in the climatic parameters driving evapotranspiration. The reason for the decreasing PET rates appears to be a decrease in solar radiation and wind speeds, while temperature actually plays a lesser role (Thomas 2000). The observed decline of sunshine duration (Stanhill and Cohen 2001) has partly reversed in the last decade (Wild et al. 2005), while terrestrial wind speeds have been observed to decrease on a global scale (McVicar et al. 2012). However, there is currently only limited knowledge which climatic variables influence the evaporative environment on global or regional scales. In order to compare the evapotranspiration potential from a climatological point of view and to allow the comparison of different climatic regions independent of actual land cover, the concept of potential evapotranspiration has been introduced, defined as ‘the amount of water transpired in unit time by a short green crop, completely covering the ground, of uniform height and never short of water’ (Penman 1956). The Penman-Monteith (PM) equation is regarded as the most reliable predictor of PET rates under all climatic conditions (Jensen et al. 1990). In the PM equation, four climatic variables (radiation, wind speed, atmospheric humidity and air temperature) are explicitly used to model the evaporative process. This approach does not allow analyzing the evaporative environment in many areas where density and duration of operation of weather stations is sparse. However, formulations relying on a single parameter such as temperature (Thornthwaite 1948; Hargreaves 1974) or a reduced set of parameters (Priestley and Taylor 1972) fail to capture the changing influence of the radiative and aerodynamic forces. From a practical point of view, the knowledge of the relative importance of contributing factors helps to determine which least data-demanding ET estimator can be used regionally. The East Asia monsoon climate offers an interesting study area with contrasting climatic conditions in the different seasons. In general, PET rates peak in the pre-monsoon season with high insolation and dry air conditions, remain on a slightly lower level during the humid summer monsoon season and decline in the cold and often overcast winter monsoon season (Thomas 2008). Several studies have reported mostly decreasing ET0 trends over China (Thomas 2000; Chen et al. 2006; Gao et al. 2006; Zhang et al. 2011). The spatiotemporal variability of these changes however is considerable, with trends even changing sign over short distances. Declining PET trends in China are mainly related to decreasing sunshine duration and wind speeds (Thomas 2000; Wang et al. 2004; Gao et al. 2006). Although Xu et al. (2006) found that local land-cover change was the primary cause for 310 Climatic Change (2013) 116:309–۳۲۵ decreasing wind speeds in China, it is largely uncertain if changes in global circulation properties or land use changes (via changing surface roughness) are responsible for the observed trends. Similarly, Guo et al. (2011) did not find conclusive evidence for decreasing wind speed in the rapidly growing urban areas of China. We restrict our analysis to Yunnan Province (YP) in the extreme Southwest of the PR China. YP is a climatically complex region, where three different circulation branches of the Asian monsoon system influence the western, eastern and northern parts of the region with seasonal varying intensity (Zhang 1988). In southern Tibet and western YP, sunshine duration is the primary driver of declining evapotranspiration on an annual basis, followed by relative humidity and wind speed (Gao et al. 2006). In Tibet (immediately to the north of YP) wind speed was found to be the primary cause for declining ET0 changes (Chen et al. 2006; Zhang et al. 2009), while Thomas (2000) identified sunshine duration at 3 stations in YP as the main drivers of declining PET rates in most of the seasons. We provide decadal (1961–۲۰۰۴) linear trends for Penman–Monteith reference evapotranspiration (ET0) rates and analyze spatiotemporal trend variability over YP. We analyze trends of climatic parameters (sunshine duration, wind speed, temperature and relative humidity) and their temporal contribution to ET0 rates. We aim to: (1) evaluate temporal trends and spatial patterns of ET0 rates and other climatic parameters over YP; (2) identify the major climate factors contributing to ET0 variability and (3) determine whether the relationship between ET0 and its driving climatological variables is stable or not. To our knowledge, this is the first study that deals with the temporal evolution of relative importance of climatic variables contributing to ET0 rates.