Spatio-temporal variation in reference evapotranspiration in recent 50 years in karst and non-karst areas in Guangxi Zhuang Autonomous Region

Reference evapotranspiration (ET₀) is the water requirement of plants driven by meteorological factors. It is critical for restoration of vegetation and regional water resources management. In Southwest China where karst landforms are common, the ecological environment is fragile. Extreme drought and flood events are usually caused by declining annual precipitation and large amounts of precipitation within short time periods. The response of ET₀ to global climate change is a key in the assessment of the effect of climate change on hydrology. Therefore, it was important to explore the spatial and temporal variations and the related driving factors of ET₀. Based on data collected from 25 meteorological stations in Guangxi for the 1960-2010, daily ET₀ was computed using the FAO-56 Penman-Monteith equation. The spatio-temporal variations and the driving factors of ET₀ in Guangxi karst and non-karst areas were analyzed separately using Kriging interpolations, cumulative anomalies and spearman rank correlation at seasonal and annual scales. The driving factors of ET₀ in karst and non-karst areas were analyzed using the path-analysis method. Results showed that ET₀ decreased gradually from southeast to northwest of the study area, with an average rate of 1 138 mm·a^-1 during the studied period. High ET₀ was mainly distributed in non-karst areas and low ET₀ in karst areas. For different seasons, ET₀ values from stations in non-karst areas were generally higher than those from stations in karst areas. Mean monthly ET₀ in both karst areas and non-karst areas tracked a unimodal curve, with relatively high ET₀ for the period from May to September. Non-karst areas had higher ET₀ variations than karst areas. The average monthly ET₀ in non-karst areas was 95 mm and that in karst areas 64 mm. Summer ET₀ was highest and winter ET₀ lowest. In the recent 51 years, annual ET₀ for both karst and non-karst areas tracked N-shaped distribution, and were the highest in the 1970s and lowest in the 1990s with an increasing trend for period 2002-2010. The trends of variation in annual ET₀ in Guangxi were also analyzed. Out of the 25 stations, 3 stations in karst and 4 stations in non-karst areas had significant decrease in trend; then 2 stations in karst and 1 station in non-karst areas showed significant increase in trend, and the other stations had no significant trends. Analysis of the relationship between ET₀ and meteorological factors showed that sunshine duration, wind speed and average temperature were the main meteorological factors driving ET₀ in non-karst areas. Relative humidity influenced annual ET₀ by interacting with other meteorological factors in karst areas. Seasonally, sunshine duration and average temperature had the highest impact on ET₀, and both were positively related with ET₀. Furthermore, the indirect effect of the coefficient between wind speed and ET₀ was negative for karst areas and positive for non-karst areas in winter and spring. Differences in geographical location and topography were the main reasons for the differences in ET₀ between karst and non-karst areas. Therefore, understanding the trend of change and the reasons for the change in ET₀ in different regions was a necessary measure for analysis of ecological water requirement.