Abstract:

This paper aims to provide a scientific basis for water-saving irrigation, the rational allocation of water resources, and drought monitoring in Tibet, China. We use the Penman-Monteith model, trend analysis, Morlet wavelet analysis, Mann-Kendall tests, the empirical orthogonal function, and gray models to explore the spatiotemporal evolution of potential evapotranspiration (ET₀) in Tibet and the central and western regions of Nagqu as well as the trend of future ET₀ changes based on daily meteorological data collected from 1981 to 2019 at 38 meteorological stations in the region. The results demonstrate that on the time scale, the average ET₀ in Tibet is 597.12 mm. It also exhibited a significant decreasing trend from 1981 to 2001 and a significant increasing trend from 2002 to 2019 (P<0.01). In these two distinct time periods, high evapotranspiration was concentrated in the central and western areas of Nagqu and along the river line. In the first mode (EOF1), the annual ET₀ change trend in Tibet had a high degree of consistency in space, mainly exhibiting a gradually decreasing distribution from the center of the central area to the southwest and southeast. The high-value center is concentrated along the river line, and the low-value center was located in the southern region. Concerning space, the positive center of central and western Tibet and the negative center of southeastern Tibet are reversed. The stations with positive Kendall trend coefficients are mainly concentrated in the central and western regions of Nagqu and the southern marginal area, where the climate has been warming and drying. The sites exhibiting abrupt changes in annual ET₀ are mainly distributed in the southern marginal area along the river line and the northeast and were concentrated in time. In the 1980s, two mutations occurred in the potential evapotranspiration at Nagqu and Dangxiong stations, whereas one or no mutations occurred at each of the remaining stations. The annual average ET₀ both of Tibet as a whole and of the five major climate zones exhibited an increasing trend (except in the southeast), and the 33 year cyclic oscillation was the first main cycle. Additionally, there was a cycle of 11 a in the whole region and along the river line. The midwest of Nagqu had cycles of 22 a and 3 a, the southern marginal area had cycles of 8 a and 12 a, the northeast had a cycle of 8 a, and the southeast had main cycles of 22 a and 12 a. The constructed gray model (GM; 1, 1) used for prediction had gray parameters a≥-0.3 and a∈(-2, 2), and the level ratio was in a smooth state, meaning it can be used to predict future ET₀. The average prediction accuracy was 87.85%, so we predicted the medium- and long-term average annual ET₀ date series for Tibet. The forecast results revealed that except for the significant decline in the forecast value of the annual ET₀ in the southeast, all other regions are exhibiting an upward trend, and the southern edge shows the steepest increase.

Key words: Penman-Monteith model, evapotranspiration, spatiotemporal analysis, grey model, Tibet