Temporal variation of reference evapotranspiration during 1961–2005 in the Taoer River basin of Northeast China

Abstract

Evapotranspiration is an important flux term in the water cycle that integrates atmospheric demands and surface conditions. Thus, analysis of the temporal variation of reference evapotranspiration (ET 0) will help us to understand climate change and its effect on hydrology. In this paper, we present an analysis of the monthly ET 0 at 15 stations during 1961–2005 in the Taoer River basin in China, calculated by the FAO Penman–Monteith method. We derived the growing season and annual total ET 0 time series and using the Mann–Kendall method, moving t test and Morlet wavelet conducted comprehensive time series analysis to characterize significance test, abrupt change and period in the ET 0 data sets. The results present that: (i) in terms of the seasonal cycle, monthly ET 0 reaches its peak in May and growing season ET 0 accounts for 60.7% of annual total. Long term growing season ET 0 fluctuates in accordance with annual ET 0, both having turning points in 1982 and 1993; (ii) with respect to the long term persistence, the trends for growing season and annual ET 0 show the same spatial patterns: high positive values in the west study area in the upper reach and negative values in the Southeast study area in the lower reach. The spatial distribution of annual tendencies suggests an influence from the altitude and latitude; (iii) for individual station, abrupt changes in annual ET 0 are more pronounced than those in growing season ET 0. The timings for the abrupt changes in ET 0 series at individual stations are consistent with those in regional ET 0. Abrupt changes detected in the early 1980s are all increasing changes, while those in earlier 1990 are all decreasing changes; and (iv) based on the Morlet wavelet analysis, there exist significant periods of 1, 3 and 7.3 years in annual ET 0 series and significant 1 year period and 7.3 years period in growing season ET 0 series. Maximum air temperature, mean air temperature, relative humidity and bright sunshine hours are main climate variables responsible for the periodicity in ET 0.