Analysis of reference crop evapotranspiration changes and meteorological factors in Heilongjiang Province

To explore the temporal and spatial variation of reference crop evapotranspiration and the change of influencing factors in mid-latitude cold regions, the response relationship between reference crop evapotranspiration and various meteorological factors was revealed. Based on the data of 34 standard meteorological stations in Heilongjiang Province, daily reference crop evapotranspiration was calculated by the Penman-Monteith formula. Using the methods of cumulative departure curve, climate tendency rate, trend analysis and mutation test (Mann-Kendall test), and Hurst exponent, the spatial and temporal variation characteristics of reference crop evapotranspiration in Heilongjiang Province and the response relationship between diversified key meteorologic factors were analyzed, and the leading causes for the dissimilitude identified. The results show that the average reference crop evapotranspiration in Heilongjiang Province from 1990 to 2019 exhibited a moderating trend as a whole; the relative humidity was the main meteorological factor affecting evapotranspiration of reference crops in spring, while the average temperature in winter was that leading to the variation of reference crop evapotranspiration. In the entire province, the high evapotranspiration areas are concentrated in the southwest, with Tailai as the center, while the low evapotranspiration areas are concentrated in the northwest, with Huzhong as the center. The main meteorological factors that affect reference crop evapotranspiration changes in the southern region of Heilongjiang Province are mean wind speed, maximum air temperature, and minimum air temperature. The main meteorological factor that affects the change of reference crop evapotranspiration in the northern region is relative humidity, which refers to the ratio of the vapor pressure of water in the air to the saturated vapor pressure of water at the same temperature and pressure. The prediction of a future variation trend proves that the Hurst exponent in Heilongjiang Province is between 0.60 and 0.69, indicating that the future change of reference crop evapotranspiration will be similar to the same present downward trend as well as a certain degree of sustainability.