Spatio-temporal change in water and thermal climatic conditions during the green grass season in the Urumqi area from 1961 to 2016

The fundamental spatio-temporal change characteristics of the start and end dates, duration, average temperature, accumulated temperature of ≥5 ℃, precipitation, and arid-wet index of the green grass season were analyzed by using linear regression and mixed spatial interpolation technology based on ArcGIS, using the daily meteorological data collected from 10 meteorological stations in Urumqi City and its surroundings area from 1961 to 2016. The first and last dates when the daily mean temperature were stably ≥5 ℃ in a year were taken as the start date and end date of the growth of natural pasture and the green grass season, respectively. The main results showed that the spatial distribution of the above-mentioned climate factors were clearly varied among several observed locations in the Urumqi area. Generally, the start dates of the green grass season were earlier in the plain desert pastures than on the mountain grasslands and meadows. However, the end dates of the green grass season were later on the plain desert pastures than on the mountain grasslands and meadows. Several thermal elements, such as duration, average temperature and ≥5 ℃ accumulated temperature of green grass season were higher on the plain desert pastures than on the mountain grasslands and meadows. Precipitation in the green grass season was higher on the mountain grasslands and meadows than on the alpine meadows and plain desert pastures. The arid-wet index of the green grass season increased with altitude. In the past 56 years, the start dates of the green grass season had shown an inconspicuous (P>0.05) advancement at the rate of -0.424 d·10 a-1, and the end dates have been significantly (P<0.01) delayed, at the rate of 1.455 d·10 a-1. At the same time, the duration, average temperature, accumulated temperature of ≥5 ℃, precipitation, and arid-wet index of the green grass season significantly (P<0.05) increased at the rate of 1.879 d·10 a-1, 0.16 ℃·10 a-1, 61.14 (℃·d)·10 a-1, 6.93 and 0.015 mm·10 a-1, respectively. However, these changes had regional differences. Comparing the last 30 a (1961-1990) with the most recent 26 a (1991-2016) the earlier days of the start of the green grass season showed little regional difference, and a greater increase in the delayed days of the green grass end dates, green grass season duration, average temperature, accumulated temperature of ≥5 ℃ was observed on the plain desert pastures than on the mountain grassland and meadow, but the increments of precipitation and arid-wet index of the green grass season were greater on the mountain grasslands and meadows than on the plain desert pastures in the Urumqi City.