Variation of surface dry and wet conditions and its influencing factors in the Hanjiang River basin from 1971 to 2016

Based on the diurnal observations at 17 meteorological stations in the Hanjiang River basin from 1971 to 2016, we used the FAO Penman-Monteith formula to calculate the potential evapotranspiration (ET₀) of all the stations above to derive the relative moist index (MI), and then discussed the temporal-spatial variation of MI and the meteorological factors that affected it. The results show that there is a weak wetting trend in the Hanjiang River basin in recent 46 years, in which decreasing trend of ET₀ is evidently shown. MI has increased gradually from northwest to southeast, with the low value center locating in Zhen'an, Shangzhou and Xixia of the north part of Hanjiang River basin. The domain of low value center increases gradually from 1970s to 2016. The high value center of ET₀ moves from the southeast of Hanjiang River basin in 1970s and 1980s to the middle of this basin from 1990s to 2016. The monthly distributions of MI show the single peak characteristic, whose maximum value is in July. The peak-value month of stations would be moved up from the upper reaches to the lower reaches of the Hanjiang River basin, showing the transformation trend from a typical single peak to the bimodal distribution. The surface wetting conditions turn better significantly. Different factors have different impacts on the MI in the Hanjiang River basin, showing a positive correlation with precipitation and relative humidity, and a negative correlation with ET₀, maximum temperature, sunshine duration and wind speed.