Abstract: In recent 50 years, a significant climate warming has occurred in China under global climate change, and it has facilitated the development of drought. Climatic drywet conditions have attracted more and more attention in recent years. The change of drywet conditions will cause the farreaching impacts to prospective agricultural production and natural environment, especially in northwest China. Located in the arid and semiarid areas in northwest China, the Heihe River Basin is the second largest inland river basin in China. Water resources have become as a “bottleneck” of social and economic development and ecological regeneration in the Heihe River Basin. Investigation on the temporal and spatial distribution of water resources in this region would be greatly helpful for the utilization efficiency improvement, appropriate redistribution of water resources, and relieving environmental deterioration. Based on the daily meteorological data observed by 16 meteorological stations in the Heihe River Basin during the period from 1960 to 2008, in this study the values of daily evapotranspiration of reference crop were calculated using the PenmanMonteith method recommended by the FAO, and then the values of monthly, seasonal and annual evapotranspiration of reference crop were obtained. In order to study the temporal and spatial change of drywet conditions in the study area, the humid index values at different time were interpolated using the data of evapotranspiration of reference crop and precipitation. By using the method of Inverse Distance Weighted (IDW) under ArcGIS, the spatial change of the drywet conditions was lucubrated so as to research the regional difference. In this study, a preliminary analysis on the main factors affecting the change of the drywet conditions was also carried out. The results indicated that the values of mean annual humid index were generally in an increase trend in a fluctuation way in the Heihe River Basin and its upper, middle, lower reaches in recent 49 years, and the surface humid status was improved to some extent. It was also found that the fluctuation of annual humid index was obvious, and the Cv (coefficient of variation) values of humid index in the Heihe River Basin and its upper, middle, lower reaches were 14.97%, 13.63%, 25.47% and 37.72% respectively. Seasonally, the drying trend occurred in spring, and the wetting trend occurred in summer, autumn and winter. According to the humid index, summer was the wettest season, then autumn and spring, and winter was the driest season. Apart from the lower reaches, the variation rate of summer humid index was the fastest, so the change of the drywet conditions was the most unstable in summer, and it was the most stable in winter. Correlation analysis between humid index and meteorological variables indicated that the change of precipitation was the dominant cause affecting the change of drywet conditions in the Heihe River Basin, and the change of other climate factors can enhance or weaken the change of drywet conditions.

Key words: evapotranspiration, humid index, temporal variation, spatial variation, precipitation, Heihe River Basin