Abstract: Abstract: This paper reviews 110 years’ history of global runoff estimation. By taking land slope and aridity index as independent variable, and employing the method of Ordinary Least Squares (OLS) regression on sample region’s runoff coefficient, the empirical formula of runoff coefficient is calculated for China. Based on the empirical formula and high-resolution precipitation grid data, by applying Grid and Spatial Analysis module in Geographic Information System(GIS), the high resolution spatial distribution field of global water resources are calculated. Main calculation results are that the total runoff volume in the world(not including Antarctic Continent) is 47,884 cubic kilometers, the average runoff depth is 359 mm, the average runoff coefficient is 0.462; the runoff depth in African is 230mm, its runoff coefficient is 0.345, both are the lowest in the 6 continents; the runoff depth in South America is 770mm, the runoff coefficient is 0.507, both are the highest; Asia’s runoff depth is 313mm, which is below the world’s average, while it’s runoff coefficient is 0.495, which is higher than the world’s average. The results are compared with relevant literatures. This study emphasis on the importance of groundwater runoff and groundwater increment in arid and semi-arid regions, and significantly increases the estimation value of runoff depth in these regions. The calculated runoff volume of Africa and Australia are 6944 cubic kilometers and 3026 cubic kilometers respectively, which is obviously higher than the average of 24 estimations between 1973 and 2014 by literatures. Moreover, the authors also calculated the detail runoff pattern of all the 31 inner flow areas in the world. The total area of the world’s inner flowing regions is 28.47 million square Km, the total runoff volume is 1,663 cubic kilometers, the average runoff depth is 58.4 mm, and the average runoff coefficient is 0.252. The runoff depth of Oceania’s inner flowing region is 35mm, and its runoff coefficient is 0.129, both are the lowest in the world. Case studies of inner flow area show that the runoff depth field calculated by the empirical formula is in accordance with the actual runoff distribution pattern in Africa, Oceania, and 4 regions of Asia, including Nura River in Kazakhstan, Hexi Corridor and western Inner Mongolia Plateau in China, Southeastern Mongolia and China’s central and eastern Inner Mongolia Plateau, and Arabian Peninsula’s inner flow region. The theoretical hypothesis is confirmed for the inner flow sub-regions, the higher is the spatial resolution; the larger is the calculated runoff values. Main conclusions of this study are as follows: the spatial resolution of this study is 0.1667 degrees, which is 3 times as much as the majority resolution used in the runoff calculation; the runoff volume is calculated from the high spatial resolution runoff field in this study, which includes the runoff in inner flow watersheds and the runoff consumed in the outflow watersheds before it inflows to the oceans; the difference between the runoff of this study and the actual measured runoff at river mouth to the oceans or inland lakes should be the intermit consumption of the evaporation of on natural water surface, wetlands, the net water consumption of agricultural irrigation, and the net dissipation of artificial water usage at urban and industrial regions in the watershed; the underground runoff, the increment of groundwater, and the increment of soil available water are the main forms of runoff in arid and semi-arid climate zones. Current literature’s underestimation on water resources mainly lies in ignoring the evaporation of natural water body and wetland, the net dissipation of artificial water usage.

Key words: global water resources, empirical formula, runoff coefficient, spatial distribution field, high resolution, geographic information system