Formation mechanism of interior drainage areas and closed drainage areas of the Ordos Plateau in the middle reaches of the Yellow River based on an analysis of the water cycle

Based on an analysis of the transformation between surface waters and groundwater on the Ordos Plateau and water resources in the Yellow drainage region and according the theory and method of the groundwater flow system, the formation mechanism of the interior drainage areas and closed drainage areas were studied and the distribution of the interior drainage areas and closed drainage areas was re-delineated. The results show that there are 2 interior drainage areas and 4 closed drainage areas on the Ordos Plateau. Although within the interior drainage areas there is no direct hydraulic connection between the surface water and the Yellow River and its tributaries, groundwater has close connection with the Yellow River and its tributaries; the interior water systems should be ascribed to the Yellow River drainage basin. Within the closed drainage areas neither surface nor groundwater has hydraulic connection with the Yellow River and its tributaries; so the closed drainage areas do not belong to the Yellow River drainage basin. The total area of the closed drainage areas is 32451.4 km2, decreasing by 9844.6 km2 as compared with that (42296 km2) given by the book The Data of Characteristic Values in the Reaches of the Yellow River” compiled in 1977, i.e.: the catchment area of the Yellow River should increases by 9844.6 km2.