The warping dams are very widely used in the Loess Plateau to control soil erosion. Unfortunately，they have a strong impact on the natural runoff process and bring enormous influence on water resources assessment and water engineering design while they play an important role in regional water and soil conservation. Therefore, it is of great importance to accurately evaluate regional water resources that how to calculate runoff impounded by the warping dams. The present study applied the reduction coefficient method and runoff coefficient method to calculate the annual water impounded by the warping dams in the Heimutouchuan Watershed (HW). By comparing the computed results of two methods, it explored the suitability and effectiveness of the reduction coefficient method for calculating impounded water in the warping dams. The results show that the average annual impounded water of the warping dams is 862.1×103 m3 in the HW during 1971-2007, which is similar with the result computed by runoff coefficient method with the only 1.36% error. In addition, we noticed something interesting that the impounded water on unit area is roughly stable, namely changing in a small range after dividing the impounded water by the catchment area. The above results indicate that the reduction coefficient method is a sort of efficient calculation method for the water impounded by the warping dams, which can be used to evaluate regional water resources in the Loess Plateau region influenced by the warping dams. When combined with the runoff coefficient method, it may be expected to get a more accurate impounded water. On the side, since the impounded water on unit area is stable in the area without human activities obviously, we can roughly calculate the impounded water by the warping dams only depending on the unit impounded water when the catchment area of warping dams is known in a catchment. It is greatly significant to regional water assessment and runoff forecasting in the Loess Plateau region in the future.