Abstract: Much attention has been paid to the cause of declining water resources in the Haihe River basin In this study,the Wutai Mountain region is chosen to assess the impact of decreasing orographic precipitation on water resources The orographic precipitation enhancement factor R_o is defined as the ratio between the hilly precipitation from Wutaishan station and the low land precipitation from 5 stations around the Wutai Mountain.A coupled water energy balance equation is used to evaluate the impact of decreasing orographic precipitation on water resources in the region.Results show that the value of the factor R_o is down by 0.53 in the past 50 years,or a reduction of 27%,indicating a significant decline in orographic precipitation and leading to a reduction of 190 million m³/y,about 26% of the mean annual runoff, received in the region,which can even account for 70% of the total reduction of water resources.In addition,the hilly precipitation is often underestimated due to the current precipitation gauge networks suffers from a shortage of precipitation gauges at the peak of mountains in many regions of China.As a result,the impact of orographic precipitation on water resources may not be properly evaluated.The reduction of water resources cou ld thus be under-evaluated when runoff is calculated from the gauge precipitation.