The forest at Maolan National Nature Reserve of Guizhou Province(MNNRG) is the largest karst primal forest area along similar latitude in the world and of great hydrological significance to the region.Its major hydrological effect,according to the tracer technology of hydrogen and oxygen stable isotope,is to increase precipitation in summer and to supply ecological water need in winter.In 2010,the precipitation was 1 710.0 mm within the reserve area and 1 066.5 mm outside.The increased rainfall in the forest mainly came from evaportranspiration of epi-karst water.The epi-karst water cycle demonstrated distinct seasonal variations including a three-month wet season and two-month dry season,suggesting the epi-karst hydrodynamics might be affected significantly by forest growth cycle.