Water level response characteristics and runoff threshold estimation of karst depressions in a valley region, western Hubei Province

Liujiaba and Longwan, Huangliang town, Xingshan county, Hubei Province are two karst depressions in valley region with elevations of 886 m and 997 m, and catchment areas 20.9 km2 and 1.83 km2, respectively. After rainfalls, these karst depressions runoff and transform the surface water into groundwater through the sinkholes which connect to the open channels at the bottom of the depressions, and finally discharge to the Bailong spring. To study such a hydrogeological process, rainfall stations, hydrological monitoring stations and soil-water monitoring equipment were installed in these depressions and a hydrological monitoring station constructed in the Bailong Spring. Rainfall stations record air temperature and rainfall, hydrological stations monitor water level and water temperature in the sinkholes, and soil-water monitoring equipment record the soil moisture content and electrical conductivity. Different methods were used to analyze these observational data, so as to reveal characteristics of water level response and runoff yield and their influencing factors under the condition of concentrated recharge. The results show that， (1) the water level responses in the sinkholes and karst springs have better synchronous relationships with the rainfall, expressing "sudden rise and steep drop" in hydrographs with large rainfall intensity and "slow rise and slow fall" in hydrographs with low rainfall intensity. When very heavy rainfalls happen, karst waterlogging is easy to occur due to the sinkhole’s limited drainage ability. On July 24, 2019 after a rainfall storm (86.5 mm in 3.5 hours), the water level (1.96 m) surpassed the open channel height (1.6 m) in the Longwan karst depression； (2) The Chey-Manning formula was used to transfer the water level of the open channel into flow, then the rainfall and the flow of the open channel was fitted by a mathematical equation to estimate the runoff threshold of the Liujiaba and Longwan depressions, yielding values of 7.4 mm and 10.6 mm, respectively； (3)Through the changes of soil moisture content and electrical conductivity before and after the rainfall event, the influencing factors of runoff threshold were analyzed, indicating the rainfall intensity decreases the runoff threshold, and large antecedent soil moisture could promote slope-runoff. In addition, the yield threshold is also related to the topographic slope and soil material of karst depressions, implying the yield threshold is a multi-factor coupling parameter.