Indicative function of karst spring temperatures on rainfall-flow response

Karst in Southwest China is located in hot and humid climate region. Strong dissolution produces various combinations of soil, rock fractures and conduits. Therefore, the hydrogeological heterogeneity in this area contributes the difficulty to the identification of various precipitation recharge formations and multiple flow components. Tracers, such as stable isotopes, electrical conductivity, and chemical ions, have been widely used to aid our understanding of hydrological processes. Compared to these tracers, the temperature is a much cheaper alternative for high spatial-temporal resolution monitoring. In this study, the dynamics of hydrograph and temperatures in atmosphere, soils and spring water were used to trace hydrological processes of precipitation infiltration, recharge into conduits and flow exchange between conduits and fractures. Taking a hillslope spring of Chenqi basin in the karst area of Southwest China as the study area, we compared variations in atmospheric temperatures, soil temperatures, spring water temperatures before and after 21 rainfall events from the middle May to the middle September in the years of 2016 and 2017. In addition, on the basis of heat-water transfer mechanism of soil, surface karst zones, and karst conduits in spring areas, we identified the infiltration and recharge modes of different types of rainfall and the effects of fast and slow flows on the decline of spring discharge, in order to reveal the formation mechanism of runoff and infiltration recharge in karst areas.

Results show that soil temperatures were much higher than spring discharge temperatures, and rainfall infiltration could sufficiently lowered soil temperatures and spring discharge temperatures in the study period. However, for the 21 rainfall events, the discharge temperatures varied in the rising phase of hydrograph because of different extents of heat mixture between the cool infiltration water and warm soils/rocks at ground surface. These differences were proven to be related with three types of precipitation infiltration and recharge, i.e. recharge by dispersed infiltration, recharge by concentrated infiltration of shallow runoff and direct recharge by concentrated rainfall. The study indicates that the recharge by dispersed infiltration occurred in the rainfall that was not heavy but lasted for a long time. In such type of rainfall, the spring discharge temperatures showed a slow rise with the increase of discharge. This phenomenon was attributed to the fact that the long-term thermal conduction in soils or small fractures heated infiltration water. However, as rainfall became more intensive but lasted for shorter time, the recharge by concentrated infiltration of shallow runoff and the direct recharge by concentrated rainfall dominated the rise of hydrograph. When the shallow runoff was developed, the spring discharge temperatures showed a decline after a rapid increase in the rising phase of hydrograph.

This research finding indicates that both thermal conduction and convection affected discharge temperatures. The thermal conduction of infiltration in soils or small fractures played a role in heating infiltration water, and thus in raising discharge temperatures. In contrast, the thermal convection via large fractures and sinkholes made the cool water (e.g., the rainfall and runoff in the flow peak) into conduits, which lowered discharge temperatures. The direct recharge by concentrated rainfall only occurred in extremely heavy but short rainfall. For such type of rainfall, the spring discharge temperatures showed a short and rapid increase in the rising phase of hydrograph, which can be inferred that the thermal convection may control discharge temperatures. Furthermore, in the recession period of hydrograph, variations in the spring discharge and temperatures can be used to distinguish mixture of the cool water in small fractures (slow flow) with conduit flow (fast flow). In the early recession period, the hydrograph maintained a high discharge and receded at a slow rate but discharge temperatures declined at a great rate, which indicated that there was a large amount of cool water in small fractures releasing to conduits. In the late recession period, the spring discharge receded to a stable state and its temperature remained steady and lowest, which can be inferred that water release from small fractures to conduits significantly reduced. The study results demonstrate that the temperature information is useful in tracing the complicated hydrological processes while more observations particularly those in epikarst and conduits are needed to increase the tracing reliability.