Relationships between springs yield dynamics and hydrological drought development

This paper presents results of the identification and assessment of relationships between river discharge dynamics and spring yield during severe hydrological drought. The study covered a weekly yield series of eight springs and the daily discharge series of river gauging stations closing catchments including these springs. The investigated area was located in the mountainous, upper reaches of the Dunajec River basin (southern Poland) and the study covered the period 1989–2018. It was assumed that river low-flow is a good indicator of hydrological drought development. Severe streamflow droughts were estimated on the basis of the threshold level method (TLM) at a truncation level of 95% on the flow duration curve (FDC). Spring yield droughts were identified in the same way, however, there were three variants of truncation criteria. Synchronicity between both types of droughts was assessed on the basis of a co- occurrence ratio. To achieve the best fit criteria analysis, time shift steps of the spring yield series in relation to the river discharge series were conducted both for individual springs and for the whole investigated group. The best results of drought co-occurrence were achieved for the spring threshold at a multiannual average yield value, especially in backward and zero time shifts for fissure springs placed in relatively small catchments. Analysis of the course of relative spring drought intensity in following time shifts allowed an indication of the typical behaviours of the aquifer spring regime in relation to hydrological drought development.