Solar-forced diurnal regulation of cave drip rates via phreatophyte evapotranspiration

Katie Coleborn,Owen Navarre,Gabriel C Rau,Mark O Cuthbert,Andy Baker

doi:10.5194/hess-20-4439-2016

Abstract

Abstract. We present results of a detailed study of drip rate variations at 12 drip discharge sites in Glory Hole Cave, New South Wales, Australia. Our novel time series analysis, using the wavelet synchrosqueezed transform, reveals pronounced oscillations at daily and sub-daily frequencies occurring in 8 out of the 12 monitored sites. These oscillations were not spatially or temporally homogenous, with different drip sites exhibiting such behaviour at different times of year in different parts of the cave. We test several hypotheses for the cause of the oscillations, including variations in pressure gradients between karst and cave due to cave breathing effects or atmospheric and earth tides, variations in hydraulic conductivity due to changes in viscosity of water with daily temperature oscillations, and solar-driven daily cycles of vegetative (phreatophytic) transpiration. We conclude that the only hypothesis consistent with the data and hydrologic theory is that daily oscillations are caused by solar-driven pumping by phreatophytic trees which are abundant at the site. The daily oscillations are not continuous and occur sporadically in short bursts (2–14 days) throughout the year due to non-linear modification of the solar signal via complex karst architecture. This is the first indirect observation leading to the hypothesis of tree water use in cave drip water. It has important implications for karst hydrology in regards to developing a new protocol to determine the relative importance of trends in drip rate, such as diurnal oscillations, and how these trends change over timescales of weeks to years. This information can also be used to infer karst architecture. This study demonstrates the importance of vegetation on recharge dynamics, information that will inform both process-based karst models and empirical estimation approaches. Our findings support a growing body of research exploring the impact of trees on speleothem paleoclimate proxies.

Highlights

Karst architecture determines the flow and storage of water from the surface to the underlying cave and is a major influence on drip discharge
In this paper we aim to increase our understanding of karst architecture by using a novel approach, the wavelet synchrosqueezed transform, to analyse drip discharge time series from 12 drip discharge sites in Glory Hole Cave, SE Australia
The drip discharge sites are spatially clustered in three groups within the cave (Fig. 1 and Table 1)

Summary

Introduction

Karst architecture determines the flow and storage of water from the surface to the underlying cave and is a major influence on drip discharge. The relationship between karst architecture and delivery of water to cave drip discharge sites has been studied to constrain uncertainty in paleoclimate studies (Bradley et al, 2010; Markowska et al, 2015), identify suitable speleothems as climate archives (McDonald and Drysdale, 2007) and, in conjunction with drip water geochemistry, determine water residence times in karst aquifers (Arbel et al, 2010; Fairchild et al, 2000; Lange et al, 2010; Sheffer et al, 2011; Tooth and Fairchild, 2003; Bradley et al, 2013). Recent research examining drip hydrology and fluctuations in drip rate have used hydrological response to characterise

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Conclusion