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

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.

Rainfall recharge thresholds in a subtropical climate determined using a regional cave drip water monitoring network

“…Despite these limitations, quantification of the spatial and temporal heterogeneity of recharge in karst would be possible using a larger network of drip loggers. Loggers deployed in vertical transects can be used to identify the depth of the zero flux plane and its variability, potentially including the identification of tree water use through diurnal drip rate variations (Coleborn et al 2016). Analysis of recharge at paired sites with different vegetation types can be used to better quantify the transpiration control on recharge thresholds.…”

A Comparison of Time‐Frequency Signal Processing Methods for Identifying Non‐Perennial Streamflow Events From Streambed Surface Temperature Time Series

“…Coleborn et al. (2016) characterized water drip rates at 12 locations in Glory Hole Cave, a karst cave located in Yarrangobilly Caves National Park in southeastern Australia. Correlations between transformed drip rate data and SCWT‐transformed barometric pressure, air temperature, and evapotranspiration data recorded at the ground surface were used to identify causal mechanisms.…”

“…Use of the SCWT in the Earth sciences has been limited to date but includes applications in hydrology (Coleborn et al, 2016), paleoclimatology (Thakur et al, 2013), seismology (Herrera et al, 2014), and oceanography (Bian et al, 2018). Coleborn et al (2016) characterized water drip rates at 12 locations in Glory Hole Cave, a karst cave located in Yarrangobilly Caves National Park in southeastern Australia. Correlations between transformed drip rate data and SCWT-transformed barometric pressure, air temperature, and evapotranspiration data recorded at the ground surface were used to identify causal mechanisms.…”

“…The theoretical development of the synchrosqueezed CWT (SCWT) was described by Daubechies and Maes (1996) and Daubechies et al (2011). Use of the SCWT in the Earth sciences has been limited to date but includes applications in hydrology (Coleborn et al, 2016), paleoclimatology (Thakur et al, 2013), seismology (Herrera et al, 2014), and oceanography (Bian et al, 2018). Coleborn et al (2016) characterized water drip rates at 12 locations in Glory Hole Cave, a karst cave located in Yarrangobilly Caves National Park in southeastern Australia.…”

Historical droughts in Southeast Australia recorded in a New South Wales stalagmite

“…Despite numerous dripwater studies at Yarrangobilly (Coleborn et al, 2016; Markowska et al, 2015; Tadros et al, 2016), Wombeyan (McDonald and Drysdale, 2007; McDonald et al, 2004, 2007) and Wellington (Cuthbert et al, 2014a, 2014b; Jex et al, 2012; Markowska et al, 2016; Rutlidge et al, 2014) caves, so far few long-term speleothem results have been produced (Goede et al, 1996; McGowan et al, 2018; Webb et al, 2014). This is in part because speleothems in the area have proven difficult to date precisely (McDonald et al, 2013).…”