SOLUTE AND STABLE ISOTOPIC RATIOS OF FOG, RAIN, SURFACE AND
SOIL WATERS AT TIMBER CREEK, CENTRAL OTAGO.
Alice Trevelyan,1Sarah Mager,1 Robert Van Hale,2 Peter Wilson,3
1
Department of Geography, University of Otago, New Zealand.
2
Department of Chemistry, University of Otago, New Zealand
3
Otago Fish and Game Council, Dunedin, New Zealand
Aims
In arid and semi-arid regions, water resources are under pressure due to increasing demand for
water and uncertain climatic futures. Most conventional water balances used in water resource
allocation and planning purely rely on rain data when determining the sources of water input into
a catchment, typically omitting precipitation inputs that are not captured in a conventional rain
gauge, such as snow and fog. The amount of water potentially derived from fog is particularly
important in summer when there is increased demand for surface waters for irrigation, recreation
and domestic use, however defining fog and rain remains problematic. Internationally, there has
been growing recognition of the potential role that fog may play in augmenting precipitation input,
with research suggesting that in most fog prone catchments the volume of fog deposition can be
twice that of rain. Additional input of precipitation as fog contributes to higher soil moisture
content and potentially increased water yield. The emphasis on the potential role fog can
contribute to stream flow is highly contested in New Zealand with debate over the significance of
fog interception, particularly in native grasslands. The aim of this study is to quantify the
movement of fog and rain thought the hydrological cycle and to determine if fog is a significant
contributor to water yield in the Timber Creek catchment, Central Otago.
Method
Stable isotopes are a technique commonly used to distinguish between fog, rain and components
of the hydrological cycle. To date only one study using stable isotopes to trace fog and rain inputs
has been conducted in New Zealand, and the assertion that fog contributes to ‘sub equal’
portions of groundwater is widely challenged by other fog studies in New Zealand using more
traditional techniques. Another tracer technique that has been used to distinguish between fog
and rain inputs is the analysis of ion composition, however, this has not yet been used in New
Zealand. The approach of this study is to combine water balance, direct fog collection, and
geochemical tracers to quantify the potential significance of fog input into a small catchment,
Timber Creek, in Central Otago. Rain water was collected in two standard rain gauges, whilst
direct capture of fog was achieved through three passive harp-shaped collectors situated over
collecting rain gauges. A layer of mineral oil was added to each collecting rain gauge to prevent
evaporation. Soil water samples were collected through 6 suction cup lysimeters. Additionally,
surface water was collected from an adjacent wetland and Timber Creek. Samples were analysed
for 18O and D using the Piccaro CRDS analyser with an analytical precision of 18O 0.09 ‰
and D 0.9 ‰. Major anion and cation analysis was undertaken on an Ion Chromatograph (IC)
and Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) at the University of
Otago with an analytical precision of 0.1 mg/L for each ion.
Results
Between 5th July 2014 and 1st April 2014 there were 62 days where fog deposition occurred
contributing 236 mm of precipitation to the catchment, representing an undercatch of incipient
precipitation in traditional rain gauges equivalent to 17% of measured rainfall. Thus, in the Timber
Creek catchment, fog is a significant source of water. Analysis of 18O and 2H shows a
difference in the isotopic ratios of fog and rain in the Timber Creek catchment, with rain
consistently more depleted than fog. When considering isotopic ratios across a year, there is no
difference in the range of values due to seasonal fluctuations and strong differences in 18O ratios
between events, however, investigation into seasonal and depositional events demonstrates a
difference in isotopic ratios does exist between fog and rain. River and wetland isotopic ratios
remain stable year round, despite fluctuations in precpitation of up to 18O 7.5‰. River water is
more depleted in stable isotopes relative to median precipation, suggesting that runoff is
generated from precipitation falling at higher elevations within the catchment and it is not possible
to determine from the isotopic data whether fog has any significant contribution to river flow.
Preliminary analysis also suggests that there is a distinct solute character between fog and
rainwater, with fog yielding higher concentrations of chloride and base cations compared to that
of rain. An examination of the isotope and solute data shows strong linkages between incipient
precipitation from fog and shallow soil water, suggesting that fogwater is important for sustaining
plant growth, but that water does not move through the hydrological system into the river. Thus,
fog is likely a small, but important contributor to incipient precipitation in Central Otago and
provides key moisture for sustaining plant growth and ecosystem services, but is not a direct
contributor to stream flow. Rather, fog offsets soil and plant water uptakes allowing a higher
portion of rainfall to contribute to river flows, and is one element that explains the higher water
yields observed from tussock grasslands.