Quantifying fog deposition,evapotranspiration,andwater yield in snow tussock grasslands

Abstract

In arid and semi-arid regions, water resources are under pressure due to increasing demand and uncertain climatic futures. In these water-scarce regions, it is recognised that fog may be a significant contributor to overall precipitation, and can be captured to supplement water supply. The amount of water potentially derived from fog is particularly important in summer when there is increased water demand. In New Zealand, the potential role of fog to augmenting stream flow is highly contested with debate over the significance of fog interception versus low evapotranspiration rates, particularly in native grasslands. The aim of this study is to determine if fog is a significant contributor to water yield in snow tussock grasslands, using a case study catchment in Central Otago. A variety of meteorological, hydrological and geochemical techniques were employed, including the direct capture of fog;isotopic and solute analysis of fog, rain, soil moisture, wetland and river water;calculation of site-specific potential evapotranspiration (PET)using the Penman–MonteithFAO-56 method;and stream discharge and soil moisture readings. From these observations it was determined that fog contributes 28.5% (372mmeq.) of total annual precipitation. Fog and rain have specific isotopic and chemical signatures, with fog found consistently more enriched in stable isotopes δ18O and δ2H relative to rain, and enriched in ionic composition relative to rain. On the basis of the geochemical analysis there is no evidence that fog water directly contributes to stream flow. Rather, fog augmented and sustained soil moisture rates and was an important source of water to sustain plant growth over summer. PET rates are demonstrably low over the tussock grassland, averaging1.52 m d−1and are an important element of the overall efficiency of water yields in tussock grasslands.Fog incidence also significantly correlated to lowered PET rates,as a result of the high humidity inducing a low specific vapour pressure gradient between the tussock and the atmosphere.Thus, the elevated water yields of tussock grasslands are due to the combined effects of low evapotranspiration rates, fog interception, and fog incidence depressing evapotranspiration. In water-scarce parts of Otago, retaining tussock-covered headwaters may be strategically valuable for sustaining river flow as a consequence of sustained soil moisture during summer, and in drought conditions.Consequently, integrated catchment management plans in Otago should focus on the protection of tussock grassland and educating farmers and landowners of the economic and ecological benefits associated with retaining a dense cover of tussock grasslands in catchment headwaters.