No abstract

The significance of thermal heterogeneities in small surface water bodies as drivers of mixing and for habitat provision is increasingly recognized, yet obtaining three-dimensionally-resolved observations of the thermal structure of lakes and rivers remains challenging. Remote observations of water temperature from aerial platforms are attractive: such platforms do not require shoreline access; they can be quickly and easily deployed and redeployed to facilitate repeated sampling and can rapidly move between target locations, allowing multiple measurements to be made during a single flight. However, they are also subject to well-known limitations, including payload, operability and a tradeoff between the extent and density over which measurements can be made within restricted flight times. This paper introduces a novel aerial thermal sensing platform that lowers a temperature sensor into the water to record temperature measurements throughout a shallow water column and presents results from initial field experiments comparing in situ temperature observations to those made from the UAS platform. These experiments show that with minor improvements, UASs have the potential to enable high-resolution 3D thermal mapping of a ∼1-ha lake in 2-3 flights (circa 2 h), sufficient to resolve diurnal variations. This paper identifies operational constraints and key areas for further development, including Water 2015, 7 6468 the need for the integration of a faster temperature sensor with the aerial vehicle and better control of the sensor depth, especially when near the water surface.

The seasonally-dry climate of Northern California imposes significant water stress on ecosystems and water resources during the dry summer months. Frequently during summer, the only water inputs occur as non-rainfall water, in the form of fog and dew. However, due to spatially heterogeneous fog interaction within a watershed, estimating fog water fluxes to understand watershed-scale hydrologic effects remains challenging. In this study, we characterized the role of coastal fog, a dominant feature of Northern Californian coastal ecosystems, in a San Francisco Peninsula watershed. To monitor fog occurrence, intensity, and spatial extent, we focused on the mechanisms through which fog can affect the water balance: throughfall following canopy interception of fog, soil moisture, streamflow, and meteorological variables.A stratified sampling design was used to capture the watershed's spatial heterogeneities in relation to fog events. We developed a novel spatial averaging scheme to upscale local observations of throughfall inputs and evapotranspiration suppression and make watershed-scale estimates of fog water fluxes. Inputs from fog water throughfall (10-30 mm/year) and fog suppression of evapotranspiration (125 mm/year) reduced dry-season water deficits by 25% at watershed scales.Evapotranspiration suppression was much more important for this reduction in water deficit than were direct inputs of fog water. The new upscaling scheme was analyzed to explore the sensitivity of its results to the methodology (data type and interpolation method) employed. This evaluation suggests that our combination of sensors and remote sensing allows an improved incorporation of spatially-averaged fog fluxes into the water balance than traditional interpolation approaches.

Intermittent fog occurrences supply significant amounts of moisture to plants in the form of fog drip onto the soil surface thereby prompting interest in their statistical behavior at multiple timescales. A comparison of rainfall and fog measurements collected at an inland tropical cloud forest in Kenya and a coastal rangeland in Northern California is presented to explore whether fog occurrences have similar intermittency characteristics as rainfall. The results confirm that both rainfall and fog show approximate power law relations for distributions of dry period and event size consistent with predictions from self-organized criticality. Moreover, the spectral exponents of the on-off time series of the fog and rainfall exhibited an approximate f −0.8 over a broad range of frequencies f , which is remarkably close to scaling exponents across sites experiencing different rainfall generation mechanisms. These results suggest that fog intermittency shares some properties of critical behavior documented in numerous rainfall studies.Plain Language Summary Beyond rainfall, intermittent fog occurrence provides water subsidy that is needed for sustaining transpiration and photosynthesis in forests. To study the connections between fog formation and rainfall, a comparison of rainfall and fog occurrence statistics is carried out at two sites: a Kenyan inland cloud forest and a coastal forest in Northern California. For durations exceeding 1 day, rainfall and fog event sizes and dry periods appear similar. Due to these similarities, intermittency in fog occurrences may abide by general laws describing critical phenomenon.