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Quantifying Fog Contributions to Water Balance in a Coastal California Watershed

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Quantifying Fog Contributions to Water Balance in a Coastal California Watershed Received: 27 February 2017 Accepted: 11 August 2017 DOI: 10.1002/hyp.11312 RESEARCH ARTICLE How much does dry-season fog matter? Quantifying fog contributions to water balance in a coastal California watershed Michaella Chung1 Alexis Dufour2 Rebecca Pluche2 Sally Thompson1 1Department of Civil and Environmental Engineering, University of California, Berkeley, Abstract Davis Hall, Berkeley,CA 94720, USA The seasonally-dry climate of Northern California imposes significant water stress on ecosys- 2San Francisco Public Utilities Commission, 525 Golden Gate Avenue, San Francisco, CA tems and water resources during the dry summer months. Frequently during summer, the only 94102, USA water inputs occur as non-rainfall water, in the form of fog and dew. However, due to spa- Correspondence tially heterogeneous fog interaction within a watershed, estimating fog water fluxes to under- Michaella Chung, Department of Civil and stand watershed-scale hydrologic effects remains challenging. In this study, we characterized Environmental Engineering, University of California, Berkeley,Davis Hall, Berkeley,CA the role of coastal fog, a dominant feature of Northern Californian coastal ecosystems, in a 94720, USA. San Francisco Peninsula watershed. To monitor fog occurrence, intensity, and spatial extent, Email: [email protected] 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 rela- tion 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. KEYWORDS ecohydrology, fog, isotope analysis, water balance 1 INTRODUCTION climatically, through reductions in radiation, temperature, and vapour pressure deficit, which collectively suppress evapotranspiration dur- Advection fog is a widespread phenomenon associated with deep ing fog events (Fischer, Still, & Williams, 2009); through direct water marine upwelling along western continental margins worldwide inputs, when fog droplets are intercepted by vegetation canopies (Garreaud, Barichivich, Christie, & Maldonado, 2008; Leipper, 1995). and form a non-rainfall water flux that reaches the soil as canopy It consists of liquid-phase water with droplet sizes ranging from throughfall (Dawson, 1998; Hutley, Doley, Yates, & Boonsaner, 1997); 1–40 m (Prada & da Silva, 2001), small enough for droplets to be or indirectly by alleviating water stress in plant leaves when absorbed advected with wind. Fog events are thus influenced by time-varying through foliar uptake (Burgess and Dawson, 2004, Wang, Kaseke, & meteorological variables such as wind speed, direction, and dew Seely, 2017) and reducing transpirational loss (Ewing et al., 2009; point (Azevedo & Morgan, 1974; Hiatt, Fernandez, & Potter, 2012; Templer et al., 2015). Fog water inputs and transpiration suppression Weathers, Lovett, & Likens, 1995; Weathers, Lovett, Likens, & Caraco, can also enhance streamflow fluxes: isotopic tracing has unambigu- 2000), which affect the size, density, and flux of these water droplets. ously identified the signature of fog water within soil moisture, Fog interaction with the terrestrial water balance occurs in three ways: groundwater, and streamflow (Ingraham and Matthews, Ingraham Hydrological Processes. 2017;1–14. wileyonlinelibrary.com/journal/hyp Copyright © 2017 John Wiley & Sons, Ltd. 1 2 CHUNG ET AL. and Matthews, 1988; 1995; Scholl, Gingerich, & Tribble, 2002). Fur- 2 MATERIALS AND METHODS thermore, dry-season streamflow fluctuations correlate with fog occurrence (Sawaske & Freyberg, 2015) and respond to interception 2.1 Site description mechanisms through changes in vegetation cover (Harr, 1980; 1982; The Upper Pilarcitos Creek Watershed is a 9.6-km2 watershed with a Ingwersen, 1985; Keppeler, 2007). Because fog zones coincide with 630 m rise in elevation, located on the San Francisco Peninsula within large arid, semi-arid, and Mediterranean climate zones, and because the Santa Cruz Mountain range and draining into Lake Pilarcitos. It is the fog season may also coincide with dry seasons in these regions one of the oldest components of San Francisco's water supply system (Fischer et al., 2009; Peace, 1969; Schemenauer & Cereceda, 1991), and is managed within the larger Peninsula Watershed by the San Fran- fog can have a significant contribution to the water balance and cisco Public Utilities Commission. The coastal, western divide of the may increase ecosystem drought resilience (Breazeale, McGeorge, & watershed is covered by dense, old-growth Douglas fir forest (Pseudot- Breazeale, 1950; Burgess and Dawson, 2004; Dawson, 1998; Haines, suga menziesii) and is characterized by highly uncompacted soil. Most 1952; 1953). of the eastern watershed is covered by chaparral rangeland; its dom- Fog advects horizontally and close to the ground, and thus, inant vegetation is coyote brush (Baccharis pilularis). The watershed terrestrial fluxes of fog water vary with topography and vegetation dis- averages approximately 100 cm of rain per year. It is also subject to tribution along its advective pathway. Typically, fog inputs peak along frequent coastal fog events in summer,which, depending on wind direc- topographic ridges and forest edges (Azevedo & Morgan, 1974; Ewing tion, can either approach the watershed from the west, rising over a et al., 2009; Fischer et al., 2009; Vogl, 1973; Weathers et al., 1995) coastal ridge, or from the north or south, flowing through the river val- and vary with vegetation canopy roughness (Dawson, 1996; ley.During these events, the canopy of understory vegetation becomes Shuttleworth, 1977; Wang et al., 2006). Heterogeneity in vegetation thoroughly wet and pools of water collect beneath trees, although cover and a topographically complex terrain are therefore likely to neighbouring grasslands remain dry (Potter, 2016); up to 150 cm of generate high spatial heterogeneity in fog water fluxes. This poses a throughfall were observed over a single month under a forest canopy challenge in making spatially integrated estimates of fog effects on a within a neighbouring watershed (Oberlander,1956). system water balance. Most methods to estimate rates of fog through- fall rely on point-scale sampling, which has a limited ability to capture important spatial heterogeneities due to the finite density and dis- 2.2 Field methods tribution of measurement locations. Moreover, the spatial patterns adopted by fog are complex, but not random. Hence, traditional meth- We deployed a suite of sensors at multiple sites in the Peninsula ods that assume negligible correlation between point observations Watershed during three summer fog seasons coinciding with the dry beyond a certain distance (e.g., Thiessen polygon or kriging (Bacchi season, spanning June–October in 2014, 2015, and 2016. The Penin- and Kottegoda, 1995)) are not suitable for interpolation between sula Watershed is a single geologic and topographic unit in the Coast sampling points. Development of spatially-integrated estimation tech- Range and experiences no geologic, vegetation, or elevation change niques that can address the complex but non-random nature of fog, as across subwatershed boundaries created by a reservoir dam. Observa- well as its interaction with a heterogeneous land surface, is needed to tion sites were located in the Upper Pilarcitos Creek Watershed and allow quantitative evaluation of the importance of fog water fluxes at its neighbouring watersheds, which share common directional gradi- watershed scales. ents in fog presence. Sites were selected to represent locations with Therefore, this study seeks to answer the following questions: different elevation, vegetation, wind conditions, and ocean exposure, along with variations in fog frequency, density, and approach path- 1. What are the dominant controls that characterize heterogeneity of ways to the watershed, as reported by on-site watershed managers. fog in a seasonally-dry,coastal watershed? Ultimately, six sites were selected covering areas of peak fog inten- 2. In what ways does fog contribute to the water balance of this sys- sity (Scarper Peak and Montara Mountain, associated with a frequently tem and how significant are its quantitative effects? occurring west-east fog pathway) and areas of less frequent fog occur- 3. How important is each sensor and its dataset in inferring fog's rence (Spring Valley, associated with some west-east and north-south watershed-scale effects? fog pathways, and Cahill Ridge, associated with a south-north fog path- In this study, we characterize and quantify spatiotemporal hetero- way) according to operator information. Site
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