A spatially distributed model for the assessment of land use impacts on stream temperature in small urban watersheds

Self Cite

We apply an integrated hydrology‐stream temperature modeling system, DHSVM‐RBM, to examine the response of the temperature of the major streams draining to Puget Sound to land cover and climate change. We first show that the model construct is able to reconstruct observed historic streamflow and stream temperature variations at a range of time scales. We then explore the relative effect of projected future climate and land cover change, including riparian vegetation, on streamflow and stream temperature. Streamflow in summer is likely to decrease as the climate warms especially in snowmelt‐dominated and transient river basins despite increased streamflow in their lower reaches associated with urbanization. Changes in streamflow also result from changes in land cover, and changes in stream shading result from changes in riparian vegetation, both of which influence stream temperature. However, we find that the effect of riparian vegetation changes on stream temperature is much greater than land cover change over the entire basin especially during summer low flow periods. Furthermore, while future projected precipitation change will have relatively modest effects on stream temperature, projected future air temperature increases will result in substantial increases in stream temperature especially in summer. These summer stream temperature increases will be associated both with increasing air temperature, and projected decreases in low flows. We find that restoration of riparian vegetation could mitigate much of the projected summer stream temperature increases. We also explore the contribution of riverine thermal loadings to the heat balance of Puget Sound, and find that the riverine contribution is greatest in winter, when streams account for up to 1/8 of total thermal inputs (averaged from December through February), with larger effects in some sub‐basins. We project that the riverine impact on thermal inputs to Puget Sound will become greater with both urbanization and climate change in winter but become smaller in summer due to climate change. Copyright © 2016 John Wiley & Sons, Ltd.

Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 94%

Climate and land cover effects on the temperature of Puget Sound streams

Cao

Sun

Yearsley

et al. 2016

Self Cite

“…The algorithm is based on that of Link and Marks (), with the exception that the model now allows the extinction coefficient ( k ) to vary monthly.

τ_{b} = e^{- λ \cdot sans-serif-italicLAI}

τ_{b} = e^{- sans-serif-italick \cdot sans-serif-italicL} = e^{- sans-serif-italick \cdot \frac{h}{sin α}}

where τ b is the fraction of shortwave radiation transmitted through the canopy, L is light attenuation length through the canopy calculated as a function of canopy height ( h ) and solar elevation ( α ), and k is the LAI ‐dependent radiation extinction coefficient, which varies monthly (Sun, Yearsley, Voisin, & Lettenmaier, ).…”

Section: Methodsmentioning

confidence: 99%

Evaluating the functionality and streamflow impacts of explicitly modelling forest–snow interactions and canopy gaps in a distributed hydrologic model

Sun

Wigmosta

Zhou

et al. 2018

Self Cite

Many plot‐scale studies have shown that snow‐cover dynamics in forest gaps are distinctly different from those in open and continuously forested areas, and forest gaps have the potential to alter the magnitude and timing of snowmelt. However, the watershed‐level impacts of canopy gap treatment on streamflows are largely unknown. Here, we present the first research that explicitly assesses the impact of canopy gaps on seasonal streamflows and particularly late‐season low flows at the watershed scale. To explicitly model forest–snow interactions in canopy gaps, we made major enhancements to a widely used distributed hydrologic model, distributed hydrology soil vegetation model, with a canopy gap component that represents physical processes of snowpack evolution in the forest gap separately from the surrounding forest on the subgrid scale (within a grid typically 10–150 m). The model predicted snow water equivalent using the enhanced distributed hydrology soil vegetation model showed good agreement (R2 > 0.9) with subhourly snow water equivalent measurements collected from open, forested, and canopy gap sites in Idaho, USA. Compared with the original model that does not account for interactions between gaps and surrounding forest, the enhanced model predicted notably later melt in small‐ to medium‐size canopy gaps (the ratio of gap radius (r) to canopy height (h) ≤ 1.2), and snow melt rates exhibited great sensitivity to changing gap size in medium‐size gaps (0.5 ≤ r/h ≤ 1.2). We demonstrated the watershed‐scale implications of canopy gaps on streamflow in the snow‐dominated Chiwawa watershed, WA, USA. With 24% of the watershed drainage area (about 446 km2) converted to gaps of 60 m diameter, the mean annual 7‐day low flow was increased by 19.4% (i.e., 0.37 m3/s), and the mean monthly 7‐day low flows were increased by 13.5% (i.e., 0.26 m3/s) to 40% (i.e., 1.76 m3/s) from late summer through fall. Lastly, in practical implementation of canopy gaps with the same total gap areas, a greater number of distributed small gaps can have greater potential for longer snow retention than a smaller number of large gaps.

“…Numerous studies have documented the impacts of urban run‐off on stream water quality. Regional case studies highlight increased loading to receiving streams of pollutants such as nutrients (Newcomer Johnson, Kaushal, Mayer, Smith, & Sivirichi, ), suspended solids (MacAvoy, Plank, Mucha, & Williamson, ), oil and grease (Sood, Sood, Bansal, & John, ), faecal coliform bacteria (Paule‐Mercado et al, ), trace metals (Ruchter & Sures, ), thermal energy (Sun, Yearsley, Voisin, & Lettenmaier, ), and salts (Corsi, De Cicco, Lutz, & Hirsch, ). These studies indicate elevated but highly variable concentrations of solutes in streams with several constituents surpassing public health standards, stressing aquatic biota, and potentially impairing any beneficial use of water resources in these landscapes.…”

Section: Introductionmentioning

confidence: 99%

Determination of subcatchment and watershed boundaries in a complex and highly urbanized landscape

Kayembe

Mitchell

2018

Urban development significantly alters the landscape by introducing widespread impervious surfaces, which quickly convey surface run‐off to streams via stormwater sewer networks, resulting in “flashy” hydrological responses. Here, we present the inadequacies of using raster‐based digital elevation models and flow‐direction algorithms to delineate large and highly urbanized watersheds and propose an alternative approach that accounts for the influence of anthropogenically modified land cover. We use a semi‐automated approach that incorporates conventional drainage networks into overland flow paths and define the maximal run‐off contributing area. In this approach, stormwater pipes are clustered according to their slope attributes, which define flow direction. Land areas drained by each cluster and contributing (or exporting) flow to a topographically delineated catchment were determined. These land masses were subsequently added or removed from the catchment, modifying both the shape and the size. Our results in a highly urbanized Toronto, Canada, area watershed indicate a moderate net increase in the directly connected watershed area by 3% relative to a topographically forced method; however, differences across three smaller scale subcatchments are greater. Compared to topographic delineation, the directly connected watershed areas of both the upper and middle subcatchments decrease by 5% and 8%, respectively, whereas the lower subcatchment area increases by 15%. This is directly related to subsurface storm sewer pipes that cross topographic boundaries. When directly connected subcatchment area is plotted against total streamflow and flashiness indices using this method, the coefficients of variation are greater (0.93 to 0.97) compared to the use of digital elevation model‐derived subcatchment areas (0.78 to 0.85). The accurate identification of watershed and subcatchment boundaries should incorporate ancillary data such as stormwater sewer networks and retention basin drainage areas to reduce water budget errors in urban systems.