Quantifying the Stormwater Benefits Provided by Mature Native Trees in the Pacific Northwest

Ben Leonard

Washington State University


The role of trees in managing stormwater runoff is an emerging area of interest in urbanizing landscapes across the world. Typically, stands of large residual native trees growing around urban areas face the threat of removal as residential and commercial development pushes outward from urban centers. Since the potential for native trees to mitigate stormwater runoff has not yet been thoroughly quantified in the Pacific Northwest region, we are currently studying tree water use at two sites near Olympia, WA in an effort to explain how mature native trees manage incident rainfall. To do this, 64 trees selected from four species common to the pacific northwest were chosen for instrumentation. The species that were selected were two evergreen species: douglas fir (Pseudotsuga menziesii) and western redcedar (Thuja plicata); and two deciduous species: red alder (Alnus rubra), and bigleaf maple (Acer macrophylum). Each tree was instrumented to measure sap flux, canopy throughfall, and stem flow. We are also measuring soil moisture and micro-climate in close proximity to the selected trees. Over two years, several rain events and intermittent dry periods will be targeted during which water budgets will be constructed for each tree. Data from these events will be used to quantify how much rainfall is prevented from becoming runoff through transpirative and canopy interception processes. These tree water budgets will provide critical information on how much urban stormwater volume can essentially be diverted away from stormwater collections systems by these four species of trees, thereby reducing downstream flooding and preventing toxic runoff from entering sensitive waters. Preliminary results from this work will be presented at the conference.

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5 thoughts on “Quantifying the Stormwater Benefits Provided by Mature Native Trees in the Pacific Northwest

  1. I may have missed it but did you measure the canopy cover of each tree? That seems like it would tie into the interception well and have implications for urban trees that have dense cover compared to more spread out.


    1. We haven’t measured our canopy areas quite yet but plan on doing so. This measurement will be combined with leaf area index (LAI) for each tree. LAI will also be used to explain transpiration measurements.

      Just to give a very rough idea of what an annual measurement might look like I calculated canopy area using mean Douglas fir crown radius from a review paper. However, none of these measurements are trivial and some of the real world complexities should be noted.

      For example, a popular method for canopy measurements is from aerial imagery (which we have access to). I actually checked a few polygons against the areas calculated from the mean crown radius to make sure I was in the ballpark. Unfortunately, even though we tried to select trees with dominant canopies there is still quite a bit of overlap between canopies which makes image segmentation difficult. To properly differentiate our canopies it may be appropriate to combine aerial orthoimagery with under-story imagery or LAI/densiometer readings. There are also a number of approaches that use structure from motion techniques or LIDAR to get a 3D canopy map, but this is probably beyond the scope of the study.

      Another problem is that crown area does not directly translate to ground-level canopy cover in terms of rainfall interception. For the mature Douglas fir, in particular, the tree’s crown is often concentrated at least 50 feet from the ground. This makes throughfall patterns much less predictable. A very practical approach that we have considered is simply using a portable TDR soil moisture probe to map soil patterns underneath each canopy after a summer rainfall event. A version of this technique has already been successfully used by one of our Evergreen student collaborators.


  2. I’m curious whether you’re going to try to quantify evaporation as well. Certainly, including trees in a landscape will increase transpirative capacity. That said, the shading provided by the trees may also reduce evaporation. It would seem you would need to include the E term in your water balance. How do you plan to do that?


    1. Evaporation is a critical component of what we are measuring with transpiration and interception. However, this process is a bit muddied by the use of terminology such as vapor pressure deficit so let me explain.

      Water is transported from a tree’s roots to its leaves where it is metabolized during autotrophic respiration, however, over 95% of this water is evaporated through transpiration and not actually used by the tree (McElrone et al. 2013). A tree must balance water loss with the need to open its stomata during photosynthesis to uptake carbon dioxide. This is because a tree’s leaf is typically fully saturated while the atmosphere may not be which creates pressure gradient that can be described by water potential. At the leaf-air interface this is usually negative representing a tendency for water to evaporate from leaf’s surface when a tree’s stomata are open. Water potential at the leaf-air interface is highly dependent on atmospheric conditions and is primarily dictated by temperature and relative humidity (RH) as explained in the Kelvin equation using vapor pressure. As a result, this negative water potential is low when the atmosphere is cold and wet (-0.65 MPa at 10°C and 99.5% RH) and high when hot and dry (-320.6 MPa at 30°C and 10% humidity) (Lambers et al. 2008). When accounting for the ability of a leaf’s cell walls to resist atmospheric vapor pressure a positive measurement called vapor pressure deficit (VPD) is used. VPD typically peaks mid-day at several kPa which correlates with high amounts of water movement within the tree’s conductive tissues.

      During precipitation events, the atmosphere is fully saturated and VPD is low which means that even if the tree is able to photosynthesize water use via transpiration is negligible. This is where interception comes into play since the tree’s canopy works as an umbrella and is able to physically intercept rainfall. However, unlike an umbrella, rainfall isn’t just simply redistributed along the canopy’s edge. A good portion of rainfall that is intercepted by a tree’s canopy is subsequently evaporated without ever reaching the ground (Xiao et al. 2000). Also, unlike an umbrella, a tree’s canopy is not homogeneous and rainfall may also be directed down the main stem. By measuring stem flow and throughfall as we are doing in this study we can calculate how much water the tree is intercepted which is proxy for an evaporative process.

      This being said, I may be missing part of your question since shaded forest floors may evaporate surface water at a slower rate ambient rate than fully exposed soils. I have found this comparison being made in studies focusing on water retention in arid environments (Raz-Yaseef et al. 2010). In contrast, it seems that this discussion is missing in many studies that focus on the stormwater benefits of urban trees (Gotsch et al. 2017). Instead, forested systems may be compared to impervious surfaces such as roadways and parking lots that generate stormwater runoff.

      To address this knowledge gap I plan on: a) Comparing soil moisture values from our weather stations that are in open fields without trees to soil moisture measurements taken from within our tree plots. b) Comparing trees to types of green infrastructure that utilize in grass-soil systems for evaporation such as bioswales and rain gardens.

      I expect there to be quite a bit of variation in these values since climate and soil type play large roles in determining the volume of evaporative loss. Considering soil type makes the direct comparison more difficult since the composition of soils in forested areas often includes a thick organic layer enriched with humic compounds. This type of soil may have higher hydraulic conductivity increasing the retention capacity of forested landscapes. Of course, many of these considerations are beyond the scope of this study to measure, but will be included in the discussion.

      Gotsch SG, Draguljić D, Williams CJ. 2018. Evaluating the effectiveness of urban trees to mitigate storm water runoff via transpiration and stemflow. Urban Ecosyst. 21(1):183–195. doi:10.1007/s11252-017-0693-y.

      Lambers, H. et al. 2008. Plant physiological ecology, Springer Verlag

      McElrone, A. J., Choat, B., Gambetta, G. A. & Brodersen, C. R. 2013. Water Uptake and Transport in Vascular Plants. Nature Education Knowledge 4(5):6

      Raz-Yaseef N, Rotenberg E, Yakir D. 2010. Effects of spatial variations in soil evaporation caused by tree shading on water flux partitioning in a semi-arid pine forest. Agricultural and Forest Meteorology. 150(3):454–462. doi:10.1016/j.agrformet.2010.01.010.

      Xiao Q, McPherson EG, Ustin SL, Grismer ME. 2000. A new approach to modeling tree rainfall interception. J Geophys Res Atmos 105:29173–29188


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