Lecture 12 Precipitation Interception 1 Interception Processes General

Lecture 12 Precipitation Interception (1) Interception Processes • General Comments • Controls on Interception • Interception in Woodlands • Interception in Grasslands • Interception by Crops • Measurement of Relative Humidity

Interception Water abstracted from gross precipitation by leaves and stems of a vegetation canopy and temporarily stored in its surfaces. Interception Loss Intercepted water lost by evaporation to the atmosphere before reaching the soil surface

General Comments • Accounts for much of the variability in evaporation and transpiration between plant species or associations • Precipitation is usually intercepted by: Tree canopy Grass Shrubs Litter Moss Built structures • Interception capacity is usually considered to be a fixed amount for a given site: Canopy Shrubs Urban • During filling and once storage is full, water passes through the canopy and reaches the soil as: Throughfall (TF) Stemflow (SF)

Net Rainfall 1. 2. 3.

Terms to Remember 1. Interception loss: Part of the rainfall intercepted by a plant canopy is evaporated back into the atmosphere and takes no part in the land-bound portion of the hydrological cycle 2. Throughfall: raindrops and snowflakes that fall through gaps in the plant canopy and water which drips from leaves, twigs and stems 3. Stemflow: Water run down the main stem or trunk from twigs and branches to the ground 4. Gross rainfall: rainfall on top of plant canopies 5. Net rainfall: The sum of throughfall and stemflow 6. Negative interception: Water intercepted from fogs and mists that contributes to stemflow

Controls on the amount of interception 1. Vegetation form/structure Shape Branch/leaf orientation Broad vs. needle leaves Number of leaves/stems Surface texture Flexibility/turgidity/stability 2. Vegetation growth pattern/physiology Seasonal growth Deciduous habit Total biomass Form/structure Age Growth rate Density of stand Leaf Area Index (LAI) 3. Meteorological Conditions Precipitation intensity and duration --Heavy and long duration precipitation will quickly exceed crown capacity leading to greater TF and SF --Conifers intercept more because they coincide with gentler rain or snow --Often possible to relate/predict losses from total P Phase of precipitation Snow/sleet/rain/hail Wind speed and turbulence Energy balance Albedo related to vegetation type

Additional Points to Note • These botanical and meteorological factors generally apply to nonbotanical surfaces as well (e. g. , urban surfaces) • Strong dependence on meteorological factors allows interception, TF, or SF to be estimated from empirical relations • Originally believed that interception losses were balanced by reduced transpiration losses. This is now believed to be incorrect • Interception is not an alternate loss, rather an additional one

Interception loss during precipitation event • Interception losses are greatest early during a precipitation event • Losses decrease when interception storage is filled • Interception ratio: (Interception loss) / (total precipitation)

Interception Loss from Woodlands • Generally: deciduous crown closure > conifer crown closure However, conifer stands tend to exhibit higher interception losses because of higher leaf area density Conifer interception losses: ~25 -35% Decid. Interception losses: ~10 -30% • Potential reasoning: needle shapes and distributions relative to broadleaves • Spatially variable: density of trees (spacing)

Interception losses from grasses/shrubs • LAI of mature, homogeneous grass cover is generally much smaller than that of forests • Higher aerodynamic resistance than tall vegetation; thus, less interception loss • Grazed or cut grasslands exhibit greatly reduced storage • Interception losses vary ~13 -26%

Interception losses from agricultural crops • Usually evenly spaced plants • Highly dependent on stage of development • Depending on LAI

Interception of snow • Difficult to measure and highly variable spatially due to wind redistribution • Idea: snow accumulation on canopy decreases aerodynamic resistance (smooth) • Thus, evaporation rates should be lower than for wet canopy • Snow often melts, slides, slips, or is blown off of vegetation • Studies indicate only ~15% of intercepted snow sublimates or evaporates • Snow-stored water can be much greater than water storage – potential for more evaporation is there but energy requirements are not always met

Fog and clouds • Deposition of fine water droplets to vegetated surfaces (e. g. , mist, fog, clouds) • Too fine to precipitate and would not be collected by rain gauges • “Negative interception” Kittredge (1948) • More common in mountainous regions and coastal areas • Can be a significant addition of moisture to local vegetation • Different process than dew, which is temperature controlled condensation of water vapor

Instrument for measuring air humidity http: //www. mtc. com. my/publication/library/drying/fig 5. gif

Relative humidity: • Ratio of the actual amount of moisture in the atmosphere to the amount of moisture the atmosphere can hold • Therefore, a relative humidity of 100% means the air can hold no more water (rain or dew is likely) • Relative humidity of 0% indicates there is no moisture in the atmosphere. eswb esdb ed Elv P Twb Tdb = = = = Saturation vapor pressure at Twb (k. Pa) Saturation vapor pressure at Tdb (k. Pa) Vapor pressure (k. Pa) Elevation above sea level (m) Air pressure (k. Pa) Wet bulb temperature (°C) Dry bulb temperature (°C)

Procedure for Calculating Relative Humidity . 1 Approximate the air pressure, P in k. Pa (kilo. Pascals). If you don't know your elevation, use P = 101. 325 k. Pa. P = 101. 325 exp(-0. 0001184 Elv) 2. Calculate a conversion factor, A. A = 0. 00066(1. 0 + 0. 00115 Twb) 3. Calculate the saturated vapor pressure at Twb. eswb = exp[(16. 78 Twb – 116. 9) / (Twb + 237. 3)] 4. Calculate the vapor pressure, or the partial pressure of water vapor, ed in k. Pa. ed = eswb – AP(Tdb – Twb) 5. Calculate the saturated vapor pressure at Tdb. esdb = exp [(16. 78 Tdb – 116. 9) / (Tdb + 237. 3)] 6. Finally, calculate the relative humidity, RH, in percent. RH = 100 (ed / esdb)