Chapter 6 Hydrologic Processes and Watershed Response Rita D. Winkler, R.D. (Dan) Moore, Todd E. Redding, David L. Spittlehouse, Darryl E. Carlyle-Moses, and Brian D. Smerdon INTRODUCTION Streamflow reflects the complex interactions be- and watershed scales. Understanding how stream- tween the weather and the biophysical environ- flow is generated is vital in evaluating the effects of ment as water flows through the hydrologic cycle. It forest disturbance on hydrologic response and in represents the balance of water that remains after all identifying best management practices in a water- losses back to the atmosphere and storage opportu- shed. Chapter 7 (“The Effects of Forest Disturbance nities within a watershed have been satisfied. This on Hydrologic Processes and Watershed Response”) chapter describes the hydrologic processes that affect describes how forest disturbances, such as insects, the generation of streamflow in British Columbia’s fire, logging, or silviculture, alter both stand-scale watersheds. These processes include precipitation, processes and watershed response as streamflow. interception, evaporation, infiltration, soil mois- The discussion in the current chapter is organized ture storage and hillslope flow, overland flow, and by surface and subsurface processes, beginning with groundwater. The processes and their spatial and precipitation and ending with streamflow. temporal variability are described at both the stand SURFACE PROCESSES Forest vegetation directly affects the amount of water evaporation. Water also returns to the atmosphere available for streamflow through the interception of through transpiration and by evaporation from the rain and snow, the evaporation of intercepted water, forest floor, soil surface, and open water bodies, such and through transpiration. Altering forest vegetation as lakes and wetlands (Figure 6.1). Forest vegetation can have an important influence on water balances at reduces the rate at which snow melts and hence the the site and watershed scale. In the hydrologic cycle rate of soil moisture recharge and downslope flow (Figure 6.1), the forest canopy is the first surface that (Spittlehouse and Winkler 2002). intercepts and stores precipitation, resulting in some A water balance equation is often used to express amount (depending on canopy characteristics) re- the contributions of these processes to streamflow. turning to the atmosphere through sublimation and At the watershed scale, the water balance equation 133 Precipitation Precipitation Interception loss Evapotranspiration Transpiration Evaporation Depression Storage Overland Flow Infiltration Subsurface Return Flow Flow Evaporation Stream Through- Stem- Flow fall flow Groundwater Recharge Root Baseflow Infiltration Lateral Zone { Flow Drainage Groundwater Flow FIGURE 6.1 The hillslope hydrologic cycle and stand water balance. links streamfl ow R( ), precipitation (P), evaporation ∆W = F – Et – Es – D (2) (E), losses to regional groundwater (G), and changes in watershed storage (∆S): where ∆W is the change in soil water storage (mil- limetres) over time. During winter in areas where R = P – E – G ± ∆S (1) precipitation falls as snow, there is a delay in infi l- where the variables are measured in water depth tration until snowmelt: Es is zero when snow covers equivalent (millimetres) over time. Th e quantity G the ground, and water content change depends only represents water that leaves the watershed as ground- on drainage from the root zone except where tree water and thus does not contribute to streamfl ow. transpiration occurs during snowmelt (Spittlehouse Watershed storage includes water stored as snow, 2002). Th ese processes are discussed in detail in the surface water (lakes, ponds, depression storage), following sections. Equation 2 does not account for glacier ice, soil moisture, and groundwater. As water lateral downslope fl ow of soil water, and therefore fl ows through a watershed, it oft en moves between may not be applicable on hillslopes. these storage compartments before discharging to a stream or being lost through evaporation and Precipitation transpiration. For example, meltwater draining from a snowpack and infi ltrating the soil represents a loss Precipitation is defi ned as liquid (rain) or frozen of snowpack storage and a gain of soil moisture, but (sleet, hail, graupel, snow) water, or a combination with no net eff ect on total watershed storage. of both, falling from the sky. Water is also depos- At smaller spatial scales, the site or stand wa- ited at the land surface through condensation (e.g., ter balance equation links soil water storage (W), dew, fog, rime, and hoar frost) (UNESCO and World precipitation (P), interception (I), overland fl ow O( ), Meteorological Association, International Hydrology infi ltration F( = P – I – O), plant transpiration (Et), Program 1998). British Columbia’s precipitation re- evaporation from the soil surface (Es), and drainage gimes are a result of weather systems that are typical from the root zone (D): of mid-latitudes on the east side of the Pacifi c Ocean 134 and that interact with the mountain ranges and val- example, these events can have a significant effect leys paralleling the coast. Alternating sequences of on vegetation (e.g., dew facilitates the growth and eastward-flowing high- and low-pressure systems, spread of disease); however, the amount of water incursions of arctic air, and flows from the midwest- deposited during these events makes a negligible ern United States interact with topography to pro- contribution to the annual water balance. duce precipitation regimes including wet coastal, dry Ice storms are more damaging than rime. These interior, moist northern, and snow-dominated high storms occur when liquid precipitation freezes on elevations (Hare and Thomas 1974; Phillips 1990; also contact with a surface and forms a glaze (layer of see Chapter 3, “Weather and Climate”). ice) (Glickman [editor] 2000). The glaze has a much Precipitation and other climatic elements vary higher density than rime and hoar frost, and its seasonally because of the combined effects of solar weight can break branches or whole trees and cause radiation and weather patterns. For example, most widespread damage (Irland 1998, 2000). coastal locations are dominated by a wet-winter/dry- Fog drip is another form of condensation precipi- summer pattern. In the lee of the Coast Mountains tation. It occurs when wind moves clouds at ground and throughout much of the interior, seasonal differ- level (fog) so that water droplets collide with and ad- ences in precipitation are smaller than on the coast here to foliage, branches, stems, and other surfaces. (see Chapter 3, “Weather and Climate”). Variations Subsequently, individual droplets coalesce and drip in sea surface temperatures in the Pacific Ocean from the tree canopy or flow down the branches and (e.g., El Niño/La Niña and the Pacific Decadal Oscil- stems to the ground. Forests on windward slopes and lation [PDO]) also influence precipitation on annual ridge tops are most prone to fog drip because of the and decadal cycles, producing large between-year large canopy area that is exposed to the cloud. On variations superimposed on long-term trends flat terrain and leeward slopes, most of the fog drip (Rodenhuis et al. 2007). occurs at the edges of stands (clearcut edges, lake In general, rainfall volume and intensity are the edges, etc.). Fog drip can be a significant component highest during the winter in coastal British Colum- of the annual water balance in some ecosystems, pre- bia. Storms that produce over 100 mm of rain in a dominantly in coastal areas (Harr 1982; Schemenau- 24-hour period are common, as are intensities that er 1986; Ingraham and Matthews 1995). For example, reach 30 mm/h. Coastal winter rainstorms can last Harr (1982) reported that fog drip in a windward- for many days with long periods of light drizzle facing watershed in coastal Oregon resulted in an intermixed with periods of high rainfall intensi- approximate 30% increase in water reaching the soil ties. Summer storms on the coast are usually much surface, which makes an important contribution to shorter in duration and lower in total precipitation summer flows. Studies in coastal British Columbia than winter storms. In contrast, rainstorms in the (Beaudry and Sagar 1995; Spittlehouse 1998a, 1998b) province’s interior and north are usually of shorter show that fog drip is not a significant component duration (less than 1 day) and occur in late spring of the water balance. Fog drip generally results in a through early fall when temperatures are above 0°C. net gain in water to the land base. Fog drip can have Storms with higher rainfall volumes and intensities negative impacts where it is combined with acidic are often convective, with durations of a few hours. deposition (Schemenauer 1986; Schemenauer et al. In the province’s interior and north, infrequent 1995). This has not been reported as a problem in the warm frontal storms in the late winter can also de- province, although localized effects may occur near liver large amounts of rain. Further information on cities and certain industrial facilities. precipitation and intensity-duration-frequency data Snow forms in saturated air at subzero tempera- is provided in Chapter 17 (“Watershed Measurement tures when a water droplet containing a conden- Methods and Data Limitations”). sation nucleus (e.g., dust, or inorganic or organic Dew forms on surfaces above 0°C, whereas hoar matter) drops below some particle-specific tem- frost forms at or below this temperature (Glickman perature and freezes. The temperature at which the [editor] 2000). These condensation events occur water droplet freezes depends on its size and chemi- overnight and are facilitated by clear skies and low cal composition, and on the ice nucleation mecha- wind speeds that cool the condensation surfaces. nism by which it freezes (Wallace and Hobbs 1977; Rime is formed by the rapid cooling of water blown Schemenauer et al. 1981). Initially, all ice crystals onto exposed frozen surfaces. Condensation pre- are very small (< 75 µm) and have simple shapes; cipitation events can be important biologically.