Hydrology

1. Hydrology Interception, effects of forestry on snow hydrology R. Hudson - VFR Research

2. Interception • Before precipitation reaches the soil, it must pass through whatever vegetation cover is present. (In urban areas, buildings and other structures act to intercept precipitation.) • Vegetation cover retains some of this precipitation and returns it to the atmosphere by evaporation and/or sublimation - this is interception R. Hudson - VFR Research

3. Precipitation interception by a coniferous canopy • Coniferous canopies intercept both rainfall and snowfall - this interception loss can be a significant component of water balance. • Studies of rainfall interception in coastal BC and elsewhere show that up to 30% of total annual rainfall does not reach the ground under a mature coniferous canopy. • A similar percentage of total annual snowfall is intercepted by mature conifers. R. Hudson - VFR Research

4. Rainfall interception • rain can fall directly through gaps in the canopy (throughfall), or it can hit the foliage and branches • once the foliage has been completely wetted by the rain, droplets will begin to cascade down through the canopy • water can drip off the canopy to the ground (throughfall) or can run down stems (stemflow) • evaporative losses can occur from the canopy, or from vegetation at the forest floor (interception) R. Hudson - VFR Research

5. Components: P = precipitation C = canopy interception F = forest floor interception T = throughfall S = stemflow Itotal = F + C Itotal = P - (T + S) P C T T S C T F R. Hudson - VFR Research

6. Rain interception has been studied in detail • a forest canopy has a saturation capacity of 0.5 to 2.0 mm of water depending on the age and structure of the stand • water can be evaporated by advective energy even during storms, so the capacity of the canopy is constantly replenished • throughfall and stemflow are monitored with appropriate collectors, subtracted from rainfall on an area basis to calculate interception loss R. Hudson - VFR Research

7. Rain interception • Spittlehouse (1998) (from Spittlehouse, D.L. 1998. Rainfall interception in young and mature conifer forests in British Columbia. In: Weather Data Requirements for Integrated Pest Management. 23rd Conference on Agricultural and Forest Meteorology. American Meteorological Society, Boston Mass. 171-174.) • interception loss is a function of storm size and the age of the stand (i.e., tree size) • asymptotic relationship: (coastal sites) • immature forest (6-10 metre trees) reaches a maximum at about 14 mm for storms > 100 mm • mature forest, interception loss maxes out at an average of 25 mm for storms > 100 mm, but in spring storms, interception loss was about twice that for winter storms

8. Rainfall interception as a function of storm size and season (coastal sites) from Spittlehouse, 1998

9. Rain interception Coast vs. Interior • For mature spruce-fir-pine forests at interior sites, results are similar: • I is much less (max of 4 mm) due to smaller rain storms. • I expressed as a percent of annual rain is similar in coastal and interior forests • Interception was 25-30% of annual rainfall for mature forest depending on species and crown density • Interception was 10-20% of annual rainfall for immature forest from Spittlehouse, 1998

10. Rain and snow interception • Studies show that interception is proportional to canopy density or canopy height for both rain and snow • Generally, interception loss of snow is greater than that of rain for coastal sites • interception for snow is as high as 50% • interception for rain is as high as 30% • I is different for mature and immature canopies due to different size and development of trees R. Hudson - VFR Research

11. Effect of Forest Density on Interception R. Hudson - VFR Research

12. Snow Interception vs. Canopy Height Gray Creek 1997 R. Hudson - VFR Research

13. Effect of canopy structure • Deciduous vs. coniferous: interception is greater for coniferous than deciduous forests • needles can hold more water than broad leaves due to greater surface area • conifers maintain foliage year-round • Mature vs. immature: • rain interception greater for mature forest • snow interception is affected by stocking density as well as canopy structure in terms of tree form R. Hudson - VFR Research

14. Water balance implications • Forest harvesting always increases water available for runoff by decreasing interception • this increased availability can be in the order of 30-50% for newly harvested coastal sites • Increases in peak flows • in rain dominated watersheds, or for spring storms, water yield and peak flow could be increased by 25-40% for a 100 mm storm R. Hudson - VFR Research

15. In snow dominated watersheds in coastal B.C., snow accumulation and average melt rates for harvested sites during the melt period can be up to twice that of uncut old growth forest. • On a watershed scale, those potential increases are proportional to the proportion of the watershed logged • e.g. 25% of watershed logged, logged areas experience 34% increase in snow catch, then there is 8.5% more water available for runoff R. Hudson - VFR Research

16. Dominance of rain vs. snow • Rain dominated zone = 0-300 metres elevation on the coast • Rain-on-snow zone (transient snow zone) • usually about 300-800 metres elevation on the coast • fluctuating freezing levels result in alternating snow accumulation and ablation in winter • rain on snow produces greater runoff rates than snowmelt or rainfall alone R. Hudson - VFR Research

17. rain-on-snow produces largest peak flows on the coast where it occurs frequently, results in flooding in the interior where it is much less frequent than on the coast • it is thought that logging in the rain-on-snow zone has the greatest potential to increase peak flows • Snow dominated areas: >800 metres on coast, most of the interior • logging results in greater water yield due to decreased interception, and increased peak flows due to accelerated melt R. Hudson - VFR Research

18. Hydrologic Recovery • We have seen that logging alters the potential to produce runoff at the site that is logged, by reducing interception and increasing rates of snowmelt • Hydrologic recovery is the process by which forest regeneration restores the runoff production to near pre-harvest condition • How quickly does a harvested area recover? R. Hudson - VFR Research

19. Hydrologic recovery for coastal B.C. R. Hudson - VFR Research

20. Equation - hydrologic recovery • Research shows that hydrologic recovery of a regenerating stand can best be expressed as an exponential function of mean canopy height of the regeneration. • There is a recovery threshold - minimum canopy height before recovery will begin, in this case the threshold is 2.25 metres • Assumes the stand is fully stocked; if patchy, the coverage must also be accounted for. R. Hudson - VFR Research

21. Equivalent Clear-cut Area • As an area that has been harvested begins to regenerate, its original area is reduced by the level of recovery • A = original opening area • e.g., a 10 ha opening has an average canopy height of 8 metres. Hydrologic recovery is 80%, so its ECA is 2 ha. • a 20 ha opening has 6 m regeneration, but in clumps with 50% coverage. Recovery is (0.5X70%) so ECA is 13 ha. R. Hudson - VFR Research