Forests and Water: A State of the Art Review for Colorado

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Executive Summary 1. Forests occupy 22.6 million acres in Colorado, or 32% of the land area. Nearly three - quarters of the forest lands in Colorado are in public ownership. About 55% of the forested area is considered as suitable for forest harvest. National forests comprise nearly half of the forested area and approximately 60% of the area considered suitable for forest harvest. There are no significant, privately- owned, industrial forest lands in Colorado. 2. Most of the runoff in Colorado comes from forest and alpine areas above approximately 9000 ft (2730 m) in eleva- tion, and is generated by snowmelt from approximately mid- April to mid -July. The dominant role of high- eleva- tion areas is due to the increase in precipitation with increasing elevation, the decrease in potential evapotranspi- ration with increasing elevation, and the concentration of snowmelt in a relatively short period of time. In lower elevation forests the amount of runoff per unit area is greatly reduced because the rainfall and snowmelt inputs are much smaller relative to potential evapotranspiration. 3. Research on the relationships between forests and water has been conducted in Colorado since 1910. Intensive studies from Wagon Wheel Gap, the Fraser Experimental Forest, Manitou Experimental Forest, and other sites provide a thorough understanding of how changes in forest cover affect evapotranspiration, soil moisture storage, and the amount and timing of runoff. This knowledge base is strongest for the higher- elevation spruce -fir and lodgepole pine forests, as these forest types generate most of the runoff from forested areas and have been studied more intensively. 4. Annual water yields in the higher elevation spruce -fir and lodgepole pine forests are inversely proportional to the amount of forest canopy as indexed by basal area. Complete removal of the forest canopy in the sub - alpine zone can increase annual water yields by as much as 8 inches (20 cm) of water per unit area. This increase in water yield is due to the reduction in winter interception losses and summer evapotranspiration. Nearly all of this additional runoff comes on the rising limb of the snowmelt hydrograph (i.e., early May to mid- June), and the increases in runoff are several times larger in wet years than dry years. This implies that water storage facilities are required if an increase in runoff is to be carried over into the summer or from year to year. 5. A reduction in the amount of forest canopy will increase the rate of spring snowmelt, and complete removal of the forest canopy will increase the size of the annual maximum peak flows by approximately 40 -50 %. Paired- water- shed studies have shown that removal of the forest canopy has no significant effect on summer low flows, as the water "saved" by reducing summer evapotranspiration is simply carried over to the following spring. 6. Reducing forest density has a progressively smaller effect on annual water yields with decreasing annual precipi- tation. Both paired- watershed studies and plot -scale research show that reducing forest density has no detectable effect on water yields when annual precipitation is less than 18 -19 inches or approximately 460 mm. Paired - watershed studies also indicate that at least 15% of the forest canopy within a watershed must be removed in order to obtain a measurable increase in annual water yields from small research watersheds. The detection of change becomes much more difficult in larger watersheds when discharge is being measured with standard techniques in natural channels. 7. The increase in water yield that results from timber harvest or other disturbances will decrease as the forest re- grows. Paired watershed studies suggest that it will take approximately 60 years until annual water yields return to their pre - disturbance levels in the higher- elevation spruce -fir and lodgepole pine forests. Hydrologic recovery is substantially faster in aspen forests due to faster regrowth and in drier forest types. 8. Historic photographs, forest stand records, and other data indicate that forest density in Colorado is generally great- er than in the mid to late 1800s. This increase in forest density is attributed to suppression of forest fires, reduced grazing, and lower rates of forest harvest for timber, fuel, and other products. The human - induced changes in forest density and composition are most pronounced in the low and mid - elevation ponderosa pine and mixed conifer for- ests, as these forests were regularly subjected to both low- intensity surface fires and high- intensity, stand - replacing iii