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

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h Chapter I Introduction and Objectives 1.1. Introduction and Background The relationship between forests and water is an impor- tant issue in Colorado because forests occupy a large percentage of the land area and forested areas supply much of the state's water. Forests and woodlands cover approximately 35,300 miz within Colorado, or approxi- mately one -third of the state (DNR, 2002). In order of prevalence, the dominant forest types are pinyon juniper (33% spruce -fir (21 %), ponderosa pine (15 %), aspen (14% lodgepole pine (10% Douglas fir (5% and other forest types (3 %) (Figure 1.1). Much of Colorado is classified as semi -arid and at lower elevations only a small proportion of the annual pre- cipitation is converted into runoff. Annual precipitation increases rapidly with elevation (Figure 1.2), and there is an even more rapid increase in the amount of runoff per unit area due to the associated decrease in potential evapotranspiration (Figure 1.3). i Y K }. Figure 1.1. Map of the major forest types in Colorado. The net result is that the higher - elevation forests (e.g., above about 9,000 feet) generate much of the state's wa- ter supply. The amount, timing, and quality of this runoff is a critical issue to the citizens of Colorado, private companies, and government agencies. Natural and hu- man- induced disturbances can greatly affect the amount and type of forest cover, and this has direct implications for the amount and quality of runoff. The effects of forest management on the quantity, qual- ity, and timing of runoff have long been of interest to land managers and water users, and a topic of consider- able debate. The ancient Greeks noted that clearing the forests could cause springs to dry up (Biswas, 1970), while a large number of recent studies have shown that forest clearing can increase annual water yields (reviews by Bosch and Hewlett, 1982; Stednick, 1996). The large variation in the hydrologic effects of forest management means that one or more studies can be found to support nearly any point of view. The resolution of these appar- Aspen - spruce -flr - Douglas fir Lodgepole pine Ponderosa pine Pinyon- juniper Mixed forest ,'l 0 60 I 100 I km 0 25 M miles i Souce Gnlorada Gap 4nalPsis