PREDICTING COLORADO FRONT RANGE STREAM DISCHARGE UNDER NATURAL CLIMATE VARIABILITY: THE CACHE LA POUDRE RIVER CASE STUDY

Natural climate variability on interannual to interdecadeal time scales can play an immediate role in the availability of surface-water resources. Along the Colorado Front Range, stream-discharge response to climate variability has particular relevance for management decisions during drought because even marginal changes in supply can have important legal and economic implications. Consequently, it is of value to understand past linkages between climate variability and the availability of surface-water supply. This enables realistic predictions of stream-discharge under future climate-variability and improved projections of the consequences of resource decisions on future water availability. Interactions between interannual and interdecadal climate cycles are known to produce a cumulative climate variability that affects precipitation and, in turn, the timing and quantity of stream discharge. Using the Cache La Poudre River, which contains one of the longest stream-discharge records (>100 years) in the US, a methodology is presented to quantify historical and future predictions of stream-discharge response to climate-variability forcings and shifts. Preliminary results indicate variability in the stream-discharge record attributed to the primary interannual to interdecadal climate cycles of the western US. These include El Nino/Southern Oscillation (ENSO) (2-6 yrs), North American Monsoon System (NAMS) (6-10 yrs), Pacific Decadal Oscillation (PDO) (10-25 yrs), and Atlantic Multidecadal Oscillation (AMO) (50-80 years). The majority of the variance in the stream-discharge record and strong correlations to the PDO and AMO indices indicate the importance of these decadal-to-multidecadal climate cycles on discharge of the Cache La Poudre River. Preliminary results indicate that the PDO and AMO dry phases of variability, the corresponding drought conditions, and the lower stream discharge across the Front Range, are likely to persist on the order of years to decades. The probabilistic prediction model of future PDO and AMO regime shift presented here can be used as a decision support tool useful to water managers in other basins along the Front Range whose principal concerns lies in accurately knowing how and when stream discharge will respond to future climate-variability shifts.