This research proposes to investigate historical river discharge and dissolved solids records from the unique landform of the Colorado River Basin (CRB). The purpose of study is to develop a detection and modeling strategy for large-scale basin response from low-frequency climate forcing (interannual and decadal time scales) and secular trends from natural and anthropogenic origin. The characteristic terrain of the CRB, represented by the Colorado plateau in the upper basin and the Basin and Range physiography in the lower basin, serves to define the natural space and time scales of fluid and solute storage across the region; while landuse practices of irrigation and reservoir operation modify the natural and hydrochemical "signal). An important question to be addressed is "what is the minimum state-space dimension of dissolved solids-discharge as a function of landuse, landform and hydrogeologic conditions, and how do they relate to long term climate forcing. The data base is: 40-60 years of monthly average streamflow and stream chemistry for 20 stations distributed from the upper Green and Colorado rivers to the Gulf of California; monthly precipitation-temperature records across the basin and adjacent plateaus and ranges; groundwater levels in irrigated and nonirrigated regions as well as digital terrain and landuse data. Two methods for extracting space-time signal from noisy hydrochemical data will be used: principal component analysis for spatial modal analysis and, singular spectrum analysis for detection of temporal oscillations. Our recent research published in Water Resources Research (Duffy, 1996; Duffy and Cusumano, 1998; Shun and Duffy, 1998) has involved formulation of dynamical models from terrain averaging of hydrochemcial variables and underlying partial differential equations. The study will form the basis for testing a practical, low-dimensional modeling strategy for the CRB which attempts to balance the degree of model complexity with a definable level of modeling skill, and to evaluate the effect of interannual and decadal climate oscillations on the water resources of the CRB.
