Snowmelt-dominated systems are a significant source of water supply for the Western United States. Changes in timing and duration of snowmelt are predicted to continue under climate change; however, the impact this change will have on water resources is not well understood. The ability to compare hydrologic processes across space and time is critical to accurately assess the physical and chemical response of headwater systems to climate change. This dissertation builds upon previous work by using long-term data from two snowmelt dominated catchments to investigate the response of hydrologic processes at different temporal and spatial scales. First, results from an hourly spatially-distributed energy balance snowmelt model were spatially and temporally aggregated to provide daily, catchment-wide snowmelt estimates, which, along with measured discharge and hydrochemical data were used to assess and compare hydrologic processes which occur on an annual scale in two headwater catchments for an eleven year study period. Second, the magnitude and timing of snowmelt, discharge fluxes and hydrochemical data were used to assess and compare inter-annual catchment response in two headwater catchments for an eleven year study period. Third, a pseudoinverse method was developed to compare mineral weathering fluxes in a series of nested sub-catchments over an eleven year study period. Advances from this work include the use of an independently-created energy balance snowmelt model for spatially-distributed hydrologic input for catchment-scale water balance, application of a quantifiable measure of catchment-scale hydrologic flux hysteresis and the development of a method to quantify and compare mineral weathering reactions between source waters across space and time. These methods were utilized to quantify and assess its role of dynamic storage in mitigating climate change response.

This study deals with the fundamental problems of hydrological design. Specifically, it explores the boundary conditions for design flood analysis. The problem of extrapolation of design relationships has been investigated by the systematic analysis of important design parameters namely unit hydrograph time to peak (tp), catchment antecedent conditions (CWI), runoff losses (PR) and the relationship between rainfall and flood return periods. In particular, this thesis investigates the combination of these variables representative of design analysis. A review of the hydrological design tools of response identification along with the physical evidence of catchment response is presented. The results of the investigation regarding time to peak (tp) show that it varies significantly between events, and the relationships developed between tp and flood magnitude (Qp) show the non-linear catchment behaviour which conform with most of the physical and field investigations. The relationships suggest that the unit hydrograph (UH) parameters derived from moderate events should be adjusted for extreme events and therefore a correction in UH tp has been developed which depends on the flood return period (Ashfaq and Webster, 2000a). The analysis of catchment wetness index (CWI) from a large number of observed events showed that antecedent conditions observed in the flood season are reasonably representative of the major events. This contrasts with the existing design recommendations which suggest consistently lower values. An alternative relationship of CWI therefore has been developed for design purposes (Webster and Ashfaq, submitted manuscript). The investigation showed that the percentage runoff (PR) characteristics of large events are consistently different than available from the existing design PR-method. The design method underestimates for the standardised conditions especially for large events because of its limited range of estimates for a wide range of return periods (e.g. 11% range in PR for 2 to 1000 year return periods). This problem is related with the PR-method itself for catchments having higher mean annual rainfall (SAAR> 800 mm) whereas for lower SAAR areas « 800 mm), it is related to the selection of design CWI values. The analysis of observed large events also revealed that a flood is generally associated with the storm of less return period than that of the flood. This contrasts with the suggested design rainfall-flood return periods relationship in the FSR (NERC, 1975; [H, 1999), but conforms with the curves presented by Webster (1998, 1999). This observation is further established by a detailed investigation of the characteristics of extreme events through a continuous model as well as the hydrological analysis of observed extreme floods of Easter-1998 (Ashfaq and Webster,2000b). The study demonstrates that the characteristics of extreme floods are different from those of small and moderate events. Relationships based on moderate events should be adjusted for the design of major events. The aggregated and integrated findings based on a comprehensive series of analysis led to the proposal of an alternative combination of design parameters. The performance of this combination showed improved flood estimates without any prior calibration in comparison to the FSR as well as FEH. A revised design methodology has therefore been proposed on the philosophy of 'independent treatment of input variables'. The application of this methodology on new catchments also provides encouraging estimates of flood quantiles. It is suggested that the methodology is equally applicable for both gauged and ungauged catchments especially where the observed data are limited or no data are available.

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In surface hydrology, much attention is paid to the effects of changing water fluxes, however there is less of a focus on the effects of changing energy fluxes. These energy fluxes are an important driver of many hydrological processes such as evapotranspiration and snow sublimation/ablation. The hypothesis that varying energy fluxes are important to the hydrological features of a catchment is tested by an experiment that involves calculating mean transit times for a number of catchments that drain different aspects of a large dome located in the Valles Caldera, New Mexico, called Redondo Peak. These catchments have different orientations and therefore receive different amounts of solar radiation. There is a general correlation between mean transit times, as determined by lumped-parameter convolution, and aspect, suggesting that in the Valles Caldera, transit times might be affected by a variety of features that are influenced by exposure to solar radiation, such as slope steepness, vegetation patterns, and soil depth. To put these transit times into context, I also used a distributed physically-based model to simulate a number of factors simultaneously to determine how hydrological features are influenced by aspect. This modeling excercise has illuminated the aspect-dependence of hydrological features such as the timing and intensity of snowmelt and soil moisture patterns, and it has quantified differences in energy and water fluxes on different aspects. These factors affect both water storage and water fluxes, and are therefore tied to transit times.