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    Climate Variability, Water Resources, and Hydrologic Extremes - Modeling the Water and Energy Budgets
    (Blackwell Science Publ, 2005) Yildiz, Osman; Barros, Ana P.
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    Elucidating vegetation controls on the hydroclimatology of a mid-latitude basin
    (Elsevier, 2007) Yıldız, Osman; Barros, Ana P.
    The rote of vegetation controls on the hydrological response to climatic variability of a mid-latitude watershed characterized by complex terrain and complex geology was assessed using a coupled surface-groundwater hydrological model. To separate infiltration and runoff production mechanisms from vegetation processes, the study was conducted with respect to both the representation of vegetation processes and the soil hydraulic properties for two different hydroclimatic regimes. The model was applied to simulate the warm season hydrological regime in the Monongahela River Basin in 1988, a major drought year, and in 1993, a wet hydrological year. Sensitivity analysis was conducted using the fractional factorial design method. Time-varying vegetation cover characteristics were directly assimilated to the model from satellite observations, and model simulations of streamflow at the outlet of various catchments were compared against observations to assess the model's ability to capture basic patterns of space-time seasonal variability within the basin. The results show that the physical controls expressed by different parameters and parameter interactions change across the basin with land-use, topography and geology on the one hand, and vary significantly between the spring and summer seasons. This is consistent with the notion of highly non-linear river-basin systems where nonstationarity emerges from the interactions among the spatially variable landscape and the temporally variable climate forcing. Above all, one key finding of this study is to elucidate the governing role of vegetation, specifically as described by Fractional Vegetation Cover and Leaf Area Index parameters in the space-time variability of hydrological response in the Monongahela River Basin for the two hydroclimatological regimes, and especially the linkage between areal. extent of vegetation and runoff production during drought. Because vegetation dynamics modulate the water and energy budgets via evapotranspiration and surface albedo, and this control is especially critical during the spring-summer transition which coincides with the greening season in mid-latitudes, we argue that these processes have far reaching implications for the predictive stability of physically-based models for hydrological change studies, and propose the notion of model calibration conditional on climate regime for operational hydrology. (c) 2006 Elsevier B.V. All rights reserved.
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    Evaluating spatial variability and scale effects on hydrologic processes in a midsize river basin
    (Academic Journals, 2009) Yıldız, Osman; Barros, Ana P.
    The impact of spatial variability and scale on the dynamics of hydrologic processes in the Monongahela river basin of USA was investigated using a physically based spatially distributed hydrologic model developed by Yildiz (2001). The hydrologic model simulations were performed at 1 and 5 km spatial scales for a 5 month period from April through August of 1993. Effects of spatial variability in topography, vegetation and hydrogeology and of spatial scale were evaluated through comparisons of the simulated and observed streamflows for the prescribed resolutions at different locations across the river basin. The evaluation of observed and simulated streamflows using the statistical measures of mean, standard deviation, coefficient of variation, root mean square error and bias showed that model statistics of streamflow followed closely the spatial patterns of those of existing observations, that is, the model captured the space-time features of the 1993 flood across the basin. The changes in the nature of the rainfall-runoff response due to changes in the spatial resolution of the model indicated that there was also a change in governing physical processes at different resolutions. Here, this change was expressed in terms of the relative contributions of surface and subsurface flows.