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[1] Despite the growing interest in hyporheic exchange and the associated stream ecosystem processes, few studies consider restoration of hyporheic exchange as a design goal. Here we study the design of three types of subsurface structures for hyporheic restoration after conceptual designs published over 40 years ago. Vaux's designs involve modifying the subsurface with low or high hydraulic conductivity material placed at the streambed or adjacent to a confining layer below the stream. In this preliminary analysis of subsurface structure design we use two-dimensional groundwater flow modeling of structures to simulate structure performance in plane bed streams for ranges of structure geometric design and hydraulic conductivities. Structure performance is evaluated on the basis of total streambed flux, physical extent of hyporheic flow paths created, and residence time distributions along flow paths modified by the structures. High hydraulic conductivity structures bend flow paths toward and through the structures themselves; performance is controlled by the structure hydraulic conductivity. Results show low hydraulic conductivity structure performance is insensitive to the structure material; hyporheic exchange is created by deflecting flow paths away from the structure itself. Time scales of simulated exchange are great enough to promote nitrification, denitrification, respiration, and thermal buffering in the subsurface, though these processes will also be controlled by site-specific chemical and biological factors. General design recommendations for specific restoration objectives are presented. Results of this study can be extrapolated to further understand the interaction of natural subsurface heterogeneities (e.g., clay and gravel deposits and bedrock knickpoints) and flow fields in creating hyporheic flow paths.Citation: Ward, A. S., M. N. Gooseff, and P. A. Johnson (2011), How can subsurface modifications to hydraulic conductivity be designed as stream restoration structures? Analysis of Vaux's conceptual models to enhance hyporheic exchange, Water Resour. Res., 47, W08512,

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An artificial neural network (ANN) was used to evaluate the hydrological responses of two streams in the northeastern U.S. having different hydroclimatologies (rainfall and snow+rain) to hypothetical changes in precipitation and thermal regimes associated with climate change. For each stream, historic precipitation and temperature data were used as input to an ANN, which generated a synthetic daily hydrograph with high goodness-of-fit (r2 > 0.80). Four scenarios of climate change were used to evaluate stream responses to climate change: + 25% precipitation, -25% precipitation, 2 x the coefficient of variation in precipitation regime, and +3"C average temperature. Responses were expressed in hydrological terms of ecological relevance, including flow variability, baseflow conditions, and frequency and predictability of floods. Increased average precipitation induced elevated runoff and more frequent high flow events, while decreased precipitation had the opposite effect. Elevated temperature reduced average runoff. Doubled precipitation variability had a large effect on many variables, including average runoff, variability of flow, flooding frequency, and baseflow stability. In general, the rainfall-dominated stream exhibited greater relative response to climate change scenarios than did the snowmelt stream.Stream ecosystems are at risk for changes due to climate change because ecological processes are strongly influenced by seasonal patterns of precipitation, runoff, and temperature (Carpenter et al. 1992;Allan 1995). If historical hydrological and thermal regimes in streams are modified by anthropogenically altered climate change, then ecosystem alteration is to be expected. Hydrological modifications may result either from changes in average conditions or from changes in the distribution and timing of extreme events such as floods and droughts. Evaluating the extent to which stream hydrographs are modified by scenarios of climate change can provide important information on the relative sensitivity of stream ecosystems to potential climate change.Modeling stream hydrological response to climate variation can be performed with a variety of techniques. If detailed watershed and climate data are available for parameterization, one can use mass balance models, such as hydrological budget models (e.g. Gleick 1987). However, for many stream systems, detailed watershed data are lacking, making the mechanistic modeling of hydrological response to climate difficult, Further, traditional empirical models (e.g. regression models) may not per- AcknowledgmentsWe thank P. Mulholland and C. P. Hawkins for constructive reviews of an earlier version of this paper.