Abstract. Drainage density reflects the signature of climate on the topography and dictates the boundary conditions for surface hydrology. Hence defining the relationship between drainage density and climate is important in assessing the sensitivity of water resources and hydrology to climate change. Here we analyze the equilibrium relationship between drainage density and climate and estimate the relative sensitivity of drainage density to climate change. We conclude that the sign of the resulting change in drainage density depends not only on the direction of the change in climate but also on the prevailing climatic regime.

Spatial variability is a basic component of all natural systems. The system being studied here is the topography of an evolving drainage basin. Spatially heterogeneous erosivity within the drainage basin influences the structure of the drainage network, the roughness of small-scale features, and the overall appearance of the simulated topography. The presence of heterogeneities does not simply blur and smear analytical relationships. The heterogeneities themselves introduce new structure to the organization of the drainage basin. Three measures, the slope-area relationship, the cumulative area relationship, and the hypsometric distribution, are all systematically affected by spatial heterogeneity. We will demonstrate how and why these relationships are affected and also how different structures in the heterogeneities themselves are manifested in the resulting simulated topography. IntroductionNumerous numerical models which simulate drainage basin evolution have been developed and described in the literature [Ahnert, 1976[Ahnert, , 1977Kirkby, 1986; Willgoose et al., 1991a, b;Howard, 1994]. The focus of these models has typically been to generate river networks, and in more recent studies, the complementary system of hillslopes have been developed, as well. The motivation of these studies was to study the roles of different sediment transport processes and the evolution of the drainage network and to discern the influence of different erosion mechanisms on the topography.The motivation of this investigation will be to examine the influence of different forms of spatial heterogeneities on the simulated topography. Spatial heterogeneities are an integral part of observed topography and must be included as a fundamental element of the formulation of any basin evolution model. The central objective of this work will be to identify patterns and structures in the topography which originate from the presence of heterogeneities and to assess whether these structures are consistent with those of observed topography. In our model, the evolution of local elevation, z i within a basin is governed by the equation: Ozi O• = U-13iO•"S• + DiV2zi(1) where U is the rate of uplift, Q is the discharge, and S is the slope. Node j lies in the steepest direction downhill from i, defining the slope from i to j. The parameters/3i, m, n, and D i will vary depending on the nature of the topography to be simulated. We are concerned here with regions that are small relative to geologic spatial scales and thus uplift is assumed spatially constant. The last two terms on the right-hand side of •Now at Office of Hydrology, National Weather Service, Silver Spring, Maryland. Paper number 95WR02036. 0043-1397/95/95WR-02036505.00 equation (1) describe erosion from flUVial processes and diffusive processes, respectively. Note that if/3i and D i are spatially constant then we have homogeneous conditions, whereas if these parameters vary in space, then the soil material is heterogeneous in its composition. We will define fluvial erosion as the process ...

Summary 1.Streams collect runoff, heat, and sediment from their watersheds, making them highly vulnerable to anthropogenic disturbances such as urbanization and climate change. Forecasting the effects of these disturbances using process-based models is critical to identifying the form and magnitude of likely impacts. Here, we integrate a new biotic model with four previously developed physical models (downscaled climate projections, stream hydrology, geomorphology, and water temperature) to predict how stream fish growth and reproduction will most probably respond to shifts in climate and urbanization over the next several decades. 2. The biotic submodel couples dynamics in fish populations and habitat suitability to predict fish assemblage composition, based on readily available biotic information (preferences for habitat, temperature, and food, and characteristics of spawning) and day-to-day variability in stream conditions. 3. We illustrate the model using Piedmont headwater streams in the Chesapeake Bay watershed of the USA, projecting ten scenarios: Baseline (low urbanization; no on-going construction; and present-day climate); one Urbanization scenario (higher impervious surface, lower forest cover, significant construction activity); four future climate change scenarios [Hadley CM3 and Parallel Climate Models under medium-high (A2) and medium-low (B2) emissions scenarios]; and the same four climate change scenarios plus Urbanization. 4. Urbanization alone depressed growth or reproduction of 8 of 39 species, while climate change alone depressed 22 to 29 species. Almost every recreationally important species (i.e. trouts, basses, sunfishes) and six of the ten currently most common species were predicted to be significantly stressed. The combined effect of climate change and urbanization on adult growth was sometimes large compared to the effect of either stressor alone. Thus, the model predicts considerable change in fish assemblage composition, including loss of diversity. 5. Synthesis and applications . The interaction of climate change and urban growth may entail significant reconfiguring of headwater streams, including a loss of ecosystem structure and services, which will be more costly than climate change alone. On local scales, stakeholders cannot control climate drivers but they can mitigate stream impacts via careful land use. Therefore, to conserve stream ecosystems, we recommend that proactive measures be taken to insure against species loss *Correspondence author. E-mail: kanelson@umd.edu Re-use of this article is permitted in accordance with the Creative Commons Deed, Attribution 2·5, which does not permit commercial exploitation. or severe population declines. Delays will inevitably exacerbate the impacts of both climate change and urbanization on headwater systems.

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