4 SAHRA (Sustainability of semi-arid hydrology and riparian areas), Global change is affecting the hydrologic response of landscapes in various ways. Understanding and predicting these effects on the water cycle are becoming increasingly critical (Jackson et al., 2001), but to date, there has been little progress in generating mechanistic relationships between climate, land use and hydrologic partitioning that can be broadly applied. Projected changes in surface temperature and precipitation dynamics will undoubtedly alter the runoff regime (Barnett et al., 2005) as well as runoff extremes (Milly et al., 2002; Dai et al., 2004). The sensitivity of annual stream flow (runoff) to changes in temperature and precipitation has been investigated empirically as well as theoretically (Arnell, 1996). Runoff sensitivity to climate change derived from the observational record has limited predictive capacity in light of non-stationarity of interdependent hydroclimatic variables and landscape features in a changing environment (Milly et al., 2008). Water balance models (Schaake, 1990; Dooge, 1992) usually consider the landscape to be invariant and their application to specific river basins relies on observations to calibrate the model parameters (Wagener, 2007). Both climate and land use are altering the landscape with concomitant changes in ecosystem structure and function (Walther et al., 2002) and regional hydrological cycles (Breshears et al., 2005). Although the hydrologic impacts of land use change imposed by humans have received considerable attention by hydrologists (e.g. Hurkmans et al., 2009), the impacts of natural large-scale vegetation shifts as a consequence of climate change are unclear (Allen and Breshears, 1998). Vegetation shifts are an important climate-ecosystem-hydrology feedback through their alteration of carbon, water and energy exchanges at the land surface (Adams et al., 2009). Although recent ecohydrological studies have focussed on understanding interactions between hydroclimatic variables and ecosystems response (e.g. Rodriguez-Iturbe and Porporato, 2004), as well as the effects of ecosystem structure on local-to-regional hydroclimate (e.g. Rinehart et al., 2008; Veatch et al., 2009), we lack a comprehensive theory of how vegetation will respond to changes in the water and energy balance of a region.Over short timescales, vegetation can respond adaptively to variations in climatic factors. Vegetation productivity, measured as net carbon uptake by the landscape or estimated from patterns of aboveground net primary production (ANPP), and growing-season actual evapotranspiration are strongly related (Webb et al., 1986). Since actual evapotranspiration typically is the largest component of many ecosystem water balances (Zhang et al., 2001), the inter-annual variability of catchment hydrologic response should be strongly related to ecosystem function and productivity. Thus, understanding vegetation response to inter-annual variability of climate and water availability should be central in any attempt t...

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The structure of the criti cal zone (CZ) is a result of tectonic, lithogenic, and climati c forcings that shape the landscape across geologic ti me scales. The CZ structure can be probed to measure contemporary rates of regolith producti on and hillslope evoluti on, and its fl uids and solids can be sampled to determine how structure aff ects CZ functi on as a living fi lter for hydrologic and biogeochemical cycles. Substanti al uncertainty remains regarding how variability in climate and lithology infl uence CZ structure and functi on across both short (e.g., hydrologic event) and long (e.g., landscape evoluti on) ti me scales. We are addressing this issue using a theoreti cal framework that quanti fi es system inputs in terms of environmental energy and mass transfer (EEMT, MJ m −2 yr −1 ) in the recently established Jemez River Basin (JRB)-Santa Catalina Mountains (SCM) Criti cal Zone Observatory (CZO). We postulate that C and water fl uxes, as embodied in EEMT, drive CZ evoluti on and that quantifying system inputs in this way leads to predicti ons of nonlinear and threshold eff ects in CZ structure formati on. We are testi ng this hypothesis in the JRB-SCM CZO, which comprises a pair of observatories-in northern New Mexico within the Rio Grande basin (JRB) and in southern Arizona within the Colorado River basin (SCM). The JRB-SCM CZO spans gradients in climate, lithology, and biota representati ve of much variati on found in the larger U.S Southwest. Our approach includes in situ monitoring of zero-order basins nested within larger CZO watersheds and measurement-modeling iterati ons. The initi al data collected at the ecosystem, pedon, and catchment scales indicates a strong role of coupled C and water fl ux in regulati ng chemical denudati on of catchments in the JRB site. Abbreviati ons: CZ, criti cal zone; CZO, Criti cal Zone Observatory; DOC, dissolved organic carbon; EEMT, environmental energy and mass transfer; EHP, ecohydrology and hydrologic parti ti oning; JRB, Jemez River Basin; LSE, landscape evoluti on; MC, mixed conifer; SCM, Santa Catalina Mountains; SSB, subsurface biogeochemistry; SWD, surface water dynamics; ZOB, zero-order basin.The functi on of the CZ as a living fi lter for Earth's hydrologic and biogeochemical cycles varies as a result of its long-term geomorphic evolution. In this portion of the terrestrial surface that extends from the outer periphery of the vegetation canopy through the vadose and saturated zones (National Research Council, 2001), heterogeneities in the fl ux of fresh water, solutes, and sediments are superimposed with biological colonization and activity leading to episodic and long-term variation in CZ structure (Lin, 2010;Wagener et al., 2010;Rasmussen et al., 2011a). Th e evolving spatial heterogeneity in the landscape aff ects the behavior of the CZ as a system, which is manifest, in part, as system-scale dynamics. Th e distribution of vegetation, weathering agents, and regolith depth along hillslope catenas, for example, aff ects the hydrochemical response at th...

[1] There is no consensus on how changes in both temperature and precipitation will affect regional vegetation. We investigated controls on hydrologic partitioning at the catchment scale across many different ecoregions, and compared the resulting estimates of catchment wetting and vaporization (evapotranspiration) to remotely sensed indices of vegetation greenness. The fraction of catchment wetting vaporized by plants, known as the Horton index, is strongly related to the ratio of available energy to available water at the Earth's surface, the aridity index. Here we show that the Horton index is also a function of catchment mean slope and elevation, and is thus related to landscape characteristics that control how much and how long water is retained in a catchment. We compared the power of the components of the water and energy balance, as well as landscape characteristics, to predict Normalized Difference Vegetation Index (NDVI), a surrogate for vegetation productivity, at 312 Model Parameter Estimation Experiment (MOPEX) catchments across the United States. Statistical analysis revealed that the Horton index provides more precision in predicting maximum annual NDVI for all catchments than mean annual precipitation, potential evapotranspiration, or their ratio, the aridity index. Models of vegetation productivity should emphasize plant-available water, rather than just precipitation, by incorporating the interaction of climate and landscape. Major findings related to the Horton index are: (1) it is a catchment signature that is relatively constant from year-to-year; (2) it is related to specific landscape characteristics ; (3) it can be used to create catchment typologies; and (4) it is related to overall catchment greenness.

[1] Primary productivity and vegetation cover are strongly related to how precipitation is partitioned into surface discharge, storage, and evapotranspiration (ET). Thus, quantifying feedbacks between changes in precipitation and vegetation at regional scales is a critical step toward predicting both carbon balance and water resources as climate and land cover change. We used a catchment-based approach to quantify partitioning of precipitation and compared these hydrologic fluxes to remotely sensed vegetation greenness (NDVI) in 86 U.S. catchments between 2000 and 2008. The fraction of precipitation potentially available to vegetation (catchment wetting; W) ranged from 0.64 to 0.99 demonstrating that up to 36% of precipitation was not available to vegetation. The ratio of ET:W (Horton Index (HI)), ranged from 0.07 to 1.0 demonstrating even greater variability in the fraction of catchment wetting used as ET. Negative slopes between annual Horton Index and maximum annual NDVI values indicated water limitation during dry years in most catchment ecosystems. Not surprisingly, grasslands were more sensitive to drying than forests. However, in nine of the wettest (HI < 0.66) catchment ecosystems, NDVI values increased as HI increased suggesting greater vegetation productivity under drier conditions. Our results demonstrate that catchment-scale hydrologic partitioning provides information on both the fractions of precipitation available to and used by vegetation. Their ratio ( HI) identifies shifts between water and energy limitation, and differential sensitivity to drying based on vegetation type within catchment ecosystems. Consequently, catchment-scale partitioning provides useful information for scaling point observations and quantifying regional ecohydrological response to climate or vegetation change.Citation: Brooks, P. D., P. A. Troch, M. Durcik, E. Gallo, and M. Schlegel (2011), Quantifying regional scale ecosystem response to changes in precipitation: Not all rain is created equal, Water Resour. Res., 47, W00J08,

[1] Feedbacks among vegetation dynamics, pedogenesis, and topographic development affect the "critical zone"-the living filter for Earth's hydrologic, biogeochemical, and rock/ sediment cycles. Assessing the importance of such feedbacks, which may be particularly pronounced in water-limited systems, remains a fundamental interdisciplinary challenge. The sky islands of southern Arizona offer an unusually well-defined natural experiment involving such feedbacks because mean annual precipitation varies by a factor of five over distances of approximately 10 km in areas of similar rock type (granite) and tectonic history. Here we compile high-resolution, spatially distributed data for Effective Energy and Mass Transfer (EEMT: the energy available to drive bedrock weathering), above-ground biomass, soil thickness, hillslope-scale topographic relief, and drainage density in two such mountain ranges (Santa Catalina: SCM; Pinaleño: PM). Strong correlations exist among vegetation-soil-topography variables, which vary nonlinearly with elevation, such that warm, dry, low-elevation portions of these ranges are characterized by relatively low above-ground biomass, thin soils, minimal soil organic matter, steep slopes, and high drainage densities; conversely, cooler, wetter, higher elevations have systematically higher biomass, thicker organic-rich soils, gentler slopes, and lower drainage densities. To test if eco-pedo-geomorphic feedbacks drive this pattern, we developed a landscape evolution model that couples pedogenesis and topographic development over geologic time scales, with rates explicitly dependent on vegetation density. The model self-organizes into states similar to those observed in SCM and PM. Our results highlight the potential importance of eco-pedo-geomorphic feedbacks, mediated by soil thickness, in water-limited systems.

72Zero-order drainage basins, and their constituent hillslopes, are the fundamental geomorphic unit 73 comprising much of Earth's uplands. The convergent topography of these landscapes generates 74 spatially variable substrate and moisture content, facilitating biological diversity and influencing 75 how the landscape filters precipitation and sequesters atmospheric carbon dioxide. In light of 76 these significant ecosystem services, refining our understanding of how these functions are 77 affected by landscape evolution, weather variability, and long-term climate change is imperative. 78 In this paper we introduce the Landscape Evolution Observatory (LEO): a large-scale 79 controllable infrastructure consisting of three replicated artificial landscapes (each 330 m 2 80 surface area) within the climate-controlled Biosphere 2 facility in Arizona, USA. At LEO, 81 experimental manipulation of rainfall, air temperature, relative humidity, and wind speed are 82 possible at unprecedented scale. The Landscape Evolution Observatory was designed as a 83 community resource to advance understanding of how topography, physical and chemical 84 properties of soil, and biological communities coevolve, and how this coevolution affects water, 85 carbon, and energy cycles at multiple spatial scales. With well-defined boundary conditions and 86 an extensive network of sensors and samplers, LEO enables an iterative scientific approach that 87 includes numerical model development and virtual experimentation, physical experimentation, 88 data analysis, and model refinement. We plan to engage the broader scientific community 89 through public dissemination of data from LEO, collaborative experimental design, and 90 community-based model development. 91 coevolution 93 94 95 1. Introduction 96Hillslopes and their adjacent hollows (i.e., zero-order drainage basins, or ZOBs) 97 constitute a large fraction of upland areas over Earth's surface and provide critical ecosystem 98 services. Within ZOBs there is exchange of water, carbon dioxide, and energy with the 99 atmosphere and transport of soil, water, and solutes into fluvial drainage networks-processes 100 that link ZOBs with the climate system and downstream water quantity and quality. The time-101 varying rates of these exchange and transport processes are integrated responses to many 102 physical and biological phenomena that occur from below the base of the soil profile to the 103 vertical extent of the atmospheric boundary layer (e.g., see discussion by Chorover et al., 2011). 104 Zero-order basins evolve as climate varies, soils form and erode, and biological 105 communities establish, compete, and change in response to environmental stimuli. Across 106 spatial and topographic gradients, these interacting processes may result in consistently 107 observable correlations between temperature and precipitation dynamics, soil depth and hillslope 108 length, and plant biomass accumulation (e.g., Rasmussen et al., 2011; Pelletier et al., 2013). 109 Coupled soil-production and ...

The interactions between atmospheric, hydrological, and ecological processes at various spatial and temporal scales are not fully represented in most ecohydrological models. This first of a two‐part paper documents a fully integrated catchment‐scale ecohydrological model consisting of a three‐dimensional physically based hydrological model and a land surface model. This first part also presents a first application to test the model over an energy‐limited catchment (8.4 km2) of the Sleepers River watershed in Vermont. The physically based hydrological model (CATchment HYdrology, CATHY) describes three‐dimensional subsurface flow in variably saturated porous media and surface routing on hillslopes and in stream channels, whereas the land surface model (LSM), an augmented version of Noah LSM with multiple parameterization schemes (NoahMP), accounts for energy, water, and carbon flux exchanges between various land surface elements and the atmosphere. CATHY and NoahMP are coupled through exchanges of water fluxes and states. In the energy‐limited catchment of the Sleepers River watershed, where snowmelt runoff generation is the dominant hydrologic flux, the coupled CATHY/NoahMP model at both 90 and 30‐m surface grid resolutions, with minimal calibration, performs well in simulating the observed snow accumulation, and melt and subsequent snowmelt discharge. The Nash–Sutcliffe model efficiency of daily discharge is above 0.82 for both resolutions. The simulation at 90‐m resolution shows a marginal improvement over that at 30‐m resolution because of more elaborate calibration of model parameters. The coupled CATHY/NoahMP also shows a capability of simulating surface‐inundated area and distributed surface water height, although the accuracy of these simulations needs further evaluation. The CATHY/NoahMP model is thus also a potentially useful research tool for predicting flash flood and lake dynamics under climatic change. Copyright © 2013 John Wiley & Sons, Ltd.