[1] We investigate the influence of hillslope aspect on landscape morphology in central New Mexico, where differences in soils, vegetation, and landforms are observed between mesic north-facing and xeric south-facing slopes. Slope -area and curvature-area relations, derived from a Digital Elevation Model (DEM), are used to characterize the opposing hillslope morphologies. In all geologies and elevation ranges studied, topographic data reveal significantly steeper slopes in north-facing aspects, and shallower slopes in south-facing aspects. North-facing slope curvatures are also greater than south-facing curvatures. Using a conceptual slope-area model, we suggest that for a given drainage area, steeper north-facing slopes imply lower soil erodibility. We argue that this interpretation, consistent with recent views of ecosystem control on semiarid erosion rates, shows the influence hillslope aspect on topography and its associated vegetation communities. Observed valley asymmetry in the region reinforces this concept and suggests a long-term legacy of aspect-modulated ecogeomorphic processes. Citation: Istanbulluoglu, E., O. Yetemen, E. R. Vivoni, H. A. Gutiérrez-Jurado, and R. L. Bras (2008), Eco-geomorphic implications of hillslope aspect: Inferences from analysis of landscape morphology in central New

Soil‐mantled pole‐facing hillslopes on Earth tend to be steeper, wetter, and have more vegetation cover compared with adjacent equator‐facing hillslopes. These and other slope aspect controls are often the consequence of feedbacks among hydrologic, ecologic, pedogenic, and geomorphic processes triggered by spatial variations in mean annual insolation. In this paper we review the state of knowledge on slope aspect controls of Critical Zone (CZ) processes using the latitudinal and elevational dependence of topographic asymmetry as a motivating observation. At relatively low latitudes and elevations, pole‐facing hillslopes tend to be steeper. At higher latitudes and elevations this pattern reverses. We reproduce this pattern using an empirical model based on parsimonious functions of latitude, an aridity index, mean‐annual temperature, and slope gradient. Using this empirical model and the literature as guides, we present a conceptual model for the slope‐aspect‐driven CZ feedbacks that generate asymmetry in water‐limited and temperature‐limited end‐member cases. In this conceptual model the dominant factor driving slope aspect differences at relatively low latitudes and elevations is the difference in mean‐annual soil moisture. The dominant factor at higher latitudes and elevations is temperature limitation on vegetation growth. In water‐limited cases, we propose that higher mean‐annual soil moisture on pole‐facing hillslopes drives higher soil production rates, higher water storage potential, more vegetation cover, faster dust deposition, and lower erosional efficiency in a positive feedback. At higher latitudes and elevations, pole‐facing hillslopes tend to have less vegetation cover, greater erosional efficiency, and gentler slopes, thus reversing the pattern of asymmetry found at lower latitudes and elevations. Our conceptual model emphasizes the linkages among short‐ and long‐timescale processes and across CZ sub‐disciplines; it also points to opportunities to further understand how CZ processes interact. We also demonstrate the importance of paleoclimatic conditions and non‐climatic factors in influencing slope aspect variations. Copyright © 2017 John Wiley & Sons, Ltd.

Relatively little is currently known about the spatiotemporal variability of land surface conditions during the North American monsoon, in particular for regions of complex topography. As a result, the role played by land-atmosphere interactions in generating convective rainfall over steep terrain and sustaining monsoon conditions is still poorly understood. In this study, the variation of hydrometeorological conditions along a large-scale topographic transect in northwestern Mexico is described. The transect field experiment consisted of daily sampling at 30 sites selected to represent variations in elevation and ecosystem distribution. Simultaneous soil and atmospheric variables were measured during a 2-week period in early August 2004. Transect observations were supplemented by a network of continuous sampling sites used to analyze the regional hydrometeorological conditions prior to and during the field experiment. Results reveal the strong control exerted by topography on the spatial and temporal variability in soil moisture, with distinct landscape regions experiencing different hydrologic regimes. Reduced variations at the plot and transect scale during a drydown period indicate that homogenization of hydrologic conditions occurred over the landscape. Furthermore, atmospheric variables are clearly linked to surface conditions, indicating that heating and moistening of the boundary layer closely follow spatial and temporal changes in hydrologic properties. Land-atmosphere interactions at the basin scale (ϳ100 km 2 ), obtained via a technique accounting for topographic variability, further reveal the role played by the land surface in sustaining high atmospheric moisture conditions, with implications toward rainfall generation during the North American monsoon.

Abstract:In semiarid complex terrain, the landscape creates spatial niches for different types of vegetation through the effects of aspect, slope and curvature on the water and energy balance at the soil surface. The ecohydrology of rangelands is defined by the interaction of soils, plants and climate occurring on a topographic surface. While these interactions have been studied for subtle terrain, little is known about the controls exerted by terrain position, in particular terrain aspect, on ecosystem processes. Furthermore, differential plant establishment can lead to measurable differences in rates of soil development, which in turn can affect soil hydraulic properties and the surface water balance. In this study, we outline the physical mechanisms affecting plant establishment, soil development and hydrologic fluxes in semiarid complex terrain. We illustrate the interactions between vegetation, root zone water fluxes and soil development using, as an example, a small drainage basin in the Sevilleta National Wildlife Refuge (SNWR), New Mexico. In the study basin, opposing hillslopes are characterized by marked differences in ecosystem composition and soil profile properties, with the north-facing hillslope dominated by one seed juniper (Juniperus monosperma) and the south-facing slope consisting of creosote bush (Larrea tridentata). We assess the effect of terrain aspect on root zone hydrologic fluxes and soil development in the two ecosystems by using soil observations, hydraulic properties from pedotransfer functions (PTFs), and numerical modelling of vadose zone fluxes. Modelling results show marked differences in root zone fluxes in the north-facing juniper and south-facing creosote ecosystems. Differences in the amplitude and frequency of soil water content and pressure correspond to changes in soil profile and vegetation characteristics. For example, soil properties of the calcium carbonate (CaCO 3 ) horizons and differential plant water uptake impact the simulated soil water pressure over an 8-year period in the opposing ecosystems. It is believed that these variations in water fluxes reinforce the development of CaCO 3 horizons present in the soil profiles, leading to a feedback between vegetation establishment, soil water fluxes and geomorphic processes in the catchment. Our results also indicate that soil properties and water fluxes compensate for large differences in evaporative demand and lead to similar actual evapotranspiration (AET) in the opposing slopes.

Climate and topographic conditions in a first‐order semiarid catchment in central New Mexico have given rise to opposing hillslopes characterized by different soil profile, vegetation and landform characteristics. In this study, we present the differential response of these two hillslope ecosystems to a geomorphically significant (GS) flood event based upon field observations of rainfall, soil moisture and peak channel discharge. We illustrate the role played by slope position, soil properties and vegetation on soil moisture dynamics and runoff production. Furthermore, we document observed geomorphic changes in the opposing slopes. Analysis of the hillslope and channel response to this exceptional event provides insights on the terrain‐soil‐vegetation interactions acting on the movement of water and sediments through the semiarid system.

Soil moisture distributions are expected to be closely tied to ecosystem processes in water-limited environments of the southwest United States. Nevertheless, few studies have addressed how soil moisture varies across grassland to forest transitions frequently observed in semiarid mountain settings. In this study, we quantify the vegetation controls on surface soil moisture by sampling a range of different ecosystems present in the Valles Caldera, New Mexico. Soil and atmospheric variables were measured during a 2-week field campaign conducted in late July to early August 2005 during the North American monsoon. Field observations were supplemented by a network of continuous instruments used to assess conditions prior to and after the sampling campaign. Results reveal that soil moisture responds directly to summer precipitation events and is mediated by plant interception, which differs across the grassland-forest continuum. The nature of the spatial and temporal variations in soil moisture changes across the different sampled ecosystems: wetlands, riparian forests, grasslands, ponderosa, deciduous and mixed conifer forests. In particular, statistical analyses of soil moisture distributions indicate that distinct regimes (e.g. probability density functions) exist along the semiarid vegetation gradient, which may not be revealed through simple metrics such as the ecosystem average. Ecosystem differences are further elucidated through comparison of the spatial variations in each vegetation type, indicating higher variability in wetland and grassland sites.

[1] In semiarid complex terrain, the combination of elevation and aspect promotes variations in the water and energy balance, resulting in slopes with distinct ecologic and hydrologic properties. Quantifying the differential energy and water dynamics of opposing slopes can provide essential information on the potential effects of climate variability on landscapes. In this study, we use observations from a network of hydrologic sensors deployed on the slopes of a semiarid catchment in central New Mexico, USA, to quantify the ecohydrologic dynamics of two coexisting and contrasting ecosystems : a juniper (Juniperus monosperma) savanna on a north facing slope (NFS) and a creosote (Larrea tridentata) shrubland on a south facing slope (SFS). Our analyses show that: (1) energy loads exert a first-order control on the dynamics of evapotranspiration and soil moisture residence times in the catchment, with vegetation imposing a second-order control at the onset of the growing season; (2) soils exhibit a characteristic progression of moisture and temperature along the slope-aspect continuum that is preserved throughout the year, going from a wetter and cooler NFS to a drier and warmer SFS; (3) there are remarkable differences in the runoff dynamics among the catchment slopes, with a smaller precipitation threshold triggering larger SFS runoff amounts than at its NFS counterpart; and (4) seasonal water balances of the NFS and SFS follow opposite trajectories in the year and point to distinct soil water pools for evapotranspiration demands. The results of this study have important implications for understanding landscape changes in areas of complex topography under current and future climate variability.

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