H43I-01
Exploring Soils and Ecohydrological Structure in Small Watersheds Using Electromagnetic Induction.
Soils, through their control over resources in drylands play a fundamental, but often unquantified role in determining the structure, function and diversity of these terrestrial ecosystems. There is an important ecological need for quantitative subsurface data, spatially distributed, that links soils, water, and plant community structure. Pressing ecohydrological questions in the Western US include the contribution of subsurface properties and processes to forest die back, in such species as pinyon pine and quaking aspen. The role of soil resources, nutrients or soil moisture is not fully understood. Soil texture patterns and the location of subsurface flow paths undoubtedly contribute to plant community structure. Understanding the soil-water-vegetation links within the ecosystem are important for understanding plant community emergent behavior and structure. We demonstrate the use of electromagnetic induction (EMI) for mapping a small watershed (41 ha) at the Reynolds Creek experimental watershed in Idaho. EMI mapping, compared to auguring soil cores, increases the number of soil measurements across a watershed that can be obtained by about 3 orders of magnitude, from 15 cores per day to perhaps 15,000 EMI measurements per day. The EMI map is highly correlated with the soil texture and illuminates the location of flow paths and subsurface colluvium accumulation zones. Vegetation community mapping combined with EMI mapping allows the identification of plant community niches within the watershed as related to subsurface properties. The information obtained, and the insight gained, helps us to understand how subsurface processes contribute to ecohydrological structure in drylands.
H43I-02
Patterns and Processes in Southwestern shrublands and grasslands: role of vegetation, soil- geomorphology, and overland flow
Pattern of variable soil properties have been linked to vegetation as well as soil-landform characteristics and processes. It has been long hypothesized that patterns of infiltration and overland flow play key roles in arid and semi-arid region ecohydrology. Specifically, the process of redistribution of water and sediments have been linked to vegetation related feedbacks that enable persistence of vegetation in water limited environments. Yet, the processes of redistribution, such as through runoff and surface ponding, have been poorly described or documented. We have documented that the spatial pattern of soil properties is dependant on the vegetation pattern as well as the type of, and in some cases location within a, landform. These patterns are likely due to feedbacks between vegetation and the surface processes that affect soil properties and therefore water availability. In this paper, we present observations and numerical simulation that show how patterns of overland flow and infiltration are affected by vegetation-topography related patterns of soil properties. We have developed a numerical model that works on 10 cm grid cells that can inform on the processes of infiltration and overland flow over continuously varying soil properties. We use this model to show how the patterns of soil properties affect runoff, as well as the conditions under which redistribution via runon and ponding can occur. Furthermore, we show using data from a central New Mexico grassland and shrubland, and an eastern Mojave Desert shrubland how climatic differences can affect the patterns of infiltration and runoff.
H43I-03 INVITED
A Complex System Perspective for Integrating Hydrologic,Geomorphologic,and Ecological Dynamics
River networks and the transport processes that take place in them provide a natural integrating framework for the hydrologic,ecologic,and geomorphologic dynamics that take place in river basins.The profound commonalities existing among all types of river basins and their drainage networks,together with the key role that these dendritic structures play in the above dynamics encourage the search for general type of statistical signatures in a variety of phenomena realted to the above intertwined dynamics. We identified the main sources of complex behaviour associated with dendritic lanscapes,relate them to the structure of the channel network and its transport characteristics and explore how these affect the patterns and dynamics of the complex systems of hydrologic,geomorphologic,and ecological character associated with them. The statistical signatures of alpha,beta,and gamma biodiversities of riparian vegetation are studied as well as the structure of the spatial organization of the vegetation in the basin.
H43I-04
Spatial organisation of vegetation in water controlled landscapes: the role of vegetation dispersion strategies.
When investigating the spatial organization of biomass resulting from plant-water feedbacks in arid ecosystems, spatial movement of plants is generally represented as a diffusive processes. Diffusive representations of plant movement have two consequences: they fix the length scale of dispersion to the immediate vicinity of the parent plant, and thus cannot explicitly represent long distance dispersal; and they treat the local gradient of biomass as the determinant of the rate and direction of dispersal. An adaptation of an existing model (Rietkerk et al. 2002) is used to investigate the significance of these assumptions on predicted spatial distributions of vegetation. Model runs were conducted separately with a diffusive and a long-distance dispersive kernel for spatial movement and the resulting spatial organization of vegetation was contrasted. The results indicate that dispersion behavior strongly influences the spatial organization of vegetation in arid landscapes, destabilizing the regular spatial patterns often predicted by models utilizing diffusive representations of biomass movement. The model results provide insight into ecological hypotheses regarding the preponderance of short-range dispersion behavior observed in arid ecosystems, and suggest that this is a consequence of the highly organized nature of the soil water resource. Rietkerk, M., M. C. Boerlijst, F. van Langevelde, R. HilleRisLambers, J. van de Koppel, L. Kumar, H. H. T. Prins, and A. M. de Roos. 2002. Self-organization of vegetation in arid ecosystems. American Naturalist 160:524-530.
H43I-05
Vegetation Dynamics and Soil Water Balance in a Water-limited Mediterranean Ecosystem on Sardinia, Italy
Semi-arid regions, such as around the Mediterranean, suffer from broad desertification processes produced by both natural and human influences. Mediterranean ecosystems are commonly heterogeneous savanna-like ecosystems, with contrasting plant functional types (PFTs, e.g., grass and woody vegetation) competing for the water use. At the same time the structure and function of the vegetation regulates the exchange of mass, energy and momentum across the biosphere-atmosphere interface, influencing strongly the soil water budget. With the objective to investigate vegetation dynamics, soil water budget and land-surface fluxes interactions in a water-limited ecosystem, an extensive field campaign in a Mediterranean water-limited field is performed, and a parsimonious and robust vegetation dynamic model (VDM) is coupled to a 3-component (bare soil, grass and woody vegetation) LSM. The case study is in Orroli, situated in the mid-west of Sardegna within the Flumendosa river watershed. Sardinia is a region that suffers from water scarcity, and the Flumendosa basin plays a primary role in the water supply for much of southern Sardinia, including the island's biggest city, Cagliari. The site landscape is a mixture of Mediterranean patchy vegetation types: trees, including wild olives and cork oaks, different shrubs and herbaceous species. An extensive field campaign started in April 2003. More than three years of data are available. Interestingly, hydrometeorological conditions of the monitored years strongly differ, with dry and wet years in turn, and a wide range of hydrometeorological conditions can be analyzed. Land-surface fluxes and CO2 fluxes are estimated by an eddy correlation technique based micrometeorological tower. Soil moisture profiles were also continuously estimated using water content reflectometers and gravimetric method, and periodically leaf area index (LAI) estimates of both plant types are made using the Accupar LP-80 by Decagon Devices Inc. Furthermore, two high spatial resolution (2.8 m) Quickbird satellite images were acquired in August of 2003 and March 2004 for defining the spatial organization of the main land cover types around the tower for two contrasting seasons of the year (Summer and Spring). A parsimonious ecohydrologic model is developed. The VDM computes the change in biomass over time as difference between the rate of production (e.g., photosynthesis) and the rate of destruction (e.g., respiration and senescence). VDM incorporates two PFTs using basic rules regarding competition for a limiting resource. The VDM is then coupled to a 3-component LSM, with the VDM providing the green biomass and the LAI evolution through time, and the LSM using this information in the computation of the land surface fluxes and updating the soil water content in the root-zone. The coupled VDM-LSM model is successfully tested for the case study, demonstrating high model performance for the wide range of eco-hydrologic conditions. The inclusion of the VDM in the LSM is demonstrated to be essential when studying the climate-soil-vegetation interactions of these water-limited ecosystems. Results demonstrate also that vegetation dynamics are strongly influenced by the inter-annual variability of atmospheric forcing, with grass leaf area index changing significantly each spring season according to seasonal rainfall amount.
H43I-06 INVITED
Diagnosing coupled watershed processes using a fully-coupled groundwater, land-surface, surface water and mesoscale atmospheric model
A variably-saturated groundwater flow model with an integrated overland flow component, a land-surface model and a mesoscale atmospheric model is used to examine the interplay between coupled water and energy processes. These processes are influenced by land-surface topography and subsurface heterogeneity. This parallel, integrated model simulates spatial variations in land-surface forcing driven by three-dimensional (3D) atmospheric and subsurface components. Spatial statistics are used to demonstrate spatial and temporal correlations between surface and lower atmospheric variables and water table depth. These correlations are particularly strong during times when the land surface temperatures trigger shifts in wind behavior, such as during early morning surface heating. Additionally, spectral transforms of subsurface arrival times are computed using a transient Lagrangian transport simulation. Macrodispersion is used to mimic the effects of subsurface heterogeneity for a range of Peclet numbers. The slopes of these transforms indicate fractal scaling of this system over a range of timescales. All of these techniques point to importance of realistically representing coupled processes and the need to understand and diagnose these processes in nature. This work was conducted under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory (LLNL) under contract W-7405-Eng-48. This project was funded by the Laboratory Directed Research and Development Program at LLNL
H43I-07
Impacts of Remotely-Sensed Vegetation Dynamics on Ecohydrological Response in a Small Mountainous Watershed
In forested mountain watersheds, the vegetation dynamics may play an important role in the seasonal distributed hydrologic response. For example, the seasonal dynamics of plant albedo and leaf area would affect the energy and mass balances in the hydrologic system through incoming radiation and rainfall interception. In this study, we attempt to capture the seasonal variations in mountain vegetation, including its albedo and areal fraction, from remotely-sensed datasets and then investigate its impact on the predictions of a distributed hydrological model. The study site is a small mountainous basin known as the Redondo Creek of the Valles Calderas National Preserve, New Mexico. Our initial focus is on distributed simulations of the 2005 summer monsoon period using the TIN-based Real-time Integrated Basin Simulator (tRIBS). We have modified tRIBS to incorporate remotely sensed vegetation parameters at prescribed temporal intervals. The vegetation observations are derived from two sensors: (1) fine spatial resolution Landsat 5 TM at 16-day intervals, and (2) is coarse spatial resolution MODIS composites at 8-day intervals. To force the distributed hydrologic model during the monsoon period, we utilize in- situ hydrometeorological forcing from a nearby weather station and gauge-corrected NEXRAD radar observations. Our model results are compared against the observed daily soil moisture obtained at a network of sites during late July of 2005 and the hourly soil moisture estimates at the weather station. We also assess model performance at a particular site relative to transpiration observations using the sapflow method. The model results using the dynamic vegetation are compared with a temporally static vegetation case to reveal the impacts of seasonal dynamics. Our study points to the importance of capturing spatiotemporal vegetation dynamics in hydrological simulations of forested ecosystems.