Measurement and Monitoring Methods in Ecohydrology II
Presiding: B D Newman, Los Alamos National Laboratory; B R Scanlon, Bureau of Economic Geology, University of Texas at Austin
H22A-01 INVITED 10:30h
Ecohydrology Applications of Atmospheric Stable Isotope Measurements via Tunable Diode Laser Absorption Spectroscopy
The stable carbon isotope composition of plant organic matter has commonly been used as an index of plant carbon-water balance, or water-use efficiency, for decades. Likewise, the oxygen isotope composition of plant organic matter has been used for understanding the interaction between climate, water source and physiology. The carbon and oxygen stable isotope composition of atmospheric carbon dioxide is recently being used to understand ecosystem-scale patterns and controls over the interaction of climate and the carbon and water cycles. However, sampling limitations have made such investigations costly both in time and finances, prohibiting extensive, continuous data sets. A new technique, Tunable Diode Laser Absorption Spectroscopy (TDLAS), allows continuous monitoring of the carbon and oxygen isotope composition of atmospheric carbon dioxide at one-minute resolution, thereby dramatically broadening the opportunities to study the patterns and controls over ecohydrologic functions of terrestrial ecosystems. This talk will describe TDLAS methodology, applications, and recent results from an ongoing ecohydrology study of a pinon-juniper woodland.
H22A-02 10:45h
Comparison of Different Approaches for Relating Ecology and Hydrology in Semiarid Regions
A variety of approaches can be used to assess the impact of linkages between ecology and hydrology on the water cycle in semiarid regions. We present a range of techniques from point to regional scales that document ecological controls on the water cycle. Nonvegetated and vegetated precision weighing lysimeters showed the importance of variations in biomass productivity in dampening the impact of elevated El Nino winter precipitation on subsurface water storage in the Mojave Desert, Nevada. Soil water storage data were compared with ground-based and satellite-based biomass productivity to regionalize the results from the lysimeter data. Soil water potential monitoring in various desert basins throughout the southwestern US shows dramatic differences in the soil water balance between winter when vegetation is dormant and spring when vegetation reactivates. Vertical profiles of soil water potential sensors also provide indirect evidence of the depth and timing of root water uptake. Soil water potential and chloride profiles in the Southern High Plains also show strong relationship between land use and subsurface water movement. Higher recharge beneath dryland (nonirrigated) relative to rangeland settings is shown by high soil water potentials and low chloride concentrations. The point unsaturated zone data are regionalized using decadal variations in groundwater levels. Combining various approaches that range in space and time scales provides a more comprehensive understanding of linkages between ecology and hydrology in these water-limited systems.
H22A-03 11:00h
Soil Zone Ecohydrology of a Semiarid Woodland at the Canopy-Intercanopy Scale
Semiarid ecosystems are often characterized by patchy vegetation distributions. The pinon-juniper (Pinus edulis and Juniperus monosperma) woodlands of the southwestern U.S. are a good example of this patchiness where grassy/bare intercanopy patches are mixed with woody, piñon-juniper canopy patches. In this study, we examined how these two patch types influence ecohydrological factors such as evaporation, downward water fluxes, and nutrient (nitrate) distributions. Examination of & ΔD values of soil waters show statistically significant differences between canopy and intercanopy patches indicating increased evaporative removal of water in the intercanopy patches. In contrast to the isotope/evaporation data, chloride based downward water fluxes are not significantly different, suggesting that while evaporation to transpiration ratios vary between canopy and intercanopy patches, total evapotranspiration may be similar (as implied by the lack of a difference in downward fluxes). This interpretation indicates the importance of understanding the partitioning of evaporation and transpiration, and how this ratio shifts within ecosystems. It may also help explain why patchy vegetation distributions develop and persist. Patchiness also appears to be important for understanding nutrient distributions. Nitrate concentrations were drastically different between the patch types, with higher nitrate concentrations and inventories occurring in the intercanopy patches. These results highlight the importance of the canopy-intercanopy scale for understanding the ecohydrology of patchy, semiarid woodlands.
H22A-04 11:15h
Asynchrony Controls on Biogeochemical Fluxes in a Mediterranean Climate
Southern California has some of the highest rates of atmospheric nitrogen deposition recorded in the world. These high rates of atmospheric deposition have resulted in elevated levels of dissolved nitrogen in some streams in southern California and may have contributed to landscape level changes in vegetative communities. The levels of nitrogen (overwhelmingly as nitrate) in streams correlate with atmospheric deposition in the region but there is also considerable spatial and temporal variability. The variability in space and time appears to be due to differences in hydrologic flowpath and biogeochemical cycling and how they affect the fate, storage and transport of nitrogen in the environment of the dominant semi-arid Mediterranean ecosystems of southern California. Catchment scale research in southern California has shown several causes for the spatial and temporal differences in nitrogen in theses ecosystems. First, interannual variability appears to be due to some level of nitrate storage within these catchments since wet years following dry years have elevated nitrate concentrations with the reverse also being true. Second, isotopic results recently published indicate that 10% of the nitrate observed at baseflow is direct throughput of atmospherically derived nitrate and during storm events nearly 40% of exported nitrate is throughput of atmospheric nitrate. These high fractions during storm events are likely due in part to direct throughfall into streams. Third, nitrate is well correlated with discharge in any stream in southern California with a significant groundwater flow component, which indicates groundwater storage of nitrate. Fourth, since the water in storm event flows bears a groundwater signature the nitrate observed in stormflows must have undergone some level of storage within the catchments vadose zone/groundwater system. Taken together the interannual variability, correlation with discharge, isotopic data and mixture modeling results indicate that the common temporal disconnect (asynchrony) between when and where nitrogen is physically available and when and where biological processes demand this nitrogen play a leading control in the export and processing of nitrogen in seasonally dry ecosystems. Additionally the increased fertility offered by the within landscape storage of nitrogen appears to contribute to a positive feedback encouraging the extirpation of coastal sage scrub and their replacement with exotic annual grasslands.
H22A-05 11:30h
Remote Sensing of Semiarid Ecosystems: Methods for Determining How Water Availability Affects Vegetation Pattern and Process
A combination of field investigations, modeling, and satellite remote sensing over spatial scales ranging from O100-105 m are used to infer how water availability governs the distribution and function of semi-arid vegetation. The Kalahari Transect (KT) in southern Africa, which spans a north-south aridity gradient from approximately 1600 mm to 200 mm of mean annual rainfall, was selected as the field setting for this investigation due to its features that are amenable for remote sensing, which include uniform sandy soils and level topography. At the landscape scale, high-resolution IKONOS images reveal that tree clustering at sites along the KT is fractal, and cellular automata modeling indicates that this spatial organization is driven by interannual variability in rainfall. The IKONOS images also allow us to assess how landscape heterogeneity affects land-atmosphere interaction in terms of the spatial variability of evapotranspiration. A large eddy simulation model applied to the remotely-sensed boundary conditions shows that water availability profoundly affects the degree of spatial coupling between the vegetation and surface fluxes. At the regional scale, 16-year time series of NDVI fields from the AVHRR sensor combined with ground-based rainfall fields enable quantification of the land cover distribution (tree, grass, bare soil) over the entire transect. The vegetation functional types respond to water availability at vastly different timescales: a soil moisture model applied to the satellite-derived land cover reveals that tree density is highly correlated with long-term mean rainfall, while the annual extent of grass cover can be successfully simulated by considering only the transient availability of near-surface soil moisture and the tree cover at a particular location. These results suggest that mixed tree/grass semi-arid ecosystems are ideally suited to reach a dynamic equilibrium with respect to the use of a fluctuating limiting resource (water) by having functional components that responds to variability in rainfall over long timescales (trees) and short timescales (grasses).
H22A-06 11:45h
Assessing Spatial-Temporal Variability of Vegetation-Precipitation Interactions with Combined NEXRAD and MODIS Data
We address the spatial and temporal variability of the land surface's response to precipitation forcing events. We focus on the Missouri River Basin from January 17-October 2, 2002. The surface data consist of the Moderate Resolution Imaging Spectroradiometer (MODIS) surface temperature (8-day composite) and the Normalized Difference Vegetation Index (NDVI, 16-day composite). Spatially distributed precipitation estimates are derived from composited Next-Generation Radar (NEXRAD) data. The dominant temporal and spatial scales of surface response to precipitation are quantified through a variety of methods. Preliminary results will be presented, focusing upon correlation and lagged-covariances. In addition, information theory metrics (entropy, mutual information content) are used to characterize the information transfer between precipitation and the remotely sensed surface conditions at different temporal scales. The implications for water and energy cycling in spatially heterogeneous landscapes will be discussed.