Biosphere-Atmosphere-Hydrology Interactions in Semiarid Environments I
Presiding: C J Watts, Universidad de Sonora; E R Vivoni, New Mexico Institute of Mining and Technology
A33A-01 INVITED 13:35h
Toward a More Plant-Centric View of the Water Budget: A Central Conundrum for Unraveling Biosphere-Atmosphere-Hydrology Interactions
Much emphasis has recently been placed on the need to more fully integrate ecology and hydrology explicitly, which implicitly also requires more fully integrating atmospheric interactions. Although environmental scientists from different disciplines have been studying aspects for the water budget for decades or longer, partitioning of the water budget in an ecologically meaningful way remains a central challenge in addressing biosphere-atmosphere-hydrology interactions. Here I provide a plant-centric view of the water budget and highlight major research challenges, drawing on examples from semiarid woodlands. Recent studies highlight how precipitation patterns can be evaluated in a more ecologically meaningful manner. Intercepted precipitation is assumed to evaporate from plant foliage, but may be absorbed and improve plant water status. Redistribution of runoff can contribute to re-concentration of water that increases the proportion of precipitation obtained by plants. Soil water content is highly spatially heterogeneous at the scale of individual plants, but usually only vertical heterogeneity with depth is considered. A central challenge is to partition soil evaporation from plant water uptake and subsequent transpiration. The proportion of cover by woody plants in particular has large effects on biosphere-atmosphere-hydrology interactions, and hence changes in woody cover through drought, fire, thinning, and succession are of key interest. A more plant-centric view of the water budget is required to address the challenging interactions and feedbacks associated with biosphere-atmosphere-hydrology interfaces.
A33A-02 INVITED 13:55h
Natural (and Managed) Islands of Hydrologically Enhanced Biotic Productivity in Arid Zones
Numerous physical characteristics influence runoff generation in arid regions such as the Jornada basin of southern New Mexico. Factors causing high spatial variations of water in deserts include precipitation, soil, physiography, and vegetation characteristics. The inherent heterogeneity of these characteristics in arid zones causes discontinuous areas of runoff and run-on which creates islands of hydrologically enhanced biotic productivity. These hydrologic islands are observed at the plant scale where they coincide with islands of fertility or resource islands. At the larger scale the islands of hydrologically enhanced biotic productivity can be observed as banded vegetation, beaded drainage networks, and playitas and playas. In the rehabilitation of degraded rangelands it is wise to mimic the natural processes which tend to slow down surface flow and allow time for infiltration, thus creating areas of high vegetation productivity. There have been several successful attempts in the Jornada basin to establish spatial discontinuities of water and vegetation similar to natural occurring patterns. The most successful treatments so far have been shallow, water ponding dikes and water spreaders.
A33A-03 14:15h
New Mexico EPSCoR: a Statewide Ecohydrology and Flux Network Within a Semi-arid Region
Semi-arid regions are often comprised of numerous biomes, from highly productive gallery forests along riparian corridors, to desert shrub or grassland steppes, and high-elevation mixed conifer forests. Each of these vegetation assemblages across the landscape regulates hydrologic and atmospheric fluxes both locally and within basins. We are introducing a recently initiated NSF-EPSCoR project in ecohydrology to integrate measurements within the Rio Grande basin at these appropriate scales. Eddy covariance flux towers are being upgraded or established in four nodes within the state of New Mexico: (1) at five riparian locations along the Middle Rio Grande, (2) four agricultural and open water locations in the Lower Rio Grande, (3) two middle-elevation sites on the Sevilleta NWR, and (4) four upper elevation sites in the Upper Rio Grande. These local measurements are to be scaled throughout the basin using a combination of integrated remote sensing and hydrologic modeling. Each site is additionally instrumented with groundwater wells, where appropriate. Previous results from existing towers illustrate species-specific variation in water and energy fluxes due to the response of vegetation to changes in groundwater depth, groundwater chemistry (e.g., nitrate and chloride), flooding, drought, micrometeorological conditions, and topographical constraints. Constructing the infrastructure for within-basin networking of evapotranspiration and fluxes of water, energy, and carbon will lead to a better understanding of the coupled responses and feedbacks between vegetation, hydrology, and the atmosphere across multiple teleconnected biomes.
A33A-04 14:30h
Vegetation-Atmosphere Interactions in Southern Sonora Associated with the North American Monsoon
The North American Monsoon (NAM) provides most of the annual rainfall in the arid conditions of northwestern Mexico and southwestern US. The onset of the rains produces a very rapid increase in the foliage of many biomes in the region. Perhaps the most spectacular example is the tropical deciduous forest (TDF) which is found on the western slopes of the Sierra Madre Occidental, south of latitude 29N. At the beginning of July 2004 a 15m tower was installed in the TDF near Tesopaco, Sonora in order to record the changes in the components of the surface energy balance as the monsoon develops and decays. Data has been collected continuously since Temperature and humidity profiles for the atmospheric boundary layer were obtained from radiosondes which were released regularly near the site and vegetation changes in the region were monitored using SPOT-VEGETATION imagery. A network of pilot balloon stations on both sides of the Gulf of California provided information about the diurnal and spatial distribution of the wind field in the region. TheThe North American Monsoon (NAM) provides most of the annual rainfall in the arid conditions of northwestern Mexico and southwestern US. The onset of the rains produces a very rapid increase in the foliage of many biomes in the region. Perhaps the most spectacular example is the tropical deciduous forest (TDF) which is found on the western slopes of the Sierra Madre Occidental, south of latitude 29N. At the beginning of July 2004 a 15m tower was installed in the TDF near Tesopaco, Sonora in order to record the changes in the components of the surface energy balance as the monsoon develops and decays. Data has been collected continuously since Temperature and humidity profiles for the atmospheric boundary layer were obtained from radiosondes which were released regularly near the site and vegetation changes in the region were monitored using SPOT-VEGETATION imagery. A network of pilot balloon stations on both sides of the Gulf of California provided information about the diurnal and spatial distribution of the wind field in the region. These data have been used to investigate the importance of feedback mechanisms between changing land surface conditions and the atmosphere. This work was carried out during the Enhanced Observation Period of the North American Monsoon Experiment (NAME) in the summer of 2004.
A33A-05 14:45h
Investigating hydrologic and atmospheric pathways to summertime convective rainfall in the Southeastern United States
The pathways to summertime convective rainfall remains a critical research topic due to the feedbacks between soil moisture content and the atmosphere. Understanding these feedbacks is particularly crucial in the Southeastern U.S. given the high productivity of these ecosystems and the role of convective precipitation in maintaining this productivity. At least 50% of the precipitation events during the summer season can be attributed to convective rainfall (even during a severe drought year). Using a combination of (i) 7 years sensible heat flux and micrometeorological time series data collected at the Duke Forest Ameriflux pine site, (ii) a simplified mixed layer slab model, and (iii) conditional analysis of half hour precipitation, we demonstrate that pathways to convective rainfall can be classified as quadrants on a soil moisture content (SM) and relative humidity (RH) plane. This 7-year record includes among the wettest and driest years within the 50-year record. For days with convective rainfall and for SM in excess of certain thresholds (~0.2), triggers to convective precipitation become primarily controlled by relative humidity (RH) and appear not sensitive to SM variations. However, during excess droughts (e.g. summer of 2002), RH and SM become highly correlated suggesting an entirely different pathway to convective precipitation. The fact that SM can control these feedbacks in the Southeastern U.S. even during drought years may have been underestimated in previous studies of convective rainfall.