Measurement and Monitoring Methods in Ecohydrology I
Presiding: B D Newman, Los Alamos National Laboratory; B R Scanlon, Bureau of Economic Geology, University of Texas at Austin
H21E-01 08:30h
Unraveling Ecohydrological Fluxes: Separating the Grain From the Chaff
In order to better understand energy, water and carbon cycling in ecosystems, ecological, hydrological, and meteorological measurements can be combined in creative ways to illuminate the myriad of component processes that control these exchanges. This talk with feature examples of this approach from experiments carried out in semiarid south-western U.S. In the first experiment, multiple eddy covariance systems were deployed to partition ecosystem water fluxes into their overstory and understory components in riparian woodland in order to partition ecosystem evapotranspiration into its groundwater or surface water sources. In the second experiment, sap flow sensors were deployed on lateral (surface) and tap (deep) roots of trees and borehole ground penetrating radar was used to quantify deep vadose zone soil moisture changes in order to assess the hydraulic redistribution activity of trees. In the final example, a combination of eddy covariance and sap flow were used to partition ecosystem evapotranspiration in a Chihuahuan desert shrubland. By determining the transpiration and evaporation, a more complete picture of the timing and interplay between these fluxes is given and the relationship between these fluxes and ecosystem carbon exchange is shown.
H21E-02 08:45h
Defining an ecohydrograph
One area of improvement in rainfall-runoff models in small catchments may be the parameterization of vegetation water storage. Recharge of water into vegetation tissues after a rain event may be of sufficient magnitude with, and occur over time periods comparable to, characteristic magnitudes and dynamics of small watershed hydrographs. Yet little work has been done to characterize how variable water storage capacity accompanying different vegetation types or statures may influence stream hydrographs. Here we show how direct characterization of vegetation hydraulic capacitance, through the use of sap flux measurements, may be used to improve the representation of vegetation in watershed hydrodynamics.
H21E-03 09:00h
An Overview of Advances in Water Content Sensing for Small Watersheds and Ecohydrological Studies
Water content sensors based on the determination of soil permittivity have become a standard method of determining soil water content. The real permittivity of a material relates to its ability to store electrical energy. The permittivity of free water is about 80 whereas air is 1, and most soil minerals are between 4.5 and 9. Therefore, the proportion of water in a composite material strongly influences the bulk permittivity. Sensors exploiting this offer high temporal resolution data that can be collected using a data logger. However, the quality of water content determination is sensor specific, this depends on both sensor quality and certain soil properties. Two main soil properties influence sensor water content determination, bulk soil electrical conductivity and soil dielectric dispersion. The former, largely due to soil salinity causes attenuation of most electromagnetic signals and leads to increasingly poor water content estimates for bulk soil electrical conductivities above 2 dS/m. Dielectric dispersion relates to the real permittivity of the material changing as a function of frequency. Dielectric dispersion occurs particularly in clay soils and can result in different calibrations for sensors operating at different frequencies. Both of these soil phenomena are impacted by soil temperature. Although these instruments are good for point measurements and for field arrays there is a need to obtain better spatial water content data. We have tried utilizing electromagnetic survey data of small watersheds to indicate zones of hydrological interest. In particular, we are interested in using this approach to rapidly appraise the spatial distribution of soil depth and soil moisture content in research watersheds, to guide sensor placement. An ultimate goal is to develop methods for improved spatial determination of water content at the small watershed scale.
http://soilphysics.usu.edu
H21E-04 09:15h
Comparison of Seven Electromagnetic Water Content Sensors Commonly Used in Ecohydrological Studies
Numerous electromagnetic (EM) sensing systems are available which employ permittivity estimates to infer soil water content, but a standard method for characterizing EM sensing system measurement capability has not been established. Our objective was to evaluate the measurement accuracy and range of seven different systems using readily available dielectric liquids. Sensor outputs of travel time or voltage were converted to real permittivity and compared to real permittivity measurements using a network analyzer. Using standard waveform analysis, three higher frequency broadband sensing systems deviated from the network analyzer by less than ± 3.0 real permittivity units across a real permittivity range of 13 to 79 in lossless liquids. Two lower frequency impedance sensing systems deviated from the network analyzer by less than ± 4.0 real permittivity units across a real permittivity range of 13 to 37 in the same media. For the higher frequency systems, imaginary permittivity values (due only to dielectric relaxation) of up to 15 resulted in real permittivity errors of ± 0.51, whereas electrical conductivity values up to 2 dS/m resulted in real permittivity errors of ± 2.7. Temperatures in the range of 5 to 40 °C resulted in errors of ± 4.9. For four lower frequency systems, dielectric relaxation, electrical conductivity and temperature effects resulted in real permittivity errors of up to ± 6.6, ± 111 and ± 3.3, respectively. These effects on measurement accuracy are to a large extent dependent on measurement frequency; with higher frequency sensing systems generally yielding better measurements.
H21E-05 09:30h
Hillslope Scale Rainfall Simulation for Understanding Water Fluxes on Rangelands
A variety of approaches are available for understanding runoff processes on rangelands but generally these approaches will involve measurements during natural precipitation events or take advantage of rainfall simulation. Rainfall simulation has many advantages but one persistent disadvantage is that scale of observation is often quite small-several square meters or less. Alternatively, runoff from natural rainfall events can be monitored at larger scales but for rangelands, runoff producing events may be few and far between. A potential compromise is to conduct rainfall simulation at the hillslope scale. In this paper we describe a methodology and approach that has been applied with considerable success on shrublands in central Texas. To date we have simulated rainfall above tree canopies at a height of over 10 meters and on an area that is 20 x 26 square meters in size. The simulator is modular and composed of individual masts that support a set of four nozzles each. An advantage of this approach is that rainfall is applied on an area sufficiently large as to incorporate processes and factors that influence runoff at the hillslope scale. During rainfall simulation experiments we make detailed measurements of the major components of the water budget including interception, stem flow, transpiration by woody plants, soil water storage, surface runoff and interflow. Since the system is modular, there is the option of expanding it and applying water on even larger scales. The major limitation is transporting and storing sufficient water for rainfall simulations on such a large scale.
H21E-06 09:45h
Development of a generic system for real-time data access and remote control of multiple in-situ water quality monitoring instruments
Currently, in-situ monitoring of water quality parameters (e.g. water temperature, conductivity, turbidity) in the Colorado River ecosystem typically consists of deploying instruments in the river, retrieving them at a later date, downloading the datalogger, then examining the data; an arduous process in the remote settings of Grand Canyon. Under this protocol, data is not available real-time and there is no way to detect problems with the instrumentation until after retrieval. The next obvious stage in the development of in-situ monitoring in Grand Canyon was the advent of one-way telemetry, i.e. streaming data in real-time from the instrument to the office and/or the world-wide-web. This protocol allows for real-time access to data and the identification of instrumentation problems, but still requires a site visit to address instrument malfunctions, i.e. the user does not have the ability to remotely control the instrument. At some field sites, such as the Colorado River in Grand Canyon, site visitation is restricted by remoteness and lack of traditional access routes (i.e. roads). Even at less remote sites, it may still be desirable to have two-way communication with instruments in order to, for example, diagnose and potentially fix instrumentation problems, change sampling parameters to save battery power, etc., without having to visit the site. To this end, the U.S. Geological Survey, Grand Canyon Monitoring and Research Center, is currently developing and testing a high-speed, two-way communication system that allows for real-time data access and remote control of instrumentation. The approach tested relies on internet access and may be especially useful in areas where land-line or cellular connections are unavailable. The system is composed of off-the-shelf products, uses a commercial broadband satellite service, and is designed in a generic way such that any instrument that communicates through RS-232 communication (i.e. a serial port) is compatible with the system. We are currently testing the system at two sites on the Colorado River in Grand Canyon and at one critical monitoring site on the Paria River where we have deployed suites of instruments for monitoring flow, sediment concentration, temperature, and conductivity. One aspect of the system that may be particularly useful for ecohydrological applications is the ability to remotely control on-site pump samplers, which allows for the collection of a water sample by the press of a button in the office.