D. I. Cooper

Seasonal estimates of riparian evapotranspiration using remote and in situ measurements

In many semi-arid basins during extended periods when surface snowmelt or storm runoff is absent, groundwater constitutes the primary water source for human habitation, agriculture and riparian ecosystems. Utilizing regional groundwater models in the management of these water resources requires accurate estimates of basin boundary conditions. A critical groundwater boundary condition that is closely coupled to atmospheric processes and is typically known with little certainty is seasonal riparian evapotranspiration (ET). This quantity can often be a significant factor in the basin water balance in semi-arid regions yet is very difficult to estimate over a large area. Better understanding and quantification of seasonal, large-area riparian ET is a primary objective of the Semi-Arid Land-Surface-Atmosphere (SALSA) Program. To address this objective, a series of interdisciplinary experimental campaigns were conducted in 1997 in the San Pedro Basin in southeastern Arizona. The riparian system in this basin is primarily made up of three vegetation communities: mesquite (Prosopis velutina), sacaton grasses (Sporobolus wrightii), and a cottonwood (Populus fremontii)/willow (Salix goodingii) forest gallery. Micrometeorological measurement techniques were used to estimate ET from the mesquite and grasses. These techniques could not be utilized to estimate fluxes from the cottonwood/willow (C/W) forest gallery due to the height (20‐30 m) and non-uniform linear nature of the forest gallery. Short-term (2‐4 days) sap flux measurements were made to estimate canopy transpiration over several periods of the riparian growing season. Simultaneous remote sensing measurements were used to spatially extrapolate tree and stand measurements. Scaled C/W stand level sap flux estimates were utilized to calibrate a Penman‐Monteith model to enable temporal extrapolation between synoptic measurement periods. With this model and set of measurements, seasonal riparian vegetation water use estimates for the riparian corridor were obtained. To validate these models, a 90-day pre-monsoon water balance over a 10 km section of the river was carried out. All components of the water balance, including riparian ET, were

Tower and Aircraft Eddy Covariance Measurements of Water Vapor, Energy, and Carbon Dioxide Fluxes during SMACEX

Abstract A network of eddy covariance (EC) and micrometeorological flux (METFLUX) stations over corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] canopies was established as part of the Soil Moisture–Atmosphere Coupling Experiment (SMACEX) in central Iowa during the summer of 2002 to measure fluxes of heat, water vapor, and carbon dioxide (CO2) during the growing season. Additionally, EC measurements of water vapor and CO2 fluxes from an aircraft platform complemented the tower-based measurements. Sensible heat, water vapor, and CO2 fluxes showed the greatest spatial and temporal variability during the early crop growth stage. Differences in all of the energy balance components were detectable between corn and soybean as well as within similar crops throughout the study period. Tower network–averaged fluxes of sensible heat, water vapor, and CO2 were observed to be in good agreement with area-averaged aircraft flux measurements.

Preface paper to the Semi-Arid Land-Surface-Atmosphere (SALSA) Program special issue.

The Semi-Arid Land-Surface-Atmosphere Program (SALSA) is a multi-agency, multi-national research effort that seeks to evaluate the consequences of natural and human-induced environmental change in semi-arid regions. The ultimate goal of SALSA is to advance scientific understanding of the semi-arid portion of the hydrosphere-biosphere interface in order to provide reliable information for environmental decision making. SALSA approaches this goal through a program of long-term, integrated observations, process research, modeling, assessment, and information management that is sustained by cooperation among scientists and information users. In this preface to the SALSA special issue, general program background information and the critical nature of semi-arid regions is presented. A brief description of the Upper San Pedro River Basin, the initial location for focused SALSA research follows. Several overarching research objectives under which much of the interdisciplinary research contained in the special issue was undertaken are discussed. Principal methods, primary research sites and data collection used by numerous investigators during 1997-1999 are then presented. Scientists from about 20 US, five European (four French and one Dutch), and three Mexican agencies and institutions have collaborated closely to make the research leading to this special issue a reality. The SALSA Program has served as a model of interagency cooperation by breaking new ground in the approach to large scale interdisciplinary science with relatively limited resources.

The Boardman Regional Flux Experiment

Abstract A field campaign was carried out near Boardman, Oregon, to study the effects of subgrid-scale variability of sensible-and latent-heat fluxes on surface boundary-layer properties. The experiment involved three U.S. Department of Energy laboratories, one National Oceanic and Atmospheric Administration laboratory, and several universities. The experiment was conducted in a region of severe contrasts in adjacent surface types that accentuated the response of the atmosphere to variable surface forcing. Large values of sensible-heat flux and low values of latent-heat flux characterized a sagebrush steppe area; significantly smaller sen- sible-heat fluxes and much larger latent-heat fluxes were associated with extensive tracts of irrigated farmland to the north, east, and west of the steppe. Data were obtained from an array of surface flux stations, remote-sensing devices, an instrumented aircraft, and soil and vegetation measurements. The data will be used to address the problem of extrapolating from a...

Evaporation and the development of the local boundary layer over an irrigated surface in an arid region

Abstract Heterogeneous landscapes common to arid and semi-arid regions are often characterized by a horizontal advection of sensible heat from dry surfaces to wet surfaces which modifies the evaporation and water balance of moist areas. This study centered around a well-watered alfalfa field surrounded locally by arid surfaces, but within a large complex of irrigated fields where both local and regional scale advection were present. Latent and sensible heat flux densities were measured using eddy correlation at four different locations within the same field. Tethered balloons were used to obtain vertical profiles of temperature, humidity, wind speed and direction from the surface to heights of 50 m at the leading and downwind edges of the field. Results show that sensible heat flux and turbulence intensity were strongly correlated indicating the dependence of sensible heat transfer on turbulence. Evaporation in excess of the equilibrium rate was governed by turbulence intensity and the advection of sensible heat. Growth of the local boundary layer above the alfalfa canopy as defined by temperature and humidity profiles averaged about 30 m over a 450 m traverse. The height of the developed local boundary layer was strongly correlated to the intensity of turbulence. Short-term evaporation rates were calculated from the upwind and downwind profiles of humidity and wind, using a modified vapor budget technique, and found to average approximately 10% lower than the measured evaporation rates. The contribution to evaporation from the advection of sensible heat was also calculated from the upwind and downwind profiles and found to range between 28–90% of the total evaporation.

Development of a scanning, solar-blind, water Raman lidar

The need for an instrument capable of measuring water-vapor fluxes over mixed canopy and large areas has long been recognized. Such a device would greatly enhance the study of evapotranspiration processes and has great practical value for water management. To address this problem, a scanning water Raman lidar has been designed and constructed. Analytical methods have also been developed to take advantage of the type of information that this lidar can generate. The lidar is able to measure the absolute water content and calculate the evaporative flux quickly over relatively large areas. This capability provides new opportunities for the study of microscale atmospheric processes. The variogram data indicate that the spatial sampling size must be of the order of 10 m if fluxes and scalars are to be properly represented. Examples of data are presented.

Spatial variability of water vapor turbulent transfer within the boundary layer

Although the physics of evaporation within the inner region of the boundary layer is believed to be well understood, observations of mass-energy exchange processes have been hindered by the limitations of point sensors. A combination of point sensors and active remote sensing, namely, water-Raman Lidar measurements, offers new opportunities to study relatively large areas at temporal and spatial scales previously unattainable. Results from experiments over uniform canopies both confirm some traditional theories and challenge some of the underlying assumptions concerning the homogeneity of the surface-atmosphere interface and the use of point sensors to characterize large areas.

The application of a scanning, water Raman-lidar as a probe of the atmospheric boundary layer

A scanning water Raman-lidar, designed and constructed to study surface-atmosphere processes with high spatial and temporal resolution is described. It is shown that the lidar is able to measure the absolute water content and then calculate evaporative fluxes and other atmospheric parameters quickly over relatively large areas. This capability provides new opportunities for the study of microscale atmospheric processes. Examples of data and analyses are presented. An analysis is presented which determines the spatial and temporal resolution which is required of a remote sensor in the boundary layer. >

Spatial and temporal properties of water vapor and latent energy flux over a riparian canopy

A scanning, volume-imaging Raman lidar was used in August 1997 to map the water vapor and latent energy flux fields in southern Arizona in support of the (Semi-Arid Land Surface Atmosphere) SALSA program. The SALSA experiment was designed to estimate evapotranspiration over a cottonwood riparian corridor and the adjacent mesquite-grass community. The lidar derived water vapor images showed microscale convective structures with a resolution of 1.5 m, and mapped fluxes with 75 m spatial resolution. Comparisons of water vapor means over cottonwoods and adjacent grasses show similar values over both surfaces, but the spatial variability over the cottonwoods was substantially higher than over the grasses. Lidar images support the idea that the enhanced variability over the cottonwoods is reflected in the presence of spatially coherent microscale structures. Interestingly, these microscale structures appear to weaken during midday, suggesting possible evidence for stomatal closure. Spatially resolved latent energy fluxes were estimated from the lidar using Monin‐Obukhov gradient technique. The technique was validated from sap-flow flux estimates of transpiration, and statistical analysis indicates very good agreement (within 15%) with coincident lidar flux estimates. Lidar derived latent energy maps showed that the riparian zone tended to have the highest fluxes over the site. In addition, the spatial variability of 30 min average fluxes were almost as large as the mean values. Geostatistical techniques where used to compute the spatial lag lengths, they were found to be between 100 and 200 m. Determination of such spatially continuous evapotranspiration from such a complex site presents watershed managers with an additional tool to quantify the water budgets of riparian plant communities with spatial resolution and flux accuracy that is compatible with existing hydrologic management tools.

The Combined Sensor Program: An Air–Sea Science Mission in the Central and Western Pacific Ocean

Abstract Twelve national research organizations joined forces on a 30-day, 6800 n mi survey of the Central and Tropical Western Pacific on NOAAs Research Vessel Discoverer. The Combined Sensor Program (CSP), which began in American Samoa on 14 March 1996, visited Manus Island, Papua New Guinea, and ended in Hawaii on 13 April, used a unique combination of in situ, satellite, and remote sensors to better understand relationships between atmospheric and oceanic variables that affect radiative balance in this climatically important region. Besides continuously measuring both shortwave and longwave radiative fluxes, CSP instruments also measured most other factors affecting the radiative balance, including profiles of clouds (lidar and radar), aerosols (in situ and lidar), moisture (balloons, lidar, and radiometers), and sea surface temperature (thermometers and Fourier Transform Infrared Radiometers). Surface fluxes of heat, momentum, and moisture were also measured continuously. The Department of Energys ...