Abstract

Spatially distributed radiometric surface temperatures over a semiarid watershed were computed using remotely sensed data acquired with an aircraft-based multispectral scanner during the Monsoon ’90 Large Scale Field Experiment. The multispectral scanner data provide watershed coverage of surface temperature at a resolution of 6.3 m and nearly daily temporal resolution. At high spatial resolution, the surface temperature values appear to be correlated strongly with surface aspect; at more coarse spatial resolution, the surface temperatures variations across the watershed appear to be correlated with background soil moisture variations caused by highly localized precipitation events. The surface temperature data were aggregated to 400-m spatial resolution for the purpose of computing spatially distributed sensible heat fluxes over the watershed. The practicality of using a spatially uniform transfer coefficient was evaluated by examining the variability of surface and meteorological factors across the watershed at the times of the aircraft overpasses. Maps of sensible heat flux over the area were computed for three aircraft overpass dates and compared to localized patterns of recent precipitation in the basin. Maps of instantaneous sensible heat flux tracked well with the spatial patterns of variable surface soil moisture that arose from the localized precipitation events.

Studies, public reports, news reports, and Web sites cite a wide range of values associated with deaths resulting from excessive heat and excessive cold. For example, in the United States, the National Climatic Data Center's Storm Data statistics of temperature-related deaths are skewed heavily toward heat-related deaths, while the National Center for Health Statistics Compressed Mortality Database indicates the reverse—4 times more people die of “excessive cold” conditions in a given year than of “excessive heat.” In this study, we address the fundamental differences in the various temperature-related mortality databases, assess their benefits and limitations, and offer suggestions as to their use. These datasets suffer from potential incompleteness of source information, long compilation times, limited quality control, and the subjective determination of a direct versus indirect cause of death. In general, these separate mortality datasets should not be combined or compared, particularly with regard to policy determination. The use of gross mortality numbers appears to be one of the best means of determining temperature-related mortality, but those data must be detrended into order to remove a persistent winter-dominant death maximum and are difficult to obtain on a regional daily basis.

Arid and semiarid rangelands comprise a significant portion of the earth's land surface. Yet little is known about the effects of temporal and spatial changes in surface soil moisture on the hydrologic cycle, energy balance, and the feedbacks to the atmosphere via thermal forcing over such environments. Understanding this interrelationship is crucial for evaluating the role of the hydrologic cycle in surface–atmosphere interactions.

This study focuses on the utility of remote sensing to provide measurements of surface soil moisture, surface albedo, vegetation biomass, and temperature at different spatial and temporal scales. Remote-sensing measurements may provide the only practical means of estimating some of the more important factors controlling land surface processes over large areas. Consequently, the use of remotely sensed information in biophysical and geophysical models greatly enhances their ability to compute fluxes at catchment and regional scales on a routine basis. However, model calculations for different climates and ecosystems need verification. This requires that the remotely sensed data and model computations be evaluated with ground-truth data collected at the same areal scales.

The present study (MONSOON 90) attempts to address this issue for semiarid rangelands. The experimental plan included remotely sensed data in the visible, near-infrared, thermal, and microwave wavelengths from ground and aircraft platforms and, when available, from satellites. Collected concurrently were ground measurements of soil moisture and temperature, energy and water fluxes, and profile data in the atmospheric boundary layer in a hydrologically instrumented semiarid rangeland watershed. Field experiments were conducted in 1990 during the dry and wet or “monsoon season” for the southwestern United States. A detailed description of the field campaigns, including measurements and some preliminary results are given.