The power of geospatial technologies comes from their ability to enable the acquisition of massive quantities of data, linked by georeferencing to specific physical locations on the planet's surface; and to permit the retrieval, analysis, and distribution of these data in a variety of combinations and permutations that can be tailored to meet a diverse array of end-user needs. The three major tools of geospatial technologies are remote sensing (RS), Global Positioning Systems (GPS), and Geographic Information Systems (GIS).

Remote sensing means the acquisition of data about an observed object without having to be in physical contact with that object. Such data are generally collected either by aerial photography or by satellite-based instruments that create images of the earth based on measurements of the radar, visual light, or infrared portions of the electromagnetic spectrum. (1)

The Global Positioning System links ground-based, usually hand-held data receivers with a network of earth-orbiting satellites to provide very precise measurements of an object's physical location on the planet's surface. GPS thus provides a way of ground-truthing or verifying remote sensing data as well as of providing original location data. (2)

Geographic Information Systems are computer-based data management systems that allow spatial, georeferenced data pertinent to a given location to be linked to non-spatial attribute data about that location (for example, a GIS system could link rainfall, streamflow and temperature data with demographic, land use and crop data to allow land managers to anticipate and plan for water demands within a certain watershed). (3)

The lead article in this issue gives several examples of how geospatial technologies can be used to understand dryland dynamics. William Stefanov describes how researchers at Arizona State University, in the context of carrying out long-term ecological research on the impacts of urban growth on surrounding ecosystems, are using these technologies to characterize land cover use and change; study urban atmospheric circulation patterns; characterize spatial distribution and abundance of vegetation in the urban area and surrounding desert; study patterns of wildfire distribution; and describe local soils and bedrock. Beyond this kind of baseline monitoring, geospatial technologies also allow them to study ways in which these various factors interact: for example, how human modification of the landscape alters rainfall runoff patterns; how desert vegetation responds to wildfires; or how urban microclimates affect surrounding temperature and rainfall patterns. Thus, this overview, while specifically referring to Phoenix, Arizona, provides a good introduction to the range and power of geospatial technologies in helping us understand the complex interactions of human and natural processes.

Water is the scarcest resource in drylands; its importance is reflected in the fact that three articles focus on the use of geospatial technologies to understand various aspects of hydrologic systems, in order to better plan for sustainable use of this most precious of drylands resources.

In the first of these articles, Robert Bryant draws on his work in Namibia to describe the use of remote sensing to monitor flooding events in arid ephemeral lake systems (or playa systems); and hence, to arrive at a better understanding of the overall hydrological regimes of such systems. His work also has implications for the understanding and prediction of potentially significant changes in regional dust emissions. Perhaps most important, his work suggests a means by which playa systems hydrology may be monitored in a cost-effective way, even in cases where climate data are sparse or unavailable (a condition that often applies to drylands).

In another example from Namibia, Heike Klock and Peter Udluft describe the use of remote sensing to map groundwater recharge and discharge zones in the Kalahari Desert, in order to enable an overall calculation of groundwater balance in the region. In arid areas, such calculations are complicated by the fact that groundwater catchments in such areas tend to be very large, with few discharging streams; and that they also contain a wide variety of surface materials that significantly affect recharge amounts. In these circumstances, remote-sensing-based mapping provides the basis for a more reliable estimate of groundwater balance than do other methods of estimation such as the chloride mass-balance method.

Finally, Eric Patrick describes two remote sensing-based approaches used to identify areas suitable for water harvesting, one in Kenya and one in Jordan. Patrick's example is particularly interesting because it reminds us that, although remote sensing provides invaluable data, the involvement of local stakeholders in the subsequent analysis and application of these data is far more fruitful than having the data analyzed by outside experts alone. As with any development projects, such involvement of local land users "helps ensure that the project design will be suitable on more grounds than just environmental criteria and will thus be more acceptable to local populations."

Of course, as evidenced by the first article in this issue, the use of geospatial technologies is by no means limited to water-related studies. The article by Pat Chavez, Jr., and colleagues describes the use of satellite and ground-based images to monitor dust storms and map landscape vulnerability to wind erosion in the U.S. Southwest. Like Patrick's paper, this one reminds us that, despite the undeniable power of remote sensing methods, ground-based observations (in this case collected remotely by digital camera) are an invaluable adjunct to satellite-based data collection. In this case it turns out that the spatial and temporal resolution provided by current-day satellite-based remote sensing instruments is not fine-grained enough to monitor smaller-scale, short-lived dust storms; for observation of such events, the digital cameras are the more powerful tool.

Finally, the article by Patricia Fanning and Simon Holdaway reminds us that geospatial technologies can help us understand human impacts on the planet as well as basic global/regional ecological processes. The article describes how Australian archeologists are using remote sensing techniques to understand the interactions of prehistoric humans with the landscape in arid regions of Australia. In this case, remote sensing has been crucial in enabling them not only to interpret a historic record that has been widely dispersed across the landscape, but also to ensure that their research only minimally disturbs the surface record; this has allowed them to respect the indigenous inhabitants' wishes for their material cultural heritage to be left in place.

These examples, of course, don't even come close to indicating the wide range of potential uses of geospatial technologies in drylands. Other potential uses include, for example, monitoring and predicting salinization; monitoring crop-growing conditions; assessing vegetation conditions; and the like. (4)

As all of these examples make clear, geospatial technologies are providing us with ever more sophisticated means of understanding and monitoring drylands dynamics. Appropriately used, they offer us powerful tools for designing sustainable solutions for many of the complex problems affecting drylands around the world.

Endnotes

(1) Some useful introductory materials on remote sensing that I found on the web include: Lectures on Remote Sensing Basics from Wageningen University in the Netherlands, at http://cgi.girs.wageningen-ur.nl/k075218/default.htm; the Remote Sensing Tutorial, from NASA, at http://rst.gsfc.nasa.gov/start.html; a special on Remote Sensing for Decision Makers (including seven case studies) from the FAO SDDimensions web site, at http://www.fao.org/sd/EIdirect/EIre0072.htm; and the Canadian Centre for Remote Sensing's Remote Sensing Glossary, at http://www.ccrs.nrcan.gc.ca/ccrs/eduref/ref/glosndxe.html. All of these sites, as well as all of the other sites listed below, were accessible as of 29 July 2002.
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(2) The Geographer's Craft web site has published a (somewhat technical) overview of GPS at http://www.colorado.Edu/geography/gcraft/notes/gps/gps_f.html.
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(3) There are several GIS-related web sites available; some that seem particularly useful are: From FAO's SDDimensions web site, a discussion of Geographic Information Systems in Sustainable Development at http://www.fao.org/sd/EIdirect/gis/EIgis000.htm; an overview of GIS from the US Geological Survey at http://www.usgs.gov/research/gis/title.html; from the Geographer's Craft web site, a document called Geographic Information Systems as an Integrating Technology: Context, Concepts, and Definitions, at http://www.colorado.edu/geography/gcraft/notes/intro/intro_f.html; and an online dictionary of GIS terms from the Association for Geographic Information, at http://www.geo.ed.ac.uk/root/agidict/welcome.html (still under development, but quite useful).
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(4) Other web sites that help illustrate the many uses of geospatial technologies in understanding drylands dynamics include: The Aral Sea Homepage at http://www.dfd.dlr.de/app/land/aralsee/; Remote Sensing of Mediterranean Desertification and Environmental Stability (RESMEDES), at http://www.iata.fi.cnr.it/resysmed/index.htm; FAO's Africover: Land cover assessment based on remote sensing for the whole African continent, at http://www.africover.org/index.htm; and, from Australia's CSIRO, Detecting and Monitoring Salt-Affected Land, at http://www.cmis.csiro.au/rsm/research/salmapmon/salmapmon.html.
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