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This Digital Atlas was developed in part with support from:
We also wish to acknowledge the utilization of data from the following individuals and organizations:
This Digital Atlas of the World Water Balance is our first attempt at creating an integrated set of data files and models in GIS format that can be used to characterize the water balance of the earth. The concept arises out of a project called the FAO/UNESCO Water Balance of Africa, sponsored by the Land and Water Development Division of the UN Food and Agriculture Organization, Rome, Italy, and the Division of Water Sciences, UNESCO, Paris, France. The goal of this project is to provide data and exercises for using the data for water resource analysis in Africa. This project involved was built around a Water Balance of the Niger Basin in West Africa. A Conference Proceedings paper prepared for presentation at the 1997 ESRI User Conference, and contained also on this CD-ROM explains the methodology. As many of the data sets were compiled for that project, they were drawn from data sets of global extent, so we decided to compile the global data into one location and add to that the data files and sample exercises that we had produced in our work on the Water Balance of Africa, to form a product with which similar tasks could be accomplished for other parts of the world. Some of the exercises were developed during a Technical Cooperation Project on the Souss Basin in Morocco in collaboration with the Direction Generale de l'Hydraulique of Morocco. Additional applications have been made to the Urubamba River Basin in Peru, and to the Aral Sea Basin in Central Asia.
The data and exercises which support this Digital Atlas are designed to be used with the Arcview Version 3.0 Geographic Information System, though Version 2.1 can be used for many of the exercises. The descriptive material contains graphic files, some of them animated, which explain the origin and processing of the data. The Water Balance of the Niger Basin paper, xplains the background and methodology
This Digital Atlas contains five types of data: precipitation, temperature, radiation, runoff and political boundaries. The precipitation, temperature and radiation data are depicted on a 0.5 degree grid spanning the earth (720 cells East-West, and 360 cells North-South, or 259,200 cells in all), covering both the land surface and the oceans. On each grid cell is presented the mean monthly and mean annual values of each variable in Arcview shapefiles. More information on these data can be found in the section on Data Sources. Mean monthly and mean annual runoff at 160 stations are found in the runoff directory. The political boundaries are used to delimit particular regions of interest in the data sets.
This Digital Atlas has been divided into data folders for each continent, and an Exercise folder. In the data directories, you can find temperature, radiation, water holding capacity, precipitation, and runoff data. The Exercise directory contains a course of six exercises showing how to run the data with ArcView Version 2.1 or 3.0, with translation of all exercises into French and some exercises into Spanish.
The data are presented in the following subdirectories:
The continental folders contain the following data:
Global data sets of mean monthly temperature and precipitation interpolated to a 0.5 grid were obtained by anonymous ftp to the University of Delaware (climate.geog.udel.edu). These data are from the "Global Air Temperature and Precipitation Data Archive" compiled by D. Legates and C. Willmott. The precipitation estimates were previously corrected for gage bias. Data from 24,635 terrestrial stations and 2,223 oceanic grid points were used to estimate the precipitation field. The climatology is largely representative of the years 1920 to 1980 with more weight given to recent ("data-rich") years (Legates and Willmott, 1990).
Global estimates of "plant-extractable water capacity" have recently become available on a 0.5 grid (Dunne and Willmott, 1996). As used in this Digital Atlas, the term plant-extractable water capacity is equivalent to water-holding capacity. One reason given for developing this global database was to eliminate the need for assuming spatially invariant plant-extractable water capacity in soil-water balance computations made over large areas. Information about sand, clay, organic content, plant rooting depth, and horizon thickness was used to estimate the plant-extractable water capacity.
It is axiomatic that the land surface water balance is connected to and bounded by the land surface energy balance since water cannot evaporate unless there is sufficient energy available to do so. A new radiation data set was recently made available by the Earth Radiation Budget Experiment (ERBE). These data are described in "GEWEX News," August, 1995, and were obtained by anonymous ftp to cloud.larc.nasa.gov. The data set includes longwave and shortwave radiation flux estimates for a 96 month period extending from July 1983 to June 1991. The data are given on the ISSCP equal-area grid which has a spatial resolution of 2.5 at the equator. Darnell and Staylor (1996) describe advances in input data and flux estimation algorithms that improve the ability to assess the radiation budget on a global scale. Input data improvements have come from the International Satellite Cloud Climatology Project (ISCCP) and ERBE. Using this satellite data, the radiation budget components that cannot be measured directly are estimated independently using physical approaches that have been validated against surface observations. According to Darnell et al., 1995, longwave flux estimates fall within +/- 25 W/m2 of surface measurements while Whitlock et al. estimate the accuracy of shortwave estimates to be within +/- 20 W/m2 of surface measurements. For comparison, the energy required to evaporate 1 mm/day of water is about 30 W/m2. In this study, net radiation (equivalent to net shortwave + net longwave) is used.
A new Surface Radiation Budget is in preparation which will contain monthly data from July 1983 through June 1995. The data set contained in the Digital Atlas of the World Water Balance is an annual average of the monthly values for the period July 1983 through June 1991.
Mean monthly runoff estimates have been calculated for 160 runoff stations were obtained from the Global Runoff Data Center, Koblenz, Germany. The attribute table displays the river, the station, the coordinates, the region, the first year of record, the last year of record, the total years of record, and the missing values in percent to total time series, as well as the monthly runoff data. One should note that the for the values recoded as 9999, the average was calculated as though there was no discharge for that month, rather than having a low discharge.
The principal financial support for the research leading to this Digital Atlas was provided by the Land and Water Development Divison and the Technical Cooperation Program of the UN Food and Agriculture Organization. Support was also provided by the Division of Water Sciences of UNESCO, and the US National Committee for the International Hydrology Program.
We are grateful for the guidance of Jean-Marc Faures and Philippe Pallas of FAO, and Alice Aureli of UNESCO in carrying out this research, and for the generous cooperation in sharing data provided by Cort Willmott of the University of Delaware (soil water holding capacity, precipitation and temperature data), Earth Radiation Budget Experiment (NASA) (radiation data), and the Direction General de l'Hydraulique, Rabat, Morocco (data for the Souss basin).
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