readme.txt 11/25/03 DESCRIPTION: GLOBAL DISTRIBUTION OF PLANT-EXTRACTABLE WATER CAPACITY OF SOIL (DUNNE) Plant-extractable water capacity of soil is the amount of water that can be extracted from the soil to fulfill evapotranspiration demands. It is often assumed to be spatially invariant in large-scale computations of the soil-water balance. Empirical evidence, however, suggests that this assumption is incorrect. This data set provides an estimate of the global distribution of plant-extractable water capacity of soil. A representative soil profile, characterized by horizon (layer) particle size data and thickness, was created for each soil unit mapped by FAO (Food and Agriculture Organization of the United Nations)/Unesco. Soil organic matter was estimated empirically from climate data. Plant rooting depths and ground coverages were obtained from a vegetation characteristic data set. At each 0.5 x 0.5 degree grid cell where vegetation is present, unit available water capacity (cm water per cm soil) was estimated from the sand, clay, and organic content of each profile horizon, and integrated over horizon thickness. Summation of the integrated values over the lesser of profile depth and root depth produced an estimate of the plant-extractable water capacity of soil. The global average of the estimated plant-extractable water capacities of soil is 8.6cm (Greenland, Antarctica and bare soil areas excluded). Estimates are less than 5, 10 and 15 cm - over approximately 30, 60, and 89 per cent of the area, respectively. Estimates reflect the combined effects of soil texture, soil organic content, and plant root depth or profile depth. The most influential and uncertain parameter is the depth over which the plant- extractable water capacity of soil is computed, which is usually limited by root depth. Soil texture exerts a lesser, but still substantial, influence. Organic content, except where concentrations are very high, has relatively little effect. COMMENTS: --------- The original data file is available in an ascii array format. The format is such that j=1 corresponds to the grid cell bounded by 90.0 and 89.5 degrees south latitude (centered on 89.75), and i=1 corresponds to the grid cell bounded by 0.0 and 0.5 degrees east longitude (centered on 0.25). No data are given for land ice grid cells, most of which occur in Antarctica and Greenland, or for other unvegetated areas. A value of -99.0 indicates either a water grid cell or a land ice grid cell. A value of -1.0 indicates that vegetation is absent (and the plant- extractable water capacity of soil is undefined). Units are cm. This data file may be read as follows: dimension whcdat(720,360) do j=1,360 read(iunit,'(36f5.1)') (whcdat(i,j),i=1,720) enddo NOTE: At the request of our user community, we have included GRID ASCII and ARC/INFO export file (*e00) versions of these data. To be readable by ARC/INFO or ArcView, the ASCII files had to be reformatted. The data was also manipulated to adhere to a more common global lat/lon presentation ( lower left corner coordinates of [-180, -90]). If you are interested in these data files, please refer to the FILES and FORMAT sections of this document. FILES: ------ COMP files in /comp: -------------------- readme.txt - this file DATA files in /data: -------------------- soilwhc.dat - the original ascii data file (see description above). dunne_soil.dat - GRID ASCII version of soilwhc.dat dunne_soil.e00 - ESRI export file version of soilwhc.dat GIS FORMATS: ------------------------------------------------------------------------------ 1. GRID ASCII - a single ASCII array with integer values. (Coordinates listed below are in decimal degrees.) The ASCII file consists of header information containing a set of keywords, followed by cell values in row-major order. The file format is: {NODATA_VALUE xxx} row 1 row 2 . . . row n where xxx is a number, and the keyword NODATA_VALUE is optional and defaults to -9999. Row 1 of the data is at the top of the grid, row 2 is just under row 1 and so on. The end of each row of data from the grid is terminated with a carriage return in the file. First six lines of file (header): ncols 720 nrows 360 xllcorner -180 yllcorner -90 cellsize 0.5 NODATA_value -9999 To import this file into ArcInfo use the following command at an ARC prompt: ASCIIGRID {INT | FLOAT} Arguments - the ASCII file to be converted. - the user specified name of the grid to be created. {INT | FLOAT} - the data type of the output grid. INT - an integer grid will be created. FLOAT - a floating-point grid will be created. Note: This data can also be imported into ArcView (with Spatial Analyst) and ArcGis as ASCII Raster data. (First replace the *.dat suffix with an *asc suffix) 2. ESRI EXPORT file (*.e00) - is a proprietary format that can be imported into ARC/INFO GRID using the following command at an ARC prompt: IMPORT