Daily 10-Day Moisture Index
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Continental Africa
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(updated 5 Feb. 2002) The moisture index (MI) is an agrometeorological indicator defined by a simple supply/demand ratio, then multiplied by 100 to provide a percent value. The MI is determined by [(PPT + SW) / PET] * 100 where PPT is the 10-day accumulated rainfall (mm), SW is the 10-day accumulated available soil water (mm), and PET is the 10-day accumulated potential evapotranspiration (mm). As a stand-alone product the daily-10-day MI images provide additional information concerning moisture available for crops, enhancing the information provided by the RFE images. In order to calculate the daily-10-day MI on a spatial basis, the RFE images are used directly for PPT, as are PET grid values computed from the GDAS (Global Data Assimilation System) analysis fields. A simple bucket model (defined by the water holding capacity (WHC) of the soil and the crop root depth) is used to calculate a new value of SW for each 10-day period, i, wherein SWi = SWi-1 + PPTi - PETi where PET is potential evapotranspiration, and SWi is subject to the constraint 0 <= SWi <= WHC. Soil water in excess of WHC is assumed to be lost as runoff or drainage out of the first 100-cm layer of soil. GDAS analysis fields, generated every 6 hours, are used to estimate previous 10 days accumulated PET on a spatial basis using the Penman-Monteith equation (the formulation of Shuttleworth (1992) for reference crop evaporation is used). GDAS fields used include air temperature, atmospheric pressure at the surface, wind, relative humidity, and radiation (long wave, short wave, outgoing, and incoming). PET is computed for each day, and appropriate sums are made to obtain the daily-10-day totals. The spatial variation of soil water holding capacity (WHC) is characterized using the FAO Digital Soil Map of the World (FAO, 1994). The scale of the original mapping is 1:5,000,000, and the soil polygons carry attributes that include an estimate of easily available water capacity in the upper 100 cm, based on soil physical characteristics. These values were adopted for calculation of soil water conditions. The FAO soil map has been rasterized at a scale that matches the 0.1-degree RFE grid. Daily-10-day MI is a dimensionless ratio of supply to demand for water, expressed as a percent value. Daily-10-day MI images are available from Jan. 21, 2001 to present. Shuttleworth, J., 1992. Evaporation. Chapter 4 in Handbook of Hydrology. (D.Maidment, ed.). McGraw-Hill, Inc., New York. |
