Daily 10-Day Soil Water Index
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Continental Africa
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(updated: Jan. 5, 2002) This image is a by-product of the Daily-10-Day Moisture Index calculation. The values in this image represent the amount of water stored in the crop root depth as a percentage of the water holding capacity (WHC) of the soil at the end of a particular 10-day period "i": SWI = (SW/WHC)* 100 where SW is soil water content. The soil water content is obtained through a simple mass balance equation where the level of soil water is monitored in a bucket defined by the WHC of the soil: SWi = SWi-1 + PPTi - PETi where PPT is precipitation and PET is Potential evapotranspiration. Application: This index is an indicator of the soil moisture status at the end of a particular 10-day period. Therefore, it may be used as a tool to assess the crop water status in the next 10-day period based on the available moisture in the soil. The index is presented in broad qualitative categories. For example, an index with 100% ("sufficient") implies that there is enough soil water in the crop root zone to support the crop through the next time period without experiencing water stress. A soil water index of "satisfactory" (60 - 99%) at the end of the 10-day period implies conditions ranging from some degree of stress (on the lower end) to areas with enough moisture to avoid crop stress in the next time period. In the "stress" range (10 - 60%), the crop is likely to experience water stress (from severe to moderate) if there is no rainfall in the next time period. In the "wilting" group (0 - 10%), the soil is already at very low moisture level such that continued drought may cause wilting of the crop. The agronomic definition of wilting is when the soil water is at 0% of WHC; thus, a plant will avoid wilting if there is rainfall before moisture is completely depleted. Spatial association (proximity) of the classes can be used to identify areas that are in the low or high side of a given class. For example, within the "satisfactory" class those areas likely to experience stress will be found adjacent to the "stress" areas. This index can potentially be used for planning activities that rely on existing soil moisture conditions in combination with forecast rainfall. Such activities may include supplemental irrigation (e.g., if current soil water index is very low and rainfall forecast for the next time period is negligible) or the identification/application of control measures for high-risk areas for malaria. Note: A value of 100% represents that the soil is at least at field capacity (a condition of soil moisture 2 to 3 days after a rain event that brings the soil water content to saturation). A value of 0.0% represents a soil moisture status at permanent wilting point. Soil water index is calculated for the current 10-day period only, with memory for soil water content carried from previous 10-day period via the soil water content parameter SW(i-1). The soil water index does not provide information about the crop condition. |
