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Monitoring Daily Vegetation-Soil-Water Balance Components
using the VegET Model

Gabriel Senay
senay@usgs.gov
(Updated: April, 2009)

The VegET model (Senay, 2008) integrates commonly used water balance algorithms with remotely sensed Land Surface Phenology (LSP) parameter to conduct operational vegetation water balance modeling of rainfed crop and grassland systems at the LSP’s spatial scaleusing readily available global weather data sets for rainfall (Xie and Arkin, 1997) or NOAA’s new precipitation product (blend of NEXRAD and station data) for the US
(http://www.srh.noaa.gov/rfcshare/precip_about.php) and reference evapotranspiration (Senay et al., 2008)

The VegET model conducts daily water balance on a pixel-by-pixels basis and produces several agro-hydrologic products. Thefollowing products are routinely posted on a website for season monitoring and early warning applications:


Product Type:
1) Soil Water Index (SWI)
2) Water Requirement Satisfaction Index (WRSI)
3) Cumulative ETa
4) Cumulative ETa Anomaly
5) Cumulative ETa Forecast Anomaly


Note: the growing season for the US is assumed to occur from April 1 through October 31 of each year. Thus,products that require cumulative values start from April 1. The model initializes the soil moisture 60 days prior to April 1st.




Graphics Label:
1) NatVeg: indicates the modeling pixel is for a landscape with natural vegetation. A mix of crop and natural vegetation under a rained influence, i.e., no irrigation is considered in this product.


2) Date Example: 2009-04-20 indicates this productis produced by taking into account agro-hydrologic processes from the beginning of an assumed growing season (April 1 for USA) till the current date of April 20 in 2009. The end-of-season for USA is considered to be end of October.


Brief Description of Operational Graphical Products, posted on the website: http://earlywarning.usgs.gov/usewem/



1. Soil Water Index (SWI)


The values inthis image represent the amount of water stored in the vegetation root depth as a percentage of the water holding capacity (WHC) of the soil at the end of a particular day “i”:

SWi
SWI = ---------- x 100
WHC

Where SW is soil water content and WHC is the water holding capacity of the soil, derived from the STATSGO database.

The soilwater content is obtained through a simple mass balance equation where the level of soil water is monitored in a bucket defined by the WHC:

SWi = SWi-1 + PPTi - ETai

Where PPT is precipitation and ETa is actual evapotranspiration. Both SW and ETa are generated within the VegET model.


Application:
This index is an indicator of the soil moisture status at the end of a particular day. The indexis presented in four broad qualitative categories. For example, an index with 90-100% (“sufficient”) implies that there is enough soil water in the crop root zone to support the crop through the next few days (~7-10 days) without experiencing water stress. A soil water index of “satisfactory” (50 – 90%) implies conditions ranging from some degree of stress (on the lower end) to areas with enoughmoisture to avoid crop stress in the next few days. In the “stress” range (10 – 50%), the crop is likely to experience water stress (from severe to moderate) if there is no rainfall in the next few days. 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 soilwater is at 0% of WHC; thus, the 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...
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