Agricultural drought is often expressed in terms of plant available soil water (PAW) content. Measuring and collecting such data is costly and cumbersome. Satellite derived relative evapotranspiration (RE) provides a good alternative. The folllowing graph compares average PAW, collected by the University of Salamaca in the Duero basin in Spain, with Meteosat derived RE values of the same area during a period of 6 years.
The PAW and RE data correspond very well. The approximate relation between the two is:
PAW = 0.3*EDI
There are several advantages of the satellite derived EDI data: the effort to collect these is much smaller and distributed data fields of the EDI can easily be obtained for very large areas. Moreover RE is a measure of actual crop water use and in this respect is more closely related to CO2 uptake and thus crop growth than PAW. During the growing season the EDI is a measure of crop production. These facts make the RE a more attractive agriculural drought indicator than plant available soil water.
The following graphs compares the soil moisture map of the province Henan in China, interpolated from about 30 soil moisture measurements and a relative evapotranspiration map of the same area and same period (1-10 May 2001). The relative evapotranspiration map provides considerable more detail.
Agricultural drought map of Henan province, China, based on soil moisture data (left) and based on the EWBMS relative evapotranspiration (right), first dekad of May 2001
Climatic drought products are particularly relevant in the framework of the United Nations Convention to Combat desertification (UNCCD). In 1958 the Russian scientist Budyko did introduce the "dryness ratio" or "aridity index"
AI = In/LR
where In is the net radiation and LR the energy required to evaporate the rainfall. In 1994 the UNCCD defined -slightly different- the Climatic Moisture Index (CMI)
CMI = LR/LEp
i.e. the ratio of rainfall and potential evapotranspiration. The CMI defines the climate types of desertification in the following way:
0.00 - 0.05
0.05 - 0.20
0.21 - 0.50
0.51 - 0.65
Arid, semi-arid and dry sub-humid area's are understood to be subject to desertification. It is therefore very important to a government to know which parts of its national territory are classified as such. However mapping these climatic zones from ground measured data is usually a laborious task. Very often this task cannot be carried out because of data insufficiency. Nevertheless on the basis of the EWBMS system such maps can ne deliverd easily and fast. A classical CMI map developed on the basis of ground data, and one on the basis of EWBMS data, both for PRC China, are shown below.
Currently used CMI map of China as derived from ground data from the 1980's
CMI map based on EWBMS rainfall and potential evapotrans-piration data fields for the year 2001