Agricultural Drought Indices - US Department of Agriculture

where Dη is the hydraulic diffusivity; Kη the hydraulic conductivity; z the depth of layer; and Sη the
water uptake by roots, estimated as a functional relation among soil moisture in the layer, its field
capacity and wilting point, and ETP. ETP is calculated by the Thornthwaite (1948) method. This
water balance model also takes into account the effective rainfall estimated by satellite; variation of
SWHC for different types of soil, classified by texture classes; leaf area index from satellite images;
and runoff. The performance of this model was tested with data from the ABRACOS/LBA project,
for pastures and forests in the Amazon region of Brazil. The results obtained by Gevaerd and
Freitas (2006) showed moderate accuracy and high precision, as can be seen in the regression
analyses presented below:
a) for pasture: SWS est = 0.65 SWS obs + 186 and R 2 = 0.82
b) for forest : SWS est = 0.87 SWS obs + 52 and R 2 = 0.90
where SWS est is the estimated soil water storage and SWS obs the observed soil water storage,
during the ABRACOS field experiment.
An example of the maps produced with this kind of technique is presented for two different soil
layers, from 0 to 19 cm and from 0 to 75 cm (Figure 10). The results are impressive since this
model allows determining soil moisture for all South America, with a resolution of 5 km, giving a
general view of the availability of water in different regions of the country, based on surface and
satellite information.
Figure 10. Soil moisture for different soil depths (19 cm – left side and 75 cm – right side) as
determined by the Gevaerd and Freitas (2006) model. Source: http://agricultura.cptec.inpe.br/.
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