Agricultural Drought Indices - US Department of Agriculture

Figure 7. Water balance for Brazil, for SWHC = 125 mm. Negative values represent water deficiency
and positive values water surplus. Source: www.inmet.gov.br.
Figure 8. Percentage of available water in the soil for Uruguay, considering variable SWHC in the
different regions of the country. Source: www.inia.org.uy.
Another model with a higher degree of complexity is presented by Allen et al. (1998). It is used to
determine ET under soil water stress conditions, through the water stress coefficient (Ks), which is
responsible for reduction of transpiration depending on the water available in the soil. The ET
under soil water stress conditions refers to the ETa, as defined earlier. To estimate ETa, the
determination of Ks is required, since ETa = ETP * Kc * Ks, where Ks is estimated by a daily water
balance computation for the root zone.
Ks is calculated as a function of SWHC, a fraction of SWHC that a crop can extract from the root
zone without suffering water stress (p fraction), and root zone depletion (Dr):
Ks = (SWHC – Dr) / [(1 - p) * SWHC] (10)
Root zone depletion (Dr) is calculated by a daily water balance, with the following components:
Dr i = Dr i-1 − (P − RO) i − CR i + ETP i + DP i (11)
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