Effective precipitation was obtained by the method proposed by the FAO bulletin no.
It was considered that the crop would not be irrigated if the effective precipitation were higher than the actual evapotranspiration of the crop, while the losses through percolation and runoff were estimated by Eq.
The effective precipitation, which refers only to the portion of the precipitation used to meet the evapotranspiration demand of the crops, showed values lower than those observed in the pluvial precipitation.
Several factors were considered to be responsible for effective precipitation
mechanisms among which fluctuating pH around bacterial cell, cell membrane, cell wall, and exopolysaccharides contributed significantly for this mechanism (Silva-Castro et al., 2015).
In this study, we have described a drought index (the standardized supply-demand water index (SSDI)) that uses crop evapotranspiration and effective precipitation and it is based on a normalization of the simple water balance.
Effective Precipitation. Effective precipitation is the fraction of the total precipitation.
The SSDI combines effective precipitation and crop evapotranspiration.
Using the method discussed in Section 2.4, we can obtain the daily crop evapotranspiration and effective precipitation. With the value for [ET.sub.c] and [P.sub.e], the difference between them for the month i is calculated:
where [X.sub.obs-i] is observed crop evapotranspiration or effective precipitation (mm) and [R.sub.sim-i] is simulated value by GCMs.
Future changes in effective precipitation, another important determinant of drought besides crop evapotranspiration, are highlighted in Figure 15(b).
The effective precipitation is rather low in the Loess Plateau.
In this diagram, soil redox status is presumed to be simply related to drainage and to effective precipitation. We agree with Maher (1998) that other factors may also influence soil drainage and therefore, ultimately, magnetic behaviour.