Rainfed rice - IRRI books - International Rice Research Institute

Rainfall and Its Agronomic
Interpretations
A
ll facets of agricultural
activities, from field
preparation to sowing,
weeding, application of
fertilisers and weedicides,
harvest and post-harvest
operations, all the way to
storage of grains, are
influenced by climate in general
and rainfall in particular. In
rainfed agriculture, especially in eastern
India, rainfall plays a vital role, either positive or negative, in crop
growth and development. Rainfall also influences the final usable
grain yield at harvest and even after harvest.
Rainwater balance
The rainwater, after touching the surface of the earth,
undergoes different processes which can be understood
using a simple rainwater balance equation.
R=DSW+RO+D+ET+M
where
R = Rainfall
DSW = Soil water change (from the pre-monsoon dry soil,
the soil water reaches field capacity/saturation point
during the rainy season)
RO = Runoff (when the intensity of rainfall is higher than
the infiltration rate, the water runs off from that area and
reaches rivers through riverlets and small canals)
D = Deep percolation-(after saturation point, the excess water that goes into the soil percolates down and
joins the groundwater)
ET = Evapotranspiration (the water over the bare soil evaporates through the surface. The soil water
works as a transporter and is finally absorbed into the atmosphere through the process of transpiration.
Thus, evaporation from the soil and transpiration through the plant occur simultaneously as ET)
M = Metabolic water (a very small portion of the soil moisture gradually accumulates in the plants as they
grow)
Thus, the DSW that is stored in the soil is used for evapotranspiration and is replenished by the rainfall.
Therefore, the ET amount is the only amount needed by the plant for its growth and development.
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