Rainfed rice - IRRI books - International Rice Research Institute

206
RAINFED RICE A SOURCEBOOK OF BEST PRACTICES AND STRATEGIES IN EASTERN INDIA
Moisture availability and crop planning
After the moisture availability analysis, weekly water balance computations
are done for each site using weekly rainfall and potential evapotranspiration
values with the help of Thornthwaite’s bookkeeping procedure. To compute
the water balance:
the field capacity of heavy-textured rice soils is assumed to be 250 mm.
weekly values of different water balance parameters for a crop season are
calculated and plotted for the crop calendar; and
this analysis provides the periods and quantities of water surplus or
deficit and moisture utilisation and recharge patterns.
The figure below shows the weekly values of water balance parameters for a
rice crop at one of the sites in eastern India as an example.
Up to the end of July, the rainfall exceeded the evapotranspiration demand
and recharged the soil moisture. From the last week of September (i.e., the
39th standard meteorological week), however, water stress conditions
prevailed at this site.
Weekly water balance during crop-growing season of 1995 at Tilda, Madhya
Pradesh, India