| dc.description.abstract | In this report, development of a computer model to simulate water use
for flooded rice production is described. Theory and practice in water
management for flooded rice production are analysed to help set the
variables used in the model. The computer model is compiled in Quick-
Basic 4.0 and tested using data for actual water intake to Hwea Irrigation
Scheme. The model is based on water use for a single crop. The
effects of staggering in land preparation and in sowing are then studied
for the whole scheme. Daily rainfall data are used. The Penman method is
used to estimate potential evaporation rates using monthly
meteorological data for the Thiba Reception Centre.
Water levels maintained in the fields during the crop season were determined
in order to help set a water management schedule for the crop-infield
phase. Data on crop height were also collected. Soil samples were
collected and analysed for porosity, bulk density and moisture content
after the crop was harvested in order to determine the nature and extent
of soil drying during the soil drying phase.
Using the HvalidatedH model, scheme irrigation and drainage requirements
for various conditions, regarding start of land preparation, maximum
length of flooding and crop staggering, are simulated for both single
and double cropping for 22 years (1967-1988) of available meteorological
records. Simulated irrigation requirements are compared with available
water from the Thiba River which supplies irrigation water to two-thirds
of the Mwea scheme.
Of the three factors that affect irrigation and drainage requirements,
the start of the land preparation affects irrigation and drainage requirements
most, followed by maximum length of flooding and, lastly, the
crop staggering period.
Maximum net scheme irrigation requirements are calculated as 11 mm/day
(1.3 lis/hal, while a net drainage modulus as high as 30 mm/day (3.5
l/s/ha) is found with the model for the Mwea scheme.
From the analysis of the simulated irrigation requirements, it is concluded
that:
i) It is not possible to extend the Hwea scheme area under the
current water supply, water management and growing the current
rice varieties because water requirements would exceed water
supply;
ii) It is also not possible to introduce double cropping in the
scheme under the current water supply, and using the current rice
varieties because water requirements would exceed water supply
for most of the cropping season;
iii) It is possible to reduce a currently experienced water stress by
slightly changing the currently fol lowed programme calendar dates. | en |
