A computer model for water management in flooded Rice production

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.