Water balance study of Thiba section of Mwea irrigation settlement
At present the Thiba and Nyamindi rivers are unable to meet the demands for the rice production. Therefore Water Balance Study was conducted in Thiba Section of Mwea Irrigation Scheme to seek more information regarding water utilizatio.n in the Scheme. The main objectives were: to determine the water require~ents at various stages of rice cultivation under present management; to determine seepage and percolation losses in unit ,feeders; to in- / vestigate the'canal damage by water erosion, human beings and animals and to recommend ways and means of improving the current water management. Each Unit had a constant-head orifice eCHO) at the outlet. Weirs were installed at the outlets of the Units. The inflow and outflow data were collected at least twice daily. Piezometers were installed in selected nuraeries of the above Units at three levels. The weather data were collected twice a day. All readings were expressed in mm/day. From this study it was f~und that rice growth and development stages consumed the most effective water - 44.8% of total effective water supplied to ) Thiba Section, followed by water topping stage 38.8%, and land preparation stage needed 16.4% of the total effective water. Of the total water supplied for the rice production in Thiba Section, river water constituted about 85.9% while rainfall water constituted 14.1%. The total irrigation water was utilised as follows:- 135.0% lost through evapotranspiration; 34.~% passed out of the fields as outflow; 7.9% was lost as deep percolation and 23.0% was stored in the soil. The tota~ estimated effective water requirements for 1979/80 season were 2667 ha-metres. The effective water requirements per hectare for the 1979/80 season was 2.34 ha-metres; effective water required per 0.405 ha holding (one-acre holding) for 3 the same season was 0.95 ha-metres (9500 m ); effective water required to produce 1 kg of paddy 3 rice was 4.19 m and effective water required to 3 produce a 75 kg bag of paddy rice was 313.95 m • Owing to canal damage the main canal capacity is 83.4% greater than original capacity; the main canal gradient has increased from 1:3500 to 1:540. The side slopes are non-uniform. The shape of the canal cross-section has changed from trapezoidal to (iii) parabolic. Similar changes have taken place in the branch canals. The canal seepage losses are negligible once the soil is saturated. However, when the canals are dry and badly cracked, the seepage losses are 7.5 mm/hour for the first twelve hours. After twelve hours the seepage losses are almost zero. From this study it was concluded that the irrigation water should be managed better than it was then; the water usage can be minimised if water topping stage can be reduced, the soils in the Units H1, H2, H3 and H20 did not have any drainage problems. Water table rises during the growing season and then falls below 150 cm during and after the harvesting stage. The water qUality for both Irrigation and drain waters was good. Although horizontal flow was prevented in the experimental plots, such flow occurred from the field to the drain in practice. To improve the present water management the following recommendations were made:- 1. The technical staff in-charge of water control needs training in water management. ( Lv ) Irrigation officers should have a sufficient knowledge of irrigation practices. 2. The tenants need to be educated in proper water utilization and management. They should be closely supervised to avoid misuse of water. 3. The existing water measuring structures need regular maintenance. 4. Additional water measuring structures should be installed in various canals and drains. 5. Water measuring scheme shoUld be worked out as a part of water management in the settlement. 6. As the maximum water requirements are at transplanting and tillering stages, the possibility of building a storage dam should be investigated. 7. At the moment the rice growth depends on the availability of irrigation. The utilization of long rains needs more studies with an aim of having two crops per year.