Water requirement for irrigated rice in a semi-arid region in West Africa

Abstract

The purpose of this study was to determine the water requirement of rice ( Oryza sativa L.) in the areas of the Office du Niger (Niono, Republic du Mali, West Africa). Average annual rainfall is 600 mm and evapotranspiration exceeds precipitation in every month except August. Alluvial soils vary in texture from heavy clay to sandy loam and have very low infiltration rates. The water use of rice was measured by establishing the water balance of four irrigation units varying in size from 12 to 26 ha. Evapotranspiration was evaluated with ‘non-weighing’ lysimeters and varied in 1980 from 5.3 mm/day shortly after flooding and 7.1 mm/day during heading to 4.1 mm/day at ripening. The crop coefficients found in this study do not always correspond to those presented by Doorenbos and Pruitt (1977). The crop coefficient depended strongly on the soil cover and was 1.0 for 10–70% soil cover and increased linearly to 1.25 for 100% soil cover. This relationship between soil cover and crop coefficient can be used to improve the estimates of rice evapotranspiration from large irrigation units with incomplete soil cover. The peak evapotranspiration may be as high as 7.8 mm/day during a 10-day period under full soil cover. The peak water requirement during the period of dry land preparation does not exceed 6 mm/day on level fields. The amount of water needed to saturate the soil profile after the dry season varied from 122 to 302 mm. A significant difference was measured between the pre-irrigation gift on a level field (175 mm) and on a non-level field (255 mm), which indicates the importance of levelling.