Impacts of different irrigation schedules on the water balance components of an onion crop in a semi-arid zone

Field experiments were performed on the site of Kamboinsé (Burkina Faso) to study the impacts of different irrigation schedules on the water balance components. Two different irrigation treatments were evaluated, one based on a scientific approach (“optimal” treatment) with different irrigation frequencies (daily and bi-weekly applications) and the second one based on the empirical practices of the farmers of the area (“farmer” treatment). The field data allowed the evaluation and calibration of a deterministic mechanistic model (HYDRUS) which was subsequently used as a prediction tool to study various irrigation management scenarios. The experimental results demonstrate a poor efficiency of the farmers' practices and that inadequate water supply leads to lower yields than in the case of “optimal” irrigation. The irrigation frequency was found to be important with irrigating twice a week instead of once a day leading to an increase of the root-zone water storage, a better crop water availability through the whole root zone and higher yields. It appears that, in the specific climatic context of the study area, frequent applications of small irrigation depths cause the irrigation water to remain near the soil surface where it is subsequently evaporated. The simulated results confirm that changes of the irrigation frequency have a strong influence on the components of the water balance. A decrease of the irrigation frequency causes an increase of the water stored in the root zone, an augmentation of the crop transpiration and a decrease of the water content in the immediate vicinity of the soil surface, leading to reduced evaporation. At the same time, drainage increases somewhat, but remains nevertheless low enough not to threat seriously the quality of the groundwater.