Optimization of Drip Irrigation in Sahel Regions by Means of Evolving Soil Matric Potential Thresholds

Abstract:

Sensor-based irrigation offers the potential to optimize water allocation in the root zone, thus maximizing crop yields while limiting water losses. Defining appropriate thresholds, e.g. for soil matric potential (SMP), to trigger irrigation is however a challenge as water stress is highly dependent on plant and site-specific conditions.

In a first phase, the onset of water stress was explored by growing a drip-irrigated crop of eggplant in Burkina Faso on which four different treatments were applied. Root growth, biomass and yield reductions were linked to measurements of SMP at different depths by only partially restoring the crop water needs and using different irrigation depths. In a second phase, field measurements were coupled with a numerical model using the software HYDRUS-2D. The soil water dynamics was modeled in the whole root zone in order to achieve a better understanding of the linkage between soil water distribution, spatial root distribution and root water uptake. Numerical simulations were then built to assess the impact of different SMP thresholds on plant water stress and water losses, using different irrigation depths and soil textures.

Results show that the value of the SMP threshold is dependent on the root distribution and the sensor depth, reflecting the spatial heterogeneity of water availability in the root zone. However, the use of a single sensor at 10 cm depth led to stable thresholds independently of the soil texture. At that depth, using evolving thresholds during the plant growth, which reflects the growing root system and changes in the plant water uptake ability, water stress could be avoided. Moreover, it appeared that most water losses could be avoided before the onset of water stress, suggesting that higher thresholds may be defined to avoid water losses rather than defining plant-specific water stress values. Thresholds values ranging from -15 kPa at early growth to -40 kPa during mid-season are recommended to be used as basis of an efficient irrigation schedule for vegetable crops.