Minimizing water losses during crop irrigation by optimizing irrigation schedule and introducing soil heterogeneity

Abstract:

Infiltration of water into soil is an essential process that determines the water available for plant growth. In arid environments, the infiltration of water into dry soil can result in a hydrodynamic instability that leads to preferential flow in the form of vertical gravity fingers. Here, we use numerical simulations to study the effect of gravity fingers on reducing water losses due to evaporation in arid and semi-arid environment with different irrigation schedules. We simulate the entire life span of a bean crop using a 2D COMSOL model, modulating the irrigation volume according to recommendations by the Food Agricultural Organization and accounting for differences during the diurnal cycle. We investigate different soil permeabilities, and we measure the total volume of transpiration, evaporation and deep drainage.

In order to maximize the water uptake by the roots and minimize evaporation and deep drainage we introduce a 10-cm layer of more permeable soil at the surface. The more-permeable layer promotes rapid percolation into the deeper soil layers, reducing total evaporation by allowing water to bypass the peak evaporative zone. In the less permeable, resident soil, water flows more slowly and the saturation in the region with the highest root density is increased. If we use an initial soil of with an average grain size 0.1 mm and we add a more permeable layer with a grain size of 2 mm, we observe 10% more water taken by the crop roots and 37% less water lost to evaporation. We evaluate how the introduction of the layered heterogeneity affects the plant water stress. In the homogeneous soil water stress starts when we irrigate less than 65% of the volume recommended, while when we add the 10 cm of more permeable soil, the threshold of water stress is 54% of the recommended. Therefore, the layered soil is less likely to reach water-stress conditions.

We also observe that the irrigation schedule has a direct impact on the partitioning of the water losses. When we irrigate more frequently, we have more evaporation and less deep drainage, while if we irrigate with a less frequently, we find less evaporation and more deep drainage. Finally, it is important to point out that reducing the evaporation will not only save water but also will reduce the increase of salinity from irrigation, which is a threat to water quality.