A flow model of groundwater-surface water interaction in a drainage trench used for irrigation

The Cuneo Plain (Piedmont, NW Italy), like the whole Po Plain, is characterized by intense agricultural activities that heavily rely on seasonal water availability, which is now challenged by the climate crisis. In the study area, groundwater resources represent a great tool to buffer temporary water scarcity and mitigate the drought risk. The connection between the irrigation network and the unconfined aquifer is made available by the historical drainage trenches, known as fontanili. They were constructed starting from the 11th century with the aim of reclaiming swamps, by lowering the water table, and to provide water for irrigation and drinking purposes. Their configuration was later improved by adding screened boreholes, known as tubi calandra, along the furrows, as they enhance the groundwater flow towards the surface.

This study presents the development of a conceptual and numerical model capable to describe the groundwater - surface water flow interaction in the presence of such structures. The model results were compared to field monitoring data of a fontanile located in the Cuneo province, Italy. The flow model was developed in Hydrus (PC-Progress), solving the Richard’s equation and allowing to model the water flow also in the vadose zone. The Finite Element Mesh consists of a network of triangular (2D) or tetrahedral (3D) elements, refined at the base of the furrow and around the tubi calandra. The 2D model of a transversal section of the trench was implemented to study the hydraulic connection to the phreatic aquifer, whereas, the 3D model was used to estimate evolution of the drainage capacity, and therefore of the discharge, along the furrow. The model was forced with head boundary conditions, applied upstream of the fontanile and at the screened boreholes, instead, an aquiclude was imposed at the bottom of the saturated thickness. Afterwards, a sensitivity analysis was conducted to determine the drainage capacity of the trench under different scenarios of aquifer hydraulic conductivity, upstream hydraulic head and, finally, length and radius of the tubo calandra.

The numerical model allowed to have a clearer picture of the mechanisms controlling the discharge in the fontanili, both in terms of their connection directly to the water table, as well as the contribution of the tubi calandra. In particular, for the latter a suitable range of granulometry and conductivity of the soil was identified to maximize their performance. The results are particularly meaningful as they contribute to the sustainable management of water resources in the area, coupling groundwater and surface water, so that a careful planning of the resource to meet the irrigation demand can be developed.