Flow routing and groundwater

Downstream routing

The model traverses the network in the direction of flow. At each node, it adds the flows arriving through upstream reaches and applies the corresponding local change.

The resulting flow is transferred to the downstream reaches.

Groundwater gains and losses

Exchanges between the river and groundwater are represented using a simplified formulation:

\[ Q^{out}_{e,m} = Q^{in}_{e,m} e^{-k_{e,m}L_e} + g_{e,m}L_e \]

where:

• \(Q^{in}_{e,m}\): flow at the start of the reach;
• \(Q^{out}_{e,m}\): flow at the end of the reach;
• \(L_e\): reach length;
• \(k_{e,m}\): infiltration loss coefficient;
• \(g_{e,m}\): groundwater gain per unit length.

The first term represents flow losses along the reach. The second represents an additive groundwater contribution.

Interaction classes

Reaches are classified into three levels:

• low;
• medium;
• high.

The classification is based on a simplified hydrogeological index that considers:

• the presence of alluvial materials;
• the soil’s water storage capacity;
• river slope.

Reaches with more alluvial material, greater storage capacity and gentler slopes are assigned a higher interaction class.

Interpretation

This formulation represents systematic spatial differences between gaining and losing reaches, but does not explicitly simulate:

• piezometric levels;
• aquifer storage;
• temporal delays in baseflow;
• effects of groundwater pumping;
• dynamic feedback between the river and aquifer.