Modeling the role of reservoirs versus floodplains on large-scale river hydrodynamics

Abstract

Large-scale hydrologic–hydrodynamic models are powerful tools for integrated water resources evaluation at the basin scale, especially in the context of flood hazard assessment. However, recent model developments have paid little attention to simulate reservoirs’ hydrodynamics within river networks. This study presents an adaptation of the MGB model to simulate reservoirs as an internal boundary condition, enabling the explicit simulation of hydrodynamic processes along reservoirs and their interaction with upstream and downstream floodplains in large basins. A case study is carried out in the Itajaí-Açu River Basin in Brazil, which has periodic flood-related disasters and three flood control dams. The model was calibrated for the 1950–2016 period forced with daily observed precipitation. The adjustment was satisfactory, with Nash–Sutcliffe metrics between 0.54 and 0.84 for the 11 gauges analyzed and with flood frequency curves also well represented. Simulation scenarios with and without floodplains and reservoirs were performed to evaluate the relative role of these factors on flood control basin-wide through evaluation of simulated discharges, water levels and flood extent. Itajaí do Oeste tributary and Itajaí-Açu mainstem present major floodplain attenuation, while in Itajaí do Sul and Itajaí do Norte tributaries the main flood control occurs due to reservoir attenuation. Downstream from the dams, results indicated that the reservoirs reach their maximum discharge reduction capacity for 5- to 10-year floods, decreasing it for larger floods. The developed model may be very useful for operational uses as flood forecasting and coordinated reservoir operation studies, as well as to enhance the comprehension of flood dynamics at basin scale.

Appendix

The MGB hydrologic–hydrodynamic model can be divided into two main modules: the rainfall–runoff (i.e., vertical hydrologic balance) and the flood wave routing (i.e., the inertial method). While the flood wave routing parameters were described in the main paper Sects. 3.2 and 3.3, the adopted rainfall–runoff parameters are described in this Appendix. Within the model, the basin is divided into unit-catchments, Hydrologic response units (HRUs) based on soil and land use types, and sub-basins.

The Itajaí-Açu River Basin was discretized into 1118 unit-catchments (Fig. 11), 11 HRU’s (Fig. 12) and eight sub-basins (Fig. 13; based on major tributaries). The HRUs were obtained through a combination of soil type and land use maps available from the Santa Catarina State’s EPAGRI/CIRAM database (available at http://ciram.epagri.sc.gov.br/).

Fig. 11

Discretization of the Itajaí-Açu River Basin into 1118 unit-catchments (gray polygons)

Fig. 12

Hydrologic response units used for model parameterization: agriculture with shallow soil (AgrShal), agriculture with deep soil (AgrDeep), forest with shallow soil (ForShal), forest with deep soil (ForDeep), grasslands with shallow soil (GrassShal), grasslands with deep soil (GrassDeep), shallow, bare soil (SoilShal), deep, bare soil (SoilDeep), urban areas (Urb), floodplains (Fp) and open water (Water)

Fig. 13

Discretization of the Itajaí-Açu River Basin into eight sub-basins for model calibration

For the model soil parameters Wm, b, Kbas and Kint, one parameter value is applied to a HRU within a sub-basin, and for the soil parameters Cs, Ci and Cb (related to linear reservoirs, i.e., hillslope routing), the same value is applied for a whole sub-basin. Table 3 presents the calibrated soil parameters for each sub-basin.

Table 3 Soil parameter values calibrated for the MGB model application in the Itajaí-Açu River Basin

The values of vegetation parameters used for evapotranspiration and canopy interception are as follows. For albedo, values of 0.2, 0.15, 0.2 and 0.3 were adopted for agriculture, forest, grasslands and bare soils, respectively. For leaf area index, values of 2, 6, 2 and 1 m²/m² were adopted for agriculture, forest, grasslands and bare soils, respectively. For vegetation height, a value of 15 m was adopted for forest and 1 m for the remaining HRUs. Penman–Monteith surface resistance parameter value was adopted as 80 s/m for all HRU’s.

Please see Collischonn et al. (2007) for a thorough description of the model parameters.