Reservoirs

Introduction

This page describes how LISFLOOD-OS models reservoirs. It starts with a description of the reservoir routine introduced in LISFLOOD-OS version 5. Once the theory behind is understood, the subsequent sections explain the input files, the output files, and how it is configured in the settings file.

Description of the routine

The LISFLOOD reservoir routine is, since version 5, an adaptaion of Hanazaki et al. (2022). In this routine, the reservoir release is a function of both the storage ($V_t$) and inflow ($I_t$) at the current time step.

As many other reservoir routines, it divides the reservoir storage capacity in several zones, and applies a different release rule to each of those zones. Specifically, this routine uses three storage values to define four storage zones:

  • A conservation zone, when the reservoir is in dry conditions ($V_t \leq V_c$). The conservation limit ($V_c$) is assumed as half of the flood limit. \(V_c = 0.5 \cdot V_f\)
  • A normal zone, where the reservoir should operate most of the time ($V_c \lt V_t \leq V_f$). The flood limit ($V_f$) is a calibration parameter; in the Hanazaki implementation, it is the 75% percentile of the observed storage time series.
  • A flood zone, when the filling is high, but the reserovir is still safe ($V_f \lt V_t \leq V_e$).
  • An emergency zone, when the reservoir is at risk of overtopping ($V_t \gt V_e$). The emergency limit ($V_e$) is assumed as the upper 20% of the storage above the flood limit. \(V_e = S + 0.8 \cdot \left( S - V_f \right)\) being $S$ the reservoir storage capacity.

Apart from the storage zone, the routine uses two characteristic release values:

  • The minimum release ($Q_{min}$) or environmental flow.
  • The normal release ($Q_n$) is the long-term average inflow ($\bar{I}$): \(Q_n = \bar{I}\)
  • The release associated with the flood outflow is a factor of the 100-year return period of inflow ($I_{100}$). The factor $\alpha$ is a calibration parameter; in the Hanazaki implementation it takes the default value of 0.30. \(Q_f = \alpha \cdot I_{100}\)

The main difference between the Hanazaki model and the routine used in previous LISFLOOD versions is the addition of the reservoir inflow ($I_t$) as a condition on the release. If the inflow is smaller than a predefined flood discharge ($Q_f$), the release is a quadratic function of storage, so the reservoir stores water if in drier conditions. If the inflow exceeds the flood discharge, the release is a linear function of storage, therefore it is larger and prevents the reservoir from filling.

The combination of those two conditions —storage and inflow— renders the following six release equations:

\[Q_t = \begin{cases} \max \left( Q_n \frac{V_t}{V_f}, Q_{min} \right) & \text{if } V_t < V_c \\ Q_n \frac{V_c}{V_f} + \left( \frac{V_t - V_c}{V_e - V_c} \right)^2 \left( Q_f - Q_n \frac{V_c}{V_f} \right) & \text{if } I_t < Q_f \text{ and } V_c \leq V_t < V_e \\ Q_f & \text{if } I_t < Q_f \text{ and } V_t \geq V_e \\ Q_n \frac{V_c}{V_f} + \frac{V_t - V_c}{V_f - V_c} \left( Q_f - Q_n \frac{V_c}{V_f} \right) & \text{if } I_t \geq Q_f \text{ and } V_c \leq V_t < V_f \\ Q_f + k \frac{V_t - V_f}{V_e - V_f} \left( I_t - Q_f \right) & \text{if } I_t \geq Q_f \text{ and } V_f \leq V_t < V_e \\ I_t & \text{if } I_t \geq Q_f \text{ and } V_t \geq V_e \end{cases}\]

where $k$ is a release coefficient that modulates the release in the emergency zone when the flood inflow is exceeded. It is a function of catchment area ($A$ in m²) and flood volume (in m³), and it reflects the amount of average rainfall over the catchment that can be stored in the flood and emergency zones. If the reservoir is able to store 200 mm of rainfall, $k$ becomes zero and the emergency release is equal to the flood outflow. Larger values of $k$ indicate a smaller regulation capacity and larger releases.

The equatiion used in LISFLOOD follows the approach explained in Casado et al., 2026:

\[k = \max \left(1 - \frac{1}{0.2} \cdot \frac{V_{tot} - V}{A}, 0 \right)\]

All the computations are done considering relative level of filling values, rather than absolute volume values. The relative level of filling is the ratio between the current storage volume and the reservoir storage capacity. In this way, both calibration parameters mentioned above (flood limit and the factor $\alpha$) are scalar values.

A visual representation of the routine can be seen in the figure below. The blue line represents the reservoir release under normal conditions, whereas the orange line the release in case of a flood event. For a given relative filling, the release is always smaller under normal conditions.

Reservoirs operation

Figure 1. Scheme of the reservoir routine.

Input files

Reservoirs are simulated as points in the channel network. LISFLOOD needs a map of the location of the dams (NetCDF or PCRaster format) which defines their reservoir ID. This ID will be used to link each reservoir with its characteristics. In the routine, LISFLOOD will calculate the reservoir inflow as the channel flow in all pixels upstream of the reservoir.

The three storage limits and two characteristic flows need to be defined for each reservoir. They are provided as space-separated TXT files, where the first column indicates the reservoir ID and the second column the value of the attribute. For instance, the example below shows the table of flood limit (relative filling):

1 0.78
2 0.75
3 0.90
...

The functioning of the reservoirs can be adjusted by tuning two reservoir parameters: the flood limit ($V_f$) and the flood outflow factor ($\alpha$). The values of these parameters can be fed either as TXT files (as in the example above) or NetCDF maps.

The table below summarises the inputs required by the reservoir routine.

Table 1. Input requirements for the reservoir routine.

Settings file Type Description Default_name Units Remarks
ReservoirSites map Reservoir location and ID reservoirs.nc - Nominal
ReservoirTotalStorage table Reservoir capacity res_storage.txt $m^3$  
ReservoirFloodLimit map/table Relative storage (level of filling) that defines the flood storage limit res_flood_limit.(txt/nc) - Calibration parameter. Default value 0.75
ReservoirFloodOutflow table Flood outflow res_flood_outflow.txt $\frac{m^3}{s}$ 100-year return period of inflow
ReservoirFloodOutflowFactor map/table Modifier of the flood outflow res_outflow_factor.(txt/nc) - Calibration parameter. Default value 0.30
ReservoirNormalOutflow table Normal outflow res_normal_outflow.txt $\frac{m^3}{s}$ Average inflow
ReservoirMinOutflow table Minimum outflow res_min_outflow_tx $\frac{m^3}{s}$ Environmental flow
ReservoirInitialFill map/table Relative filling at the beginning of the simulation -9999 - -9999 assumes 80% of the flood limit
upAreaTrans map Upstream area upArea.nc $m^2$ Used to estimate the release coefficient $k$

Output files

If the option repsimulateReservoirs is active (see Section Settings), the reservoir routine produces 3 additional time series (TSS format) and one map, as listed in the following table:

Table 2. Outputs of the reservoir routine.

Settings file Format Description Default_name Units
ReservoirInfowTS TSS Inflow time series res_inflow.tss $\frac{m^3}{s}$
ReservoirOutflowTS TSS Outflow time series res_outflow.tss $\frac{m^3}{s}$
ReservoirFillTS TSS Time series of reservoir filling res_fill.tss -
ReservoirFillState NetCDF Reservoir filling at the end of the simulation res_fill.nc -

Note. The map of reservoir filling at the end of a simulation can serve as the initial condition for a subsequent simulation by using the ReservoirInitialFill option (see Section Settings).

Settings

The reservoir routine is optional and can be activated with the simulateReservoirs option in the lfoptions section of the settings file in the settings file. To save the outputs of the reservoir simulation, activate the option repsimulateReservoirs.

<lfoptions>
    [...]
    <setoption choice="1" name="simulateReservoirs"/>
    <setoption choice="1" name="repsimulateReservoirs"/>
    [...]
</lfoptions>

The values of the two reservoir parameters and the reservoir initial condition are defined within the lfuser section of the settings file:

<lfuser>
    [...]
    <!--Reservoir model parameters-->
    <textvar name="ReservoirFloodStorage" value="$(PathMaps)/res_flood_limit.nc"/>
    <textvar name="ReservoirFloodOutflowFactor" value="$(PathMaps)/res_flood_outflow_factor.nc"/>
    
    <!--Reservoir initial condition-->
    <textvar name="ReservoirInitialFill" value="res_fill_19900102.nc">
        <comment>
            Initial reservoir fill fraction
            -9999 sets initial fill to normal storage limit
        </comment>
    </textvar>
    [...]
</lfuser>

The reservoir characteristics and the output files are defined in the lfbinding section of the settings file:

<lfbinding>
    [...]
    <!--Reservoir characteristics-->
    <textvar name="ReservoirSites" value="$(PathMaps)/reservoirs_Global_03min_new.nc"/>
    <textvar name="ReservoirTotalStorage" value="$(PathTables)/res_storage.txt"/>
    <textvar name="ReservoirFloodOutflow" value="$(PathTables)/res_100yr_inflow.txt"/>
    <textvar name="ReservoirNormalOutflow" value="$(PathTables)/res_avg_inflow.txt"/>
    <textvar name="ReservoirMinOutflow" value="$(PathTables)/res_min_outflow.txt"/>
    
    <!--Reservoir outputs-->
    <textvar name="ReservoirInflowTS" value="$(PathOut)/res_inflow.tss"/>
    <textvar name="ReservoirOutflowTS" value="$(PathOut)/res_outflow.tss"/>
    <textvar name="ReservoirFillTS" value="$(PathOut)/res_fill.tss"/>
    <textvar name="ReservoirStorageM3TS" value="$(PathOut)/res_storage.tss"/>
    <textvar name="ReservoirFillState" value="$(PathOut)/res_fill_20221231.nc"/>
    
</lfbinding>

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