GR4J¶
At a glance…¶
This node uses a GR4J rainfall-runoff model to represent catchment inflows from a catchment of a fixed area. The model takes rainfall and potential evapotranspiration data and determines inflows. The GR4J model has 4 parameters representing catchment characteristics.
[node.my_gr4j_node]
type = gr4j
loc = 20, 30
area = 165
rain = data.rex_rain_csv.by_name.value
evap = data.rex_mpot_csv.by_name.value
params = 1500, 4, 65, 0.38
ds_1 = my_other_node
Node properties¶
| Property | Description |
|---|---|
| [node.?] (compulsory) | Start of node declaration. This says we are creating a node, and also defines the name of the node. Node naming conventions are discussed at . Example: [node.my_gr4j_node] |
| type (compulsory) | The node type, which is “gr4j” in this case. type = gr4j |
| loc (compulsory) | The location of the node in cartesian coordinates. Example: loc = 20, 30 |
| area (compulsory) | The catchment area [km2]. Example area = 165 |
| rain (compulsory) | Rainfall data [mm]. Example: rain = data.rex_rain_csv.by_name.value |
| evap (compulsory) | Potential evapotranspiration data [mm]. Example: rain = data.rex_mpot_csv.by_name.value |
| params (compulsory) | The four GR4J model parameters: X1, X2, X3, X4. Example: params = 1500, 4, 65, 0.38 |
| variant (optional) | Selects the model formulation: gr4j (classic daily model, used by default if omitted) or gr4h (suitable for sub-daily timesteps). Example: variant = gr4h |
| ds_1 (optional) | Name of the downstream node. This property defines a downstream link. Gr4j nodes may only have 1 downstream link. Example: ds_1 = my_other_node |
Results associated with this node¶
| Result | Description |
|---|---|
| dsflow | Downstream flow [ML] |
| usflow | Upstream flow [ML] |
| ds_1 | Downstream flow on link ds_1 [ML] |
| ds_1_order | Order on link ds_1 [ML] |
| runoff_volume | Catchment runoff volume from GR4J model [ML] |
| runoff_depth | Catchment runoff depth from GR4J model [mm] |
| rain | Input rainfall [mm] |
| evap | Input evapotranspiration [mm] |
How the node works¶
The GR4J node simply adds inflows from a GR4J model (Perrin et al., 2003) to the system. The downstream flow is
dsflow=usflow+runoff\_volume
The GR4J model uses four parameters:
- X1 (Production Storage Capacity): This parameter represents the maximum capacity of the production store in millimetres (mm).
-
Bounds: A common range for X1 is from 0 mm to 6000 mm.
-
X2 (Groundwater Exchange Coefficient): This parameter controls the exchange of water between the groundwater and the routing store, with units of mm/day.
-
Bounds: The typical range for X2 is -20 mm/day to 10 mm/day. Negative values indicate groundwater infiltration, and positive values indicate water leaving the aquifer.
-
X3 (Routing Storage Capacity): This parameter refers to the 1-day-ahead maximum capacity of the routing store in mm.
-
Bounds: A common range for X3 is 0 mm to 4000 mm.
-
X4 (Unit Hydrograph Base Time): This parameter is the time base of the unit hydrograph in days.
- Bounds: A typical range for X4 is 0.04 days to 20 days.
Considerations:
-
Variability: While these are common bounds, the exact parameter ranges for GR4J can vary depending on the specific study, the characteristics of the catchments being modelled, and the calibration dataset used.
-
Calibration: These bounds are used during the model calibration process to help find the optimal parameter values for a given hydrological model application.
Daily (GR4J) and sub-daily (GR4H) formulations¶
By default the node uses the standard daily GR4J model. Setting variant = gr4h switches it to GR4H, the sub-daily formulation from the airGR package (INRAE). GR4H differs from GR4J only in two constants that were recalibrated for short timesteps: the unit-hydrograph shape exponent (2.5 in GR4J, 1.25 in GR4H), and the percolation coefficient (9/4 in GR4J, 21/4 in GR4H).
GR4H is appropriate for sub-daily timesteps (e.g. 15-minute, 1-hour, etc). Time-related quantities are expressed per timestep rather than per day — i.e. X2 is in mm/timestep and X4 is a number of timesteps.
References¶
Perrin, C., C. Michel, et al. (2003). "Improvement of a parsimonious model for streamflow simulation." Journal of Hydrology 279(1-4): 275-289