Rainfall-Runoff Model: parameters The rainfall-runoff model is a dynamic (daily) lumped (semi-distributed) water budget model for small to medium-sized basins.
The model parameters are grouped into three sets:
- Basin parameters
- Model parameters
- Basin specific constants
Of these three sets, the model parameters can be used for calibration, for which an automatic calibration routine based on a constraint satisficing approach is implemented as an efficient Monte-Carlo scheme, is also available; basin characteristics, within the limits of conceptual uncertainty are supposed to be constant; the site specific model constants may vary from location to location, but are again assumed to be constant at least between scenarios for one and the same basin.
Basin Parameters: These describe the primary physical properties of the basin; a special case is the orography (area/elevation distribution) that is defined with a specific editor.
Basin parameters should be inherited from a corresponding river basin object (not yet implemented) describing such physical (as opposed to model scenario) entities.
Model parameters are the primary candidates for calibration; they express the specific properties of a basin, as interpreted and used by the model.
| Basin Parameters |
| Basin Size |
km² |
Overall size of the hydrographic catchment or basin to be modeled; please note that this only used for consistency checking, the actual area used for computations id derived from the area/elevation distribution. |
| Minimum Elevation |
masl |
Elevation of the lowest point (the outflow) of the basin; used for consistency checking, and to estimate average gradients. |
| Maximum Elevation |
msal |
Elevation of the highest point in the catchment; the altitude range (as defined by the area/elevation editor) is used to correct vertical temperature distribution and account for the distribution of snow accumulation and snow melt |
| Basin Length |
km |
Approximate basin length (maximum dimension along the main direction of the river) |
| Main Channel Length |
km |
Approximate length of the main (first order) river channel; used for routing |
| Drainage Length |
km |
the total length of all perennial channels; for basins with only intermittent, seasonal flow, use the total length of all significant channels that contribute to runoff; used to determine the average travel distance of the interflow |
| Average Channel Width |
m |
an approximate measure (due to possibly large seasonal variation), representing average (surface) width, possibly approximated by the dimension 1/3 upstream from the river mouth or confluence; used to estimate routing parameters |
| Channel type |
symbolic |
selection of channel types (symbolic) with associated roughness parameters for the routing |
| Land use/land cover a class residual (other) is constructed as the difference to make up 100% |
| Forest |
% |
area and fraction of the area covered by forest |
| Agriculture |
% |
area and fraction of the area covered by agricultural fields, including both rain fed and irrigated agriculture |
| Meadows |
% |
area and fraction of the area covered by meadows, uncultivated |
| Model Parameters |
| Precipitation Factor |
% per 100m |
precipitation is assumed to change (usually increase) with elevation; the parameter describes the relative increase with every 100 m layer |
| Precipitation Scaling |
|
used to adjust the precipitation input which is assumed to represent values characteristic for the lower end of the basin (see above) |
| Lapse Rate |
deg/100m |
The adiabatic lapse rate describes the decrease of temperature with elevation, usually in the order of up to one degree Centigrade per 100 m |
| Temperature Shift |
deg |
ad additive shift used to adjust the temperature input to be representative of the lower end of the basin, see above |
| Field Capacity |
mm/m |
soil water holding capacity, average, of the soil system; this will be corrected by the rootzone thickness of the individual land use classes to obtain a basin wide (weighted) average |
| Maximum Percolation |
mm/day |
maximum speed of percolation from the unsaturated zone (virtual drainage store) to the shallow groundwater |
| Routing parameters, control segment outflow |
| Delay, Interflow |
- |
weight factor, defines the relative weight of inflow into a "hydraulic segment" |
| Delay, Surface |
- |
weight factor, defines the relative weight of inflow into a "hydraulic segment" |
| Delay, Channel |
- |
weight factor, defines the relative weight of inflow into a "hydraulic segment" |
| Storage, Interflow |
- |
weight factor, defines the relative weight of content of a "hydraulic segment" |
| Storage, Surface |
- |
weight factor, defines the relative weight of content of a "hydraulic segment" |
| Storage, Channel |
- |
weight factor, defines the relative weight of content of a "hydraulic segment" |
| Initial Conditions (to be simplified as: very wet - wet - normal - dry - very dry) |
| Snow-pack |
mm |
initial water content of the snow pack at the begin of the simulation. Please note that this is expressed in mm water equivalent and assumed to be geometrically distributed, with 50% in the topmost layer, 25% in the second from the top, etc. |
| Interception Storage |
mm |
if the simulation is started on or immediately after a rainy day, interception storage could be full, i.e., all surfaces are wet. |
| Soil Moisture |
% |
specified as a % of field capacity depending on the season and the weather at or immediately before the start of the simulation, soils can contain more or less moisture - to minimize dependency on initial conditions, starting the simulation in a dry period is advisable. |
| Groundwater (shallow, GWS) |
mm |
initial water content of the shallow groundwater system - the groundwater is represented as a non-linear reservoir, contributing base flow as a function of its content or level. |
| Groundwater (deep, GWD) |
mm |
initial water content of the deep groundwater system - the groundwater is represented as a non-linear reservoir, contributing base flow as a function of its content or level. |
| Groundwater parameters |
| Deep Percolation |
mm/m/day |
percolation from the shallow to the deep aquifer |
| Response Coefficient (shallow) |
|
multiplier a in baseflow = a * GW ** b |
| Response Exponent (shallow) |
|
exponent b in baseflow = a * GW ** b |
| Response Lag (deep) |
days |
describes the (half) time it would take to drain (half) the deeper groundwater reservoir, by contributing to the baseflow |
Model Constants These are considered constants, but may need adjustment for different climatic zones, of basins of very different type (urban, alpine, wadis, etc).
| Basin specific Model Constants |
| deg/day evaporation |
mm/deg |
evapotranspiration is estimates with a simple degree day factor, that describes how many mm of water will evaporate for each degree of average air temperature; applies to interception storage |
| deg/day snowmelt |
mm/deg |
snowmelt is described similar to evapotranspiration: for every degree average air temperature, a certain amount of snow (in mm) is melted and added to the runoff |
| heavy rain limit |
mm |
this threshold defines conditions of heavy rain and associated high levels of humidity, that affect (reduce) evaporation |
| evpt reduction factor for heavy rain |
|
under conditions of heavy rain (defined above) and high (saturated) relative humidity, evaporation will slow down. |
| reduction factor infiltration |
|
under conditions of heavy rain and very wet (saturated) soils, infiltration speed can be reduced due to the swelling of soil particles. |
| reduction factor percolation |
|
under conditions of heavy rain and very wet (saturated) soils, infiltration speed can be reduced due to the swelling of soil particles. |
| evtp temperature threshold |
deg |
evapotranspiration is a non-linear function of temperature; at very low temperatures, physiological water demand will slow down. |
| warm rain coefficient snowmelt |
mm/mm*deg |
snow melt is primarily driven by air temperature; however, rain falling on snow will also contribute to snow melt due to the high heat capacity of the (warm) rain water; assuming that the temperature of the rain is close to air temperature, rain will effectively increase the degree day factor that describes the effect of temperature only. |
| Land-use specific constants (one set for each land cover class including residual) |
| average rootzone |
m |
rootzone thickness defines the vertical extent of the zone for effective infiltration, water holding in the soils, as well as evapotranspiration |
| deg/day coefficient evpt |
mm/deg |
evapotranspiration is estimates with a simple degree day factor, that describes how many mm of water will evaporate for each degree of average air temperature |
| Interception storage capacity |
mm |
before precipitation reaches the ground, it first is intercepted by land cover: this constant defines the land use specific amount |
| maximum infiltration |
mm/day |
the maximum amount of infiltration (during a day) depends on the land cover and thus porosity of the ground, soil, root zone thickness, soil moisture, etc. |
|
