Geoscience Reference
In-Depth Information
between the elements. Mostly applied at the lumped scale, but some models use ESMA components
to represent distributed hydrological response units [Section 2.4]
Evaluation
see Validation
Explicit Solution
The independent calculation of predicted variables at a time step given values of the
variables at the previous time step (see also Implicit solution) [Section 5.1.1; Box 5.3]
Field Capacity
An imprecisely defined variable normally expressed as the water content of the soil
when it is allowed to drain from saturation until rapid drainage has ceased (see Soil Moisture Deficit)
[Box 6.1]
Finite Difference
The approximate representation of a time or space differential in terms of variables
separated by discrete increments in time or space [Box 5.3]
Finite Element Method
The approximate representation of time or space differentials in terms of inte-
grals of simple interpolation functions involving variables defined at nodes of an irregular discretisation
of the flow domain into elements [Box 5.3]
Fuzzy logic
A system of logical rules involving variables associated with a continuous fuzzy measure
(normally in the range 0 to 1) rather than the binary measure (right/wrong, 0 or 1) of traditional
logic. Rules are available for operations such as addition and multiplication of fuzzy measures and
for variables grouped in fuzzy sets. Such rules can be used to reflect imperfect knowledge of how a
variable will respond in different circumstances [Sections 1.7, 5.2.2]
Gain
A multiplier applied to a transfer function to scale inputs to outputs in a linear systems analysis.
May be made adaptive in real-time forecasting [Box 8.1]
Geomorphological Unit Hydrograph
A unit hydrograph derived from the structural relationships
of the catchment geomorphology, in particular the branching structure of the channel network
[Sections 2.3, 4.7.2]
Global optimum
A set of parameter values that gives the best fit possible to a set of observations
[Sections 1.8, 7.2]
Head
An expression of pressure as energy per unit weight commonly used in hydrology and hydraulics
since it has units of length [Section 5.1.1]
Heteroscedastic Errors
A time series of model residuals that exhibit a changing variance over a
simulation period (see also Autocorrelated Errors) [Section 7.3; Box 7.1]
Hortonian model
Runoff production by an infiltration excess mechanism. Named after Robert E.
Horton [Section 1.4, Box 1.1, see also Partial Area Model]
Hydrological Response Unit
A parcel of the land surface defined in terms of its soil, vegetation and
topographic characteristics [Sections 1.7, 2.6, 3.8, 6.6]
Hyperparameter
Used to describe the parameters of a structural model of the rainfall-runoff
model residuals in statistical calibration of the rainfall-runoff model parameters (see also
Model Inadequacy Function). Hyperparameters are estimated as part of the calibration process
[Box 7.1]
Hysteretic function
A function between two hydrological variables that is different under wetting and
drying conditions. Might refer to storage-discharge at the catchment scale or soil moisture-capillary
potential at the point scale. Extreme wetting and drying values are called the primary wetting and
drying curves; when there is a sequence of wetting and drying periods, the function might also follow
intermediate secondary or scanning curves
Implicit Solution
The simultaneous solution of predicted variables at a time step given values of the
variables at the previous time step, usually by an iterative method (see also Explicit solution) [Section
5.1.1; Box 5.3]
Importance Sampling
Methods for sampling the model space with a view to obtaining samples with
a density that depends on the local value of the sampled model output. Where the output is a like-
lihood in model calibration, the sample density should reflect the local magnitude of the likelihood
[Section 7.10.2]
