7/7/031 PC-IHACRES is HOMS Component K and is downloadable, free of charge, from the World Meteorological Organization’s HOMS * website at * Hydrological Operational Multipurpose System (HOMS) WORLD METEOROLOGICAL ORGANIZATION It is also downloadable from the CEH website at

7/7/032 IHACRES – what is it? PC-IHACRES is a Windows-environment computer package (software and User Guide) for catchment-scale continuous rainfall-streamflow modelling Identification of unit Hydrographs And Component flows from Rainfall, Evaporation and Streamflow data

7/7/033 possible applications of PC-IHACRES include …  identification of unit hydrographs (UHs)  continuous time series streamflow modelling  environmental impacts detection - hydrological regime studies  runoff event modelling  hydrograph separation (e.g. to assist with water quality investigations)  derivation of a Slow Flow Index (SFI) – analogous to Base Flow Index (BFI)  derivation of Dynamic Response Characteristics (DRCs)  DRC - Physical Catchment Descriptor (PCDs) studies (regionalization)  aid for teaching UH theory and its applications  hydrometric data quality assurance/control (QA/QC)  infilling missing streamflow data

7/7/034 PC-IHACRES requires only catchment size (km 2 ) and unbroken time series of rainfall, streamflow and a surrogate variable representing evaporation (air temperature, t k, is usually used) … rainfall r k is converted to effective rainfall u k by a non-linear loss module … followed by a linear unit hydrograph (UH) module to convert effective rainfall to streamflow x k rkrk ukuk xkxk (non-linear)(linear) (loss module, SVAT)(unit hydrograph) tktk

7/7/035 The specific loss module used in PC-IHACRES has been employed for many humid/temperate region catchments where streamflow is perennial … please see the research literature for other loss modules that have been applied with the UH module, e.g. for modelling ephemeral streams and snow-affected catchments but it may not be suitable for all hydrological analyses in such regions research of IHACRES loss modules and UHs is ongoing at CEH and ANU is one of many that have been applied with the IHACRES UH module

7/7/036 depending on which configuration of linear storages in the UH module is prescribed by the operator, the model structure in PC-IHACRES has either 5 or 6 parameters … giving separate ‘quick’ and ‘slow’ UHs and a corresponding hydrograph separation … the following configurations of linear storages in the UH module are allowed in PC-IHACRES a single storage two in series two in parallel … with adequate data quality and a suitable data timestep, a best UH configuration is often ‘two linear storages in parallel’ (6 parameters) …

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8 when the UH module configuration is ‘two linear storages in parallel’ the model has six parameters known as Dynamic Response Characteristics (DRCs) …

7/7/039 Non-linear 'loss' module (3 parameters) Linear UH module (3 parameters) 6 parameters (or DRCs) C - volume of a conceptual catchment wetness storage (mm)  w – catchment drying time constant (days) f – temperature modulation factor ( o C -1 )  (q) – quick flow response decay time constant (days)  (s) – slow flow response decay time constant (days) (s) – proportional volumetric contribution of slow flow to streamflow = SFI (analogous to BFI) rkrk ukuk xkxk tktk

7/7/0310 for a more detailed description of the IHACRES modelling methodology, please refer to the PC- IHACRES User Guide, references cited therein, and recent papers in the research literature the model calibration procedure described in the User Guide gives provisionally ‘best’ catchment models … … the operator can apply further diagnostic checks and try to adjust a catchment model to make it better ‘fit for purpose’...

7/7/0311 e.g. for additional use of flow duration curves to calibrate a model that fits low flows well, while preserving good model-fit at high flows, see Littlewood, I.G. (2003). Improved unit hydrograph identification for seven Welsh rivers: implications for estimating continuous streamflow at ungauged sites. Hydrological Sciences Journal, 48(5),

7/7/0312 PC-IHACRES software, its User Guide and set-up instructions can be downloaded either from the HOMS Reference Manual (Component K ) at happy modelling! or from the CEH website at

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