SURM is a conceptual rainfall-runoff model that estimates daily stream flow from daily rainfall and areal potential evapotranspiration data. It is a simplified version of SIMHYD. The model contains three stores for interception loss, soil moisture and groundwater and model has seven parameters.

SURM is the default rainfall-runoff model in music (Model for Urban Stormwater Improvement Conceptualisation).

Scale

SURM operates at a daily time-step in Source. This model is applied at the functional unit scale.

Principal developer

Cooperative Research Centre for Catchment Hydrology. SURM is a variant of SIMHYD, developed for use in urban areas (Chiew et al. 1997).

The current model was developed by eWater CRC.

Scientific provenance

The antecedant models of SURM, SimHyd and Hydrolog, have been widely applied in catchment modelling across Australia. SURM has been widely used in Australia, and to a lesser extent in other countries, as the default model in the music urban stormwater modelling framework.

Version

Source v3.8.10

music version 4 (Oct 13, 2009)

Dependencies

None.

Availability/conditions

SURM is automatically installed with Source.

SURM is provided as the rainfall-runoff model in music version 4.

Flow phase

The structure of SURM is shown in Figure 1. The model allows for separate runoff generation processes on impervious and pervious portions of a catchment.

Figure 1. Conceptual structure of SURM

In SURM, all rainfall on the impervious area becomes runoff once a small storage capacity or initial loss is exceeded. The initial loss storage is emptied each day.

Rainfall on the pervious part of the catchment is subject to infiltration, with the infiltration rate of the soil being defined as an exponential function of the soil moisture storage. The infiltration rate is at a maximum when the soil moisture store is empty, and gradually decreases to a minimum when the soil moisture store is full. Runoff from the pervious area occurs when the rainfall exceeds the infiltration rate of the soil (infiltration excess runoff).

Evapotranspiration is subtracted from the soil moisture store, and is dependent on the amount of water in the soil store and the areal potential evapotranspiration rate.

Soil moisture recharges groundwater whenever the soil moisture store exceeds field capacity. Recharge is calculated as a constant percentage of the storage above field capacity.

Base flow from groundwater is simulated as a constant percentage of the groundwater store. Deep seepage is also calculated as a constant percentage of the groundwater store. Base flow becomes part of the catchment outflow, but deep seepage is permanently lost from the catchment.

This model is the equivalent of a model structure with one soil moisture store over the pervious area (a1 = 1, a2 = 0).

Input data

The model requires daily rainfall and potential evapotranspiration data (PET). The rainfall and PET data sets need to be continuous and overlapping.

Daily rainfall data may be obtained from rain gauges or rainfall represented as a spatial layer, eg rainfall grids, but will need to be converted to a time series record that is spatially representative of the whole catchment. Note that the time that rainfall data is collected may be important. Very often rainfall data is collected in the morning, the usual time is 9am, and may be more representative of the previous day’s rainfall.

Daily PET is an estimate of the spatially averaged areal potential evapotranspiration rate of the catchment being modelled. This estimate is subject to a number of climatic and land use/land cover variables. This may be estimated by applying a crop/land use factor to daily pan data or extracted directly from maps of calculated areal potential evapotranspiration data.

Selecting stream flow data to use in a river-basin-scale simulation study needs information about the reliability of the data. It is best to use data which are most representative of the stream flow from the catchment. Observed data would normally be selected, except where the data are of poor quality or of unknown reliability.

Parameters or settings

The parameters for SURM are summarised in Table 1.

Table 1. SURM Model Parameters

Parameter	Description	Units	Default	Min	Max
bfac	Base flow coefficient		n.a.	0	1
Coeff	Infiltration coefficient		n.a.	0	400
dseep	Deep seepage		n.a.	0	1
Frac. field capacity	The field capacity, expressed as a fraction of the maximum soil moisture capacity		n.a.	0	1
Fimp	Impervious fraction		n.a.	0	1
initgw	Initial groundwater level	mm	n.a.	0	500
Initial moisture	Initial soil moisture content, as a fraction of the maximum store capacity		n.a.	0	1
Rfac	Recharge coefficient		n.a.	0	1
smax	Soil moisture store capacity (mm)	mm	n.a.	1	500
sq	Infiltration shape		n.a.	0	10
thres	Impervious threshold	mm	n.a.	0	5

Many authorities in Australia have guidelines that specify default values for parameters of SURM models for use within music models. These default parameter sets should be consulted to provide guidance for your rainfall runoff model if you are using SURM in an area that is covered by one of these guidelines.

Output data

The model outputs daily surface and base flow. This may be saved in ML/day, m³/s or mm/day. In addition, the variable listed in Table 2 may be recorded.

Table 2. Recorded variable

Variable	Parameter	Frequency
soilMoistureLevel	The soil moisture depth, in mm. Note that this is not the equivalent depth averaged over the catchment, but the one averaged over the pervious area	time step

Reference list

Chiew, FHS, Mudgway, LB, Duncan, HP & McMahon, TA 1997, Urban Stormwater Pollution, Industry Report 97/5, Cooperative Research Centre for Catchment Hydrology, Canberra.

Bibliography

eWater 2009, music v4 by eWater User Manual, eWater, Canberra.

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