UHM

The UHM (Unit Hydrograph) model constitutes an alternative to the NAM model for flood simulation in areas where no streamflow records are available or where unit hydrograph techniques are already well established.

The model includes a number of simple unit hydrograph models to estimate the runoff from single storm events by the use of the well-known unit hydro­graph technique. The models divide the storm rainfall in excess rainfall (or runoff) and water loss (or infiltration).

More details on the UHM model can be found in the ‘MIKE 1D Reference’ manual.

UHM parameters

The following parameters are used for all types of UHM models:

·         Adjustment and baseflow

·         Hydrograph

·         Enlargement and loss model

·         Lag time

Adjustment and baseflow

·         Area adjustment factor. This adjustment factor (different from 1.0) may be applied if the catchment rainfall intensity is assumed to differ from the input rainfall data series by a proportional factor.

·         Baseflow. If different than zero, the value specified represents a con­stant baseflow that is added to the runoff.

Hydrograph

The distribution of the runoff in time can be described using different meth­ods.

Three different options can be selected for the hydrograph type:

·         SCS triangular. This is a standard hydrograph in which the time to peak is assumed to be half the duration of the excess rainfall plus the lag time t_l.

·         SCS dimensionless. This type of hydrograph has been derived from a large number of natural unit hydrographs from catchments of varying size and location. The flow values are expressed in Q/Qp, where Qp is the peak discharge, and the time in T/Tp, where Tp is the time from the start of the hydrograph rise to the peak.

·         User defined. This hydrograph should be specified in its dimensionless form, i.e. Q/Qp as a function of T/Tp, as for the SCS dimensionless hydrograph above. But in this case this information has to be provided in the Hydrograph table (Dimensionless from Q/Qp as a function of T/Tp). The hydrograph table must be provided in a time series file, where the axis type is either ‘Equidistant Relative Axis’ or ‘Non-Equidistant Relative Axis’ and containing one item with the type ‘Dimensionless factor’.

Enlargement and loss model

Four different options are available to represent the loss model, each of them requiring different inputs.

These types of loss models are:

·         Constant loss. In this case the infiltration is described as an Initial Loss at the beginning of the storm followed by a Constant Loss term caused by infiltration.

·         Proportional loss. If this model is selected, a Runoff coefficient should be specified as the ratio of runoff to the rainfall.

·         SCS method. The SCS Loss Model uses a Curve number that charac­terises the catchment in terms of soil type and land use characteristics. The model also requires an Initial AMC (antecedent moisture conditions) parameter.

·         SCS generalised. The SCS generalised loss model does not make use of the concept of an antecedent moisture content (AMC) but applies a storage Initial abstraction depth. Like in the SCS method, the Curve number should be specified for the SCS generalised.

Lag time

The Lag time type for the UHM model can be specified in two different ways:

·         User specified. In this case the lag time is specified directly in hours.

·         Curve number method. In this case the lag time is calculated by the standard SCS formula and will appear in the Derived Lag Time box. When this method is selected, three parameters need to be specified: Hydraulic length, the Slope of the catchment and the Curve number.

Time series

In this section the input time series of the rainfall-runoff model are entered. The UHM only requires two input time series:

·         Rainfall. This time series represents the average catchment rainfall. The time interval between values, may vary through the input series. The rainfall specified at a given time should be the rainfall volume accumu­lated since the previous value.

·         Evaporation. The potential evaporation is typically given as monthly val­ues. Like rainfall, the time for each potential evaporation value should be the accumulated volume at the end of the period it represents. The monthly potential evaporation in June should be dated 30 June or 1 July.

Weighted time series may be used by enabling ‘Use weighted time series’. This adds a new tab ‘TS weighted rainfall/evaporation’ where time series, their corresponding weights and distribution in time may be defined (see ‘Weighted time series’ previous to this section).