A network of consultants in: water resources, hydrology, hydraulics, hydrogeology and water quality

Hydrological Modelling

Hydrological models serve a range of purposes but they are used primarily to estimate runoff from sequences of rainfall and the meteorological information needed to estimate potential evaporation. They can be used to estimate river flows at ungauged sites, fill gaps in broken records or extend flow records with respect to longer records of rainfall.

Powerful areally-distributed models are based on physical principles governing the movement of water within a catchment area, but they need detailed high-quality data to be used effectively.

More commonly, simpler conceptual models are used to represent the basin as a whole. The main controls on water movement are represented by quasi-physical model elements whose action is governed by a set of model parameters. In some circumstances, these parameters can be adjusted to represent changes to land-use in the catchment area. The flow diagram illustrates HYSIM, one of this type of model used by WRA, which is also capable of simulating many of the artificial influences on a river basin.

Other classes of model include stochastic models that can relate runoff to rainfall or be used to synthesise contemporary flows in a number of basins by preserving the spatial and temporal correlation structure defined from a common record.

WRA has experience in the development and use of many of these different types of model. We are aware of the limitations of all forms of modelling and of the need to take account of the limitations of data quality or data availability in a particular river basin and to select a model that meets the precise needs of the project.

UK
The River Rother in Kent is an important river for water resources. Its flow record is incomplete and, for certain periods, of doubtful accuracy. The HYSIM model was calibrated to three sub-catchments in the basin. The input data to the model (precipitation and PET) were checked for consistency and the simulated flow record was used to provide a consistent basis for evaluating changes to the observed flow record and for infilling missing data.

UK
The river Trent, a complex drainage area of 20,000 km2, rises in the English Midlands and flows into the North Sea. Its upper reaches are highly urbanised with much industry. In the past this river has been considered unsuitable for potable use but recent improvements in quality have led to that decision being revised. This led to a need for a long-term naturalised flow record. The HYSIM rainfall/runoff model was calibrated to six years of recent flow data for which data on abstractions and discharges were also available. An extended naturalised flow sequence was produced using rainfall and evaporation data going back to 1920.

Ethiopia
The Omo-Gibe basin, in southern Ethiopia, had a drainage area of 80,000 km2 and a population of six million. The basin is at present largely undeveloped. As part of a Master Plan Study a flow sequence was need to determine the potential of the basin for irrigation and hydropower development. A monthly rainfall-runoff model was developed to produce extended flow records for those sub-catchments with flow data and synthetic flow series for ungauged sub-catchments.

Chad
Lake Chad plays an important part in the regional economy. The lake is filled by the rivers Logone and Chari which flow from wetter areas of Cameroon, Nigeria and the Central African Republic. As the time lag between rainfall in the upper basin and the lake is 3 or 4 months it was possible to develop a monthly hydrological model for flow forecasting purposes. One

Kenya
A river flow model was developed to investigate the environmental impact of increased abstractions from the Mzima springs in the Tsavo National Park on the Tsavo, Athi and Sabaki river system in eastern Kenya. The model operates on a five day time step over the period 1932 to 1991 and was used to evaluate different pipeline capacities, pipeline operating strategies. It also simulates the effects of climate change and man-made influences on the Athi river.

India
Development of river basin models capable of routing the estimated runoff through a series of irrigation control structures as part of a UNDP-funded programme to strengthen the capability of the Institute for Water Studies in Madras.

Thailand
Use of rainfall-runoff models to investigate the impact of deforestation, changes in cropping and irrigation and hydropower development on the hydrology of the Mekong basin.