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5. AN OVERVIEW OF ERITREA FROM AN HYDROLOGICAL PERSPECTIVE
5.2 Previous Attempts at Stormflow Modelling in Eritrea
According to a report by Euroconsult (1997), there appear to have been very few previous attempts at rainfall-runoff modelling in Eritrea. One of the first attempts at quantification of the available surface water resources of the country was that undertaken by Natural Resources Consulting Engineers (NRCE) in 1992, who estimated average monthly and annual flows using a weighed average runoff coefficient dependent upon catchment rainfall. This approach may be adequate for reconnaissance studies, but cannot yield sufficient information on the reliability of runoff or on the inter-annual variability of flows.
In a feasibility report by NRCE (1994) on the rehabilitation and expansion of the Aligidir irrigation project, a statistical analysis of recorded flows on the lower Gash was employed to derive the "median" (50% exceedance) and "firm yield" (80% exceedance) annual flows by using the log-Normal distribution. These design annual discharges were found to be distributed log-Normally and hence daily flows could be estimated using a random number generator. Single representative "unit seasonal hydrograph" was produced. This was scaled using the annual design flows derived earlier. According to Euroconsult (1997), this approach may be valid for a site such as the lower Gash where there is a reasonable quantity of flow data, but the method would be difficult to transpose to an ungauged area. Not only would it be difficult to derive suitable annual runoff volumes of the required exceedance probability, but
disaggregation into realistic daily flows would be almost impossible. This modelling approach is too site specific to be more generally applicable throughout Eritrea.
The NRCE (1996) attempted to employ the SCS method to derive a dimensionless unit hydrograph using limited data available from Wadi Laba and Ruba Haddas to calibrate the model. The method could possibly be transferred to other ungauged parts of Eritrea. However, a number of uncertainties remain as to how the model was applied, particularly regarding the estimation of initial abstractions (losses of rainfall before runoff commences), which is one of the most difficult hydrological processes to model. As a result, the simulated runoff failed to approximate the observed runoff. The model, as described, was not thought to be suitable for wider application (Euroconsult, 1997).
Halcrow (1997) attempted to simulate surface runoff from storm rainfalls by plotting 3-day rainfalls over the catchment of the eastern escarpment against resulting peak flows.There was no discernible relationship at all, with the largest recorded rainfall ofjust over 80 mm yielding a flood peak of about 32 m3.S·I, whereas rainfalls of only 30 mm on other occasions produced peak flows of 400 and 512 m3.s- l• Halcrow's (1997) modelling approach was not meant to simulate runoff volumes or peaks from individual storm events,but rather to generate a series of flood events which reproduced the statistics of the historical data as realistically as possible. Thus, the intention was to generate the correct number of flood events for different times of year, and then to scale these flood events so that their magnitudes appeared to be representative of historical events (Euroconsult, 1997).
Colombo and Sarfatti (1996) utilised the SCS method to simulate the runoff volume of two subcatchments of the Mereb River, viz. the Shiketi and Emni- Tzelim ,situated on the Eritrean highland. The Curve Numbers (CNs) were determined from remotely sensed information and used together with conventional field data. However, the results could not be verified against streamflow observations and,therefore, cannot be extended to other ungauged areas.
A more recent modelling study was attempted by Euroconsult (1997) for an assessment of the surface water resources throughout most of Eritrea. A SCS-based semi-distributed model which consists of two components was employed: one component to compute daily runoff volumes from daily rainfall inputs, and the second to distribute this runoff volume between flood and baseflow runoff. The approach adopted was to select as wide a range of catchments for their study and to use common adjustments to the calibration parameters across all
catchments. As such, the calibration sites covered catchments in all the main river basins of the country.
The model was tested on catchments ranging from 200 to 21 000 km2 and shown to give realistic representations of some complex hydrological processes over the wide variety of spatial scales. According to Euroconsult (1997), the model can be used for preliminary assessment of surface water resources and for assisting in the planning and development of surface water projects. However, it is not possible to apply this model for the design of hydraulic structures. The reasons are as follows:
• The flow information employed for model calibration and verification at the selected sites was insufficient and a subjective approach was used in model calibration and verification instead of using more objective calibration techniques. Hence, the model accuracy remains doubtful.
• The model used statistically generated rather than observed rainfall series as input and that could reduce the value of resulting flow series.
• The physical properties of the catchments from which final Curve Numbers, CNfi were generated were highly oversimplified, especially when the antecedent soil moisture conditions of the catchments were considered.
• More than 90% of the Setit basin is outside of Eritrea and it was not possible to obtain the rainfall data needed to model this basin.
As a conclusion, therefore, it may be stated that previous hydrological modelling attempts in Eritrea cannot be used to simulate design floods from small catchments throughout most of the country because of the following reasons:
• First, there is at present no recommended hydrological model in Eritrea which has been widely verified.
• Secondly, most of the models used in previous studies were developed to solve particular problems and the solutions are too site specific for general use.
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The next chapter deals with testing the SCS-ACRUstormflow modelling approach in Eritrea.
This approach was previously proven to be successful under wide ranging environmental