Runoff generation

In earlier sections of this chapter, the methodology and results of the measured precipitation (section 5.4.1) and the measured runoff (section 5.4.3) were explained and discussed. These measurements have been used in calibrating and validating the hydrological model (section 5.7). Here, the quantitative results of the model concerning the generated runoff and the transmission losses for the runoff events, which had been used in the calibration process, will be discussed in addition to the spatial distribution of the generated runoff.

The runoff results will be presented and discussed on an event basis, in which the volumes will be given for the whole catchment area as a mean value in millimeters.

The generated runoff is either a product of IEOF or SEOF. A certain volume of the generated runoff (either due to IEOF or SEOF) will be lost during the routing process which has been referred to as the transmission losses. Therefore, the total amount of transmission loss and the final volume of runoff, which reaches downstream, form together the generated runoff.

Measured and simulated runoff events are quantitatively given in table 5.10. Values given in table 5.10 are final runoff values while in this section; the total generated runoff (including the value of transmission losses) is discussed.

5.9.4.1 Storm event 21-22 February 2009

It has been shown in section 5.4.1 that precipitation amount was lower than average in the water year 2008/2009. As mean area rainfall, only 2.5 mm were measured over the catchment area of Wadi Kafrein in January 2009. During February and prior to the main rainfall event of 21-22.02.2009, five rainfall days were recorded with total area rainfall of 34.7 mm, out of which 22.8 mm were recorded in the 10^th and 11^th of February 2009.

The generated runoff during the 10^th and 11^th of February 2009 was mostly due to IEOF (98.7%) while the transmission losses presented around 44%. During the main storm event on the 21^st and 22^nd of February, 2009, a total area rainfall of 34.5 mm were recorded (around 5.6 MCM). As the runoff in the prior storm event on the 10^th of February was generated by IEOF, the actual soil storage was low with a mean area value of no more than 17 mm. Also, considering the high intensity of this storm event, it is expected that the generated runoff would be a result of IEOF. The model results show that 77.3% of the generated runoff was due to IEOF and 22.7% was due to SEOF. The transmission losses present 30.4% of these values.

It can be noticed from Fig.5.52, that runoff was mainly generated in the northeastern and southeastern part of the study area where urbanization is taking place. Also runoff was generated on the bare rock area in the western part near the Kafrein reservoir.

Fig. 5.52: Spatial runoff distribution on Feb. 21^st, 2009.

5.9.4.2 Storm event 27 February-03 March 2009

The second storm event, which has also been used in the calibration process, had a total area rainfall of 82.4 mm (around 13.3 MCM). Out of this amount, 70 mm precipitated in the first 3 days. In this storm event, most runoff was generated due to SEOF in contrary to the 21-22 of February event. This is due to the antecedent soil moisture from the earlier event where 22.7% of the generated runoff was due to SEOF. Also, the relatively short lag time of 5 days between the two storm events contributed as well. The SEOF was 83% of the runoff while 17% was due to IEOF.

The transmission losses present 25% of the generated runoff by the two mechanisms.

Fig. 5.53: Spatial runoff distribution on Feb. 28^th, 2009.

5.9.4.3 Storm event on March 23^rd, 2009

During the third storm event, on the 23^rd and 24^th of March, 2009, a total area rainfall of 38.7 mm (around 6.2 MCM) was recorded. This event comes almost after 3 weeks of no rain and most of the rain took place during midnight of the first day and high rainfall intensity. The runoff was generated as IEOF with 63.7% and 36.3% as SEOF. The transmission losses present 17.8% of these values.

In the western part of the study area and despite relatively low rainfall, most runoff was generated (Fig. 5.54). Similar to the first storm event, the spatial distribution of the generated runoff is mainly in the urban areas and the bare rock areas in which the infiltration rate is not high. Consequently, after a specific amount of precipitation of high intensity, these surfaces do not have the capacity to infiltrate any additional amount of rain and the runoff will be generated due to exceeding the infiltration capacity of these surfaces.

Fig. 5.54: Spatial runoff distribution on Mar. 23^rd, 2009.

5.9.4.4 Runoff coefficients

The runoff coefficient is the percentage of runoff from the precipitated rainfall. The mean daily values of the generated runoff and the precipitated rainfall were used to calculate the runoff coefficients for the entire modelling period of 2007-2009. The water year 2007/2008 had low runoff coefficients, not exceeding 4% (as shown in Fig.

5.55) compared to runoff coefficients of more than 10% in the water year 2008/2009 (Fig. 5.56).

0%

1%

2%

3%

4%

5%

6%

7%

8%

16.11.07 01.12.07 16.12.07 31.12.07 15.01.08 30.01.08 14.02.08 29.02.08 15.03.08 30.03.08 Time (days)

Runoff coefficient (%)

0 5 10 15 20 25 30 35 40 45 50

Accumulated rainfall (mm)

Runoff coefficient Rainfall

Fig. 5.55: Runoff coefficients for all events during the water year 2007/2008.

0%

25.10.08 14.11.08 04.12.08 24.12.08 13.01.09 02.02.09 22.02.09 14.03.09 03.04.09 23.04.09 Time (days)

Fig. 5.56: Runoff coefficients for all events during the water year 2008/2009.

In the calibration year, 2008/2009, the second storm event of 27.02-03.03.09 had the highest runoff coefficients (Feb. 28^th) with a value of 10.6%. As stated earlier, the runoff during this day was mainly generated due to saturation conditions of the soil and all the additional rain was transformed to runoff. During the third storm event on Mar. 23^rd, a high magnitude of runoff was also generated (9.2%) due to the high intensity of the precipitation and prevailing infiltration excess conditions.