2.6 Climatology and hydrological processes
2.6.2 Hydrological processes
Most of the precipitation over the catchment area (as well as over Jordan) takes the form of rainfall; around 28 frontal depressions per year reach the eastern Mediterranean originating from Mediterranean or Polar regions. They center over or near Cyprus then move northeast (10.5 depressions), east-northeast (11 depressions), southeast (2 depressions), and 5 depressions fill up (HMSO, 1962). Figure 2.15 shows the frontal depressions track in the Mediterranean basin.
Fig. 2.15: Main winter frontal depressions track in the Mediterranean Basin with annual averages frequencies given between brackets (HMSO, 1962, p.33).
Snow and hail are not common phenomena and are limited exclusively to the highlands. Snow occurs once or twice a year over the eastern highlands of the study area with a mean number of snowy days in Salt station (1991-2000) and Sweileh station (1985-2000) of 3.9 and 4 days, respectively (Jordan Meteorological Department, 2002). Hail occurs even less frequently with long term average of 0.1 and 0.7 days in Salt and Sweileh stations respectively, (calculated from the same period of time as snow).
The Salt station is located around 7 km northwest of the study area (320 20‟ N, 350 44‟
E) while Sweileh station is located around 4 km to the northeast of the study area (320 00‟ N and 350 54‟ E). Both stations are operated by the Jordan Meteorological Department.
The long term annual average precipitation over Wadi Kafrein ranges from 514 mm in the upper eastern part near Wadi As Sir to 179 mm near the Kafrein dam. The average area precipitation of Wadi Kafrein is 387 mm calculated from historical data of 10 stations covering the period from 1980-2008. Figure 2.16 shows the stations network locations and the spatial precipitation distribution over the study area. Data were obtained from Ministry of Water and Irrigation/ Jordan. The interpolation was done using the ordinary Kriging method (Spatial analyst tool, ArcGIS).
Fig. 2.16: Average annual rainfall over Wadi Kafrein catchment area.
Three rainfall stations where selected to present the spatial distribution of precipitation over Wadi Kafrein; Wadi As Sir station, located in the eastern high lands (720 m asl) of the study area, Adassiya Janoubiya station, located in the southern part (530 m asl), and South Shuna station (-230 m asl), located 5 km northwest of Kafrein Dam and which represents the precipitation pattern in the Jordan Valley (Fig. 2.16).
Spatial variation in precipitation amounts is clear with respect to elevation as shown in Figs. 2.16 and 2.17.
The highest precipitation amount in the analyzed period was recorded in Al Salt station with 1,188 mm in the wet year 1991/1992, and also all other stations in the area recorded the highest amounts of precipitation in the same year, while the lowest value of precipitation was recorded in South Shuna station with 58 mm in the dry year 1998/1999. This spatial and temporal variation of precipitation is reflected on the
Precipitation (mm)
annual water storages in Kafrein dam and has a direct impact on runoff generation and concentration zones. Also it affects the groundwater recharge which in turn affects positively or negatively the agricultural sector.
Fig. 2.17: Spatial variations of precipitation over Wadi Kafrein catchment with respect to elevation.
2.6.2.2 Temperature
The topography variations also affect the temperature over the study area but with a reversed effect compared to precipitation; temperature increases with lower elevations. Records are available for the two climatological stations Wadi As Sir AL0057, located in the upper eastern part of the study area, and South Shuna station AM0007, which lies in the Jordan Valley. The highest maximum temperature in Wadi As Sir station was recorded in the summer of 2002 with 39 °C while the lowest minimum temperature was recorded in the winter of 2003 with -11 °C.
The South Shuna station‟s highest maximum temperature was recorded to be 49 °C in the summer of 2003 and the lowest minimum of 1 °C was recorded in the winter of 2002; the average temperature in the highlands is 17.2 °C and in the Jordan Valley it is 25.4 °C. Figure 2.18 summarizes the spatial and temporal variations of temperature in Wadi Kafrein catchment. It can be noticed from Fig. 2.18 that the maximum temperature ranges of Wadi As Sir station is more or less similar to the minimum temperature range of South Shuna; this indicates an obviously large variation of temperature distribution over the study area. Data was available for Wadi As Sir station from 2002-2006 and for South Shuna from 1997-2006.
0 200 400 600 800 1000 1200
precipitation (mm)
80-81 82-83 84-85 86-87 88-89 90-91 92-93 94-95 96-97 98-99 00-01 02-03 04-05 06-07 Date (years)
Wadi As Sir Adassiya Janoubiya South Shuna
Station name Elevation (m) Annual average (mm) -Wadi As Sir 720 502 -Adassiya Janoubiya 530 231 -South Shuna -230 164
39
Highest Max. and Min. T Lowest Max. and Min. T Average T
Wadi As Sir station South Shuna station
Fig.2.18: Spatial and temporal variations of temperature in Wadi As Sir and South Shuna stations.
2.6.2.3 Evaporation
Evaporation rates depend on several factors like air temperature, water temperature, and absolute humidity of layers of air just above the free-water surface. Also, it is affected by the wind which carries away the vapor from the free water surface and keeps the absolute humidity low. The driving force behind evaporation is solar radiation because it warms both the water and the air (Fetter, 1988). Air temperature and the above mentioned factors vary in space and time (the variations of climatic conditions) and affect directly the potential of evaporation and the evaporation rates.
This leads in the Kafrein area to an increase in the potential evaporation rates when moving westward to the Jordan Valley.
The A-pan evaporation values measured in Wadi As Sir and South Shuna stations were used to verify the spatial and temporal variations. Wadi As Sir station represents the prevailing climatic conditions over the highlands while South Shuna station the prevailing climatic conditions in the Jordan Valley. Figure 2.19 shows the spatial effect where higher evaporation rates are measured with lower elevations. The temporal variation is caused by increases in solar radiation during summer months leading to higher values in both stations. The highest monthly evaporation rates over the eastern northeastern part of the catchment area occur during July with an average value of 335 mm with gradual increases toward west-southwest of the catchment to an average of 450 mm. The lowest rates of evaporation occur during January with an average of 65 mm in Wadi As Sir station up to105 mm in South Shuna station. It is worth mentioning that the available data series for Wadi As Sir station is rather short (June 2002-April 2006) and could not be considered as representative for a long term evaluation; though it was presented here to compare these values with those recorded in South Shuna to emphasize the strong effect of the spatial variations caused by elevation differences.
0 50 100 150 200 250 300 350 400 450 500
Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.
Months
Pan A evap. (mm)
South Shuna station Wadi As Sir station
Fig. 2.19: Evaporation rates (class A land pan).
The monthly evaporation rates for Wadi Kafrein catchment were calculated by using the A-pan measured values in Baq‟a station (with a coefficient of 0.74) and Penman equation by Salzgitter and JCE (1992). They calculated the annual evaporation in Wadi Kafrein catchment as 1,928 mm with the highest monthly evaporation rates being 240-260 mm during the period June to August and the lowest evaporation rates being 64-66 mm during the period December to January.
2.6.2.4 Runoff generation and concentrations
The vegetation cover in the middle and upper part of the study area intercepts some of the rainfall before it reaches the ground “Interception;” a portion of the intercepted rain goes back to the atmosphere via “Transpiration” while the other portion falls to the ground via “Throughfall”. The amount of rain which reaches the ground either evaporates, infiltrates into the soil, forms puddles (depression storage), or flows over the surface as thin sheet of water forming what is known as “Overland flow” or runoff. All these processes are illustrated in Fig. 2.20, while more details on the runoff processes is given below.
Horton (1933) was the first who discussed the overland flow and defined it as:
“Neglecting interception by vegetation, surface runoff is that part of rainfall which is not absorbed by the soil by infiltration. If the soil has an infiltration capacity “f”, expressed in inches per hour, then when rainfall intensity “i” is less than “f”, the rain is absorbed and there is no surface runoff. It may be said that a first approximation that if “i” is greater than “f”, surface runoff will occur at a rate (i-f)”.
Fig. 2.20: Physical processes involved in runoff generation (Tarboton, 2003).
This mechanism of runoff generation is called Infiltration Excess Overland flow (IEOF) or the Hortonian overland flow (Fig. 2.22a). It occurs when the precipitation rate (rainfall intensity) exceeds the infiltration rate of the soil (Horton, 1933; Freeze, 1972; Dunne, 1983; Fetter, 1988). The Hortonian overland flow is considered to be the dominant runoff generation type in semi arid regions where rainfall intensities are high and vegetation cover is sparse (Puigdefabregas et al., 1998; Guentner and Bronstert, 2004).
Several conditions could promote the occurrence of IEOF by the reduction of the near surface saturated hydraulic conductivity (Buttle, 1998). Such conditions are related to urbanization activates and the associated sealing of surfaces (Ziegler et al., 2004) or to the crusting of soil surface (Patrick, 2002). This type of overland flow is applicable in zones with low infiltration capacity and the impervious surfaces in urban areas. The prevailing conditions in the lower part of the study area where the Aridic soil moisture regime prevails (see Section 4.2.4) with thin layer of crusted soil surface and no vegetation cover (see Section 2.3) suggest that IEOF is most probably the runoff mechanism which dominates these zones.
The spatial variability of soil properties, which affects the infiltration capacity and the spatial variability of surface water inputs, limits the infiltration excess runoff to some parts of the drainage basin during a rainfall event. A new concept was presented by Betson (1964) who pointed out that only a part of the catchment could contribute to Hortonian overland flow. This refinement of the Hortonian overland flow is known today as the partial area concept as illustrated in Fig. 2.22b.
Another mechanism which may produce overland flow in semi arid regions is the Saturation Excess Overland Flow (SEOF) see Fig. 2.22c. It occurs when the storage capacity of the soil is completely filled, leading any additional rainfall, regardless of
intensity, to flow over the surface (Kirkby, 1988). In zones where the soil texture is coarse and the soil permeability is high, the SEOF is the dominant runoff generation process. In semi arid regions, this mechanism occurs under specific conditions like rainfall in the valley bottom (Ceballos and Schnabel, 1998) or soils where a relatively permeable topsoil layer overlies less permeable material, which was defined by Gerits et al. ( 1990) as a separate type of overland flow called Topsoil saturation overland flow. This is a typical mechanism in case of plough pans, shallow profiles over bedrock, sand and gravel overlying layers of compact structures, etc. (Bergsma, 1983). In some parts of the study area, cultivation is common where several agricultural activities take place, and it is usually above a shallow soil layer overlying compacted bedrocks with low hydraulic conductivity. Figure 2.21a shows a thin layer of soil overlying massive limestone rocks while Fig. 2.21b shows a thin soil layer not exceeding 20 cm overlying compacted bedrock.
In their study, Hewlett and Hibbert (1967) observed high rates of runoff generated in their research catchment without any saturation areas being formed. To explain this, they related the runoff to a subsurface storm flow (Fig. 2.22d). Some of the subsurface water returns to the surface and adds to the overland flow; this is called the return flow and it is illustrated in Fig. 2.22c as (qr).
It is possible in the same catchment area to have infiltration excess, saturation excess or only subsurface responses at different times or different locations due to different antecedent conditions or soil characteristics or rainfall intensities (Beven, 2000). This might lead to the generation of a perched water table and even to saturation at the surface of a soil that may be unsaturated at depth (Fig. 2.22.e).
Runoff generation types and processes were not studied earlier in Wadi Kafrein; this research is the first to deal with these processes and explains the different mechanisms leading to runoff. The previous hydrological studies carried out on Wadi Kafrein catchment refer to quantitative amounts of runoff calculated using synthesized data and lumped methods (e.g. Soil Conservation Service- Curve Number method “SCS- CN”).
Fig. 2.21a : Thin layers of soil overlaying massive limestone (310 59’ 13” N, 350 49’ 06” E).
Fig. 2.21 b: thin layer of soil overlaying compacted Bedrock (310 59’ 25” N, 350 48’ 33” E).
P
Fig. 2.22: Classification of runoff generation mechanisms (after Beven, 2000).
P
a) Infiltration excess overland flow
(Hortonian overland flow) P P
P f
f q0
P
P P q0
f
b) Partial area infiltration excess overland flow
P P
P
qs
d) Subsurface storm flow
P P
P qs
Horizon 1 Horizon 2 e) Perched subsurface storm flow
P P
P q0
qs
c) Saturation excess overland flow
qr
2.6.2.5 Transmission losses and runoff routing
In arid and semi arid regions the generated overland flow is characterized by a general decrease in volume in the downstream direction. Some of the flash flood water infiltrates into the sediments, which comprise the channel bed; these losses of water are called transmission losses and they can be a significant portion of the total runoff volume (Lane, 1983). These losses are also considered to be an important source of groundwater recharge in arid and semi arid regions (Shentsis and Rosenthal, 2003;
Goodrich et al., 2004). The duration of flow at a particular location in the channel network depends on the velocity and size of the flood affected by the hydrodynamics of the flow and the cumulative transmission losses upstream of that location (Mudd, 2006).
Transmission losses were studied by Renard and Keppel (1966) at the Walnut Gulch experimental Watershed located in Arizona, United States, where large flood volume losses were reported as measurements progressed downstream (Table 2.2). Further studies about transmission losses are cited by Pilgrim et al. (1988).
Table 2.2: Effect of transmission losses on a flood at Walnut Gulch, Arizona (Renard and Keppel, 1966; Pilgrim et al., 1988).
Drainage basin area (km2)
Volume of flood (1000 m3)
Peak discharge (m3/s)
95 92.3 41.9
114 79.9 27.2
149 40.1 15.6
By studying the runoff generation process, the amount of water which reaches the stream and flows downstream towards a catchment outlet within a period of time is measured. However, the routing of the runoff from the source areas to the outlet is an important component which must be taken into consideration. The boundary between runoff generation and runoff routing is not a very precise one and it is generally not possible to predict the volume and timing of the inflows, precisely making the routing problems one of the velocities of surface and subsurface flows on the Hillslope as well as in the stream channel (Beven, 2000).
There is no available data or information about the volumes of transmission losses or runoff routing in the catchment area of Wadi Kafrein despite their role and affect on the total runoff volume as discussed above. In this study, the transmission amounts will be calculated for the catchment area due to its importance and influence on runoff and the routing component to taken in consideration in the modelling process.