RAINFALL CHANGES AND WATER EROSION OF SOIL BY USING THE WEPP MODEL IN LATTAKIA , SYRIA

Changes of soil erosion and rainfall have been simulated by using the Water Erosion Prediction Project (WEPP) model between 2016 and 2039 in Lattakia, Syria. This study was conducted in 6 locations that are characterized by two different ecosystems (agricultural, forest). The results show a linear decrease in rainfall amount of about 7.11 mm per year (170 mm for the whole studying period). For the years 2025, 2026 and 2030, three snowy storm events have been predicted, while the year 2030 will record the highest rainfall amount of 1816.1 mm. According to the WEPP model, the average of estimated soil erosion amount in Alhamara has reached 19 t/ha/y for the agricultural system while it is estimated to be 2.03 t/ha/y for the forest system. The general average of soil erosion in the study area (taking into consideration the variety of slope) within the agricultural system has reached 14.086 t/ha/y, which indicates that there will be a dangerous impact of future erosion on the sustainability of natural sources (soil, water) in the study area.


Introduction
Soil erosion is one of the most serious land degradation and soil fertility deterioration (Morgan, 1995;El-Swaify, 1997).It occurs under diverse circumstances and impacts soil features (crops and water resources).Erosion increases over the years to the detriment of cultivated land.Its main consequences are loss of precious soil resources for cultivation, and siltation of reservoirs and streams (Jain and Dolezal, 2000).Soil erosion is a physical process, but the underlying causes are related to the socio-economic, political and cultural environment in which land users operate (Stocking and Murnaghan, 2001).General relationships between climate and erosion are well established (Langbein and Schumm, 1958).And these have been applied by many researchers to hypothesise about the probable impact of climate change on soil erosion.Several global and regional models have been developed for this purpose.
However, if both rainfall amount and intensity change together in a statistically representative manner, predicted erosion rate will increase by 1.7% for every 1% increase in total rainfall (Pruski and Nearing, 2002a).
In recent years, the Water Erosion Prediction Project (WEPP) model has been used to simulate the potential impacts of climate change on soil erosion and surface runoff in different places around the world (Pruski and Nearing, 2002a,b;O'Neal et al., 2005;Zhang and Nearing, 2005;Zhang et al., 2009;Li et al., 2010).
Soil erosion in the Syrian costal region has main environmental impacts like soil degradation and impoverishment and deterioration of surface water quality in basins.A quick and quite accurate method to assess the erosivity potential in a particular region is provided by the application of soil loss prediction models, which take into account the assets and liabilities of erosion factors.The Arab Center for the Studies of Arid Zones and Dry Lands (ACSAD) has developed a map of soil degradation which demonstrates that about 6% of the Syrian land is threatened by water erosion with varying degrees (85% of this land is exposed to slight degradation, 12% is exposed to moderate degradation, while 3% suffers from extreme degradation) (ACSAD, 2007).There is also another map published by FAO in 2008 and United Nations Environment Programme, which clearly shows high rates of water erosion in the coastal area which goes over 100 t/ha/y.Hussien (2014) studied the changes in the amount of soil erosion into three systems (forests, burned forests, and agricultural lands) in Lattakia.The study concluded to record erosion, which reached 121.55 t/ha/y for cultivated lands, 47.5 t/ha/y in burned forests, and 10.55 t/ha/y in the forests.According to a study done by Kbibo et al. (2014) in fifteen locations that have different slopes and agricultural use in the coastal area in Syria using field experiments, the rate of erosion varies between 12 to 200 t/ha/y.This research aims to stimulate the changes in the amounts of soil erosion and estimated rainfall by using the WEPP model throughout 24 years (2016-2039) including 6 selected locations that have various slopes in two eco-systems (agricultural, forests) in Lattakia governorate in Syria.

Study area
The study area is located in Lattakia governorate, in the north west of Syria.The case study is dominated by the Mediterranean climate which is characterized by hot and humid summers, and relatively mild to warm wet winters (Eid, 2004).The annual rainfall rate is considered as the first-level stable area.In the coastal region, throughout the period 1989-1998, the minimum temperature reached 12.5°C while the maximum temperature reached 21.69°C.The coastal areas are characterized by complex topography whereas from the east, the coastal line is generally less steep.On the other hand, the highlands begin to appear at the eastern coastal plan in the forms of low level hills at 1700m above sea level where steep mountains are threatened by soil erosion (Kbibo et al., 2014).The study was applied to 6 different locations that were classified as the most exposed to soil erosion as the following: • The agricultural system: the surface of the soil is ruined and highly exposed to water erosion.The agricultural operations include tilling, seeding, crop service operations (in some areas, it is restricted only to tillage in order to prevent weed growth and this in its turn increases the erosion seriously).
• The woodland system: it includes the woodland cover in the study area.
Geomorphologically, the area is highly complicated and it is characterized by its various morphological components.Generally, two different groups of rocks exist; calcareous and non-calcareous, and they lead to the composition of three major groups of soil which are as the following: terra rossa, calcareous rendzina, and brown calcareous.Land-cover types vary from natural vegetation (forests) mixed with crop land.Five types of land use/cover exist in Lattakia: residential and commercial areas (10.26%), forest (37.12%), mixed agricultural (45.79%), open rangeland (0.72%) and bare soil (4.26%).

Estimation of soil erosion amounts using field experiments
This study relied mainly on the field results obtained by Kbibo et al. (2014).The study was carried out in the coastal area through the seasons 2011-2013.After determining the study areas according to the adoption and vegetation cover, small experimental plots were installed to measure the amounts of soil erosion (Kbibo and Nesafi, 1997), and these plots were modified by Kbibo et al. (2014).Small plots of 4 m 2 in the area with a regular shape (generally 1.5 m wide and 2.5 m long, measured horizontally) were formerly used in the rolling loess regions (Kbibo et al., 2014) and the plot led to a container of 200L to collect the runoff and the erosion outputs.

Estimation of soil erosion by using the WEPP (Version 2012.800) model
The WEPP (Nearing et al., 1989) is a new generation, process-based, soil erosion prediction model based on fundamentals of infiltration theory, hydrology, soil physics, plant science, hydraulics, and erosion mechanics.It is a continuous simulation model for predicting daily soil loss and deposition from rainfall, snowmelt, and irrigation.
The WEPP hill slope profile erosion model uses a steady-state sediment continuity equation (Flanagan et al., 2001) to calculate erosion as follows: D i : inter rill sediment delivery to the rill (kgs -1 m -2 ) G: sediment load (kgs -1 m -1 ) D r : rill erosion rate (kgs -1 m -2 ) x: distance downslope (m) The hill slope component of the WEPP erosion model requires a minimum of four input data files to run.These are: climate, slope, soil and crop/management files.

Climate file
The climate generator program (CLIGEN-V5.3) was used to stimulate the climate of the study area.However, the program recognizes only the climatic data that has GDS format.Authentic climatic data has been obtained for all the Syrian climatic stations by using the same program between the period 1987 to 1993.The file (NM-40016.GDS) was uploaded from USDA-ARS website: (http://hydrolab.arsusda.gov/nicks/nicks.htm).The file represents Lattakia's station which is considered as the main station to stimulate the climate of the study area.After the file was uploaded to the program, a comparison was made between the file data and databases of the similar climate stations.The compatible station that was approved by the program was Lahaina 3GL in the American Hawai state.Certain modifications in the data were done in order to fit to the study area.These modifications are: • The maximum rainfall rate within 30 minutes was 48 mm (General Directorate of Meteorology, 1990Meteorology, -1999)).
The program simulates the daily climate of the study area for 100 years (daily and monthly averages of rainfall, maximum temperature, minimum temperature, wind speed, dew point, freezing point, and the number of wet and dry days).Based on these modifications, the WEPP program includes all the necessary climatic data to represent the conditions of the study area, by using the climate generator (CLIGEN), which generates daily rainfall pattern, daily temperature (maximum, minimum, and dew point), solar radiation, and wind speed and direction (Nicks et al., 1995).Precipitation occurrence is generated using a firstorder, two-state Markov chain based on conditional transition probabilities of a wet day following a wet day (P w/w ) and a wet day following a dry day (P w/d ).Daily precipitation amounts are generated using a skewed normal distribution, and daily temperature and radiation are generated using normal distributions.Furthermore, the other input files (soil database editor, slope file editor, management editor) were established as shown in Table 1.

Model Calibration
The model has been calibrated by comparing the results of field experiments conducted by Kbibo et al. (2014) and the model outputs.The comparison between field results and estimated values for agricultural plots gave Nash-Sutcliffe efficiency (NSE) value of 0.84 and Pearson correlation coefficient (r) of 0.96.Furthermore, NSE reaches 0.7 and (r) 0.69 in forest plots, which indicates the good performance of the WEPP model in simulating soil erosion in the two ecosystems (agricultural, forest).

Rainfall changes between 2016 and 2039
The WEPP model gives the expected estimations of both rainfall and storm events on a time scale with the use of CLIGEN V5.30.Depending on the Markov chain, the results of the analysis of expected rainfall changes from 2016 to 2039 demonstrate a linear decline with 7.11 mm for each year and with a total decline of 170 mm for the entire studied period (Figure 1).According to Haleme and Qrah Falah (2015), there was a decline of rainfall rates in Lattakia with 5 mm for each year between 1968 and 2008.Also, temperature decreased by 1.3 between 1992 and 2008, and this could explain the increase in the drought rates in the last decades.

Changes of soil erosion amounts
The agricultural system Agricultural lands form 44% of the total area of Lattakia governorate which illustrates the highly dangerous effects of water erosion on the environmental sources represented by the contaminated water sources in addition to the loss of top soil layer which is rich in nutrients.
The analysis of the estimated erosion by the WEPP between 2016 and 2039 showed a slight increase in the expected soil erosion.The amounts of soil erosion increase alongside with the slope.The highest proportions were recorded in Hamra (35%) and reached 150 t/ha/y in 2027, while a minimum value was found in Sabahea (10%) with 27 t/ha/y in 2022.The slow development of soil, the nature of source rocks and soil depth in the coastal area were taken into consideration.These numbers constitute a threat to the sustainable agriculture in the study area.
According to Kbibo et al. (2014), the critical rate of erosion must not exceed 5 t/ha/y.The highest ratio of erosion was recorded in 2027 within all the study area locations.Going back to the results of the rainfall rate analysis, the model has anticipated that in 2027 there will be 64 rainfall events and 28 storm events and total rainfall amount will reach 1777 mm.However, the WEPP model has expected that in 2018, there will be 66 rainfall events and 23 storm events a total amount of 1291 mm.Covering soil surface, particularly canopy cover, is considered a major factor in protecting soil aggregates against the kinetic energy of rain drops.Based on this, a study has been carried out to study the relationship between rainfall and canopy cover of soil surface (Figures 3 and 4).The model shows that most rainfall periods are uncovered (there is no vegetarian growth).This is due to the nature of the agricultural cycle and specifically for the cultivation of winter wheat where farmers till the soil in this period of the year.Then sowing the seeds to begin the vegetarian growth by the end of the winter and the beginning of the spring when the soil is exposed to most rainfall periods.Pieri et al. (2007) pointed to a rise of soil erosion amounts in wheat fields when they followed the agricultural cycle wheat-corn in the Apennines Mountain Range in Italy.This is due to leaving the soil bare and exposed to the devastating impact of rain drops.

The Forest system
Forest composes about 9% of the total area of Lattakia.However, this proportion has declined due to the continuous burning operations and this in its turn causes high erosion in these areas.The analysis of the estimated erosion predicted by the WEPP for the period 2016-2039 indicates a total decrease in the amount of soil erosion in Fronlk area (25%) -0.2 t/ha/y, Sabahia (10%) -0.1 t/ha/y, Shoh (11%) -0.5 t/ha/y, Hamra (35%) -0.07 t/ha/y, Haraa (13%) -0.02 t/ha/y, and Hsamoo (20%) -0.1 t/ha/y.This is related to the role of forest cover in reducing the direct impact of rain drops.
However, in Hamra (35%), which is characterized by the slope of 35%, it has been noticed that there is no significant decline in the amount of soil erosion.This is due to the major role of slope factor in affecting the erosion.Furthermore, the nature of soils in the study area will affect the soil erosion process, that is, the high organic matter content will reduce the soil erosion like in Haraa -5% while the higher content of silt will contribute negatively to the erosion process like in Shoh -44 (Figure 5).
Canopy cover of soil surface plays a significant role in reducing erosion threat.Based on this, a study has been conducted to examine the relationship between rainfall and canopy cover of the soil surface (Figure 7).The model has shown that the soil was covered in most rainfall periods, and thus a significant reduction of soil erosion amounts happened, and run-off rate decreased due to the effect of canopy cover (Figures 6 and 7).

Conclusion
In this study, changes of soil erosion and rainfall have been simulated in Lattakia in the coastal area of Syria during the period 2016-2039.The results showed a linear decline in the general direction of rain in which it reached 7.11 mm/year.The results have also showed that the agricultural system is more affected by soil erosion than other ecosystem.A decline in the amount of soil erosion was noticed in the forest system.It is expected that 2027 will be one of the most dangerous years for the natural resources where it has been observed that there will be a rise of erosion amounts within the two studied systems.Here, preventing measures should be taken by the Ministry of Agriculture (decision makers) to reduce the impact of water erosion.It is important to take into account an appropriate land use and agricultural cycle, offering residue soil cover or increasing the soil organic matter content.This contributes to reducing the potential effects of rain drops and thus to reducing the erosion.In these areas, where the erosion rate is higher than 8 t/ha/y, it is necessary for agriculture to apply soil conservation procedures to reduce the effects of surface erosion.

Figure 2 .
Figure 2. Predicted soil erosion changes between 2016 and 2039 in the agricultural system.

Figure 3 .
Figure 3. Predicted canopy cover and rainfall (WEPP) between 2016 and 2039 in the agricultural system.

Figure 4 .
Figure 4. Predicted canopy cover and run-off (WEPP) between 2016 and 2039 in the agricultural system.

Figure 5 .
Figure 5. Predicted soil erosion changes between 2016 and 2039 in the forest system.

Figure 6 .
Figure 6.Predicted canopy cover and rainfall (WEPP) between 2016 and 2039 in the forest system.

Figure 7 .
Figure 7. Predicted canopy cover and run-off (WEPP) between 2016 and 2039 in the forest system.

Table 1 .
Soil characteristics of locations in the study area.