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Water erosion assessment and control in Northern Iraq
Soil & Tillage Research 45 Ž1998. 161–173
Water erosion assessment and control in Northern Iraq Mohammad H. Hussein Dept. of CiÕil Eng., College of Eng., P.O. Box 61160, Hoon, Libya Accepted 3 March 1997
Abstract The semiarid region of northern Iraq consists of about 12 millions ha of forest, grazing and farmland areas. Water erosion is a serious problem on forest and grazing lands due mainly to land mismanagement. On cropland, conventional farming practices and intensive cropping increased water erosion on marginal land. To assess the damage caused by water erosion in the region, the Universal Soil Loss Equation was used to predict the potential annual soil loss. Present erosion and erosion forms were determined by site visits, remote sensing and from soil survey reports. These information were then compiled in a soil degradation map for the region. The map consists of nine units. Each map unit characterizes potential and present water erosion, erosion forms and a land-use capability index. Overall, about 23% of total land area has slight water erosion. About 22% of total land area which is mostly mountainous land located to the north and northeast of the region has severe water erosion. The rest of the region has a slight to moderate water erosion. An exception are the areas of Sinjar mountain, Adhaim valley and the Hemrin and Makhool mountains where severe rill and gully erosion were identified; these areas form about 8% of total land area in the region. Further land deterioration in the region is expected unless the proper soil conservation measures are implemented. q 1998 Elsevier Science B.V. Keywords: Universal soil loss equation; Land use capability index; Erosion map
1. Introduction Soil erosion causes a loss in the topsoil and the less fertile subsoil will be exposed. Since the topsoil is more favoured to plant growth, a decline in the soil productivity potential usually results from soil erosion. Sediment and chemicals resulting from the erosion process are ones of the most important pollutants of water resources. This pollution causes a deterioration in water quality with respect to domestic use, irrigation and recreation in addition to the damage to fish habitat. 0167-1987r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 3 3 - 3 6 3 0 Ž 9 7 . 0 0 0 0 7 - X
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Due to agricultural expansion and land mismanagement, soil erosion by water has increased in the semiarid region of northern Iraq during the past four decades. However, official and public awareness of the problem remained low. Some areas in the region have become so badly eroded that it will be difficult to restore them to their original land use. The purpose of this study was to use the universal soil loss equation ŽUSLE. ŽWischmeier and Smith, 1978. and other suitable techniques like remote sensing to assess soil loss and erosion pattern associated with water erosion in the region. Through various maps and tables, the seriousness of the erosion problem and methods for erosion control are presented to help in decision making and planning concerning soil conservation and land management in the region.
2. Background The region of northern Iraq has a semiarid Mediterranean type climate. Rainfall season in the region normally extends from October to May. The region is often divided into high, medium and low rainfall zones according to the mean seasonal rainfall. The high rainfall zone receives more than 600 mm of mean seasonal rainfall. The medium and the low rainfall zones receive respectively between 400 and 600 mm, and below 400 mm of mean seasonal rainfall. High mountain peaks located to the north and northeast of the region are usually covered with snow during much of the rainfall season. Differences in mean temperature between winter and summer months are usually above 208C. During the winter months, minimum daily temperature occasionally drops below the freezing point. During the summer months, maximum daily temperature is usually above 358C and may reach 458C or more in the southern part of the region. Total annual Pan A evaporation in the region exceeds 2000 mm. Total monthly evaporation during the winter months of December, January and February is usually below 50 mm which is lower than the normal monthly rainfall depth during these months. However, total monthly evaporation during the summer months of June, July and August may reach 450 mm or more especially in the western and southern parts of the region. With respect to land use, the forests are located almost exclusively within the high rainfall zone and they cover a total area of more than 1.5 millions ha of mostly degraded oak forests. Topography is mostly hilly to steep. The grazing land extends through all rainfall zones with a total area of more than 8 millions ha. All types of topography can be found within this land. Overgrazing is common, resulting in a degraded rangeland. Dryland farming is practised within all rainfall zones with a total area of more than 2 millions ha, from which 60% is located within the low rainfall zone. Topography is level to rolling. Field size varies considerably and ranges from few hectares to several hundreds hectares. Wheat, barley and legumes are the major crops grown. Conventional tillage of mouldboard ploughing andror discing is normally practised. In the low rainfall zone, Aridisols are dominant. In the medium and high rainfall zones, Vertisols, Mollisols and Aridisols are dominant on gently sloping land normally
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used for dryland farming. On more sloping land, Entisols are dominant. Soil profile depth is normally below 1 m for Aridisols and Entisols and more than 1.5 m for Vertisols and Mollisols ŽAl-Tae et al., 1969.. 3. Study methods USLE was the tool used to assess potential water erosion in the region. The simplicity of the equation and the availability of data to apply the equation on a regional scale were the primary reasons for this choice. USLE is: A s RKLSCP Ž 1. where A s average annual soil loss Žt hay1 ., R s rainfall-runoff erosivity factor ŽMJ hay1 mm hy1 ., K s soil erodibility factor Žth MJy1 mmy1 ., L s slope length factor, S s slope steepness factor, C s cover-management factor and P s supporting practice factor. 3.1. EÕaluation of the USLE factors in the region 3.1.1. The R-factor Due to the lack of extensive soil loss records and recording raingauge network in the region, a modified Fournier index was used to have an estimate for the R-factor in Iraq ŽHussein, 1986.. In this case average annual EI 30 Žrainfall energy = max. 30 min. intensity. expressed in MJ hay1 mm hy1 is calculated from: 1.93
n
EI 30 s 0.3
ž
Ý pirP is1
/
Ž 2.
where pi s mean monthly rainfall Žmm., P s mean annual rainfall Žmm. and n s number of rainy months. Monthly distribution of annual erosivity is given by: n
Ž PE. i s pi2r Ý pi2 = 100
Ž 3.
is1
where ŽPE. i s contribution of the month i to annual erosivity Ž%.. In a recent study ŽHussein, 1996., it was shown that Eq. Ž2. satisfactorily approximates average annual EI 30 in the low rainfall zone of northern Iraq. The rainfall erosivity map for northern Iraq based on Eq. Ž2. is given in Fig. 1. 3.1.2. The K-factor Available measurements from natural runoff plots in the region indicate that the soil erodibility nomograph ŽWischmeier et al., 1971. overpredicts K-value in the region by nearly one order of magnitude ŽHussein, 1997.. Samples were collected from more than fifty sites, distributed throughout the region to estimate K-value by using the nomograph ŽHussein et al., unpublished data.. These nomograph values were then divided by 10 to obtain approximate values for the K-factor. Obtained values for the K-factor in the region were mapped in Fig. 2. 3.1.3. The LS-factor Since soil erosion by rainfall and runoff in northern Iraq is active, natural land slopes are largely determined by water induced erosion and deposition. In this case, slope
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Fig. 1. The rainfall erosivity factor in northern Iraq.
length normally decreases as slope steepness increases. This is because runoff forces increase with slope steepness with the channel flow becoming dominant after shorter overland flow paths.
Fig. 2. The soil erodibility factor in northern Iraq.
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Table 1 Slope classes and their topographic factor a Symbol
Class
Range of slope Ž%.
Charact. slope length Value Žm. Factor Ž L.
Charact. slope steepness Charact. Value Ž%. Factor Ž S . topog. factor Ž LS .
A AB B C CD D
Nearly level Level Rolling to rolling Hilly Hilly to steep Steep
0–5 0–10 5–10 10–30 )10 ) 30
223 61 50 22 8 6
2.5 5 7.5 20 35 60
2 1.67 1.5 1 0.75 0.67
0.22 0.46 0.77 3.60 5.84 11.8
0.44 0.77 1.16 3.60 4.4 7.8
a
Ls Ž l r22.1. m , l s length of slope Žm. and msslope length exponent, ms 0.3 for class A, 0.5 for classes AB, B, and C and 0.3 for classes CD and D. Ss65.41 sin2u q4.56 sin u q0.065 for classes A, AB, B and C; Ss Žtan u r0.09.1.3 for classes CD and D ŽWischmeier and Smith, 1978; McCool et al., 1982.. Since the highest slope steepness in McCool et al. Ž1982. data was 60%, tan u was assumed 0.6 for class D and 0.35 for class CD.
Soil erosion increases sharply with slope steepness until about 208 Ž36%.. At steeper slopes and until about 408 Ž84%., the curve flattens off to rise gently ŽHorton, 1945.. Data used to develop the slope effect chart in the USLE were from slopes between 3–25% for natural rainfall studies and down to about 0.5% for simulated rainfall studies. For the length of slope Ži.e., distance of overland flow., the range was from 9 to 90 m for natural rainfall studies and from 11 to 200 m for simulated rainfall studies. Different relationships may be used for conditions outside the above mentioned ranges. Table 1 shows slope classes suggested for northern Iraq. The classes were mapped in Fig. 3. Application of the USLE in northern Iraq indicates that the slope length factor Ž L. ranges from about 0.67 for steep land ŽClass D. to about 2 for nearly level land
Fig. 3. Dominant slope classes in northern Iraq.
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Fig. 4. Land use in northern Iraq.
ŽClass A.. Since soil erosion on natural land slopes increases with slope steepness, both S and LS factors will increase when the slope steepness increases. For slope classes AB, B, and C, the characteristic slope length factors were selected by knowing that the LS factor increases with slope steepness and the rate of increase is also increasing with slope steepness; this rate of increase is reduced for slope class CD ŽTable 1.. 3.1.4. The C-factor Fig. 4 summarizes the present land use in northern Iraq ŽHussein and Kariem, 1988.. Typical C-factor values in the region are given in Table 2. These C-factor values were estimated from soil loss ratios derived for major land uses in the region. The ratios were derived by dividing the cover-management effect into subfactors ŽWischmeier, 1975; Laflen et al., 1985. and evaluating these subfactors for the conditions of northern Iraq.
Table 2 The cover-management factor for major land uses in northern Iraq Žafter Hussein and Kariem, 1988. Land use
Major crops or forest trees
Condition or rotation
C-factor
Forests Grazing land
Oak trees –
Cropland
Wheat and barley
Degraded Poor Fair Good Continuous 2 yrs Crops-fallow rotation
0.01a 0.30 0.16 0.07 0.50 0.40
a
For severely degraded oak forests, poor grazing land condition was assumed.
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Table 3 Water erosion classification used in northern Iraq Symbol
Description
Typical annual soil loss Žt hay1 .
1 2 3 4 5
None to slight Slight to moderate Moderate to severe Severe Very severe
0–5 5–10 10–50 50–100 )100
3.1.5. The P-factor Mechanical erosion control practices are nearly absent in the region. If present erosion rate has to be estimated at a site, this factor is assumed unity. Contour cultivation in the region is not practised in a way that a proper P-factor can be assigned. 3.2. The water erosion map The maps shown in Figs. 1–4 were overlaid to obtain an estimate for the potential annual soil loss at locations distributed throughout the region. This potential annual soil loss ŽAp. is given by: Ap s RKLS
Ž 4.
Lines of equal potential annual soil loss were drawn and zoning was done according to the guidelines given in Table 3. These zones were then divided into subzones according to present erosion, erosion forms and the land use capability index ŽTables 4 and 5.. Site visits and remote sensing were the main criteria used to determine present erosion and erosion forms at a location. I travelled through most accessible roads in the region to record erosion severity and forms. Landsat MSS images of scales between 1r10 5 –1r10 6 were also used to record erosion severity and forms especially at locations not covered by the field trips. If Landsat images were not available for such locations, the value RKLSC was considered as the present erosion rate. In this case, the C-factor was taken from Table 2 according to the present land use ŽFig. 4.. Erosion
Table 4 Maximum permissible soil loss a Soil rooting depth Žm.
T b Žt hay1 yry1 .
)1 0.5–1 0.25–0.5 - 0.25
10 8 4 2
a
Modified from the SCS guidelines ŽLogan, 1977.. For underlying materials other than bedrock Že.g., sand, gravel, fragipan. 2 t hay1 yry1 is added to the values in column 2.
b
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Table 5 The land use capability index ŽLUCI. Class LUCI ŽT r RKLS . Proposed land use I II III
)1r2 1r15<1r2 -1r15
Cropland—No water erosion hazard when cultivated. Cropland or grazing land—Soil conservation measures are needed when cultivated. Restricted—Pasture, forest or basin protection.
forms at such locations were guessed using mainly the available soil survey reports. The final water erosion map is shown in Fig. 5.
4. Description of erosion map units Table 6 lists water erosion map units and total area occupied by each unit. A brief description of units is presented below: 4.1. The unit (4 4 SRG r III) This unit comprises about 21.5% of total land area in northern Iraq, occupying mainly the northern and the northeastern parts of the region. Potential water erosion is severe due to the high annual erosivity and the rugged terrain. Present water erosion rate is also considered severe due to land mismanagement especially during the past four decades. Sheet, rill and gully erosion are all active in the region. On the high mountain peaks, snow cover during most of the rainfall season protects the soil from excessive erosion. Severe water erosion, shallow soils, and rugged terrain limit land use within this unit to forest and grazing land. In areas where bedrock is nearly exposed, afforestation is not possible. These areas are considered a wasteland Ži.e., rocks and barren land. which may be used for basin protection. Hilly areas not severely affected by erosion can be used for controlled grazing. With, the proper mechanical protection Že.g., terracing., these hilly areas can be used to grow various fruit trees in the high rainfall zone. An alternative is the afforestation of these slopes with well adapted tree species. Gully erosion within this unit requires special attention. High runoff forces on the steep slope cause continuous enlargement and widening of gullies. For this reason, gully control and stabilization should receive a high priority in any erosion control program designed for this area of northern Iraq. 4.2. The unit (2 3 SRG r II) The land occupied by this unit covers about 21% of total land area in northern Iraq. It is located in the southwestern corner of the region to the west of the Tigris river. Wadis and depressions like Tharthar, Snaisla and Ashkar are all located within this unit. Runoff water accumulates in these depressions during the rainfall season which usually extends from October to May. During the summer, most of this accumulated water will evaporate back into the atmosphere. This accumulated runoff causes serious rill and
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Fig. 5. The water erosion map for northern Iraq.
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Table 6 The water erosion map units for northern Iraq Map unit
Total area Žmillions of ha.
% of total land area
4 4 SRGrIII 2 3 SRGrII 2 2 SRrII 1 1 SrI 3 3 SRrII 2 1 SrI 2 4 RGrIII 3 4 RGrIII 3 5 RGrIII
2.6 2.5 1.6 1.6 1.5 1.1 0.6 0.26 0.14
21.5 21 13.5 13.5 12.6 9.5 5 2.2 1.2
gully erosion especially within the basins of these depressions. Due to the low seasonal rainfall, the land should be used mainly as grazing land, but overgrazing should be avoided. However, crop production and afforestation are possible when irrigation water can be secured. Wind erosion is active especially during spring months. Active sand dunes are located in the south and southwestern parts of this unit. 4.3. The unit (2 2 SR r II) This unit occupies about 13.5% of total land area in northern Iraq. The Mosul– Arbil–Karkuk plain is the name that this unit is usually called by. In this unit, the sloping topography increased present erosion to level 2 and reduced the land suitability for crop production due to the active sheet and rill erosion. Conservation practices like contouring and conservation tillage are needed when the land is used for crop production. The lack of such practices in the past caused an increase in water erosion. Grazing should be under control when this area is used as a grazing land. 4.4. The unit (1 1 S r I) This unit is located mainly in the southern part of the region; it occupies about 13.5% of the total land area in northern Iraq. Due to the low annual erosivity and gentle topography ŽFigs. 1 and 3., water erosion is not a problem within this unit. Wind erosion is rather active due to the low seasonal rainfall. This low seasonal rainfall limits agricultural expansion unless some type of irrigation is used. 4.5. The unit (3 3 SR r II) This unit occupies about 12.6% of total land area in northern Iraq. It forms a transition zone between cropland and forestland in the region. Potential erosion is moderate to severe due mainly to the moderately high annual erosivity and the hilly terrain ŽFigs. 1 and 3.. Present erosion is moderate to severe consisting mainly of sheet and rill erosion. At the present time, the land is mainly used as a grazing land with overgrazing and mismanagement common. The area is not suitable as a cropland unless some types of soil conservation measures are adopted. Among these measures are contour cultivation and conservation tillage. Terraces may also be used on more sloping
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land. However, the later option is capital intensive and unless irrigation water can be secured, this option might not be economical. 4.6. The unit (2 1 S r I) This unit occupies about 9.5% of total land area in northern Iraq. The mountain plains and the nearly level cropland south and east of the Sinjar mountain are classified within this unit. It is similar to the Ž1 1 SrI. unit except higher annual erosivity increased the potential erosion level to 2. This unit forms an excellent agricultural land. However, due to the moderate to high annual erosivity ŽFig. 1. cultivation should be always on the contour. Use of conservation tillage to check soil erosion and preserve soil productivity provides an additional land protection. 4.7. The unit (2 4 RG r III) This unit occupies about 5% of total land area in northern Iraq. The gullied land in the Adhaim river valley and the eroded slopes of the Hemrin and Makhool mountains are the major parts of this unit. The exact cause of gully formation in the Adhaim valley is unknown; but a probable cause seems the soil high susceptibility to crusting which speeds up runoff generation during rainfall events. These gullies are a major source of sediment to the Tigris river in which the Adhaim river channels it’s water. After heavy showers, especially during the fall and winter months where the grass cover is a minimum, the sediment-laden Adhaim river flow raises the turbidity in the Tigris river to a level that occasionally threatens the municipal water facilities in Baghdad and the surrounding areas. At the present time, the government is building a dam to check sediment flow from the Adhaim river. The Hemrin and Makhool slopes are severely eroded due to overgrazing and mismanagement. The entire unit should be kept as a basin land which requires special protection to prevent further deterioration. Recreation may be allowed but grazing should be kept to a minimum or prevented. 4.8. The unit (3 4 RG r III) This unit occupies about 2.6% of total land area in northern Iraq. It is similar to the previous unit except that the higher annual erosivity and the hilly slopes raised the potential erosion to level 3 Žmoderate to severe.. However, present erosion is severe consisting mainly of rill and gully erosion. The Sinjar mountain in the northwest has been covered with oak forests until the beginning of this century where overcutting and mismanagement left the mountain slopes virtually without forest cover. As a consequence, severe rill and gully erosion occurred on these slopes. Except for limited afforestation and terraced orchards, the mountain slopes remained without erosion control measures. Rapid afforestation and grazing control are two important steps to avoid irreversible soil removal and bedrock exposure on the mountain slopes. The rest of this unit consists of hilly eroded spots which extends from Kanaqin in the south east to Karkuk. These areas should be treated as a basin land and grazing should be prevented.
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4.9. The unit (3 5 RG r III) This unit occupies about 1.4% of total land area in northern Iraq. Present erosion is very severe on these hilly eroded slopes. These areas should be treated as a basin land and grazing should be prevented.
5. Discussion Gibbs Ž1954. made a soil erosion survey in Iraq. He estimated that 12% of total land area in Iraq Žabout 5 millions ha. had a serious water erosion problem while 10% Žabout 4.4 millions ha. had a moderate water erosion problem. In Table 6, it is shown that the area of moderate water erosion has increased over the figure given by Gibbs. The main reason is related to the mismanagement of cropland and rangeland in northern Iraq during the past four decades. Livestock number has increased far beyond the capacity of rangeland in the region resulting in overgrazing on most of these lands. On cropland, which has been expanded, crop residues are normally removed after harvest leaving the soil without protective cover. Conventional tillage practices normally used to prepare the seed bed cause a substantial water erosion on more sloping cropland. For the 12 millions ha of land in northern Iraq, potential soil loss will be in the hundreds of millions of tons annually. Records of suspended sediment flow in the Tigris river before the major dams were built were in the millions of tons annually ŽAl-Ansari and Ali, 1986.. The majority of this sediment flow occurs during the period of peak floods. As snow melts, runoff carries the previously detached sediment in the Tigris basin. At the present time, dam siltation in the region is high due to the relatively high rate of water erosion in the Tigris basin. In addition, water erosion causes a substantial economic loss due to the loss in cropland productivity and the damage to forest and range lands in the region. However, any effort to reduce sediment flow in the Tigris and it’s tributaries in the region should include that part of their basins which is located outside the national boundaries. Methods to combat water erosion suggested for each map unit are in general sense. Research on soil erosion and improved methods for its control that incorporate technical, economic or socioeconomic and environmental factors in the region should receive a high priority. Developed erosion control methods should utilize recent thoughts in soil conservation which give greater emphasis to biological control and control through improved farming practices ŽHudson, 1992.. The fact that the area is mostly semiarid,with moisture deficiency not uncommon during the growing season, plays a role in selecting the proper method for water erosion control ŽFAO, 1987..
6. Conclusions Ž1. Of the 12 millions ha which is the total land area in northern Iraq, one third has a serious water erosion problem. This area is mainly located within the mountain region.
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Ž2. The rangeland in northern Iraq is usually overgrazed and water erosion is moderate to severe depending mainly on topography. Grazing control should be the major objective on this land. Ž3. Water erosion on cropland in the region is slight to moderate depending on topography. Proper soil conservation measures are needed on more sloping cropland. Ž4. Severely gullied land and land where bedrock is exposed should be kept for basin protection.
Acknowledgements Thanks go to Tariq H. Kariem, Ibrahim M.A. Ahmedi, Sattar J. Yassin and Kasim M. Al-Saadi for their help in carrying out this project.
References Al-Ansari, N.A., Ali, J.L., 1986. Suspended load and solute discharge in river Tigris within Baghdad. J. Wat. Resources 5, 51–65. Al-Tae, F.H., Sys, C., Stoops, G., 1969. Soil groups of Iraq: Their classification and characterization. Pedologie XIX, 65–148. FAO, 1987. Soil And Water Conservation in Semiarid Areas. Soils Bulletin 57, FAO, Rome. Gibbs, G.K., 1954. Report to the Government of Iraq on Soil Conservation. Report No. 242, FAO, Rome. Horton, R.E., 1945. Erosional development of streams and their drainage basins: hydrological approach to qualitative morphology. Geo. Soc. Am. Bull. 56, 275–370. Hudson, N., 1992. Land Husbandry. BT Batsford, London, 192 pp. Hussein, M.H., 1986. Rainfall erosivity in Iraq. J. Soil Wat. Cons. 41, 336–337. Hussein, M.H., 1996. An analysis of rainfall, runoff and erosion in the low rainfall zone of northern Iraq. J. Hydrol. 181, 105–126. Hussein, M.H., 1997. Soil erodibility in regions of low intensity rain: A new approach, in preparation. Hussein, M.H., Kariem, T.H., 1988. Deriving the cover-management factor for major land uses in northern Iraq. Iraqi J. Agric. Sci. ŽZANCO. 6, 73–86. Laflen, J.M., Foster, G.R., Onstad, C.A., 1985. Simulation of individual storm soil loss for modeling the impact of soil erosion on crop productivity. In: El-Swaify, S.A., Moldenhauer, W.C., Lo, A. ŽEds.., Soil Erosion And Conservation. Soil Cons. Soc. Am., Ankeny, Iowa. Logan, T.J., 1977. Establishing soil loss and sediment yield limits for agricultural land. In: Soil Erosion and Sedimentation, Proc. of the Nat. Symp. on Soil Erosion and Sedimentation by Water. Am. Soc. Agric. Eng., Chicago, ILL., 12–13 Dec., 1977. McCool, D.K., Wischmeier, W.H., Johnson, L.C., 1982. Adapting the universal soil loss equation to the Pacific Northwest. Trans. Am. Soc. Agric. Eng. 25, 928–934. Wischmeier, W.H., 1975. Estimating the soil loss equation cover and management factor for undisturbed areas. In: Present And Prospective Technology For Predicting Sediment Yields And Sources. USDA-ARS-S-40. Wischmeier, W.H., Johnson, C.B., Cross, B.V., 1971. A soil erodibility nomograph for farmland and construction sites. J. Soil Wat. Cons. 26, 189–193. Wischmeier, W.H. and Smith, D.D., 1978. Predicting rainfall erosion losses. Agric. HB No. 537, USDA, Washington, D.C.
Water erosion assessment and control in Northern Iraq Mohammad H. Hussein Dept. of CiÕil Eng., College of Eng., P.O. Box 61160, Hoon, Libya Accepted 3 March 1997
Abstract The semiarid region of northern Iraq consists of about 12 millions ha of forest, grazing and farmland areas. Water erosion is a serious problem on forest and grazing lands due mainly to land mismanagement. On cropland, conventional farming practices and intensive cropping increased water erosion on marginal land. To assess the damage caused by water erosion in the region, the Universal Soil Loss Equation was used to predict the potential annual soil loss. Present erosion and erosion forms were determined by site visits, remote sensing and from soil survey reports. These information were then compiled in a soil degradation map for the region. The map consists of nine units. Each map unit characterizes potential and present water erosion, erosion forms and a land-use capability index. Overall, about 23% of total land area has slight water erosion. About 22% of total land area which is mostly mountainous land located to the north and northeast of the region has severe water erosion. The rest of the region has a slight to moderate water erosion. An exception are the areas of Sinjar mountain, Adhaim valley and the Hemrin and Makhool mountains where severe rill and gully erosion were identified; these areas form about 8% of total land area in the region. Further land deterioration in the region is expected unless the proper soil conservation measures are implemented. q 1998 Elsevier Science B.V. Keywords: Universal soil loss equation; Land use capability index; Erosion map
1. Introduction Soil erosion causes a loss in the topsoil and the less fertile subsoil will be exposed. Since the topsoil is more favoured to plant growth, a decline in the soil productivity potential usually results from soil erosion. Sediment and chemicals resulting from the erosion process are ones of the most important pollutants of water resources. This pollution causes a deterioration in water quality with respect to domestic use, irrigation and recreation in addition to the damage to fish habitat. 0167-1987r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 3 3 - 3 6 3 0 Ž 9 7 . 0 0 0 0 7 - X
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Due to agricultural expansion and land mismanagement, soil erosion by water has increased in the semiarid region of northern Iraq during the past four decades. However, official and public awareness of the problem remained low. Some areas in the region have become so badly eroded that it will be difficult to restore them to their original land use. The purpose of this study was to use the universal soil loss equation ŽUSLE. ŽWischmeier and Smith, 1978. and other suitable techniques like remote sensing to assess soil loss and erosion pattern associated with water erosion in the region. Through various maps and tables, the seriousness of the erosion problem and methods for erosion control are presented to help in decision making and planning concerning soil conservation and land management in the region.
2. Background The region of northern Iraq has a semiarid Mediterranean type climate. Rainfall season in the region normally extends from October to May. The region is often divided into high, medium and low rainfall zones according to the mean seasonal rainfall. The high rainfall zone receives more than 600 mm of mean seasonal rainfall. The medium and the low rainfall zones receive respectively between 400 and 600 mm, and below 400 mm of mean seasonal rainfall. High mountain peaks located to the north and northeast of the region are usually covered with snow during much of the rainfall season. Differences in mean temperature between winter and summer months are usually above 208C. During the winter months, minimum daily temperature occasionally drops below the freezing point. During the summer months, maximum daily temperature is usually above 358C and may reach 458C or more in the southern part of the region. Total annual Pan A evaporation in the region exceeds 2000 mm. Total monthly evaporation during the winter months of December, January and February is usually below 50 mm which is lower than the normal monthly rainfall depth during these months. However, total monthly evaporation during the summer months of June, July and August may reach 450 mm or more especially in the western and southern parts of the region. With respect to land use, the forests are located almost exclusively within the high rainfall zone and they cover a total area of more than 1.5 millions ha of mostly degraded oak forests. Topography is mostly hilly to steep. The grazing land extends through all rainfall zones with a total area of more than 8 millions ha. All types of topography can be found within this land. Overgrazing is common, resulting in a degraded rangeland. Dryland farming is practised within all rainfall zones with a total area of more than 2 millions ha, from which 60% is located within the low rainfall zone. Topography is level to rolling. Field size varies considerably and ranges from few hectares to several hundreds hectares. Wheat, barley and legumes are the major crops grown. Conventional tillage of mouldboard ploughing andror discing is normally practised. In the low rainfall zone, Aridisols are dominant. In the medium and high rainfall zones, Vertisols, Mollisols and Aridisols are dominant on gently sloping land normally
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used for dryland farming. On more sloping land, Entisols are dominant. Soil profile depth is normally below 1 m for Aridisols and Entisols and more than 1.5 m for Vertisols and Mollisols ŽAl-Tae et al., 1969.. 3. Study methods USLE was the tool used to assess potential water erosion in the region. The simplicity of the equation and the availability of data to apply the equation on a regional scale were the primary reasons for this choice. USLE is: A s RKLSCP Ž 1. where A s average annual soil loss Žt hay1 ., R s rainfall-runoff erosivity factor ŽMJ hay1 mm hy1 ., K s soil erodibility factor Žth MJy1 mmy1 ., L s slope length factor, S s slope steepness factor, C s cover-management factor and P s supporting practice factor. 3.1. EÕaluation of the USLE factors in the region 3.1.1. The R-factor Due to the lack of extensive soil loss records and recording raingauge network in the region, a modified Fournier index was used to have an estimate for the R-factor in Iraq ŽHussein, 1986.. In this case average annual EI 30 Žrainfall energy = max. 30 min. intensity. expressed in MJ hay1 mm hy1 is calculated from: 1.93
n
EI 30 s 0.3
ž
Ý pirP is1
/
Ž 2.
where pi s mean monthly rainfall Žmm., P s mean annual rainfall Žmm. and n s number of rainy months. Monthly distribution of annual erosivity is given by: n
Ž PE. i s pi2r Ý pi2 = 100
Ž 3.
is1
where ŽPE. i s contribution of the month i to annual erosivity Ž%.. In a recent study ŽHussein, 1996., it was shown that Eq. Ž2. satisfactorily approximates average annual EI 30 in the low rainfall zone of northern Iraq. The rainfall erosivity map for northern Iraq based on Eq. Ž2. is given in Fig. 1. 3.1.2. The K-factor Available measurements from natural runoff plots in the region indicate that the soil erodibility nomograph ŽWischmeier et al., 1971. overpredicts K-value in the region by nearly one order of magnitude ŽHussein, 1997.. Samples were collected from more than fifty sites, distributed throughout the region to estimate K-value by using the nomograph ŽHussein et al., unpublished data.. These nomograph values were then divided by 10 to obtain approximate values for the K-factor. Obtained values for the K-factor in the region were mapped in Fig. 2. 3.1.3. The LS-factor Since soil erosion by rainfall and runoff in northern Iraq is active, natural land slopes are largely determined by water induced erosion and deposition. In this case, slope
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Fig. 1. The rainfall erosivity factor in northern Iraq.
length normally decreases as slope steepness increases. This is because runoff forces increase with slope steepness with the channel flow becoming dominant after shorter overland flow paths.
Fig. 2. The soil erodibility factor in northern Iraq.
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Table 1 Slope classes and their topographic factor a Symbol
Class
Range of slope Ž%.
Charact. slope length Value Žm. Factor Ž L.
Charact. slope steepness Charact. Value Ž%. Factor Ž S . topog. factor Ž LS .
A AB B C CD D
Nearly level Level Rolling to rolling Hilly Hilly to steep Steep
0–5 0–10 5–10 10–30 )10 ) 30
223 61 50 22 8 6
2.5 5 7.5 20 35 60
2 1.67 1.5 1 0.75 0.67
0.22 0.46 0.77 3.60 5.84 11.8
0.44 0.77 1.16 3.60 4.4 7.8
a
Ls Ž l r22.1. m , l s length of slope Žm. and msslope length exponent, ms 0.3 for class A, 0.5 for classes AB, B, and C and 0.3 for classes CD and D. Ss65.41 sin2u q4.56 sin u q0.065 for classes A, AB, B and C; Ss Žtan u r0.09.1.3 for classes CD and D ŽWischmeier and Smith, 1978; McCool et al., 1982.. Since the highest slope steepness in McCool et al. Ž1982. data was 60%, tan u was assumed 0.6 for class D and 0.35 for class CD.
Soil erosion increases sharply with slope steepness until about 208 Ž36%.. At steeper slopes and until about 408 Ž84%., the curve flattens off to rise gently ŽHorton, 1945.. Data used to develop the slope effect chart in the USLE were from slopes between 3–25% for natural rainfall studies and down to about 0.5% for simulated rainfall studies. For the length of slope Ži.e., distance of overland flow., the range was from 9 to 90 m for natural rainfall studies and from 11 to 200 m for simulated rainfall studies. Different relationships may be used for conditions outside the above mentioned ranges. Table 1 shows slope classes suggested for northern Iraq. The classes were mapped in Fig. 3. Application of the USLE in northern Iraq indicates that the slope length factor Ž L. ranges from about 0.67 for steep land ŽClass D. to about 2 for nearly level land
Fig. 3. Dominant slope classes in northern Iraq.
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Fig. 4. Land use in northern Iraq.
ŽClass A.. Since soil erosion on natural land slopes increases with slope steepness, both S and LS factors will increase when the slope steepness increases. For slope classes AB, B, and C, the characteristic slope length factors were selected by knowing that the LS factor increases with slope steepness and the rate of increase is also increasing with slope steepness; this rate of increase is reduced for slope class CD ŽTable 1.. 3.1.4. The C-factor Fig. 4 summarizes the present land use in northern Iraq ŽHussein and Kariem, 1988.. Typical C-factor values in the region are given in Table 2. These C-factor values were estimated from soil loss ratios derived for major land uses in the region. The ratios were derived by dividing the cover-management effect into subfactors ŽWischmeier, 1975; Laflen et al., 1985. and evaluating these subfactors for the conditions of northern Iraq.
Table 2 The cover-management factor for major land uses in northern Iraq Žafter Hussein and Kariem, 1988. Land use
Major crops or forest trees
Condition or rotation
C-factor
Forests Grazing land
Oak trees –
Cropland
Wheat and barley
Degraded Poor Fair Good Continuous 2 yrs Crops-fallow rotation
0.01a 0.30 0.16 0.07 0.50 0.40
a
For severely degraded oak forests, poor grazing land condition was assumed.
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Table 3 Water erosion classification used in northern Iraq Symbol
Description
Typical annual soil loss Žt hay1 .
1 2 3 4 5
None to slight Slight to moderate Moderate to severe Severe Very severe
0–5 5–10 10–50 50–100 )100
3.1.5. The P-factor Mechanical erosion control practices are nearly absent in the region. If present erosion rate has to be estimated at a site, this factor is assumed unity. Contour cultivation in the region is not practised in a way that a proper P-factor can be assigned. 3.2. The water erosion map The maps shown in Figs. 1–4 were overlaid to obtain an estimate for the potential annual soil loss at locations distributed throughout the region. This potential annual soil loss ŽAp. is given by: Ap s RKLS
Ž 4.
Lines of equal potential annual soil loss were drawn and zoning was done according to the guidelines given in Table 3. These zones were then divided into subzones according to present erosion, erosion forms and the land use capability index ŽTables 4 and 5.. Site visits and remote sensing were the main criteria used to determine present erosion and erosion forms at a location. I travelled through most accessible roads in the region to record erosion severity and forms. Landsat MSS images of scales between 1r10 5 –1r10 6 were also used to record erosion severity and forms especially at locations not covered by the field trips. If Landsat images were not available for such locations, the value RKLSC was considered as the present erosion rate. In this case, the C-factor was taken from Table 2 according to the present land use ŽFig. 4.. Erosion
Table 4 Maximum permissible soil loss a Soil rooting depth Žm.
T b Žt hay1 yry1 .
)1 0.5–1 0.25–0.5 - 0.25
10 8 4 2
a
Modified from the SCS guidelines ŽLogan, 1977.. For underlying materials other than bedrock Že.g., sand, gravel, fragipan. 2 t hay1 yry1 is added to the values in column 2.
b
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Table 5 The land use capability index ŽLUCI. Class LUCI ŽT r RKLS . Proposed land use I II III
)1r2 1r15<1r2 -1r15
Cropland—No water erosion hazard when cultivated. Cropland or grazing land—Soil conservation measures are needed when cultivated. Restricted—Pasture, forest or basin protection.
forms at such locations were guessed using mainly the available soil survey reports. The final water erosion map is shown in Fig. 5.
4. Description of erosion map units Table 6 lists water erosion map units and total area occupied by each unit. A brief description of units is presented below: 4.1. The unit (4 4 SRG r III) This unit comprises about 21.5% of total land area in northern Iraq, occupying mainly the northern and the northeastern parts of the region. Potential water erosion is severe due to the high annual erosivity and the rugged terrain. Present water erosion rate is also considered severe due to land mismanagement especially during the past four decades. Sheet, rill and gully erosion are all active in the region. On the high mountain peaks, snow cover during most of the rainfall season protects the soil from excessive erosion. Severe water erosion, shallow soils, and rugged terrain limit land use within this unit to forest and grazing land. In areas where bedrock is nearly exposed, afforestation is not possible. These areas are considered a wasteland Ži.e., rocks and barren land. which may be used for basin protection. Hilly areas not severely affected by erosion can be used for controlled grazing. With, the proper mechanical protection Že.g., terracing., these hilly areas can be used to grow various fruit trees in the high rainfall zone. An alternative is the afforestation of these slopes with well adapted tree species. Gully erosion within this unit requires special attention. High runoff forces on the steep slope cause continuous enlargement and widening of gullies. For this reason, gully control and stabilization should receive a high priority in any erosion control program designed for this area of northern Iraq. 4.2. The unit (2 3 SRG r II) The land occupied by this unit covers about 21% of total land area in northern Iraq. It is located in the southwestern corner of the region to the west of the Tigris river. Wadis and depressions like Tharthar, Snaisla and Ashkar are all located within this unit. Runoff water accumulates in these depressions during the rainfall season which usually extends from October to May. During the summer, most of this accumulated water will evaporate back into the atmosphere. This accumulated runoff causes serious rill and
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Fig. 5. The water erosion map for northern Iraq.
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Table 6 The water erosion map units for northern Iraq Map unit
Total area Žmillions of ha.
% of total land area
4 4 SRGrIII 2 3 SRGrII 2 2 SRrII 1 1 SrI 3 3 SRrII 2 1 SrI 2 4 RGrIII 3 4 RGrIII 3 5 RGrIII
2.6 2.5 1.6 1.6 1.5 1.1 0.6 0.26 0.14
21.5 21 13.5 13.5 12.6 9.5 5 2.2 1.2
gully erosion especially within the basins of these depressions. Due to the low seasonal rainfall, the land should be used mainly as grazing land, but overgrazing should be avoided. However, crop production and afforestation are possible when irrigation water can be secured. Wind erosion is active especially during spring months. Active sand dunes are located in the south and southwestern parts of this unit. 4.3. The unit (2 2 SR r II) This unit occupies about 13.5% of total land area in northern Iraq. The Mosul– Arbil–Karkuk plain is the name that this unit is usually called by. In this unit, the sloping topography increased present erosion to level 2 and reduced the land suitability for crop production due to the active sheet and rill erosion. Conservation practices like contouring and conservation tillage are needed when the land is used for crop production. The lack of such practices in the past caused an increase in water erosion. Grazing should be under control when this area is used as a grazing land. 4.4. The unit (1 1 S r I) This unit is located mainly in the southern part of the region; it occupies about 13.5% of the total land area in northern Iraq. Due to the low annual erosivity and gentle topography ŽFigs. 1 and 3., water erosion is not a problem within this unit. Wind erosion is rather active due to the low seasonal rainfall. This low seasonal rainfall limits agricultural expansion unless some type of irrigation is used. 4.5. The unit (3 3 SR r II) This unit occupies about 12.6% of total land area in northern Iraq. It forms a transition zone between cropland and forestland in the region. Potential erosion is moderate to severe due mainly to the moderately high annual erosivity and the hilly terrain ŽFigs. 1 and 3.. Present erosion is moderate to severe consisting mainly of sheet and rill erosion. At the present time, the land is mainly used as a grazing land with overgrazing and mismanagement common. The area is not suitable as a cropland unless some types of soil conservation measures are adopted. Among these measures are contour cultivation and conservation tillage. Terraces may also be used on more sloping
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land. However, the later option is capital intensive and unless irrigation water can be secured, this option might not be economical. 4.6. The unit (2 1 S r I) This unit occupies about 9.5% of total land area in northern Iraq. The mountain plains and the nearly level cropland south and east of the Sinjar mountain are classified within this unit. It is similar to the Ž1 1 SrI. unit except higher annual erosivity increased the potential erosion level to 2. This unit forms an excellent agricultural land. However, due to the moderate to high annual erosivity ŽFig. 1. cultivation should be always on the contour. Use of conservation tillage to check soil erosion and preserve soil productivity provides an additional land protection. 4.7. The unit (2 4 RG r III) This unit occupies about 5% of total land area in northern Iraq. The gullied land in the Adhaim river valley and the eroded slopes of the Hemrin and Makhool mountains are the major parts of this unit. The exact cause of gully formation in the Adhaim valley is unknown; but a probable cause seems the soil high susceptibility to crusting which speeds up runoff generation during rainfall events. These gullies are a major source of sediment to the Tigris river in which the Adhaim river channels it’s water. After heavy showers, especially during the fall and winter months where the grass cover is a minimum, the sediment-laden Adhaim river flow raises the turbidity in the Tigris river to a level that occasionally threatens the municipal water facilities in Baghdad and the surrounding areas. At the present time, the government is building a dam to check sediment flow from the Adhaim river. The Hemrin and Makhool slopes are severely eroded due to overgrazing and mismanagement. The entire unit should be kept as a basin land which requires special protection to prevent further deterioration. Recreation may be allowed but grazing should be kept to a minimum or prevented. 4.8. The unit (3 4 RG r III) This unit occupies about 2.6% of total land area in northern Iraq. It is similar to the previous unit except that the higher annual erosivity and the hilly slopes raised the potential erosion to level 3 Žmoderate to severe.. However, present erosion is severe consisting mainly of rill and gully erosion. The Sinjar mountain in the northwest has been covered with oak forests until the beginning of this century where overcutting and mismanagement left the mountain slopes virtually without forest cover. As a consequence, severe rill and gully erosion occurred on these slopes. Except for limited afforestation and terraced orchards, the mountain slopes remained without erosion control measures. Rapid afforestation and grazing control are two important steps to avoid irreversible soil removal and bedrock exposure on the mountain slopes. The rest of this unit consists of hilly eroded spots which extends from Kanaqin in the south east to Karkuk. These areas should be treated as a basin land and grazing should be prevented.
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4.9. The unit (3 5 RG r III) This unit occupies about 1.4% of total land area in northern Iraq. Present erosion is very severe on these hilly eroded slopes. These areas should be treated as a basin land and grazing should be prevented.
5. Discussion Gibbs Ž1954. made a soil erosion survey in Iraq. He estimated that 12% of total land area in Iraq Žabout 5 millions ha. had a serious water erosion problem while 10% Žabout 4.4 millions ha. had a moderate water erosion problem. In Table 6, it is shown that the area of moderate water erosion has increased over the figure given by Gibbs. The main reason is related to the mismanagement of cropland and rangeland in northern Iraq during the past four decades. Livestock number has increased far beyond the capacity of rangeland in the region resulting in overgrazing on most of these lands. On cropland, which has been expanded, crop residues are normally removed after harvest leaving the soil without protective cover. Conventional tillage practices normally used to prepare the seed bed cause a substantial water erosion on more sloping cropland. For the 12 millions ha of land in northern Iraq, potential soil loss will be in the hundreds of millions of tons annually. Records of suspended sediment flow in the Tigris river before the major dams were built were in the millions of tons annually ŽAl-Ansari and Ali, 1986.. The majority of this sediment flow occurs during the period of peak floods. As snow melts, runoff carries the previously detached sediment in the Tigris basin. At the present time, dam siltation in the region is high due to the relatively high rate of water erosion in the Tigris basin. In addition, water erosion causes a substantial economic loss due to the loss in cropland productivity and the damage to forest and range lands in the region. However, any effort to reduce sediment flow in the Tigris and it’s tributaries in the region should include that part of their basins which is located outside the national boundaries. Methods to combat water erosion suggested for each map unit are in general sense. Research on soil erosion and improved methods for its control that incorporate technical, economic or socioeconomic and environmental factors in the region should receive a high priority. Developed erosion control methods should utilize recent thoughts in soil conservation which give greater emphasis to biological control and control through improved farming practices ŽHudson, 1992.. The fact that the area is mostly semiarid,with moisture deficiency not uncommon during the growing season, plays a role in selecting the proper method for water erosion control ŽFAO, 1987..
6. Conclusions Ž1. Of the 12 millions ha which is the total land area in northern Iraq, one third has a serious water erosion problem. This area is mainly located within the mountain region.
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Ž2. The rangeland in northern Iraq is usually overgrazed and water erosion is moderate to severe depending mainly on topography. Grazing control should be the major objective on this land. Ž3. Water erosion on cropland in the region is slight to moderate depending on topography. Proper soil conservation measures are needed on more sloping cropland. Ž4. Severely gullied land and land where bedrock is exposed should be kept for basin protection.
Acknowledgements Thanks go to Tariq H. Kariem, Ibrahim M.A. Ahmedi, Sattar J. Yassin and Kasim M. Al-Saadi for their help in carrying out this project.
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