Soil erosive power of rainfall in the different climatic zones of Sri Lanka

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1 Soil erosive power of rainfall in the different climatic zones of Sri Lanka W. D.Joshua Abstract Erosivity (R) is a quantitative measure of the erosive power of rainfall. Erosivity as defined by KE>1 was calculated for nine locations which are representative of the different rainfall patterns and soils of Sri Lanka. More than 50 per cent of the total rainfall in the lowlands was erosive and this proportion decreased with increasing elevation. In the uplands, less than 25 per cent of the total rainfall was erosive. In the wet zone, southwest monsoon rains were more erosive than the northeast monsoon rains. Rainfall intensities up to 4 in./h are experienced throughout the country. Average annual erosivity values range from 781 X 10 2 ft-tons/acre in the wet zone to 154 X 10 2 ft-tons/acre in the dry zone. Erodibility (AT) of different soils in Sri Lanka was estimated, using the nomograph developed by Wischmeier et al., to assess the relative susceptibility of the different soils to erosion. Erodibilities thus calculated varied from 0.17 to The relative erosion hazard for the nine locations was assessed by comparing the magnitudes of the factor R X K. Experimental evidence suggest that erosivity defined by KE>1 may not be appropriate for some soils in Sri Lanka and the basic infiltration rate of soil may have to be taken into consideration when calculating erosivity. Pouvoir érosif de la pluie dans différentes zones climatiques du Sri Lanka Résumé. L'érosivité R est la mesure quantitative du pouvoir érosif de la pluie. L'érosivité, exprimée par KE>1, a été calculée pour neuf sites du Sri Lanka représentatifs des régimes pluviaux et des sols. Plus de 50 pour cent de la pluie totale est érosive dans les basses terres et cette proportion décroit avec l'altitude (moins de 25 pour cent). Dans la zone humide, les pluies de mousson du sud-ouest sont plus érosives que celles du nord-est. Des intensités pluviales allant jusqu'à 4 inches/h sont courantes dans la région. L'érosivité moyenne annuelle va de 781 X 10 2 ft-tons/acre dans la zone humide jusqu'à 154 X 10 2 ft-tons/acre dans la zone sèche. L'érodibilité (K) des différents sols du Sri Lanka a été estimée grâce au nomographe de Wischmeier: elle varie de 0.17 à Le danger d'érosion des neuf sites étudiés a été évalué en comparant les grandeurs du facteur R XK. L'expérimentation fait penser que l'érosivité définie par KE> 1 peut ne pas être appropriée pour tous les sols du Sri Lanka et que la vitesse d'infiltration dans le sol doit être prise en considération. INTRODUCTION The impact of rain on soil is the primary cause of soil erosion. However, other factors such as gradient and length of slope, vegetative cover and erosion control practices modify the erosive power of rainfall to determine the actual quantity of sediment transported by rainfall erosion. The universal soil loss equation (Wischmeier and Smith, 1965) which was developed in the USA through extensive research, expresses the relationship between these factors in the form, A=RKLSCP in which A is the mass of soil loss per unit area, R the rainfall factor, K the soil erodibility factor, L the length of slope factor, S the slope gradient factor, C the cropping management factor and Pis the factor related to erosion control practices. The rainfall factor R, is the potential ability of rain to cause erosion and is termed 51 ^

2 52 W. D. Joshua the erosivity index or simply erosivity. Experimental results have shown that the kinetic energy of rain satisfactorily estimates the erosive power of rainfall. Erosivity, which is a quantitative measure of the erosive power, has been defined by the I 30 index (Wischmeier, 1959) or by KE>1 index (Hudson, 1971). These indices can be calculated from rainfall data alone. The erodibility factor K is a measure of the soil's susceptibility to erosion. Therefore it will largely depend on the physical characteristics of the soil. The best estimation of the erodibility factor for a given soil can be obtained only through field experiments. It quantitatively expresses the amount of soil loss per unit of erosivity under specified standard conditions. Attempts to evaluate erodibility from the physical properties of soil have not been wholly successful. However, the method developed by Wischmeier et al. (1971) to evaluate erodibility using measurable soil physical properties may be usefully adopted in inferring the relative erodibility of soils. The factors L, S, C and P are associated with the catchment geometry and vegetation. Reliable estimates of these factors can be obtained from published data from other countries. For quantitative estimation of soil erosion, factors such as erosivity (R) and erodibility (K) have to be evaluated under local soil and rainfall conditions. In spite of serious erosion hazards very little information is available on erosivity and erodibility for tropical soils in general and for Sri Lanka in particular. Therefore as a first step, erosivity values for the different climatic zones of Sri Lanka were calculated from available data on rainfall intensity. A thorough knowledge of the characteristics of the rainfall patterns in the country, the seasonal variation in erosivities and the total annual erosivity, are of immense value for the development of sound soil conservation and crop management practices. This information can also be used to predict relative erosion hazards and soil loss for different times of the year for any particular location. CLIMATIC ZONES AND SOILS OF SRI LANKA Sri Lanka can be divided into three climatic zones, namely, the wet zone, the intermediate zone and the dry zone (Fig. 1). This classification is based mainly on the annual rainfall, seasonal rainfall distribution, physiography, soils and the type of vegetation. The rainfall regime in Sri Lanka can be divided into four main periods: the southwest and the northeast monsoon periods and the intermonsoon periods. The southwest monsoon occurs from May to September bringing rains to the upland and lowland areas in the wet zone. Rainfall is a minimum in the dry zone during this period. During the northeast monsoon between December and February, the rainfall is fairly heavy in the dry zone and moderate in the wet zone. The intermonsoon period during October and November is marked by heavy rainfall over the entire country. The other intermonsoonal activity occurs during March and April and the rainfall though small is significant especially in the dry zone. The soils of Sri Lanka have been classified and mapped at the Great Soil Group level (Moorman and Panabokke, 1961). In Fig. 1 these soils have been grouped very broadly into soil regions whose erodibility values may be assumed to be uniform. The stations for which erosivity is being presented in this paper are generally representative of the different rainfall patterns and soils occurring in Sri Lanka. The stations are shown in Fig. 1 and some particulars regarding each station are given below. Station no. 1 Station no. 2 Station no. 3 Ratnapura : wet zone - lowland - red-yellow podzolic soil region. Badulla : intermediate zone - upland red-yellow podzolic soil region. Katugastota : wet zone ~ upland reddish-brown latosolic soil region.

3 Soil erosive power of rainfall in Sri Lanka LEGEND A j Reddish-Brown Earth Region j B 1 Lato sol Region C Red-Yellow Podzolic Soil Region D Reddish-Brown Latosolic Soil Region E [ Regosol Region (Sandy) F Non Calcic Brown Soil Region Red-Yellow Podzolic Soil Region (hard latérite) Rainfall Statk Hambantota FIGURE 1. Agroclimatic zones and major soil regions in Sri Lanka.

4 54 W.D.Joshua 20 " RATNAPURA r BADULLA KATUGASTOTA t^jjui 16 " KATUNAYAKA ANURADHAPURA KANKESANTURAI l8 ~ BATTICALOA MANNAR HAMBANTOTA I I I I 1 I I! I I I I I I I 1 1 I I I I i i i i i i i Weeks Weeks O «O 48 FIGURE 2. Distribution of total and erosive rain (in inches). Erosive rain tbb Station no. 4 Station no. 5 Station no. 6 Station no. 7 Station no. 8 Station no. 9 Total rain I \ Katunayake : wet zone lowland sandy regosol region. Anuradhapura : dry zone lowland reddish-brown earth region. Kankesanturai : dry zone - lowland - latosol region. Batticaloa : dry zone lowland non-calcic brown soil region. Mannar : dry zone (very dry) - lowland - latosol region. Hambantota : dry zone (very dry) lowland reddish-brown earth region.

5 SEASONAL DISTRIBUTION OF RAINFALL Soil erosive power of rainfall in Sri Lanka 55 The average 4-weekly totals of rainfall for 52 standard weeks are shown in Fig. 2. The results were computed from the daily rainfall records from each station taken for a continuous 5-year period. The average values of total rainfall agree satisfactorily with the monthly averages calculated over a 30-year period. It is apparent from Fig. 2, that for the whole country in general, the rainfall pattern is bimodal with peaks occurring during the monsoonal or the intermonsoonal periods. Within the wet zone there is considerable variation in the total annual rainfall and in its distribution. The region represented by station no. 1 is characterized by high annual rainfall (about 140 in.) and uniform distribution. Whereas, stations no. 3 and 4 experience lower annual rainfalls (76 in. and 91 in.) and have a bimodal distribution. In the dry zone the total annual rainfall is about 50 to 60 in. and it is concentrated largely during the intermonsoonal and the northeast monsoonal periods (stations nos. 5, 6, 7, 8 and 9). EROSIVE POWER OF RAINFALL The erosive power of rainfall can be assessed by calculating the erosivity, R, for a particular location. Wischmeier(1959) has shown that the best estimator of erosivity (i.e. soil loss due to the rainfall factor alone) is a compound parameter, the product of the kinetic energy of the rainfall and the greatest average intensity experienced in any 30-min period. The erosivity calculated in this way is called the 7 30 index. However, Hudson (1971) studying soil erosion in Africa found that for tropical rains the erosivity defined as the total kinetic energy of rain falling at intensities greater than 1 in./h(ke>l) estimates soil loss better than the 7 30 index. The erosivities for the different stations were calculated using the KE>1 method and consequently all rainfall whose intensity is equal to 1 in./h or more is considered erosive. The kinetic energy of the rainfall was calculated from the expression: kinetic energy = log 10 / where kinetic energy is in ft-tons/acre-inch and the intensity /, is in inches per hour. Erosive rain in relation to total rain The amount of erosive rain for the 4-weekly periods is also shown in Fig. 2. In general, the seasonal distribution of erosive rain follows the same trend as the total rainfall. Comparison of total rainfall and erosive rainfall for stations nos. 1, 3 and 4, shows that in the wet zone more than half the rainfall for each 4-weekly period is erosive. For these stations, the southwest monsoon rains during April, May, June have a higher erosive proportion when compared to the northeast monsoon rains between September and December. Further, throughout the year stations nos. 1 and 4 in the lowlands experience higher proportions of the total rainfall as erosive rain than station no. 3 in the uplands. Therefore, it appears from the data that in the wet zone, the southwest monsoon rains are more erosive, and in general, the rainfall in the lowlands is more erosive than that in the uplands. In the upland intermediate zone (station no. 2) a significant feature is that only a small fraction of the rainfall is erosive (less than 25 per cent). This trend was also observed for wet zone stations at higher elevations. These results tend to suggest that the proportion of erosive rains decreases with increasing elevation, even when the rainfall is appreciable. In contrast to the wet and intermediate zones, most of the rain which falls during the course of the year in the dry zone is erosive. The rainfall during the dry period between June and September is almost entirely erosive as can be seen from Fig. 2, for

6 56 W. D. Joshua RATNAPURA 40- o 35- BADULLA KATUGASTOTA CD KATUNAYAKA ANURAOHAPURA (D I l-n-n KANKESANTURAI Qfl tra 1 I J BATTICALOA MANNAR HAMBANTOTA r f HoA o- Ltm HTh IO I* Intensity Class (ins/hr.) Intensity Class (ins/hr.) FIGURE 3. Distribution of rainfall at different intensities. ) fr O 0-5 IO I* Intensity Class (ins/hr.) stations nos. 5, 6, 7 and 8. However, since the total quantity of rainfall is low in this region, the erosivities are also low. Rainfall intensity Figure 3 shows the distribution of rainfall in the different intensity classes. It can be seen that a large proportion of the erosive rain falls in the in./h intensity class.

7 TABLE 1. Soil erosive power of rainfall in Sri Lanka 57 Mean annual rainfall and erosivity for selected locations in Sri Lanka Rainfall station Mean annual rainfall [in.] Erosive rain [% of total] Mean annual erosivity KE>1(R) [ft-tons/ac] 1. Ratnapura 2. Badulla 3. Katugastota 4. Katunayake 5. Anuradhapura 6. Kankesanturai 7. Batticaloa 8. Mannar 9. Hambantota X X X X X X X X X 10 2 The amount of rainfall in this class even exceeds that in the other lower intensity classes. Unlike the intensity distribution of rainfall in temperate countries, a significant proportion of the rainfall occurs in the higher intensity classes of up to 4 in./h, thus contributing substantially to the total erosive power. However, the proportion of rainfall in the higher intensity class decreases as you proceed towards the upland. More than 55 per cent of the total annual rainfall in the lowlands is above 1 in./h and this progressively decreases to less than 25 per cent as the elevation increases (Table 1). Seasonal distribution of erosivity The total amount of erosive rain (in inches) will not reflect the total erosive power of the rain for a particular station. The measure of the total erosive power of the annual rainfall is the annual erosivity as defined earlier and therefore will depend on the actual amount of rain falling in the different intensity classes which are greater than 1 in./h. Figure 4 shows the 4-weekly distribution of erosivity for all the stations. Table 1 shows the average annual rainfall, the percentage of annual rainfall which is erosive and the annual erosivity index. For all stations except no. 9, the distribution of erosivity follows a similar trend, having pronounced peaks during the intermonsoonal rainy seasons. However, the magnitude of the erosivity during the peak period varies considerably in the different climatic zones; the erosivity in the wet zone being about 2 to 3 times greater than in the dry zone. For station no.l whose mean annual rainfall is high, the average erosivity for the 4-weekly interval is about 6000 ft-tons/acre with the maximum about 9500 ft-tons/ acre and minimum about 2000 ft-tons/acre. Erosivities for stations nos. 3 and 4 range between 8000 and 2000 ft-tons/acre with an average of about 3000 ft-tons/ acre. In the dry zone, the average erosivity for the 4-weekly interval is in the region of 2250 ft-tons/acre with the maxima for the stations varying from 4000 to 6000 ft-tons/acre and the minimum less than 1000 ft-tons/acre. The minimum erosivity for the whole country occurs in January when the rains are mostly non-erosive. Comparison of the annual erosive power of rains for the different climatic zones can be made from the relative magnitude of the erosivities shown in Table 1. The annual erosivity for station no.l is 781 X 10 2 ft-tons/acre which is relatively high compared to the other stations and is fairly evenly distributed throughout the year. On the other hand, stations nos. 3 and 4 have erosivities (338 X 10 2 and 463 X 10 2 ft-tons/acre) only about half as high as station no. 1 and these are confined mainly to two periods. In the intermediate zone and the dry zone (stations nos. 2, 5, 6 and 8) the annual erosivity values are often less than 250 X 10 2 ft-tons/acre and the distribution follows a bimodal pattern in most cases. Station no. 7 which is also in the dry zone is unique 3

8 58 W. D. Joshua RATNAPURA BADULLA KATUGASTOTA a. ë 1 r it S! 4000' KATUNAYAKE n I 1 I \ i \ \ 1 A / I \ A / / w ANURADHAPURA KANKASANTURAI BATTICALOA HAMBANTOTA weeks Weeks 0 FIGURE 4. Seasonal distribution of rainfall erosivity, in that the total erosivity (368 X 10 2 ft-tons/acre) is higher than that of the other stations and has a maximum in December. The variation of erosivity during the year as shown in Fig. 4 is a valuable tool in assessing the soil loss due to erosion. For a particular location, where the land use and vegetative cover remain unchanged, the soil loss will be directly proportional to the erosivity of the rain in that location. Therefore, the variation in the relative amount of sediment transported will be in direct relation to the variation in erosivity. With a few

9 Soil erosive power of rainfall in Sri Lanka 59 TABLE 2. Erodibility of some sois of Sri Lanka estimated by use of the nomograph prepared by Wischmeier et al. (1971) Rainfall Soil Erodibility (R X K) X 10~ 2 ition (JO Ratnapuia Badulla Katugastota Katunayake Anuradhapura Kankesanturai Batticaloa Mannar Hambantota red-yellow podzolic red-yellow podzolic reddish-brown latosolic sandy regosol reddish-brown earths red-yellow latosol non-calcic brown red-yellow latosol reddish-brown earths measurements of sediment transported in the runoff for different intensity rains, it should be possible to estimate, at least approximately, the quantity of soil loss during different periods of the year. Similarly, information given in Fig.4 is useful in determining the period during which the erosion hazard is minimal so that activities which involve high erosion risk such as land clearing and levelling may be confined to these periods. ERODIBILITY OF SOILS The erodibility factor, K, for a given soil is such that when multiplied by the erosivity R, the product equals the soil loss from that soil under specified conditions. Consequently, the magnitude of the erodibility factor will depend on the method of calculation of erosivity as well as on the specified standard conditions. A method was developed by Wischmeier et al. (1971) for evaluating soil erodibility from soil properties such as particle size distribution, organic matter content, structural index and permeability. In this method a soil erodibility nomograph graphically solves an abridged equation that incorporates these soil physical properties. Such methods are useful in comparing erodibilities so that the relative susceptibility of a soil to erosion can be approximately assessed until experimental data becomes available. Table 2 shows the annual erosivity, the soil erodibility calculated according to Wischmeier's nomograph and the product of erosivity and erodibility (i? X K) for all the stations. Table 2 shows that the calculated erodibility for the major soil regions varies from 0.17 to 0.48 and the relative susceptibility of these soils to erosion follows the order regosols > non-calcic brown soils > red-yellow latosols > reddish-brown earths > red-yellow podzolic soils > reddish-brown latosolic soils. The magnitude of the factor obtained by multiplying erosivity by erodibility i.e. RX K gives us a means of comparing the relative degree of erosion hazard or soil loss between locations as influenced by rainfall and soil alone. For example, if the product R X K for location A is twice as much as in B, then under similar conditions of landform and vegetation, the soil loss due to erosion at A will be twice as much as in B. It is apparent from Table 2 that for stations nos. 1 and 3 where the erosivities are high, the erosion hazard has been somewhat reduced due to the low erodibility value

10 60 W. D. Joshua of the soils. Fortunately for areas such as the reddish-brown earth and red-yellow latosol regions where the erodibilities are high, the erosivities are low and the terrain is flat or only gently undulating, thus reducing the danger of soil erosion. For stations nos. 4 and 7, where both erosivity and erodibility are high, the actual soil erosion observed is low due to the flat nature of the topography and the high infiltration rates of the sou (5 in./h). In Sri Lanka the highest amount of soil erosion is observed in the upland area in spite of the relatively low soil erodibility. This is because of the long and steep slopes existing in this area. Nevertheless, the factor JR X K (Table 2) gives an indication of the expected erosion hazard and the conservation methods that have to be adopted if landform and vegetation are changed due to construction or due to agricultural activity. An idea of the relative erosion hazard for the different stations can be had from the magnitude of the factor K X R in Table 2. The erosivity values as defined by KE>1 assume that rain falling at intensities lower than 1 in./h does not cause any significant erosion. However, data from experiments carried out in the reddish-brown-earth region (Allés, 1958) indicate that there is significant erosion at intensities between 0.5 and 1.0 in./h. The basic infiltration rate of these soils is about 0.5 in./h, and it has been observed that there is appreciable runoff if the rainfall intensities are above 0.5 in/h. Similar data for red-yellow-podzolic soils (Manipura, 1972) which have a basic infiltration rate of 1 2 in./h showed significant runoff and soil loss only when rainfall intensities exceeded 1 in./h. Further, the reddish-brown earths and the red-yellow latosols occur under very similar conditions of climate, landform and vegetation. Therefore, one could expect higher erosion on the red-yellow latosols because of their higher erodibility. However, the runoff and erosion on the reddish-brown earths is always considerably higher than on the redyellow latosols. The reason being that the infiltration rate of the red-yellow latosols is about 15 in./h and the rainfall is almost completely absorbed by the soil without any runoff. However, when the vegetation is removed and the infiltration rate becomes reduced drastically by surface sealing, the soil erosion is severe in the latosols as indicated by the higher erodibility value. From the foregoing discussion it would appear that the threshold value of rainfall intensity for calculating erosivity is closely linked to the basic infiltration rate of the soil. This is reasonable to expect since there should be sufficient runoff to transport the soil particles which are detached due to splash erosion. Therefore, the most appropriate threshold intensity would probably be equal to the basic infiltration rate of the soil, provided it is high enough to cause splash erosion. However, field experimentation is necessary to test the practical applicability of this concept. From the limited experimental evidence already available and from field observations, it seems that some modification may be necessary in the estimation of these parameters if soil losses are to be predicted with reasonable accuracy. However the erosivity and erodibility values presented in this paper have many useful practical applications in Sri Lanka. For example, the most economical and realistic soil conservation practices can be formulated according to the relative erosion hazards of a region, rather than by giving a blanket recommendation to cover all the plantation areas as is presently done. Similarly, operations involving erosion hazards such as replanting of old tea lands and clean weeding by mechanical implements, can be timed for periods of minimum erosivity. Soil conservation measures, such as establishment of cover crops and mulches, can be adopted according to information on soil erodibility. These are but a few of the many uses to which information on erosivity and relative erodibility can be applied for formulating measures to minimize soil erosion. Experiments have already been initiated to study erosivities and erodibilities for different soil regions in Sri Lanka. When more information becomes available, it will be possible to apply the universal soil loss equation in a more quantitative manner for practical purposes.

11 REFERENCES Soil erosive power of rainfall in Sri Lanka 61 Allés, W. S. (1958) Tropical Agriculturist CXW, Hudson, Norman (1971) Soil Conservation: Cornell University Press, Ithaca, New York. Manipura, W. B. (1972) Tea Quarterly 45 (3), Moorman, F. R. and Panabokke, C. R. (1961) Tropical Agriculturist CXVII, Wischmeier, W. H. (1959) Soil. Sci. Soc. Amer. Proc. 23, Wischmeier, W. H., Johnson, C. B. and Cross, B. V. (1971) /. Soil Water Cons Wischmeier, W. H. and Smith, D. D. (1965) Agric. Handbook No. 282: USDA, ARS.