Study of Effective Rainfall for Irrigated Agriculture in South-Eastern Part of Bangladesh

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1 World Journal of Agricultural Sciences 4 (4): , 2008 ISSN IDOSI Publications, 2008 Study of Effective Rainfall for Irrigated Agriculture in South-Eastern Part of Bangladesh 2 2 M.M. Rahman, M.O. Islam and M. Hasanuzzaman Rajshahi University of Engineering and Technology, Rajshahi, Bangladesh 2 Department of Agronomy, Sher-e-Bangla Agricultural University, Dhaka, Bangladesh Abstract: The major source of water available either for agriculture or for human consumption is obtained from the rain that falls on the earth's surface. Agricultural production in Bangladesh is very much dependent upon rainfall. But, not all the rain that falls on the earth's surface is useful for agricultural purpose. This study deals with "effective rainfall", i.e. the rainfall that directly satisfies crop water requirements. In this study four different methods have been used for estimating effective rainfall in Bangladesh. Effective rainfalls have been estimated for the ten meteorological stations of Bangladesh, covering South - Eastern part of the country. By using different climatological data, effective rainfall have been estimated for two crop growing seasons, one is Kharif (July to October) and the other is Rabi (December to March). It has been observed that the values of effective rainfall percentage for Kharif season varies from 3.94% at Teknaf to 47.27% at Chandpur by Renfro Equation Method, from 8.79% at Teknaf to 46.33% at Chandpur by U.S. Bureau of Reclamation Method, from 52.94% at Sandwip to 00% at several stations by Potential Evapotranspiration/Precipitation Ratio Method and from 22.58% at Teknaf to 64.4% at Chandpur by U.S.D.A, SCS Method. Effective rainfall percentage for Rabi season varies 63.30% at Chittagong to 76.79% at Maizdi Court by U.S.D.A, SCS Method and by other methods the values of effective rainfall percentage is near about 00%. It has been observed that as the distance from sea increases the value of effective rainfall percentage also increases. While designing an irrigation project, optimum utilization of irrigation water can be achieved by using the effective rainfall values. Key words: Effective rainfall Agriculture Climate Bangladesh INTRODUCTION due to large scale thunderstorm activity called Norwesters. These rains are of substantial importance for The major source of water available either for the early spring crop of rice and for tea crop. agriculture or for human consumption is obtained Precipitation during the cold weather season is due to from the rain that fall's on the earth's surface. Bangladesh low pressure system. The amount of precipitation during is a land where it rains in abundance. Rainfall in our the cold season is very little. But, these rains are of country varies greatly in space and time. Maximum substantial importance in raising rainfed Rabi crops rainfall occurs in the monsoon period extending from [2]. the month of June to September []. Due to high The primary source for agriculture production for temperature of summer, the moisture - laden south - west most of the world is rainfall. The most important monsoon originate from the vast expanse of the Bay of characteristic of rainfall vary from place to place, Bengal. By June this monsoon winds moves all over day to day, month to month and also year to year. In Bangladesh and precipitates heavily. The withdrawal of spite of voluminous data on weather, all is not yet the monsoon starts in September. The crucial months known that should be known about rainfall. Certain for agriculture are July and August and the fate of rainfed. simple entities have baffled planners right up to present. Kharif crop largely depends upon the amount and One of these is effective rainfall". In its simplest distribution of rain especially during these two months sense, effective rainfall means useful or utilizable [2]. rainfall. Rainfall is not necessarily useful or desirable at In Bangladesh the hot weather rainfall occurs in the time, rate or amount in which it is received. Some of it between the months March and May, which are mainly may be unavoidably wasted while some may even be Corresponding Auhtor: Md. Obaidul Islam, Department of Agronomy, Sher-e-Bangla Agricultural University, Dhaka-207, Bangladesh 453

2 destructive. Just as total rainfall varies, so does the amount of effective rainfall [3]. The useful portion of rainfall is stored and supplied to the user, the unwanted parts need to be conveyed or removed speedily. The effective rainfall has been interpreted differently not only by specialists in different fields but also by different workers in the same field. To a canal irrigation engineer, the rain which reaches the storage reservoir directly and by surface runoff from the surrounding area indirectly is the effective portion. Agriculturists consider that portion of the total rainfall as effective which directly satisfies crop water needs and also the surface runoff which can be used for crop production on their farms by being pumped from ponds or wells. Most rain water is used in agriculture for crop production. But some part of rainfall is lost. These losses includes water intercepted by living or dry vegetation, that lost by evapotranspiration during growth of crops, that lost by evaporation from the soil surface, that fraction which contribute to leaching and percolation. Meteorologists can neither solve nor evaluate the problem of effective rainfall merely from the tables of frequency, amount and intensity of rainfall or from physical phenomena in the atmosphere [4]. It is a task in which several disciplines and sub disciplines overlap. For example, in the field of agriculture, soil types, cropping patterns and social, economical and management factors all have a direct impact on the extent of effective and ineffective rainfall [3]. In view of these facts the study was conducted to review the various methods of determining effective rainfall; to calculate the effective rainfall for Bangladesh at different meteorological stations for Rabi and Kharif crop season, using selected methods and to compare the different methods of estimating effective rainfall. MATERIALS AND METHODS This study was conducted in different climatic zones of the south-eastern parts of Bangladesh during The rainfall was measured from different meteorological stations of those regions. The following four methods have been used to calculate rainfall for the thirty meteorological stations of Bangladesh covering the entire country: Renfro Equation method U.S. Bureau of Reclamation method Potential Evapotranspiration/Precipitation Ratio method U.S.D.A., SCS method Effective rainfalls have been calculated for two crop growing seasons, one is Kharif season (July to October) and the other is Rabi season (December to March). Three different values of net depth of irrigation water requirement during the crop growing season, have been assumed for estimating effective rainfall by Renfro Equation method and USDA, SCS method. The values are 50, 75 and 00 mm. For the estimation of effective rainfall by Potential Evapotranspiration/Precipitation Ratio method, three different values of seepage and percolation losses have been assumed. The values are 3, 6 and 9 mm/day. RESULTS AND DISCUSSION The effective rainfall values for both Kharif and Rabi season have been worked out using the mean monthly consumptive use data of different meteorological stations. The effective rainfall values are tabulated. From Table it can be observed that the values of effective rainfall are higher for Rabi season than those of Kharif season. The rainfall characteristics of Bangladesh are the prime reason behind this difference. A soil has a definite and limited water intake rate and moisture holding capacity. Hence greater intensities of rainfall normally reduce the effective rainfall. Higher intensities increase the run-off and reduce infiltration. Similarly, uneven distribution decreases the extent of effective rainfall while an even spread enhances it. A well distributed rainfall in frequent light showers is more conducive to crop growth than heavy downpours. Benjamin et al. [5] also observed similar results in China. In Bangladesh the intensity, frequency and amount of rainfall are high in Kharif season. As the intensity, frequency and amount of rainfall in Rabi season is low, a little amount or almost no water is lost through surface runoff, deep percolation and seepage. As the temperature is low during the Rabi season the evaporation loss from the land surface is negligible. The methods that have been used for estimation of effective rainfall calculation neglect retention losses. So, almost 00% of rainfall is effective for Boro rice grown in Rabi season. 70% to 80% of total annual rainfall occurs during the Kharif season. The temperature during this season is high. So, the percentage of effective rainfall during the Kharif season is low. The Renfro Equation Method gives the lowest value of effective rainfall, among the four methods which have been used. Another aspect of this method, the more the 454

3 Table : Percentage of effective rainfall calculated by various methods in various stations in South-East zone of Bangladesh Effective Rainfall as Percentage of Growing Season Rainfall (%) Renfro Equation PET/Precipitation Ratio Method U.S.D.A, SCS Method When Depth of Irrigation When Seepage and When net Depth of Growing Water Application Percolation Loss Irrigation Application Season Rainfall = 50 = 75 =00 U.S.B.R = 3 = 6 = 9 = 50 = 75 =00 Station Season (mm) (mm) (mm) (mm) Method mm d mm d mm d (mm) (mm) (mm) CND KHA RABI FNI KHA RABI MZD KHA RABI STD KHA RABI RNT KHA RABI SNP KHA CHD= Chandpur; FNI= Feni; MZD= Maizdi Court; STD= Sitakunda; RNT= Rangamati; SND= Sandwip KHA = Kharif; RABI = Rabo average depth of irrigation water application the higher is and distribution of rainfall. It also does not consider the the effective rainfall. With this method one can only effect of aridity. This method has been developed only calculate the total growing season (such as July - considering surface runoff. This method is recommended October, December-March) effective rainfall, but not for for arid and semi arid regions. Among the values of a short period (such as 0 days period, monthly period). effective rainfall obtained for Kharif season, the highest But, some times it is required to find out effective rainfall value is 46.33% at Chandpur and the lowest value is for a short period. So, from that point of view this 8.79% at Teknaf. The U.S. Bureau of Reclamation method method is not suitable. The Renfro Equation Method is is not suitable for use under all sorts of condition. too empirical. It may not suit many situations. The "E" (a Patwardhan et al. [7] also supported this method. constant dependent upon the ratio of consumptive use of Potential Evapotranspiration/Precipitation Ratio water to rainfall during the growing season) values which method is a simple semi empirical method. With the have been used to work out the effective rainfall are given help of this method the total growing season effective without considering the runoff and type of crop. It only rainfall as well as the effective rainfall for a certain group considers aridity and makes assumption about the of days can be computed. The number of days in a group soil. So, it can be concluded that the accuracy of this is based on the soil type or soil moisture properties as method is too low. Dastane [6] also concluded similar well as general weather conditions or evapotranspiration findings. So, for quick determination of effective rainfall rates. The maximum number of days in a group is with the net depth of irrigation water application of 5 during warm weather and 30 during cool weather. 00 mm, is 5.89% at Teknaf and the lowest value with the But in this study one month has been used as a group net depth of irrigation water application of 50 mm, is for both the Kharif and Rabi seasons due to the non 3.94% at Teknaf. availability of required data. The lowest water With the help of U.S. Bureau of Reclamation method holding capacity of the soil and the highest the the seasonal effective rainfalls as well as monthly evapotranspiration rate, the shortest the period in the effective rainfall have been calculated. Quick estimation group. These factors had not been considered in this of effective rainfall is the principal feature of this method. study due to the lack of necessary data. But the U. S. B. R. method is not considered to be In the PET/Precipitation Ratio method effective satisfactory. Because this method does not take into account the type of soil, nature of the crop and frequency rainfall varies with the potential evapotranspiration and also with the seepage and percolation losses. The highest 455

4 the values of these losses, the highest will be the value of exhibit higher value of effective rainfall. Simmers et al. effective rainfall. Three values of seepage and percolation [0] and Patwardhan et al. [7] also observed similar losses have been assumed which are 3, 6 and 9 mm/day. findings. In this method the rainfall lost through seepage and Teknaf have the lowest value of effective rainfall. The percolation are considered as part of effective rainfall. As growing season rainfall for Teknaf is 2566 mm. Intensity a result, with the seepage value of 9 mm/day the of rainfall at this station is very high. So, water lost effective rainfall for both Kharif and Rabi season is near through surface runoff is high. Because of that reason about 00% throughout the South Eastern part of this station has the lowest value of effective rainfall Bangladesh. But, the highest value of effective rainfall among all the meteorological stations. with the seepage and percolation loss of 3 mm/day is The effective rainfall is dependent upon various 94.94% at Chandpur and the lowest value is 52.94% at factors, such as land characteristics, soil characteristics, Sandwip. With a loss of 6 mm/day the highest value of soil water characteristics, ground water characteristics etc. effective rainfall is 00% at Chandpur and the lowest These factors are different for the 0 stations of value is 63.43% at Teknaf. This method is the most Bangladesh. But, the factors that have been considered in suitable among the four methods. In this method this study are only consumptive use and rainfall. So, we approximation have been made about the runoff and soil cannot expect a high degree of accuracy from this study. condition. Aridity also has been considered. This method Zimmermann [] and Uribe et al. [2] observed similar was found to be laborious but most accurate. This result result. was supported by Mohan et al. [8]. Rainfall should satisfy not only the consumptive In the USDA, SCS method, the effective rainfall needs of crops but also leaching, land preparation needs, varies with the net depth of irrigation water application. percolation needs etc. It is more appropriate and precise The greatest the depth of water, the highest is value of to use the total water needs of the crop in computing effective rainfall. It is also dependent upon the effective rainfall. The PET/Precipitation Ratio method consumptive use of crop. A higher value of gives effective rainfall considering the seepage and consumptive use results in a greater value of effective percolation losses. Effective rainfall has been calculated rainfall. The method has been developed after a with this method by assuming three different values of comprehensive analysis at different experimental stations seepage and percolation losses. More accurate values of representing different climatic and soil conditions. So, the effective rainfall can be calculated with this method accuracy of this method is high. Obreza and Pitts [9] also by determining the actual seepage and percolation found similar observation. losses [2]. With the USDA, SCS method the highest value of A method used for estimating effective rainfall effective rainfall in Kharif season is 64.4% at Chandpur must account satisfactorily for surface runoff, with the net depth of irrigation water application of evapotranspiration and crop characteristics. For field use 00 mm and the lowest is 23.68% at Sandwip with the net the method should be simple, inexpensive, rapid and depth of irrigation water application of 50 mm. It has been accurate. It should be useful for broad regional planning found that this method is good for areas with low or precise irrigation scheduling under a given set of intensity of rainfall and high soil infiltration rate. conditions. The methods that have been used are simple, The meteorological stations at Chandpur have the inexpensive and rapid. A high degree of accuracy can be highest value of effective rainfall, as determined by attained with these methods by carrying out various field different methods. The rainfall in Kharif season for experiments to suit different conditions. A high degree of Chandpur are 887 mm. This value of rainfall is the precision can be attained with these methods by lowest among the 0 meteorological stations of considering various influencing factors. Information Bangladesh. The values of consumptive use for on effective rainfall can be used for design of Chandpur are 743. mm, which is the highest among all the irrigation projects, design and operation of drainage 0 meteorological stations. So, the above mentioned this systems, leaching of salts, rice cultivation, planning of stations has very low value of growing season rainfall irrigation systems using groundwater and for rainfed and high value of consumptive use. With the combination agriculture. This result was corroborated with of these two factors this meteorological station Mohan et al. [8] 456

5 CONCLUSION 3. FAO, 975. Production végétale et protection des plantes. Surveillance agrométéorologique pour la It may be concluded that effective rainfall is prévision des récoltes, N 7. directly proportional to consumptive use and inversely 4. Manalo, E.B., 976. Agro-climatic Survey of proportional to the amount and intensity of rainfall. Bangladesh. International Rice Research Institute, Amount of effective rainfall increases with the increase in Manila, Philippine. the depth of irrigation water applied to the field. In case of 5. Benjamin, N.N., M. Jacques and S.R. Jean, Potential Evapotranspiration/Precipitation Ratio method, Groundwater Recharge from Rainfall in the Southern effective rainfall increases with the increase in seepage Border of Lake Chad in Cameroon, World Applied and percolation losses. Effective rainfall for Kharif season Sci. J., 2(2): (July-October) varies widely from 3.94% at Teknaf to 6. Dastane, N.G., 974. Effective Rainfall in Irrigated 00% of several stations throughout South-East zone of Agriculture. F.A.O. irrigation and drainage paper 25, Bangladesh. Effective rainfall percentage for Rabi season Flood and Agriculture Organization of the United (Dec - March) is almost 00%. Very little experimental Nations, Rome, Italy. information is available in Bangladesh, for planning 7. Patwardhan, A.S., J.L. Nieber and E.L. Johns, purposes of irrigation projects on the basis of effective Effective rainfall estimation methods, J. Irrigation rainfall. These experimental data may be collected by Drainage Eng., 6(2): setting up crop lysimeters in the field. The four methods 8. Mohan, S., B. Simhadrirao and N. Arumugam, use in this study for estimating effective rainfall was Comparative study of effective rainfall estimation mainly developed for U.S.A. So, these methods therefore, methods for lowland rice. Water Res. Mang., 0(): should be modified and improved if necessary, to suit the climatological patterns of Bangladesh by comparing these 9. Obreza, T.A. and D.J. Pitts, Effective Rainfall in results with practical values. Rapid empirical methods can Poorly Drained Microirrigated Citrus Orchards, Soil also be developed for different irrigated areas in Sci. Soc. Am. J., 66: Bangladesh. This study has been carried out from the 0. Simmers, I., J.M.H. Hendrickx, G.P. Kruseman and irrigation point of view. The same study can be carried out K.R. Rushton, 997. Recharge of Phreatic Aquifers in for other purposes, such as, flood control engineering, (Semi-) Arid Areas. International Association of hydro-electrical engineering, reservoir engineering, etc. Hydrogeologists (9), A. A. Balkema Rotterdam, pp: 277. REFERENCES. Zimmermann, E., Bayesian approach to daily rainfall modelling to estimate monthly net. BMD (Bangladesh Meteorological Department), 2000, infiltration using the Thornthwaite water budget and Mean Monthly Rainfall Data of Bangladesh. curve number methods, Hydrogeol. J., 4: Bashar, M.K., 987. Study of Potential 2. Uribe, H.L., J.A. Moraga, F.S. Zazueta and Evapotranspiration and Consumptive use of Water A.G. Smajstrla, 995. A comparison of effective for Different Crops over Bangladesh. An rainfall calculations using the SCS TR-2method and unpublished B.Sc. Engineering project report, AFSIRS. no. FL-95-0, Am. Soc. of Agric. Eng., St. Rajshahi Univ. Eng. Technol., Rajshahi. Joseph, MI. 457