PRATUMMINTRA Somjate (1), Van RANST Eric (2), VERPLANKE Ir (3), SHAMSUDDIN Jusop (4)

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1 Scientific registration n o : 1421 Symposium n o : 33 Presentation : poster Evaluation crop coefficient for rubber with the application of soil water balanced method Coefficient d'évaluation de la croissance du caoutchouc par l'application de la méthode du bilan de l'eau PRATUMMINTRA Somjate (1), Van RANST Eric (2), VERPLANKE Ir (3), SHAMSUDDIN Jusop (4) 1. Rubber Research Institute of Thailand, Department of Agriculture, MOAC 2. ITC. Laboratory of Soil Science, Faculty of Science, University of Ghent, Belgium. 3. Department of Soil Management and Soil Care, Faculty of Agricultural and Applied Biological Sciences, University of Ghent, Belgium. 4. Department of Soil Science, Faculty of Agriculture, Universiti Putra Malaysia. ABSTRACT Many Projects have been earmarked for rubber cultivation in Northeastern Thailand although the area is regarded as unsuitable. People question the viability of growing rubber in the region where rainfall is low and dry period is long. This study was assigned at using rubber growth models to evaluate crop coefficient for rubber. Four soil series along a toposequence in Maha Sarakharm Province were studied using RRIM 600 clone. The growing period of rubber tree was related with humidity index. The index was obtained from the crop evapo-transpiration (Etcrop) which was calculated from actual evaporation (PAN A, ETp) and crop coefficient for young rubber (Kc), Etcrop = ETp x Kc. The Kc was evaluated, using field soil moisture data based on soil water balance and law of constancy of mass. The Kc was modified around the year with the relative with soil moisture content in each soil averaging 0.71 g/cm 3. The girth of rubber trees showed highly significant correlation with the land index which calculated from this Kc value. The growth models tested were GI = LI (R 2 = 0.98) and TY = Erreur! where GI is girth increment, LI is land index and TY is tapping year. The models were used to evaluate the establishment of rubber in Ubon Ratchathanee province. INTRODUCTION Northeast Thailand has serious Agro-ecosystem problem because of deforestation. Thai Government had set up a project to help farmers grow new economic crops in the area. Rubber is the preferred crop but its viability is uncertain. People question its suitability because of hot and dry climate prevailing in this area. Farmer would want to know the immature period and the expected yield. Studied on establishing rubber in three provinces in the Northeast Thailand stated since Land evaluation methodology 1

2 may provide some answers to the questions posed. Crop coefficient (Kc value) for young rubber is related to growing period of an immature period. However it is difficult to find the methodology to determine crop coefficient for rubber. According to the calculation based on FAO methodology by using Malaysian climatic data (Sys, cited by Yew, 1982; Sinthurahut 1985, 1992), the Kc for immature stage is 1.1 and the immature stage is 1.0. The Kc is related to the evapo-transpiration of a diseases-free crop grown in large fields under optimal soil, water and fertility conditions, and achieving full production potential under the given growing environment. The Kc is based on extrapolation, when the mean relative humidity is more than 70 percent and mean wind speed is 0-5 m/sec. Sys (1980) defined the relationship between crop evapo-transpiration and water availability as follows: WA = Erreur! where, WA = Water availability (%) ET C = Crop evapo-transpiration d = Water deficit comparing ET C and effective rainfall, taking into account the storage capacity of the soil The method used in this study is based on monitoring natural evaporation in soil water content and soil water pressure head. So the water balance is an itemised statement off all gains, losses and changes of storage occurring in a given field within specified period of time. It is a detailed statement of the law of conservation of matter, which states simply that matter can neither be created nor destroyed, but can only change from one state or location to another. The field soil water balance is intimately connected with the energy balance since it involves processes that require energy. The energy balance is an expression of the chemical Law of Conservation of Energy, which states that, in a given system, energy can be absorbed from, or released to the outside, and that along the way it can change after, but it cannot be created or destroyed. The content of water in the soil affects the way the energy flux reaching the field is portioned and utilised likewise, the energy flux affects the state and movement of water. Material and Methods Field soil moisture content A monthly field soil moisture was determined in each profile pit by thermo-gravimetric method at,, 40, 80, 1, 150 and 180 cm depth from ground level or consolidated layer. The results were used to study field soil moisture fluctuation, which were used to calculate the matric head, hydraulic head profile and flux in the profile pit. Soil Water Characteristics Soil samples were collected from each horizon of a soil profile. Water content at low matric suction (between -150 cm of water height) was determined by using a sand box, but that at high pressure (at 1/3, 1 and 15 bar) were determined by pressure plate apparatus. Test Crop The test crop is RRIM 600 rubber clone. The rubber is planted on 4 soil series along the toposequence in 1989, using deep planting technique. The field management of rubber field was based on the recommendation of the Rubber Research Institute of Thailand. Determination of Crop Coefficient for Rubber Crop coefficient for rubber can be determined from crop evapo-transpiration for a period of time and different situations and flux plane which are present in the profile. Under 2

3 such condition, either a continuous drainage or upward flows take place within the profile. The principles of calculating the flux and crop evapo-transpiration are as following is explained below. The upward flows take place within the profile. When the hydraulic head gradient is positive, the negative flux is directed upward. The downward or positive flux is determined as the drainage component during the time period: q zr = K θ Erreur! where q zr = soil flux K θ = Hydraulic conductivity dh = Hydraulic head gradient dz = Gravitation head gradient Soil samples were collected for moisture determination by thermo-gravimetric with the interval of 1 month. Rough analytical data from laboratory were used to interpolate soil moisture content (cm 3 /cm 3 ) into matric head, the reference level for determining the gravitational head use at soil surface. The hydraulic head gradient in each period was studied and used to evaluate the direction of the flux in soil profiles as shown in Figure 1. Crop evapo-transpiration (Etcr) was determined from the flux and climatic factors, using the formula: ETcr = P + I - ( S) 0 zr - q zr t - R where ETcr = Evapo-transpiration in rubber Plantation P = Precipitation during the sampling period I = Irrigation water ( S) 0 zr= Change of water storage = (Erreur!θdz) t 1 - (Erreur!θdz) t 2 ; (S = Erreur!θdz) q zr = flux t = time duration R = Run off water; in this study considered that no run off Crop coefficient value for rubber was determined by the formula: Kc = Erreur! where Kc = Crop coefficient for rubber ETcr = Evapo-transpiration ETo = Evaporation, which was measured by PAN A Rubber Growth Models Rubber growth models was obtained by running linear regression analysis on land index and rubber growth. Land index for rubber cultivation was calculated based on crop requirement for rubber. The humidity index, which indicated the growing period when it higher than 0.5, was calculated from Kc value,actual monthly evaporation and evaerage monthly rainfall. Parametric approach was used to calculated the index as follow: LI = Erreur! where LI = Land index A = Indices of each characteristics n = Number of characteristics 3

4 The rubber growth models were validated using data from Ubon Ratchathanee Province. The index was used to calculated a standard growth of each plantation. After the actual growth was observed, a management index was calculated from a relationship between actual girth and standard growth. Plane of zero flug Figure 1 Soil water content and hydraulic head profiles. After Verplanke (1993). Results and Discussion Determination of Crop Coefficient for Rubber in Maha Sarakham Crop coefficient for rubber in Maha Sarakham by using soil water balance equations, was determined. Four soil were investigated, named Warin, Satuk, Korat and Roi Et. Soil water characteristics curves The soil water characteristics curves determined by sand box and pressure plates apparatus are presented in Figure 2. (a) (b) (c) (d) θ % volume Ap E B21t B22t B23t θ %volume Ap E S21t S22t S23t θ %volume Ap E B1 B21 B22t θ % volume Ap E B21 B22t B23t Fig 2 Soil water characteristics of Warin (a), Satuk (b), Korat (c) and Roi Et (d) Field soil moisture fluctuation 4

5 The moisture fluctuation on Warin soil series is shown in figure The drainage upper limited (DUL) is in August and drainage lower limited is in May. The figure 3.1 show the soil moisture in the measurement time interval, figure 3.2 show the drainage upper and lower limited and the figure 3.3 show the field soil moisture extractable /12/ /11/ /08/ DLL Extractable DUL /05/ Water Content (%Volume) Figure 3.1 Figure 3.2 Figure 3.3 The drainage upper limited (DUL) on Satuk soil series is in November, but Korat and Roi Et soils series are in August. The drainage lower limited of all soil series are in May. Determination the Flux The flux has been calculated and used to indicate the upward flow within the profile, when the hydraulic head gradient is positive, then the negative flux is directed downward and define as a part of Et crop. or the drainage component during the period. Determination of Crop Coefficient of Rubber The crop evaporation is calculated from soil water storage and crop coefficient is calculated. The average Kc value is varied from 1.85 in mid rainy season to 0.24 at the end of dry season. Rubber Growth Model The climatic index which is applied the FAO Kc value, is 16.6 and classified as nonrecommended for rubber cultivation. The limitation is long dry period and the total water deficit is very high in the dry season (729 mm/year). But the Kc value in this study give better result. The index is 31.3 which classified as a marginal climate for rubber. (Table 5). The land index of Warin soil series, which is the best soil in the study area and Roi Et series is the lowest, the index is 47 and 31. The growth of RRIM 600 rubber tree in 4 soil series (Table 6) show the best girth increment in Warin soil series, which is average 7.79 cm per year and the lowest annual growth in Roi Et soil series is about 5.52 cm. The growth model for immature rubber is calculated as TY = Erreur! when GI = Girth increment (cm / year) LI = Suitability land index TY = number of tapping years Table 3 Calculation for Crop Coefficient of Rubber. Period Warin Soil Series Rainfall Eto (Pan) DS z q Zr. Dt Et crop mm mm mm mm mm K c 5

6 Aug.-Sep Sep.-Nov Nov.-Dec Dec.-Jan Jan.-Mar Mar-May Average/Total Satuk soil series Aug.-Sep Sep-Nov Nov.-Dec Dec.-Jan Jan.-Mar Mar-May Average/Total Korat soil series Aug.-Sep Sep.-Nov Nov.-Dec Dec.-Jan Jan-Mar Mar-May Average/Total Roi Et Soil series Aug.-Sep Sep.-Nov Nov.-Dec Dec.-Jan Jan-Mar Mar-May Average/Total Table 2 The Average of Kc value in 4 soil series Periods Wartin Satuk Korat Roi Et Average Aug.-Sep Aug.-Nov Nov.-Dec Dec.-Jan Jan-Mar Mar-May Average Conclusion and Discussion Crop coefficients of rubber base on soil water balance method show that they vary with the soil moisture content, drainage component and soil characteristics. The difference soil series in the same climate have the same pattern. In case study, the high value of crop coefficient (Kc = 1.85) is in August to September, which are in the mid of rainy season and the soil has the highest water content. The lowest value (Kc = 0.24) is in 6

7 January to March, which are in the hot and dry season; We can compare with the FAO. method as the following; 1. Climatic data for determining ETo Method Temperature Humidity Wind Sunshine Evaporation Environment FAO measure measure measure measure - estimate Pan A - estimate estimate - measure measure 2.Soil data In FAO, Kc value relates to evapo-transpiration of a disease-free crop, under optimum soil water and fertility conditions. But soil water balance method estimates Kc with the real data which characteristics are present in soil profile. 3.Crop coefficient of rubber In FAO method, the crop coefficient for rubber were calculate and given the value of 1.1 for immature rubber. In soil water balance method, crop coefficients of rubber vary from the high value in rainy season to low value in dry season. References Ninane,F Les Aspects Ecophysiologiques de la Productivite Chez Hevea braziliensis. Muell.Arg. Au Cambodge.Univ.Catholique Louv. Dise. Presentee in vue de obtention dugrade de doctor in Sc. Agro. 390 pp. (Self contact) Pratummintra, S., Suwanmongkol, P., Dolkit, P., Somnak, S. and Ninane, F Land Evaluation on Potential of Rubber Establishment in the North-East of Thailand. Tech.Doc. of RRIT, Bangkok Thailand. 214pp. Pushparajah, E Problem and Potentials for Establish Hevea under Different Environmental Conditions. Paper presented on the Inter. Rubb. Res. and Dev. Board Symp. Hat Yai. Thailand. on 8-9 May, Sinthurahut, S., Elaboration of a Land Evaluation Model for Rubber Cultivation in Peninsular Thailand. Dr. in Soil Sci. Thesis. ITC. Univ. of Ghent. Belgium. 261 pp. Sys, C The Outlook for Practical Application of Land Evaluation Standard for Rainfed Agriculture. WSRR. No. 49: FAO. Rome. Sys, C Land Evaluation. Part I to III. The Lecture note for ITC. Post-Graduate Soil Scientist. Univ. of Ghent. Belgium. 24 pp Verplancke, Ir. H Soil Physics. Faculty of Agriculture, Ghent univ. Belgium. Wright,H Para Rubber : Its Botany, Cultivation, Chemistry and Disease. 4 th Mc.Laren and Sons Ltd. London, England. 542 pp. Dr. in Agri. Sci. Thesis. ITC. Univ. of Ghent. Belgium. 330 pp. Yew, F.K Contribution towards the Development of a Land Evaluation System for Hevea brasiliensis Muell.Arg. Cultivation in Peninsular Malaysia. Keywords : rubber, land evaluation, crop coefficient, growth models Mots clés : caoutchouc, évolution régionale, coefficient de croissance, modèles de croissance 7