CROP GROWING PERIODS AND IRRIGATION NEEDS AT SOME STATIONS IN ALAGOAS.

Transcription

1 CROP GROWING PERIODS AND IRRIGATION NEEDS AT SOME STATIONS IN ALAGOAS. Kamada Karuna Kumar, Tantravahi venkata ramana Rao, Cristiana Vidal Accioly 2, Maite Duarte Leal Coutinho 3 RESUMO: Resultados de um estudo climatológico de umidade do solo no estado de Alagoas são apresentados neste trabalho. Os dados de temperatura média mensal e precipitação diária, para um período mínimo de vinte e cinco anos, para dez localidades no estado foram utilizado neste estudo. Valores diários de água disponível são computados para quatro valores de capacidade de água disponível. Modelo de cadeia de Markov de primeira ordem é aplicado para os dados diários de água disponível e as probabilidades inicial e condicional de dias secos e úmidos foram obtidas. Os valores médios e probabilidades de umidade do solo foram usados para avaliar as estações de cultivo e as necessidades de irrigação para os localidades escolhidos. Palavras-chave: Temperatura média mensal, precipitação diária, modelo de cadeia de Markov. ABSTRACT: Results of a climatological study of soil moisture conditions in the state of Alagoas are presented in this paper. Monthly mean temperature and daily precipitation data for a minimum period of twenty five years at ten stations in the state are used in the study. Daily values of available soil moisture are computed for four available water capacity values. A first order Markov chain model is applied to the daily soil moisture data and the initial and conditional probabilities of dry and wet soil days are derived..soil moisture averages and probabilities are used to evaluate the crop growing periods and irrigation needs at the stations. Key words: Mean monthly temperature, daily precipitation, Markov chain model. INTRODUCTION Agroclimatic studies based on soil moisture information can yield better results than those based on precipitation data since soil moisture is more directly related to crop growth than precipitation. Longterm soil moisture records are not often available and models of varying degrees of complexity have been developed in the past for the estimation of soil moisture conditions (De Jong and Shaykewich, 98; Baier and Robertson, 966; Holmes and Robertson, 959). For agroclimatic purposes it is preferable to use models which are simpler than the complex Tel. (083) (O); (083) (Res.) e.mail: ramana@dca.ufcg.edu.br

2 mathematical models and still yield better results than those based on averages of rainfall and potential evapotranspiration. In the present study Thornthwaites water balance procedure (Thornthwaite 948,Thornthwaite and Mather 957) is used to compute daily values of soil moisture content for a minimum period of twenty five years at ten stations in the state of Alagoas. The soil moisture data obtained is used to evaluate crop growing periods and irrigation needs at the stations. METHODOLOGY The evaluation of daily soil moisture values is based on the procedure suggested by Thornthwaite and Mather (957). The variation of mean monthly potential evapotranspiration (PE) values during the year is used to obtain PE values for each decade of the year.each month is divided into three decades for this purpose the last decade having 8,9,0 or days depending on the month.from the decadal PE values daily values are obtained and these together with daily precipitation data are used to evaluate the daily soil moisture values.at each station daily soil moisture values are evaluated for the entire study period for each of four available water capacity (AWC) values (25,00,50 and mm). Soil moisture values based on AWC of 25 mm are used to determine the start of the growing period when moisture content only in the speedbed is of importance. A first order Markov chain model is applied to the estimated soil moisture data and the initial and conditional probabilities P(D), P(W), P(D/D) and P(W/W) are determined for each decade of the year. Here P(D) is the probability of soil on a given day being dry, P(W) the probability of soil being wet, P(D/D) the probability of soil being dry given that the previous day is dry and P(W/W) is the probability of soil on a given day being wet given that the previous day is wet. The threshold soil moisture content separating a dry from a wet day is taken to be 50% of the AWC value assumed. Using the initial and conditional probabilities the probability of five consecutive wet days in a decade (P(5W)) are obtained for each decade of the year. It is assumed that (a) five consecutive wet days in each decade during the growing period are necessary for crop growth, (b) successful agriculture is based on good crops being produced in atleast seven out of ten years and (c) sowing is normally done after rain has moistened the soil and five successive wet days are needed for germination and early seedling growth. Based on these assumptions and using the initial and conditional probabilities the start and duration of crop growing periods at the stations are evaluated for different AWC values. Tel. (083) (O); (083) (Res.) e.mail: ramana@dca.ufcg.edu.br

3 The amounts of irrigation required to maintain the soil moisture content above 70% AWC during the growing period are evaluated by means of a simple modification of the program for daily water balance computations (Karuna Kumar and Bezerra, 996). RESULTS Crop growing periods at the stations for two AWC values are shown in Table. Some of the significant results of the study are as follows: The length of the growing period increases with increase in the AWC value assumed. This implies that with a given soil type the growing period for deep rooted crops will be longer than for shallow rooted crops. There is a significant phase difference between the variation during the year of mean decadal precipitation and available soil moisture. At nine stations the phase difference between the maximum decadal values of available soil moisture and precipitation is 50 to 60 days while at Traipu the difference is 70 days for AWC 00 and 80 days for AWC.This suggests that crop growing periods evaluated on the basis of precipitation data alone may lead to erroneous conclusions. Irrigation needs at the stationd during the optimum growing periods are given in Table 2. It is found that to maintain similar moisture levels in the soil less irrigation is necessary for AWC than for AWC 00. This feature has also been reported by De Jong (985) and Karuna Kumar and Silva (996) Comparison of irrigation needs based on daily soil moisture balance with climatological monthly mean values of P and PE suggests that the later can be of little use in assessing the climatic suitability of a region for agriculture. At Atailia for example the precipitation for the six month growing period(08 mm) is more than the potential evapotranspiration ( 64 mm) and during five months P is more than PE. However significant amount of supplementary irrigation is necessary to maintain the soil moisture above 70%of the AWC during this period. CONCLUSIONS Crop growing periods at a station for deep rooted crops will be longer than for shallow rooted crops. Growing periods estimated using precipitation data alone may lead to erroneous conclusions. Climatological mean monthly values of precipitation and potential evapotranspiration can be of little use in determining the climatic suitability of a region for agriculture. Tel. (083) (O); (083) (Res.) e.mail: ramana@dca.ufcg.edu.br

4 REFERENCES Baier,W and G.W.Robertson,966: A new versatile soil moisture budget.can.j.plant.sci.,46, De Jong,R and C.F.Shaykewich,98: A soil water budget model with a nearly impermeable layer.can.j.soil.sci,63, De Jong, R., 985: Soil water modelling using daily and mean daily data derived from historical monthly values. Atmosphere-Ocean. 23(3), Holmes, R. M., and G. W. Robertson, 959: A modulated soil moisture budget. Mon. Weather Rev., 87, Thornthwaite, C. W. and J. R. Mather, 957: Instructions and tables for computing potential evapotranspiration and the water balance. Climatology 0(3), Drexel, Insr. of Technol, N.J., P Kumar, K. K. ; Silva, J. A. T. ; Bezerra, V. F. A climatological study of soil moisture under corn crop at Campina Grande (NE Brazil). mausam, new delhi, india, v. 48, n. 3, p , 997. Karuna Kumar, K. and V. de F. Bezerra, 996: Irrigation requirements of corn crop at some stations in northeast Brazil. Proceedings of the 0th Brazilian meteorolical congress. Campos do Jordao Tel. (083) (O); (083) (Res.) e.mail: ramana@dca.ufcg.edu.br

5 Table. Crop growing periods. STATION AWC(mm) CROP GROWING PERIOD START END DURATION (DAYS) ATALAIA 00 TRAIPU 00 0 OCT 3 OCT 0 JUN 30 AUG 30 SEP MIGUEL CAMPOS DOS 00 0 OCT 3 OCT 80 VIÇOSA 00 0 OCT 0 NOV 90 LIMOEIRA ANADIA DE SEP 20 OCT 30 S. LUIS DO QUITUNDE OCT 3 OCT 40 PENADO SEP 20 OCT 40 SANTANA IPANEMA DO SEP 0 OCT 0 40 MACEIO SEP 3 OCT 90 ANADIA OCT 0 NOV Tel. (083) (O); (083) (Res.) e.mail: ramana@dca.ufcg.edu.br

6 Table 2. Irrigation requeriments. STATION PERIOD AWC (mm) WATER NEED (mm) ATALAIA -OCT 00 TRAIPU JUN-SEP MIGUEL DOS CAMPOS -OCT VIÇOSA -NOV LIMOEIRA DE ANADIA S. LUIS DO QUITUNDE -OCT 00 -OCT PENADO -OCT SANTANA DO IPANEMA JUN-SEP MACEIO -OCT 00 ANADIA -NOV Tel. (083) (O); (083) (Res.) e.mail: ramana@dca.ufcg.edu.br