S Mohan

Dastane (1974) recommended four different methods, namely evapotranspiration- rainfall ratio method, Indian-1 and Indian-2 methods and Vietnam method for estimating effective rainfall for rice crop. These methods together with the USDA-SCS method were compared against the physically-based water balance method. The analysis was carried out using daily data of evapotranspiration (ET) and rainfall pertaining to thirteen seasons with special reference to lowland paddy rice in South India. Among the different methods, the ET-rainfall ratio and Indian-2 methods estimate the effective rainfall values nearly closer to those by the water balance method. The USDA-SCS method underpredicts values of effective rainfall compared to other methods. The ET-rainfall ratio and Indian-2 methods can be used as alternatives to the water balance method for determining effective rainfall which in turn be used in estimating irrigation requirements for lowland rice. © 1996 Kluwer Academic Publishers.

Crop coefficients (Kc) were estimated for cotton, sorghum and millet for tropical South India based on lysimeter measured actual evapotranspiration (ET) and reference crop ET computed by the modified Penman method. The estimated crop coefficients were 0.46, 0.70, 1.01, and 0.39 in the four crop stages for cotton. Kc value of the last stage was significantly lower than the FAO Kc value. The estimated Kc values were considerably lower than FAO crop coefficients in the third and fourth stages for sorghum, for which the estimated stagewise Kc values were 0.42, 0.71, 0.62 and 0.23. For millet, the estimated Kc values for four stages were 0.51, 0.78, 0.87 and 0.50 and found to differ markedly from FAO Kc values. Crop coefficient relationships with respect to time were developed using mean crop coefficients derived from multi-year data. The developed crop coefficients and relationships would be of great use for the estimation of crop water requirements under tropical climatic conditions. © 1994.

Evapotranspiration is the most important variable next to rainfall in the context of irrigation to crops and it is a multivariate phenomenon as it is influenced by many hydrological variables. In this paper, the relative importance of different variables involved in evapotranspiration has been investigated through a multivariate tehnique, namely factor analysis. Monthly data pertaining to eight meteorological stations located in two tropical states of India are used in this analysis. Factor analysis was applied to determine the relative effect of rainfall, temperature (maximum and minimum), wind speed, relative humidity, and sunshine duration on evapotranspiration. The analysis was performed in two stages: in the first stage, the principal components were obtained and in the second stage, a set of factors was derived using Varimax factor rotation. Three factors were extracted which explained more than 85% of the total variation in the original data set. The identified factors were related to physical processes influencing the evapotranspiration. The results of the study reveal that the variables, relative humidity, temperature, and wind speed, are those with the most influence in the evapotranspiration process, in that order. The variables rainfall and sunshine duration have been found to have less influence on the ET process. © 1996 Kluwer Academic Publishers.

Evapotranspiration (ET) is an important parameter needed by water managers for the design, operation and management of irrigation systems. Since there are many methods to compute ET, based on climatic data, an inexperienced engineer or hydrologist is perplexed with the selection of an appropriate method. In this context, an expert system (ES) would be of great use to aid in selection of a suitable ET estimation method, given the location, data availability and climatic conditions. In this paper an intelligent front-end expert system (ETES), that has been developed to select suitable ET estimation methods under South Indian climatic conditions, is presented. Ten meteorological stations located in different climatic regions and thirteen ET estimation methods have been considered in this ES. Like a human consultant, the system asks the user for detailed information regarding the details of the project site such as location, season, climatic zone, and data availability. It then makes a recommendation based on this information and the system's own knowledge of such a situation. Along with the recommended method, ETES will also suggest suitable correction factors for converting the resulting ET values to those of methods that result in accurate estimation. The developed ES would be of much potential use in irrigation management in developing countries like India. © 1995.