S Mohan

Evapotranspiration (ET) is an important process in the hydrological cycle and needs to be accurately quantified for proper irrigation scheduling and optimal water resources systems operation. The time variant characteristics of ET necessitate the need for forecasting ET. In this paper, two techniques, namely a seasonal ARIMA model and Winter’s exponential smoothing model, have been investigated for their applicability for forecasting weekly reference crop ET. A seasonal ARIMA model with one autoregressive and one moving average process and with a seasonality of 52 weeks was found to be an appropriate stochastic model. The ARIMA and Winter’s models were compared with a simple ET model to assess their performance in forecasting. The forecast errors produced by these models were very small and the models would be promisingly of great use in real-time irrigation management. © 1995 Taylor & Francis Group, LLC.

Meteorological and lysimetric data for a period of nine years were used to develop crop coefficients for rice grown under lowland conditions in a sub-humid tropical climate in India. The estimated crop coefficients were found to be higher than those values recommended by FAO. A crop coefficient model with basal coefficient, moisture availability coefficient and surface wetness coefficient terms has been proposed and found suitable. On most counts, the moisture availability coefficient was found to be near unity and the wetness coefficient was found to be significant. The basal crop coefficients for lowland rice have also been presented for practical use with the proposed models. © 1994 Kluwer Academic Publishers.

Irrigation systems must be operated efficiently and optimally to manage them under practical situations. Much complexity is involved in respect to system operation and management which puts water managers and operators in a predicament. An expert system (ES) would be an appropriate tool to address these complex problems. In this paper, a hybrid expert system that has been developed for operation of a tank irrigation system in South India, is presented. The heuristics and optimal knowledge are integrated with algorithmic techniques to operate the system under real-time conditions.

Tank irrigation systems contribute nearly one third of the total irrigated area in south India and hence their management is an important practical need. This article examines some major considerations related to tank irrigation systems in south India. A comprehensive view of the problems affecting the sustainability of these systems is presented. The major deficiencies that influence the sustainability are inadequate maintenance. reduction in storage capacity, heavy seepage losses in the delivery system, and poor water management techniques. There is a pressing need to evolve and implement appropriate strategies that must be sound on technical, social, economical, institutional, and economical dimensions for sustainable development and management of tank systems. Important practical solutions are discussed for urgent action in the context of sustainable development and management of these irrigation systems. © 1997, Taylor & Francis Group, LLC.

Planning and management of irrigated cropping systems are of paramount importance to effectively utilize the resources namely water, land, labour, and other inputs. Irrigation systems, in nature, are different from place to place and from time to time. Irrigation environment becomes more complex as it undergoes rapid changes due to natural and man-made actions. Changes in climate, field and soil, farmer and other conditions make the environment more ill-structured and it would be difficult for a farm manager to make decisions in the above circumstances. Decision-making would better be done, based on his own experience and getting expertise from others regarding the dynamic nature of irrigation environment. Hence expert systems have a great potential for providing decision support to irrigation systems managers and farmers. In this paper, an expert system that has been developed for crop planning and managing irrigated cropping system is discussed. The expert system development consists of two phases. In the first phase, given the field, climate and farmer related conditions, suitable crop(s) would be selected. In the second phase, management decisions regarding irrigation and fertilizer schedules are recommended. Twenty crops which require decision support for crop planning are included in the rule-base. This rule-base is put into the knowledge base of INSIGHT 2+ shell and used for decision making.

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.