Crop Water Requirement for Krishnai Irrigation Project – A research report by National Institute of Hydrology

In this report an attempt has been made to estimate the mean monthly reference crop evapotranspiration (ETo) for the Krishnai Irrigation Project in Assam. To undertake this analysis, the mean monthly climatological and physiographical data for the period of 1986 to 1994 has been collected from the Meteorological Department, Borjhar, Assam.

ETo value depends on a large number of climatological, physiographic and other data but its manual estimation is highly tedious and time consuming. The use of computer software is very essential. In this project, a computer program presented in FAO’s Publication Number 24 developed by Doorenbos and Pruitt (1977) has been modified. The modified model has been used for estimation of ETo by seven different empirical methods viz, Blaney & Criddle, Radiation, Penman, Modified Penman, Pan Evaporation, Hargreave and Thornthwaite.

Of these methods, the Modified Penman Method is the most elaborate and rational but requires a large amount of data. In the absence of such data empirical methods are used. For that reason, the study is further extended to evaluate regional relationships for Krishnai Irrigation project relating the ETo values of Modified Penman Method with that derived from the six other climatological methods. The developed relationships are useful in determining the rational value of ETo, corresponding to the ETo value obtained by the Modified Penman Method with scarce data.

The study calculates the crop water requirements on the basis of mean monthly ETo values received from modified Penman method. The report concludes that –

• Reliable crop evapotranspiration can be determined even if all climatological data are not available for Modified Penman Method, by making use of the developed regional relationships of different methods with the most rational approach.
• In this study, it is observed that monthly relationship is most suitable for the determination of ETo values when there is a lack of data for the Modified Penman Method.
• The methods Blaney & Criddle, Radiation and Penman have a good correlation with the Modified Penman Method for development of linear relationship between the two.
• To improve management practices there is a need of installing automatic weather stations in the command area. This will also reduce energy, time and error in collecting data pertaining to rainfall, humidity, radiation, wind velocity and wind direction.

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