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Water Deficit Period for Irrigated Agriculture Based on Evapotranspiration and Dependable Rainfall

M.B. Masud
Agricultural Systems and Engineering, Asian Institute of Technology, Thailand

A. K. Ghosh
Department of CSM, Bangladesh Agricultural University

Mesbauddin Ahmed
Department of IWM, Bangladesh Agricultural University

J.Ferdous
Agricultural Systems and Engineering, Asian Institute of Technology, Thailand

Abstract/ Executive Summary:

Actual estimation of potential evapotranspiration (ETp) over large areas is needed for hydrologic studies and other practices related to efficient utilization of water resources. A simple method of estimating monthly ETp was therefore identified to test the reliability of traditional methods for estimating ETp at Bangladesh Agricultural University (BAU) campus. Major climatologic parameters like temperature, precipitation, relative humidity, wind speed and sun shine hours for a period of 30 years (1977-2006) were collected from weather station, BAU, Mymensingh. Research was conducted to find out the best method of measuring evapotranspiration (ETp) from seven different methods for the study area. Also the research was conducted to find out the water deficit period based on best method of evapotranspiration and dependable rainfall for irrigated agriculture. Of the seven methods, we compared ETp rates estimated by Blaney-Criddle method was found to be the most suitable and accurate method and Class-A (pan evaporation) method is more similar to the actual Pen evaporation method. According to Pan-evaporation, Hargreaves Class-A pan evaporation, Thornthwaite, Lowry-Johnson, Modified penman, Hargreaves and Blaney-Criddle method corresponding to the highest values of ETp are 5.43, 5.7, 7.5, 9.7, 7.12, 4.79 and 6.69 and the lowest values are 2.37, 2.49, 1.52, 9.6, 3.096, 2.88 and 4.15 mm/day, respectively. Corresponding highest values occur in April and June and lowest values occur in January and December. That is, Potential evapotranspiration higher in the month of April and June and lower in the month of January and December. Dependable rainfall was calculated by using Normal distribution and Extreme value type 1 (EV1) at 75% probability from precipitation data over 30 years. According to Normal and EV1 distribution, at 75% probability corresponding highest values of dependable rainfall are 10.87 and 11.84 mm/day in the month of July and corresponding lowest values are 0 mm/day in the month of November to February for Normal and November to December for EV1 distribution. Values of these two distributions were more or less same and lower than mean rainfall. Considering potential evapotranspiration (ETp) and dependable rainfall, irrigation is needed from November to April.

Keywords: Evapotranspiration, Climatologic parameters, Dependable rainfall, Normal distribution and Extreme value type 1 (EV1) distribution

Location: Weather station, BAU, Mymensingh

Start Date: 00-00-1977

End Date: 00-00-2006

Program Area: Crop-Soil-Water Management

Commodity: Irrigation scheduling

Objectives:

1. To estimate water deficit period for irrigation based on evapotranspiration and dependable rainfall by using climatic data as well as best method of measuring evapotranspiration based on suitability and simplicity.

Methodology:

Site selection and data collection - Bangladesh Agricultural University (BAU) (latitude 24.460 N, longitude 90.260 E, elevation 15 m above mean sea level) was chosen as the study area for analysis of climatic parameters. All climatic data were collected for the period of 30 years (1977-2006) from weather station, BAU, Mymensingh for estimating potential evapotranspiration and dependable rainfall. Methods used for estimating potential evapotranspiration (ETp) - Blaney-Criddle method,Hargreaves method, Thornthwaite method, Lowry-Johnson method, Hargreaves Class A pan evaporation method, Modified Penman method were used for estimating potential evapotranspiration (ETp). Distribution used for estimating dependable rainfall - For dependable rainfall analysis, the recommended distribution is normal, lognormal, extreme value type-1 (EV1) and Gamma. In this study, the Normal and EV1 distributions were used. Extreme Value Type 1 Distribution - This is a two parameter distribution and is also referred to as the double exponential and/or Gumbel distribution. Its form arises from consideration of the statistical properties of samples extreme values and it was first introduced by Gumbel.

Source of Information: BANGLADESH RESEARCH PUBLICATIONS JOURNAL; ISSN: 1998-2003, Volume: 5, Issue: 4, Page: 321-328, July -August, 2011

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Dependable rainfall was calculated by using Normal distribution and Extreme value type 1 (EV1) at 75% probability from precipitation data over 30 years. According to Normal and EV1 distribution, at 75% probability corresponding highest values of dependable rainfall are 10.87 and 11.84 mm/day in the month of July and corresponding lowest values are 0 mm/day in the month of November to February for Normal and November to December for EV1 distribution. Values of these two distributions were more or less same and lower than mean rainfall. Considering potential evapotranspiration (ETp) and dependable rainfall, irrigation is needed from November to April.

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