Crop co-efficient, Irrigatiion scheduling, Lysimeter, Onion
Onion bulbs (BARI Piaz-1) were planted on 17 November, 2012 in four lysimeter tanks maintaining row to row and plant to plant spacing of 20 cm × 15 cm, respectively. Also, to maintain a favorable atmospheric condition around the lysimetric tanks, the same crop was grown in the fields. Onion seeds were harvested on 31 March 2013. The soil was sandy clay loam with field capacity and bulk density, 28% and 1.5 gm/cc, respectively. The selected treatments were; T1= Irrigation at 10 days interval allowing drainage;T2= Irrigation at 15 days interval allowing drainage; T3= Irrigation at 20days interval allowing drainage; T4= Irrigation at 25 days interval allowing drainage.
The micro-lysimeter contains 4 tanks, each having area of 1 x 1 m. Climatic data such as, maximum and minimum temperatures, air humidity, sunshine hours and wind speed together with the location information like elevation, latitude and longitude were incorporated in the software, CROPWAT, to estimate the potential evapo-transpiration.
The crop was irrigated in accordance with the design of the treatments. Measured quantity of water was applied to the tanks to ensure drainage. Prior to irrigation, soil moisture was measured to determine the depleted soil water. Therefore, only the stored soil moisture during the period under consideration were subtracted from the applied water to obtain crop evapo-transpiration (ETc) i.e.,
ETct = Wa + Re– (Dw + Ss)
Where, ETct = crop evapo-transpiration in mm for time period, t ; Wa = applied water, mm, Re = effective rainfall, mm, for time t , Dw = drainage water, mm, for time, t; Ss = stored soil moisture, mm, for time, t ; Then from the potential evapo-transpiration (ETo) estimated for the specified period, the value of Kc for the period was determined from the ratio, ETc / ETo.
The crop coefficient values of onion seed at initial, development, mid season and late season stages were found 0.68, 0.93, 1.23 and 0.78, respectively, from the lysimeter study. As the lysimeter provides a control environment, further study is not needed for confirmation.