Determination of evapotranspiration and crop coefficient of onion during growing season at Tabriz plain.

High area devoted to onion in East Azarbayjan province especially Tabriz plain and use of excess low quality water to irrigation by onion producers in this region, has led to improper use of this vital liquid. Other results of this over irrigation are leaching of essential elements especially nitrogen and salinization of groundwater and subsequently irrigated lands with these waters. This reason has caused that some lands in the area to be unusable after several years. Several studies have been showed that total water application for an onion crop varies from 350 to 1800 mm of water, depending upon climatic condition and crop variety. This lysimetric study have been conducted to determination of water requirement (evapotranspiration) of onion (Allium cepa L, Azarshahr Red) at Tabriz plain and estimating it's crop coefficient during growing season by water balance method during 2003-2005. After planting of onion inside and outside of the drainage lysimeter in 3600 m2 area at early Farvardin, parameters of water balance equation was measured regularly and evapotranspiration (ETc) and crop coefficient (Kc) of onion was computed from 1st of Farvardin to 10th of Aban in ten days interval and monthly each year. Cultural practices such as irrigation, fertilizations, and controls of weeds, pests and insects were carried out according to potential ETc definition (growth without any shortage and limitations). Applied fertilizers were 180, 90 and 130 kg.ha-1 for N, P and K respectively. Whole P and K fertilizer were applied before planting and N fertilizer was used during growing season every 15 days. Results indicated that seasonal (from 21 March until 1 November) ETc of onion was 1016.5 mm. 114.7 mm are provided by precipitation and remainder (901.8 mm) should provide by irrigation. Mean seasonal evaporation from class A pan (Ep) for growing season of onion was 1857.1 mm. So the mean ETc/Ep ratio for total growing season will be 0.55. Average Kc computed based on reference evapotranspiration (ETo) obtained from another simultaneously lysimetric study was 0.93. Based on FAO recommended development stages, Kc was 0.55, 0.81 1.06 and 0.78 for initial stage, development, mid season and late season respectively. Average bulb yield was obtained 43.28 ton/ha.