The effect of submergence depth on evaporation losses in paddy fields

Background and objective: Evaporation is one of the main components of water losses in submerged irrigation method in paddy fields. The amount of evaporation is a function of temperature, relative humidity, wind speed, vegetated surface, submerges depth, water table level and other elements. In different intermittent irrigation managements, paddy fields frequently are under submerged and non-submerged situation. In each irrigation practice, the water level changes from submerged to capillary crack. This research aims measuring of evaporation rate during rice growth in different submerged depths in Guilan Province paddy fields in Rice Research Institute near meteorological research station in 2013.
Material and methods: Five different water level treatments (5, 2.5, 0, -5, -10 cm) where applied to the farm in three repetition and using mini Lysimeters the evaporation is measured in daily scale in the middle of pig plots.
Results: The results show that evaporation in different submerged levels is significantly different in 5%.The most and least evaporation amounts are consequently seen in 0 cm and 10 cm treatments respectively 120.8 millimeter and 94 millimeter. In all treatments the evaporation reduces during the time to the half. Precipitation minimize also evaporation rate till 75 %. Neglecting precipitation dates also does not change the difference between treatments. The comparisons show that higher levels of water on the soil surface cause higher evaporation losses. By reducing water level and narrowing water depth on soil surface, especially in vegetation period evaporation reduces. If the thickness of this layer reduces and reaches to zero or soil became semi saturated, evaporation increases again. If the thickness of this layer reduces and reaches to zero or soil became semi saturated evaporation increases again. Then when the soil became dryer and the water level stays at -10 cm below soil level, the evaporation decreases significantly.
Conclusion: The results of evaporation measurements and its fluctuations are highly strongly to fluctuations of soil temperature in every treatment (in depth of 5 and 10 cm under the top soil) and water temperature and treatments which have higher records of temperature in the soil and water environment, have severer evaporation rates. In case of enough available water, presence a thin layer of water on top soil surface can reduce effectively evaporation. But in the absence of water necessary to maintain submergence, to reduce evaporation losses it is recommended to keep water level table in lower than 5 cm from top soil surface.