Tham khảo tài liệu 'evapotranspiration remote sensing and modeling part 3', khoa học tự nhiên, công nghệ sinh học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Evapotranspiration Estimation Using Soil Water Balance Weather and Crop Data 49 end In the case of canola the end of season Kcb does not need adjustment since it is which is less than . Soil evaporation coefficient Similar to Kcb soil evaporation coefficient Ke needs to be calculated on a daily basis. Ke is a function of soil water characteristics exposed and wetted soil fraction and top layer soil water balance Allen et al. 2005 . In the initial stage of crop growth the fraction of soil surface covered by the crop is small and thus soil evaporation losses are considerable. Following rain or irrigation Ke can be as high as 1. When the soil surface is dry Ke is small and even zero. Ke is determined using Eq. 14 . Ke min Kr Kc max - Kcb fw Kc max 14 Where Kc max maximum value of crop coefficient Kc following rain or irrigation Kr evaporation reduction coefficient which depends on the cumulative depth of water depleted and few fraction of the soil that is both wetted and exposed to solar radiation. Kc max represents an upper limit on evaporation and transpiration from the cropped surface. Kc max ranges Allen et al. 2005 . Its value is calculated for initial development mid-season or late season using Eq. 15. K L max ff z_ r h .3 i z max w2 -2 RHmin -45 PI Kci I V 3 JI 15 Evaporation occurs predominantly from the exposed soil fraction. Hence evaporation is restricted at any moment by the energy available at the exposed soil fraction . Ke cannot exceed few x Kc max. The calculation of Ke consists in determining Kc max Kr and few. Kc max for initial development midseason and late season stages were calculated to be and respectively. Evaporation reduction coefficient The estimation of evaporation reduction coefficient Kr requires a daily water balance computation for the surface soil layer. Evaporation from exposed soil takes place in two stages an energy limiting stage Stage 1 and a falling .
