Estimating soil evaporation in dry seeded rice and wheat crops after wetting events

Soil evaporation (Es) is quite challenging to measure directly, and researchers most commonly resort to using mini-lysimeter's to measure Es from both bare soil and from soil beneath the crop canopy. However, this technique is unable to measure Es during and shortly after periods of irrigation or rain, and an approach to fill data gaps during these periods would be valuable. One possible approach is the use of the relationship between Es, pan evaporation (Ep) and gravimetric soil water content (theta g). To this end, a mini-lysimeter field study was carried out during periods of low evaporative demand (Ep <= 3 mm) and high evaporative demand (Ep > 3 mm) to determine the relationship between Es, Ep and theta g. There was a highly significant multi-linear relationship between Es, Ep, 8g and time after wetting (R-2= 0.78; p < 0.001). Direct measurement using mini-lysimeters and gap-filling using Ep on days of irrigation or rainfall, and the multi-linear relationship for up to 6 d following wetting events, showed that significant amounts of water were lost as Es from both wheat (168 mm) and dry seeded rice (455 mm) crops. This study suggested that both theta g and evaporative demand influence Es collectively and are not separate influences as has been implied by Ritchie's two-stage evaporation model (Ritchie, 1972). The high significance and high R-2 of the relationship between Es, theta g, Ep and time suggests that theta g can be used to calculate Es after irrigation or rainfall when empirical measurements of Es are unavailable. Further, the strong significance of regression between theta g and Ep suggests that water content and energy potential are important components of Es during both stage 1 and stage 2 evaporation.