In Northeast Thailand lowlands with shallow saline watertable, rainfed paddy fields often present high salt concentration in the dry season, forming patches or spots of salt crusts on the soil surface. In this context, the mechanisms implied in salt concentration during dry season were studied by establishing salt budget with evaporation and drainage estimates inside and outside a saline patch. Drainage was estimated by Hydrus-1D modelling constrained by an hydrodynamic characterization and the profile of water contents at the end of dry season. Evaporation rates at the end of the dry season were computed by interpreting natural detailed profiles of deuterium (D) and chloride (Cl) contents. Because of the drastic diminution of hydraulic conductivity at saturation with depth and the decrease of groundwater level at the end of the cropping season, simulated hydrological balance with Hydrus-1D pointed out zero cumulated fluxes for depths of 39.5 cm (outside the saline patch) and 37.5 cm (inside the saline patch). Therefore, ail the chloride accumulated in the very upper layers during dry season comes from the chloride that was present in the 0-39.5 cm layers before the beginning of the drying. Inside the saline patch, the tentative Cl budget is coherent with the hypothesis of saturation of the profile by aquifer saline water during the flooding. Evaporation rates computed from the diffusion of chloride and deuterium at the end of the drying season, when the aquifer level was 1.4 m deep, range between 0.121 and 0.378 mm d(-1). This does not sustain the assumption of a considerable salinity contribution from the aquifer during the dry season. Moreover, evaporation estimates based on Cl and D diffusion equilibrium showed depleted rates (38-63%) inside the saline patch due to salt accumulation in the first 12 cm of the soil. In the vapour transfer Layer, estimated evaporation rate based on the vapour movement of D was in the same order of magnitude than computed rate assuming liquid Cl diffusion. This coincidence is attributed to the liquid fluxes that occurred during the expansion of the vapour transfer Layer during the progression of the evaporation front.