Evaluation of Evaporation Related Diurnal Change from Dielectrically Measured Soil Moisture

Separation of physical and apparent diurnal fluctuation from dielectrically measured soil water content (SWC) is critical for accurate estimation of temperature related inaccuracies. Evaporation process at soil surface was simulated under synthetic and semi synthetic conditions in HYDRUS-1D environment. At synthetic conditions daily evaporation rate was assumed as 12 mm while evaporation rates at semi synthetic conditions were estimated using meteorological data. This study used 12 soil moisture monitoring stations which represent various climatic and soil conditions to characterize high and low evaporation levels. Simulated SWC at semi synthetic conditions were agreed well with the observed SWC with hourly NASH value ranging from 0.45-0.6. As a consequence of less evaporation related diurnal fluctuations, amplitude of simulated SWC always lesser than that of the observed SWC after eliminating the rainy days and freezing period. For Mongolian sites 6-16% and 3-9% for Unites States’ sites consists of evaporation related amplitude variations of dielectrically measured SWC. Synthetic experiment results indicated that when ground water table getting deeper than 1m evaporation related diurnal fluctuations are disappearing from the near surface soil moisture.