Evaluation of Evaporation Related Diurnal Change from Dielectrically Measured Soil Moisture

Abstract

Abstract-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.

Assessment of the Temperature Effects in SMAP Satellite Soil Moisture Products in Oklahoma

“…Moreover, the significance values of the correlation between the satellite and in situ data are very small, and they have improved after removing the temperature effects. According to Lu et al [30] and Kapilaratne et al [29,37], the corrected SWC value is much closer to the actual SWC and can be viewed as the ground truth. Therefore, it can be said that the removal made the relationship between satellite soil moisture and in situ soil moisture better.…”

“…Their experiments indicate that the ε b sensor reading changes with temperature while the actual value of soil water content is kept unchanged. Schanz et al [33] have verified these effects on soil water content with time domain reflectometry (TDR) measurement with experiments at different temperatures in a range between 20 • C and 80 • C. Kapilaratne and Lu [29] confirmed that fluctuations in soil water content, synchronized with those of soil temperature, exist and are mainly caused by temperature effects. From their study, though the evaporation also causes diurnal variability, it differs from the soil moisture fluctuation in time series.…”

“…While in situ data have been used to calibrate the algorithm parameters and validate the satellite soil moisture data in many studies [23,[25][26][27], several lines of evidence indicate that these datasets, especially dielectrically measured ones, contain errors relating to temperature fluctuation, known as temperature effects [28][29][30][31][32]. They are widely observed in dielectrically measured volumetric soil water content (hereafter referred to as "SWC").…”

Temperature Effects in AMSR2 Soil Moisture Products and Development of a Removal Method Using Data at Ascending and Descending Overpasses

“…The ascending SWC data in the daytime are more significant affected by the surface temperature than the descending nighttime ones. Similar to dielectrically measured in situ SWC and SMAP SWC products, this phenomenon can be considered as an apparent diurnal change caused by temperature effects because it cannot be reasonably explained by other physical factors, such as evapotranspiration [47,48].…”

“…These diurnal changes differ from the ones caused by evaporation [47] because evaporation usually reduces the daytime SWC. Kapilaratne and Lu [48] also showed an opposite diurnal change in SWC to the potential evaporation. This phenomenon implies that TEs exist in both the in situ SWC and AMSR2 SWC and are more dominant than changes caused by evaporation.…”