New evidence for the links between the local water cycle and the underground wet sand layer of a mega-dune in the Badain Jaran Desert, China

Scientists and the local government have great concerns about the climate change and water resources in the Badain Jaran Desert of western China. A field study for the local water cycle of a lake-desert system was conducted near the Noertu Lake in the Badain Jaran Desert from 21 June to 26 August 2008. An underground wet sand layer was observed at a depth of 20–50 cm through analysis of datasets collected during the field experiment. Measurements unveiled that the near surface air humidity increased in the nighttime. The sensible and latent heat fluxes were equivalent at a site about 50 m away from the Noertu Lake during the daytime, with mean values of 134.4 and 105.9 W/m2 respectively. The sensible heat flux was dominant at a site about 500 m away from the Noertu Lake, with a mean of 187.7 W/m2, and a mean latent heat flux of only 26.7 W/m2. There were no apparent differences for the land surface energy budget at the two sites during the night time. The latent heat flux was always negative with a mean value of –12.7 W/m2, and the sensible heat flux was either positive or negative with a mean value of 5.10 W/m2. A portion of the local precipitation was evaporated into the air and the top-layer of sand dried quickly after every rainfall event, while another portion seeped deep and was trapped by the underground wet sand layer, and supplied water for surface psammophyte growth. With an increase of air humidity and the occurrence of negative latent heat flux or water vapor condensation around the Noertu Lake during the nighttime, we postulated that the vapor was transported and condensed at the lakeward sand surface, and provided supplemental underground sand pore water. There were links between the local water cycle, underground wet sand layer, psammophyte growth and landscape evolution of the mega-dunes surrounding the lakes in the Badain Jaran Desert of western China.

Experimental study of the effect of shallow groundwater table on soil thermal properties

In plains areas with semi-arid climates, shallow groundwater is one of the important factors affecting soil thermal properties. In this study, soil temperature and water content were measured when groundwater tables reached 10 cm, 30 cm, and 60 cm depths （Experiment I, II, and III） by using sensors embedded at depths of 5 cm, 10 cm, 20 cm, and 30 cm for 5 days. Soil thermal properties were analyzed based on the experimental data using the simplified de Vries model. Results show that soil water content and temperature have fluctuations that coincide with the 24 h diurnal cycle, and the amplitude of these fluctuations decreased with the increase in groundwater table depth. The amplitude of soil water content at 5 cm depth decreased from 0.025 m^3·m^-3 in Experiment II to 0.01 m^3·m^-3 in Experiment III. Moreover, it should be noted that the soil temperature in Experiment III gradually went up with the lowest value increasing from 26.0℃ to 28.8℃. By contrast, the trends were not evident in Experiments I and II. Results indicate that shallow groundwater has a ＂cooling＂ effect on soil in the capillary zone. In addition, calculated values of thermal conductivity and heat capacity declined with the increasing depth of the groundwater table, which is consistent with experimental results. The thermal conductivity was stable at a value of 2.3 W.cm^-1·K^-1 in Experiment I. The average values of thermal conductivity at different soil depths in Experiment II were 1.82 W.cm^-1·K^-1, 2.15 W.cm^-1·K^-1, and 2.21 W. cm^-1·K^-1, which were always higher than that in Experiment III.

The Simulation of L-Band Microwave Emission of Frozen Soil during the Thawing Period with the Community Microwave Emission Model(CMEM)

One-third of the Earth’s land surface experiences seasonal freezing and thawing.Freezing-thawing transitions strongly impact land-atmosphere interactions and,thus,also the lower atmosphere above such areas.Observations of two L-band satellites,the Soil Moisture Active Passive(SMAP)and Soil Moisture and Ocean Salinity(SMOS)missions,provide flags that characterize surfaces as either frozen or not frozen.However,both state transitions—freezing and thawing(FT)—are continuous and complex processes in space and time.Especially in the L-band,which has penetration depths of up to tens of centimeters,the brightness temperature(T_(B))may be generated by a vertically-mixed profile of different FT states,which cannot be described by the current version of the Community Microwave Emission Model(CMEM).To model such complex state transitions,we extended CMEM in Fresnel mode with an FT component by allowing for(1)a varying fraction of an open water surface on top of the soil,and(2)by implementing a temporal FT phase transition delay based on the difference between the soil surface temperature and the soil temperature at 2.5 cm depth.The extended CMEM(CMEM-FT)can capture the T_(B)progression from a completely frozen to a thawed state of the contributing layer as observed by the L-band microwave radiometer ELBARA-III installed at the Maqu station at the northeastern margin of the Tibetan Plateau.The extended model improves the correlation between the observations and CMEM simulations from 0.53/0.45 to 0.85/0.85 and its root-mean-square-error from 32/25 K to 20/15 K for H/V-polarization during thawing conditions.Yet,CMEM-FT does still not simulate the freezing transition sufficiently.

An Air-to-Soil Transition Model for Discrete Scattering-Emission Modelling at L-Band

Topsoil structures and inhomogeneous distribution of moisture in the soil volume will induce dielectric discontinuities from air to bulk soil,which in turn may induce multiple and volume scattering and affect the microwave surface emission.In situ ELBARA-Ⅲ L-band radiometer observations of brightness temperature T_(B)^(p) (p=H or V polarization)at the Maqu site on the Eastern Tibetan Plateau are exploited to understand the effect of surface roughness on coherent and incoherent emission processes.Assisted with in situ soil moisture(SM)and temperature profile measurements,this study develops an air-to-soil transition(ATS)model that incorporates the dielectric roughness(i.e.,resulted from fine-scale topsoil structures and the soil volume)characterized by SM and geometric roughness effects,and demonstrates the necessity of the ATS model for modelling L-band T_(B)^(p).The Wilheit(1978)coherent and Lv et al.(2014)incoherent models are compared for determining the dielectric constant of bulk soil in the ATS zone and for calculating soil effective temperature T_(eff).The Tor Vergata discrete scattering model(TVG)integrated with the advanced integral equation model(AIEM)is used as the baseline model configuration for simulating L-band T_(B)^(p).Whereafter,the ATS model is integrated with the foregoing model for assessing its performance.Results show the ATS-based models reduce the underestimation of T_(B)^(p)(≈20-50 K)by the baseline simulations.Being dynamic in nature,the proposed dielectric roughness parameterization in the ATS model significantly improves the ability in interpreting T_(B)^(p) dynamics,which is important for improving SM retrieval at the global scale.