Earth observation by the Advanced Microwave Scanning Radiometer 2 (AMSR2) aboard the Global Change Observation Mission 1st-Water (GCOM-W) satellite is a promising method for monitoring the global daily soil moisture.
The brightness temperatures observed at 10 and 36 GHz are used to estimate the soil moisture through
a dielectric model that relates water content and permittivity of soils, for which the Dobson model has been widely used.
In this study, the performance of the Dobson model was compared with those of other models and a novel model was proposed.
The models were validated using the soil samples taken in Japan and Cambodia measured at 25 MHz -- 4 GHz and that of Toyoura Sand and bentonite measured at 1 -- 18 GHz.
Large differences were observed among estimates by Dobson, Topp, Wang-Schmugge, and Mironov models.
The estimates by the Wang-Schumugge and Mironov models were in fair agreement and comparable with the measurements.
However, there was a large difference in the estimated quantity and permittivity of bound water between the two models, and the permittivity for both bound and free soil water estimated using the Mironov model was higher than that estimated by the other models.
In our novel model, the quantity of bound water was defined as the water content at air-drying under a relative humidity of 50 %.
The permittivity of bound water was obtained under a matric potential of -500 MPa.
The matric potential was calculated using a pedotransfer function and the van Genuchten model, and the wet-soil permittivity was obtained using Birchak's mixing model.
The estimated permittivity was fairly accurate for the soil types and frequencies examined in this study.
The proposed model is an effective means of applying soil physics concepts to improve soil moisture estimates by a satellite for the wide range of soil types around the world.
To simulate soil water contents and tempera-tures in a root zone, understanding of root uptake charac-teristics and unsaturated hydraulic conductivity K is nec-essary. In this study, numerical simulations of soil water and heat transport were conducted for a soil drying pro-cess with evapotranspiration in a soybean ﬁeld, using a coupled program of HYDRUS-1D and a canopy energy budget model. Impacts of K, compensatory water uptake, and water uptake distribution β (z) on simulated soil water contents and temperatures were evaluated comparing with ﬁeld observed data. Two β (z) distributions, uniform dis-tribution βuni and observed root density based-distribution βRD, were tested. K, controlling soil surface evaporation rate Eg, indicated a considerable impact on soil tempera-tures. In a case with large surface coverage, simulated tem-peratures agreed well with observed ones when relatively large K was used and Eg under the canopy occurred with the potential evaporation rate. By simulating with com-pensatory water uptake, water uptake rates S increased at well-watered lower part of the zoot zone. Although S dis-tributions were different between βuni and βRD, simulated water contents agreed well with observed ones for both cases when root adaptability factor ωc = 0 was used (fully compensated). It indicated that the sensitivity of β (z) to soil water content and temperature was much smaller than K and compensatory uptake. To simulate soil water con-tent changes for relatively shallow root zones, using a uni-form distribution for β (z) and full compensatory uptake (ωc = 0) can be one simple and useful assumption. And applying soil water and heat transport model to ﬁeld ob-served data to determine average K of the root zone is a promising method for further detailed analysis of soil sur-face evaporation and transpiration.
Methane is an important greenhouse gas. Aerated forest soils are assumed to be an atmospheric methane sink. However, methane emission from small riparian wetlands and/or tree-mediated methane emission have the potential to offset the soil methane sink and may convert the forest from a net sink to a net source. In this paper, I focus on the methane emission from the stem and the ecosystem-scale methane flux in tropical peat swamp forest. These topics are particularly uncertain in forest methane dynamics. The soil water condition was an important controlling factor of methane fluxes in forests in all cases. Finally, how integrated analysis using an international database of ground-based observation data can be used for understanding global methane dynamics is discussed.