土壌水分変動とその大気へのインパクト : スリランカにおける事例

Time series analyses of soil moisture, rainfall, air temperature and other hydrometeorological components observed at inland area of Sri Lanka in 1994 were carried out. Auto-correlation analysis gives a time scale from 10 to 30 days as persistency of soil moisture anomaly, which is considerably small relative to that under midlatitude temperate climate and is shorter than that for atmospheric temperature. Two dominant periodicities are detected from spectral analysis : 45-60-day period (especially for large rainfall and soil moisture at all depths) and 22.5-30-day period (for small rainfall and shallow soil moisture). The two intraseasonal variations are also detected for satellite-remote-sensed outgoing longwave radiation (OLR) over and around Sri Lanka. Band-pass filtered OLR anomaly for the former periodicity propagates from around Sri Lanka to the west and is strengthened through the process of its westward propagation. On the other hand, OLR anomaly for the latter periodic component has its origin at east ocean-region and propagates westward to Sri Lanka. For the 45-60-day period, band-pass filtered anomaly propagates from rainfall to soil moisture, soil temperature, wet-bulb temperature and back to rainfall, indicating a feedback loop. In contrast, propagation of anomaly through hydrometeorological variables is not found for the 22.5-30-day period. These statistical results indicate that external forcing causes the latter periodic and also that the former periodicity is introduced by an enhancement of the latter variation once in its two cycles due to negative feedback between soil moisture and rainfall. The selective enhancement against different periodic variations appears to be associated with characteristic time-scale of the soil moisture persistence

A data-driven method to remove temperature effects in TDR-measured soil water content at a Mongolian site

As a convenient, easy-to-use tool, time-domain reflectometry (TDR) is becoming extensively used to measure soil water content. Used not only in hydrological applications, the measurements are also used as ground truth for satellite remote sensing of soil moisture. However, TDR measurements usually include diurnal fluctuation caused by diurnal change of temperature. Though this is an old problem, there is not a general solution. The purpose of this study is to develop an algorithm to remove temperature effects of TDR measurements by analyzing its relationship with meteorological variables. From data observed at a Mongolian site, it is found that impact of soil temperature on soil water content is nearly proportional to soil temperature itself and soil water content. An algorithm is developed and applied to the Mongolian data set. The temperature effects can be effectively removed under dry and wet conditions