Variations of soil moisture over Nagqu at different time scales based on network observations

Based on the soil observations under the support of the Third Tibetan Plateau project, the seasonal and diurnal variations of soil moisture measured at 2, 5, 10, 20 and 30 cm depths at 28 stations in Nagqu are analyzed. The results show that in the winter and spring, the soil moisture decreased from 20 cm depth upward; in the summer and autumn, the soil moisture decreased with the increase of depth. Two peaks of soil moisture occurred in early-to-mid-July and from early September to early October, respectively, followed by a decreasing period. Soil temperature and soil moisture changes were closely related to each other. Freezing-thawing process occurred within a certain range of soil temperature, causing changes in soil moisture. Soil temperature was affected by soil surface evaporation. The discreteness of soil moisture observed at different stations was larger in the summer and autumn than in the spring and winter. The diurnal variation of soil moisture above 10 cm depth in the spring was obvious, and the minimum and maximum values appeared at 08:00 to 10:00 and 19:00 to 20:00 BT, respectively. The diurnal variation of soil moisture was relatively weak in the summer and became obvious at 2 cm depth in the autumn. The linear fit results show that soil moisture and soil temperature were positively correlated in January, April and October, and negatively correlated in July. The soil moisture discreteness among the sites indicates that observations from only a few observational sites cannot reflect the comprehensive status of land surface in the Tibetan Plateau. The current study provides a basis for the validation of satellite observations and numerical model parameterizations of soil moisture in the Tibetan Plateau.