Characteristics of Surface Soil Moisture Drydown in the Heihe River Basin

Abstract

The process of soil drying is referred to as soil moisture drydown.It usually occurs after precipitation， especially for surface soil， and we address this process in three dominant regimes： gravity drainage， energy-limited regime and water-limited regime.The Heihe River Basin originates from the Qilian Mountains， and the landscape zonality here is clear from its upstream， midstream and downstream.By exploring the characteristics of surface soil moisture drydown in this area， researchers were able to understand the patterns and factors associated with soil evaporation under different conditions.The soil drydown events were investigated using data on soil moisture， soil temperature， and air temperature collected between 2016 and 2020 from 10 meteorological observation stations located in the Heihe River Basin.The relationships between soil moisture （SM）， potential evaporation （PE）， the normalized difference vegetation index （NDVI）， soil texture and surface soil drying rates were explored.Using the exponential model for fitting drydown events， the seasonal variation of the drying rate at each site was compared.Results showed that： （1） There were 362 effective drydown episodes between 2016 and 2020， totaling 3584 days.After entering the drydown stage， as the soil gradually became dry， the drying rate also slowed down and became stable after 12 days.The upstream site settled around 0.04 cm3·cm-3·day-1， whereas the middle and downstream sites tended to 0.（2） In upstream， midstream， and downstream， the values of surface soil moisture evaporation efficiency （90%） are lower than 0.25， 0.15， and 0.12， respectively， which can only partially meet atmospheric demand.The evaporation efficiency may be close to 1 if the evapotranspiration of soil profiles below 5 cm is taken into account.（3） Because the surface soil in upstream locations of the Heihe River is relatively wet， the soil drydown process is limited by the atmosphere.The middle and lower reaches of the Heihe River are drier and limited by water.As for the surface soil evaporation rate， the upstream was mainly affected by atmospheric water demand； it increases along with PE.When SM exceeded 0.4 cm3·cm-3， PE and SM had no significant impact on soil drying rate.In the middle and downstream of the Heihe River， the soil evaporation rate increases with SM.In comparison to upstream， the impact of PE on soil evaporation rate is less significant.（4） The type of vegetation and the dry and wet states of the soil both affect how quickly the surface SM dries； the drying rate is very slightly influenced by soil texture.（5） The drydown events fitted by the exponential model have distinct season features.The highest drying rate （τ）， the highest drying range （A）， and the lowest lower limit of soil moisture （θw） were observed in summer.As for desert region， θw is more affected by precipitation and is higher in the summer.For the entire basin， the drying rate of surface soil moisture from high to low was summer， spring， autumn， and the seasonal variability of drying rate increased with the change in surface coverage （meadow， farmland， shrub）.

Keywords

• soil moisture
• drydown
• soil evaporation
• in situ observation
• heihe river basin