Abstract
Abstract. The lack of direct measurement of root-zone soil moisture poses a challenge to the large-scale prediction of ecosystem response to variation in soil water. Microwave remote sensing capability is limited to measuring moisture content in the uppermost few centimetres of soil. The GRACE (Gravity Recovery and Climate Experiment) mission detected the variability in storage within the total water column. However, root-zone soil moisture cannot be separated from GRACE-observed total water storage anomalies without ancillary information on surface water and groundwater changes. In this study, GRACE total water storage anomalies and SMOS near-surface soil moisture observations were jointly assimilated into a hydrological model globally to better estimate the impact of changes in root-zone soil moisture on vegetation vigour. Overall, the accuracy of root-zone soil moisture estimates through the joint assimilation of surface soil moisture and total water storage retrievals showed improved consistency with ground-based soil moisture measurements and satellite-observed greenness when compared to open-loop estimates (i.e. without assimilation). For example, the correlation between modelled and in situ measurements of root-zone moisture increased by 0.1 (from 0.48 to 0.58) and 0.12 (from 0.53 to 0.65) on average for grasslands and croplands, respectively. Improved correlations were found between vegetation greenness and soil water storage on both seasonal variability and anomalies over water-limited regions. Joint assimilation results show a more severe deficit in soil water anomalies in eastern Australia, southern India and eastern Brazil over the period of 2010 to 2016 than the open-loop, consistent with the satellite-observed vegetation greenness anomalies. The assimilation of satellite-observed water content contributes to more accurate knowledge of soil water availability, providing new insights for monitoring hidden water stress and vegetation conditions.
Highlights
Water is a growth-limiting resource that impacts over 40 % of Earth’s vegetated surface (Nemani et al, 2003)
soil moisture (SMOS) soil wetness and W3 top-layer soil wetness from open-loop and data assimilation were compared with the in situ near-surface soil wetness observations from International Soil Moisture Network (ISMN) (Fig. 3)
The joint assimilation of both SMOS and GRACE observations (Fig. 3b) shows improved correlation with in situ measurements compared with the model open-loop over the majority of the sites where SMOS observations better correlated with in situ measurements
Summary
Water is a growth-limiting resource that impacts over 40 % of Earth’s vegetated surface (Nemani et al, 2003). Vegetation productivity and water stress are strongly coupled by the interactions between soil moisture, photosynthesis, transpiration, interception, and hydraulic redistribution (Porporato et al, 2004). The amount of water available to support plant growth and buffer against rainfall deficiencies largely determines the length of the growing period (Leenaars et al, 2018). Some vegetation species have roots that can grow to tens of metres in depth (Canadell et al, 1996), most plants have roots that are contained in the upper 2 m of the soil column and cannot access the deeper water stores (Tokumoto et al, 2014).
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