Abstract. In recent years, the amount of water used for
agricultural purposes has been rising due to an increase in food demand.
However, anthropogenic water usage, such as for irrigation, is still not or
poorly parameterized in regional- and larger-scale land surface models (LSMs). By contrast, satellite observations are directly affected by, and hence potentially able to detect, irrigation as they sense the entire integrated soil–vegetation system. By integrating satellite observations and fine-scale
modelling it could thus be possible to improve estimation of irrigation
amounts at the desired spatial–temporal scale. In this study we tested the potential information offered by Sentinel-1
backscatter observations to improve irrigation estimates, in the framework
of a data assimilation (DA) system composed of the Noah-MP LSM, equipped
with a sprinkler irrigation scheme, and a backscatter operator represented
by a water cloud model (WCM), as part of the NASA Land Information System
(LIS). The calibrated WCM was used as an observation operator in the DA
system to map model surface soil moisture and leaf area index (LAI) into
backscatter predictions and, conversely, map observation-minus-forecast
backscatter residuals back to updates in soil moisture and LAI through an
ensemble Kalman filter (EnKF). The benefits of Sentinel-1 backscatter observations in two different
polarizations (VV and VH) were tested in two separate DA experiments,
performed over two irrigated sites, the first one located in the Po Valley
(Italy) and the second one located in northern Germany. The results confirm
that VV backscatter has a stronger link with soil moisture than VH
backscatter, whereas VH backscatter observations introduce larger updates in the vegetation state variables. The backscatter DA introduced both
improvements and degradations in soil moisture, evapotranspiration and
irrigation estimates. The spatial and temporal scale had a large impact on
the analysis, with more contradicting results obtained for the evaluation at the fine agriculture scale (i.e. field scale). Above all, this study sheds light on the limitations resulting from a poorly parameterized sprinkler irrigation scheme, which prevents improvements in the irrigation simulation due to DA and points to future developments needed to improve the system.