Resolving the Daily Water Cycle over Land with Radar Satellites

Short Description

Starting point / motivation

Observing and understanding processes of the daily water cycle (DWC) over land with a high spatial resolution is one of the outstanding grand challenges in Earth Observation (EO). This challenge has been taken up by the G-Glass mission, which is a geosynchronous C-band radar satellite that was recently selected by ESA as one of the candidate missions for its 10th Earth Explorer (EE10).

Among other variables, G-Class will observe soil moisture (SM) and rainfall (RF), which are key to understand the DWC. Accurate SM and RF estimates are of paramount importance as they play a key-role in many fields as, to cite a few, natural hazard assessment (floods and landslides), drought management, weather forecasting, agriculture, and disease prevention.

Both variables can be retrieved from spaceborne radar observations in a sequential manner: 

  • Firstly, SM can be estimated by inverting a backscatter model that describes the interaction of the microwave pulses with the vegetation and soil surface. 
  • Secondly, RF can be derived from the soil moisture data by inverting a soil water balance model.

Research has already demonstrated the feasibility of this approach, and a number of operational services are already running (for SM) or are about to be introduced soon (for RF). However, existing services do not properly resolve the DWC as they rest on polar-orbiting radar satellites that capture SM at best twice a day at their ascending and descending passes respectively.

Even though daily RF can be estimated from such data, estimates remain rather noisy and the model needs auxiliary data for calibration. To more densely sample SM across one day, and to improve model calibration and reduce the noise in the daily RF estimates it is essential to collect several backscatter measurements per day.

This may either be achieved by a constellation of polar-orbiting satellites or much better still, as proposed by G-Class, by a geosynchronous satellite that observes the same land surface areas every few hours (1-3 hours).

Contents and goals

The goal of the DWC-Radar project is to exploit the first-time availability of five contemporary C-band radar instruments in space (three Metop ASCAT instruments, two Sentinel-1 Synthetic Aperture Radar (SAR) sensors) to retrieve SM and RF estimates at 1km resolution to support Phase 0 activities for G-Class, demonstrating scientific algorithms and data products, and highlighting known strengths and weaknesses of this ground-breaking technology.

Furthermore, the project will demonstrate the high practical utility of this technology by using the developed sub-daily SM and improved RF data as input for three applications targeted by G-Class, namely irrigation water use mapping, flood forecasting and landslide risk assessment.


The data records and three use cases will be tested and validated over the larger Mediterranean region, which is particularly vulnerable to climate change, and the main target area of G-Class. The data records of sub-daily backscatter, SM, and RF will be made publicly available through a visualization tool at the Earth Observation Data Centre for Water Resources Monitoring.

Expected results

The results of DWC-Radar will directly feed into the ESA EE10 Phase 0 activities closely interacting with the G-Class Science Team, addressing science requirements for G-Class and providing algorithms for sub-daily SM and RF documented in two separate Algorithm Theoretical Baseline Documents.

Project Partners


Vienna University of Technology - Department of Geodesy and Geoinformation

Project partner

  • Earth Observation Data Centre for Water Resources Monitoring GmbH
  • Research Institute for Geo-Hydrological Protection (CNR)

Contact Address

Vienna University of Technology
Department of Geodesy and Geoinformation
Univ.Prof. Wolfgang Wagner
Wiedner Hauptstraße 8-10/E120.1
A-1040 Vienna