Forecasting drought using SPEI at the 10m level with ERA5 and Sentinel-2 spectral albedo products as part of ESA-GTIF project for the UK, Ireland and France.

Drought is defined in a variety of different ways. One method is through the use of the SPEI (Standardised Precipitation and Evapotranspiration Index) derived from the HadUK4 by the UK CEH (Centre for Ecology and Hydrology) and made available at 1km for the UK only and globally through a new product, the Global Multi-Index Drought (GMID) at 0.1º . After an exhaustive test of various dependent variables, three variables were chosen for deep learning training, 1km LST from the VIIRS instrument onboard the NASA-NOAA satellites is combined with Precipitation and Soil-evapotranspiration monthly datasets then downscaled to 1km from 0.1º to train a deep learning model to forecast 1 km SPEI. These forecasts can then be compared with the aforementioned 1km (UK only) and 0.1º (UK, Ireland and France) SPEI and GMID datasets respectively. Examples will be shown of the UK , Ireland and France regions. Farmers, NGOs, government scientists and policy makers require drought forecasts at near human scale and as far ahead as possible for water conservation planning. These 1km results need to be downscaled to human level at 10m.

An unique processing system for generating 10m spectral and broadband albedo which is part of the Copernicus Global Land Monitoring Service called S2GM (Sentinel-2 Global Mosaic) has been employed to generate 10m products [1,2]. From these spectral albedos, simple vegetation indices such as NDVI can be derived over a monthly time period and NDVI can be employed to downscale the 1km forecasts up to 10m. This application of a composited product eliminates the problems of cloud cover at mid-latitudes which Sentinel-2 sampling every 5-daily has. Examples from the 2022 and 2025 droughts will be shown for the UK, Ireland and France (UKIF). The monitoring of drought through the water extent of reservoirs using S2GM monthly composite spectral albedos will also be shown as an independent method of drought assessment.

The GTIF-UKIF drought capability results will be shown in the context of crop-type and vegetation productivity at the 10m level using an unique webGIS system developed for all the Green Transition Information Factory (GTIF) capabilities (gtif-uk-ireland-france.net). These results indicate that this drought monitoring and forecasting method may have the potential to be rolled out across the rest of Europe and southwards across Africa to provide forecasts 3-6 months ahead of a drought.

The authors would like to thank ESA for contract no. 4000144118/24/I-NS, Burak Bulut of UK CEH for the SPEI and GMID datasets and Gillian Watson for the NDVI processing and downscaling of the SPEI.

Cited references
[1] Muller J.P., Song R. Brockley D., Whillock M., 2023. Sentinel-2 Global Mosaic HR-Albedo Algorithm Theoretical Basis Document S2GM-UCL-ATBD-v3.1 https://s2gm.land.copernicus.eu/help/documentation

[2] Muller, J-P., Song, R., Griffiths, P., 2025. Bi-facial PV solar power systems for mixed use of arable and grassland, an evaluation over GB and Ireland taking into account environmental exclusion areas. DOI: 10.5194/egusphere- EGU25-18951