Drought and water stress monitoring through satellite observation
The early detection of drought and water stress situations constitutes an operational requirement for public administrations, managers of critical infrastructure and territorial planning bodies. Earth observation from satellite platforms makes it possible to derive, from multispectral and hyperspectral imagery, vegetation indices that reflect the physiological state of the vegetative cover. Among those most widely used in operational contexts are those that relate near-infrared reflectivity to that of the visible red band, which are sensitive to photosynthetic activity and, by extension, to water availability in the soil-plant system. When these indices are analysed in time series spanning weeks or months, it is possible to identify anomalies with respect to historical reference values and to delineate zones with accumulated water deficit. In parallel, the estimation of soil moisture from active and passive microwave data provides a complementary variable of high diagnostic value. Unlike vegetation indices, which respond to integrated effects on the plant, surface soil moisture offers information on the state of the water resource in the first centimetres of the soil profile, with different response latencies. The combination of both sources, together with land surface temperature data obtained in the thermal infrared, makes it possible to construct water stress indicators with greater robustness than any of them in isolation. Nevertheless, the operational effectiveness of these systems is conditioned by two structural limitations that must be taken into account in the design of any monitoring service. The first is persistent cloud cover, which prevents the acquisition of optical and thermal imagery over prolonged periods, a factor particularly relevant in certain Atlantic regions or during episodes of moist air mass advection. The second is the satellite revisit frequency, which determines the temporal resolution of the final product. Both constraints can be mitigated through the fusion of data from constellations with different orbital geometries and complementary sensors, although this integration introduces complexity in the processing and validation of the resulting products. Complementarity with ground station networks remains an indispensable element in operational drought monitoring systems. Pluviometric, piezometric and actual evapotranspiration data make it possible to calibrate and validate satellite products, correct systematic biases and provide physical context for the detected anomalies. For operators of water supply, irrigation or hydroelectric generation infrastructure, the integration of both sources into geospatial analysis platforms improves the capacity to anticipate water deficit situations and supports decision-making in environments governed by European regulatory frameworks for water management and climate change adaptation.
NASSAT - Network Satellite Systems