Pierre Laluet, Luis Enrique Olivera-Guerra, Víctor Altés, Giovanni Paolini, Nadia Ouaadi, Vincent Rivalland, Lionel Jarlan, Josep Maria Villar, Olivier Merlin

Irrigation is the most water consuming activity in the world. Knowing the timing and amount of irrigation that is actually applied is therefore fundamental for water managers. However, this information is rarely available at all scales and is subject to
large uncertainties due to the great diversity of existing agricultural practices and associated irrigation regimes (full irrigation, deficit irrigation, or over irrigation). To fill this gap, we propose a two-step approach based on 15 m resolution Sentinel-1 (S1)
surface soil moisture (SSM) data to retrieve the actual irrigation at the weekly scale over an entire irrigation district. As a first step, the S1-derived SSM is assimilated into a FAO-56-based crop water balance model (SAMIR) to retrieve for each crop type both the irrigation amount (Idose) and the soil moisture threshold (SMthreshold) at which irrigation is triggered. For this, a particle filter method is implemented, with particles reset each month to provide time varying SMthreshold and Idose. As a second step, the retrieved SMthreshold and Idose values are used as input to SAMIR for estimating the weekly irrigation and its uncertainty. The assimilation approach (SSM-ASSIM) is tested over the 8000 hectare Algerri-Balaguer irrigation district located in north-eastern Spain, where in situ irrigation data integrating the whole district are available at the weekly scale during 2019. For assessment, the performance of SSM-ASSIM is compared against that of the default FAO-56 irrigation module (called FAO56-DEF), which sets SMthreshold to the critical soil moisture value and systematically fills the soil reservoir for each irrigation event. During 2019, with a yearly observed irrigation of 687 mm, SSM-ASSIM (FAO56-DEF) shows a Root Mean Square Difference between retrieved and in situ irrigation of 6.7 (17) mm week-1, and a Pearson correlation coefficient of 0.88 (0.47). The SSM-ASSIM approach shows great potential for retrieving the weekly water use over extended areas for any irrigation regime, including over irrigation.