Application of a satellite-based soil moisture retrieval technique to irrigated areas in Piemonte

Water is an indispensable resource for human activities and agriculture. Its scarcity has severe consequences, including impacts on hygiene, sanitation, nutrition, and food security. Access to clean drinking water is a basic human right, yet it remains a challenge in many parts of the world. This calls for a paradigm shift in our approach to water management, which includes adopting a proactive approach that goes beyond our present needs and aims to safeguard the resource for the well-being of the environment and future generations. As numerous studies have already highlighted, all solutions towards better management begin with the same step: achieving a good quantitative knowledge, primarily through significant technological improvement. This is fundamental as it would enable an accurate assessment of the current situation in addition to tracking its evolution and evaluating any proposed solutions in the future. Given the complexity of the water cycle and the large scale over which it occurs, in-situ point measurements have been shown to be quite insufficient in addressing this issue. In contrast, the large scales provided by radars and satellites has led researchers to regard them as sources of great potential. Given the fact that irrigation practices are estimated to be responsible for more than 70% of global water withdrawals, using these large-scale studies in order to estimate irrigation quantities, in particular, has proven to be of critical importance. In Europe, the large majority of such studies rely on the Sentinel-1 mission provided by the Copernicus Program thanks to its open access. The loss of one of the two twin radars, Sentinel-1B, at the end of 2021, doubled the mission’s revisit period and, thus, greatly impacted studies relying on this data. Unfortunately, all the following efforts to make up for this loss, namely in the attempts to launch Sentinel 1-C and Sentinel 1-D, have faced either delays or failures. In this context, the first part of this study includes an investigative analysis of radar missions that could potentially be used, as separate alternatives or complementary sources to Sentinel-1, for radar data. In addition to this, this study attempts to build on the latest research on the topic of obtaining irrigation estimates based on the inversion of soil moisture data obtained from radar data by relying on adapted versions of two main algorithms: SM2Rain and RT1. SM2Rain is an algorithm originally developed for the retrieval of rainfall estimates from soil moisture data. RT1, on the other hand, is a new radiative-transfer modeling framework for the retrieval of soil moisture estimates, which originally relied on scatterometer data. Therefore, this study also presents a regional-scale, high-resolution (∼1 km) irrigation data set obtained using the above-mentioned methods. The study area consists of 73km2 of agricultural areas in Piedmont, Italy, falling within the Canavese area, between Chivasso and Caluso. The time period considered spans over the years of 2020 to 2022. Radar data was obtained from the Sentinel-1 mission through the Copernicus website. Meteorological data was obtained from ARPA Piemonte, specifically from the Caluso station, which is centered within our area of interest. The analysis also made use of other sources, such as ERA5-Land hourly data from 1950 to present for Leaf Area Index (LAI) and benchmark soil moisture data, in addition to irrigation calendar data specific to the considered agricultural areas.