How much water is used for irrigation? A new approach exploiting coarse resolution satellite soil moisture products

Knowledge of irrigation is essential for ensuring food and water security, and to cope with the scarcity of water resources, which is expected to exacerbate under the pressure of climate change and population increase. Even though irrigation is likely the most important direct human intervention in the hydrological cycle, we have only partial knowledge on the areas of our planet in which irrigation takes place, and almost no information on the amount of water that is applied for irrigation. In this study, we developed a new approach exploiting satellite soil moisture observations for quantifying the amount of water applied for irrigation. Through the inversion of the soil water balance equation, and by using satellite soil moisture products as input, the amount of water entering into the soil, and hence irrigation, is determined. Through synthetic experiments, we first assessed the impact of soil moisture measurement uncertainty and temporal resolution, also as a function of climate, on the accuracy of the method. Second, we applied the proposed approach to currently available coarse resolution satellite soil moisture products retrieved from the Soil Moisture Active and Passive mission (SMAP), the Soil Moisture and Ocean Salinity (SMOS) mission, the Advanced SCATterometer (ASCAT), and the Advanced Microwave Scanning Radiometer 2 (AMSR-2). Nine pilot sites in Europe, USA, Australia and Africa were used as case study to test the method in a real-world application. The synthetic experiment showed that the method is able to quantify irrigation, with satisfactory performance from satellite data with retrieval errors lower than similar to 0.04 m(3)/m(3) and revisit times shorter than 3 days. In the case studies based on real satellite data, qualitative assessments (due to missing in situ irrigation observations) showed that over regions in which satellite soil moisture products perform well, and which are characterized by prolonged periods without rainfall, the method shows good results in quantifying irrigation. However, at sites in which rainfall is sustained throughout the year, the proposed method fails in obtaining reliable performances. Similarly, low performances are obtained in areas where satellite products uncertainties are too large, or their spatial resolution is too coarse with respect to the size of the irrigated fields.