Value of microwave soil moisture and thermal-infrared evapotranspiration retrievals for the mapping of irrigation coverage

To better monitor global water resources, and understand how they will vary in the future, it is critical to track the extent and intensity of irrigated agriculture. Within the past decade, there has been increased interest in the satellite-based detection of anomalous soil moisture (SM) and/or evapotranspiration (ET) signals associated with irrigation. However, little comparative information is available concerning the relative merits of available ET versus SM satellite products for this purpose. Such uncertainty has hampered the development of optimal monitoring strategies that appropriately integrate information acquired across a range of remote sensing resources. Here, using relatively more mature irrigation products derived from a combination of ground data and visible/near-infrared remote sensing as a reference, we compare the skill of microwave (MW) SM and thermal-infrared (TIR) ET satellite products for mapping coarse-scale (36-km) spatial variations in the proportion of land irrigated across the conterminous United States. Results suggest that, while spatial mapping skill exists in both products, the irrigation signal in satellite-based ET products is stronger, and easier to interpret, than the analogous signal in SM products. Since MW-based SM and TIR-based ET products appear to possess approximately equal spatial precision (i.e., spatial correlations with respect to true SM and ET, respectively), this difference is attributed to the stronger impact of irrigation on spatial ET patterns versus surface SM.

Plain Language Summary: To better monitor global water resources, and understand how they will vary in the future, it is important to track the extent and intensity of irrigated agriculture. To do this, different satellite-based methods have been proposed to track irrigation from space. Unfortunately, there has been little comparison of these methods, and it is unclear which ones work best. Here, we compare methods for monitoring irrigation based on two different remote sensing techniques: thermal infrared and passive microwave. While information derived from both types of remote sensing is useful for irrigation monitoring, results show that thermal-infrared remote sensing contains more information. This insight will help improve future efforts to globally monitor irrigation using satellite-based sensors.