Irrigation Impacts on Scaling Properties of Soil Moisture and the Calibration of a Multifractal Downscaling Model

Irrigation is an anthropogenic factor that can introduce significant spatial heterogeneity in the distributions of soil moisture (θ) and potentially affect the performance of downscaling models of coarse satellite products. In this paper, we propose a framework to: 1) quantitatively analyze and compare the scale invariance and multifractal properties of θ in the presence of irrigation; and 2) filter out the effect of irrigated croplands in the application of a multifractal downscaling algorithm based on the hypothesis of spatial homogeneity. For this aim, we use the θ data sets of the National Airborne Field Experiments 2005 (NAFE05) and 2006 (NAFE06) campaigns in Australia. Results show that irrigation affects the scale-invariance properties (tested from 32 to 1 km) in a large high-density agricultural district in the semi-arid NAFE06 site, although it does not have a significant impact on the sparser agricultural districts of the temperate NAFE05 region. The multifractal downscaling model was calibrated using a strategy that attenuates the effect of irrigation on θ fields, thus mimicking natural settings. Performances, tested against aircraft and, for the first time, ground-based observations, are adequate in most cases. Some deficiencies are found for drier conditions in regions with a higher percentage of irrigated fields, suggesting the need to further refine the techniques for detecting irrigated croplands. Overall, the findings of this work reveal that the impact of irrigation on the soil moisture statistical variability and downscaling is larger in drier regions or conditions, where irrigation creates a drastic contrast with the surrounding areas.