Modeling irrigation and land surface dynamics: comparing AquaCrop and Noah-MP over the Po Valley

Abstract. In this study, irrigation was estimated over the Po Valley (Italy) at a 1-km² spatial resolution using (i) a crop model, AquaCrop, and (ii) a land surface model, Noah-MP. Both models were run with sprinkler irrigation using a similar setup within NASA's Land Information System. Irrigation estimates were evaluated at the pixel and basin scale, using in situ and satellite-based reference data. In addition, surface soil moisture (SSM), vegetation, and evapotranspiration (ET) estimates were compared with satellite retrievals.

Noah-MP shows on average higher annual irrigation rates (434 mm yr^-1) compared to AquaCrop (268 mm yr^-1), mainly because more irrigation water losses (not consumed by transpiration) are simulated and compensated for in Noah-MP (runoff, interception, and soil evaporative losses), whereas AquaCrop only accounts for soil evaporative losses. When taking into account representative application water losses for AquaCrop, and conveyance water losses for both models, the irrigation estimates fall within reported ranges of 500–600 mm yr^-1. For the field-based evaluation, Noah-MP presents large irrigation events (> 100 mm per event) and less interannual variability compared to AquaCrop. Two-week averaged SSM estimates from both models agree well with downscaled estimates from the Soil Moisture Active Passive (SMAP) mission, with spatially averaged unbiased root mean square differences of 0.05 and 0.04 m³ m^-3 for AquaCrop and Noah-MP, respectively. Both models show limitations in terms of vegetation and ET modeling, mainly due to the generalization of vegetation parameters for AquaCrop and a possible sub-optimal vegetation parametrization for Noah-MP. The results of this study highlight the differences between the models which have been created for distinct original purposes and scales, as well as the complexity of irrigation modeling due to its anthropogenic nature, emphasizing the need for observations.