## 4 Discussion  
This study represents a systematical analysis of the influence of spatial resolution of the selected sensor on the mapped irrigated area. The study confirms the findings of [PERSON] et al. [24], in that the mapped irrigated area generally decreases when moving to a coarser spatial resolution. The magnitude of change in the irrigated area with spatial resolution shows a strong linear relation with the LSI in all three regions and seems to be regionally independent. However, many factors influence the scaling relation, with the characteristics of the regional farming system being the most obvious. These characteristic farming systems result in the spatial formation of the irrigated fields and were affected differently according to their shape and their spatial arrangement in the coarser spatial resolution. While the mapping result in China stayed constant, the analyzed regions in Sudan and USA showed large discrepancies in the mapped irrigated area at different spatial resolutions. This implies a high complexity of the irrigation patterns which affect the spectral upscaling to a coarser resolution, while the irrigated area in China is ordered mainly in irrigation agglomerations with a low complexity in shape. That means the determination of irrigated areas in regions of small and scattered fields is more affected when moving to coarser resolutions than in regions of larger, connected fields in areas which are completely used for agriculture. As soon as single fields are embedded in a non-irrigated surrounding of fallow fields or barren land, the identified irrigated area is highly sensitive to a decrease in resolutions. The upscaling of the spectral information smooths the NDVI signal and influences the mapping method. This leads to a significant change of the average NDVI in case of a high NDVI variation at the underlying resolution. High NDVI variations are caused by different growth phase or by a mix of different land uses at one 300 m-resolution pixel. This effect is shown in the example of Sudan, where the landscape is characterized by a mix of small fields, meadows and settlements interrupted by streets and fallow land. At coarser  
Figure 7: Loss of the mapped irrigated area from 10 m to 300 m spatial resolution as a function of the Landscape Shape Index (LSI) in the three regions: Sudan, USA and China. resolutions (>100 m), this leads to high variations in the NDVI within one coarse resolution pixel and, thereby, influences the mapping result (Figures 3 and 5). The largest absolute changes in identified irrigated area with decreasing spatial resolutions were found in the USA. The differences may have several reasons: The annual precipitation in the region is 329 mm/a, which is the highest of the three compared regions. For rain-fed agricultural systems, this precipitation amount is very low, causing supplemental irrigation systems to be widely used in this area. Precipitation events might occur very locally and in summer as heavy thunderstorms, which have comparable effects to technical irrigation. The high-resolution images show small water ponds and water channels used for storage and transportation of water from wells or water bodies to the irrigated fields. The greening effect around the water storage and transportation bodies are part of the high-resolution images but are too small to be resolved in the coarser resolution images. Decreasing the resolution, therefore, affects the recognition of the areas around the water bodies and results in a smaller irrigated area. The most decisive reason is the structure and the complexity of shape of the agricultural fields in this area. The irrigated fields in the north of the scene are distributed spatially, separated by barren land, pastures or unmanaged land. This leads to low NDVI values at the coarser resolutions, which reduce the identified irrigated area. However, the NDVI is limited regarding the fast saturation in case of active vegetation and does not provide details about biomass or LAI [41].