Studies published so far either use a per-capita approach that assigns a value derived from the global threshold, whether it can have consequences on the stability of the Earth System or not, or they calculate a local boundary that ignores the global relevance of the concept. Only Zipper et al. (2020) have developed a framework for the regional application of the freshwater Planetary Boundary. This framework is able to combine both a fair share based on the global boundary and a local safe operating space based on locally relevant control and response variables. They divided the water Planetary Boundary into six sub-boundaries as per Gleeson et al. (2020), which reflect the different functions of water within the Earth System, and represent five different stores of water (atmospheric water, soil moisture, surface water, groundwater and frozen water). Each store of water can either have a boundary only at the global/local level, in which case only the relevant boundary will be used, or it can be relevant at both scales. In this case, if the control variable of the boundary is different for the global and the local scale, two boundaries will result, with two different control variables. If the control variable is the same, the more conservative boundary will be
This cpatures the key challenges to downscaling esp to regional levels, along with a possible solution