Calculation of the latter will require use of a hydrological model and will therefore need to be done outside the CGE model. Given the differing characteristics of water supplied from surface and ground sources, and in the absence of a more elaborate model of conjunctive use, it is appealing to treat these two sources of water as imperfect substitutes in the irrigation production function, as proposed by Sue-Wing and Lanzi (2014). The common interest shared by this community is the quantity (and sometimes quality) of water resource.