Te main limits regarding the regional hydrogeological model (HM) of Latvia (see Fig. 1) are, as follows: - HM will be used for management of drinking groundwater resources of Latvia; - HM is created by the Environment Modelling Centre team of the Riga Technical University (RTU) ; the project is co financed by the European Fund of Regional Development; - the project duration is 24 months; in 2013, HM must be established; - geological and hydrogeological information, needed for establishing HM, is provided by the Latvian Environment, Geology and Meteorology Centre (LEGMC); - principal parameters of HM must be agreed between RTU and LEGMC; - data carried by HM must be publicly available as a part of Latvia environment information system; the system is supported by LEGMC; - during five years (till 2017), RTU and LEGMC cannot use HM commercially. Fig. 1: Location of Latvia HM HM of Latvia will generalize geological and hydrogeological information accumulated by LEGMC. HM will also serve as the base for creating more detailed local HM. It is not possible to incorporate into regional HM all data that can be provided by LEGMC. Reasonable reduction of HM complexity can be achieved by implementing the following presuppositions: - complexity and dimensions of HM must not exceed feasibility of a modern personal computer used to run HM; HM simulates the steady state average regimes of the groundwater flow; the HM area size is 475km300km; the HM volume is approximated by the finite difference method; its plane approximation step is 500 meters; the spatial HM grid contains 25 planes; therefore, the grid consists of 95160125=14.86106 nodes; the HM volume represents the active groundwater zone that is bedded by the regional Narva aquitard; - to ensure compatibility with models and software tools of other countries, the commercial program “Groundwater Vistas” (GV) is used for running of HM; the program is being regularly updated (GV- 6 version is available); it contains software tools MODFLOW, MODPATH, MT3D applied for groundwater modelling worldwide; - at the present, HM consists of its active and passive parts; the active part includes the land territory of Latvia and the Gulf of Riga; the passive part represents border areas of neighboring countries. However, HM is open for trans boundary modelling projects; then a neighboring country provides data for activating the HM area involved; - although, buried valleys may be of considerable importance, they are not accounted for by the current HM version; it is difficult to create them geometrically, a filling material of valleys may be unknown; - in HM, only the Narva aquitard is continuous; the other geological layers are discontinuous, because they include areas with a zero thickness; for the model, these areas have the thickness 0.02 meters, their permeability is 1.0m/day; - three elevation surfaces of HM are especially important: - the hydrogeological relief relh that represents the ground surface where the hydrographical network is incorporated; - the geological relief relg that gives land surface elevations; - the sub Quaternary surface subQ that covers the system of basic geological layers. The difference mw=relh-relg is the thickness of surface water bodies. (in HM, mw >0 for the sea area and for the Daugava river with its three lakes of hydroelectric power stations); for other water bodies (lakes, rivers), mw =0. The difference ∆=relg-subQ is used for obtaining the Quarternary system thickness mQ: mQ=∆ if ∆>1.0; mQ=1.0 if ∆1.0 and relg=subQ+1; by correcting relg, along the river valleys where ∆<0, the subQ surface remains unchanged (no deep valleys are cut into it); otherwise, the grid nodes will be lost where river long lines elevations must be connected (option River of GV): - the relh map serves as the piezometric boundary condition, on the HM top; due to this condition, HM automatically creates a feasible infiltration flow distribution; - no real thicknesses of bogs, of the aeration zone and of the unconfined Quaternary aquifer are used during the HM calibration; the aeration zone of the thickness 0.02 meters acts as a formal aquitard that controls intensity of the infiltration flow; the bogs are located within this formal layer; if necessary, the real thicknesses of the above mentioned layers can be restored; - in HM, real thicknesses are used for layers (bogs, the aeration zone and the quaternary unconfined layer are exceptions); to account for admixtures that exist in the layers, the maps of their permeability are corrected; - for aquifers, along the borderline of the HM active part, piezometric boundary conditions (heads) are applied; an impervious border surface cannot be used, because the cross border groundwater flow is notable everywhere; - as the piezometric boundary condition, on the HM bottom, the Pernava aquifer map of its head distribution is used. Most of the above described measures can be used, if complex hydrogeological models must be created.