The study area is a sub catchment of Abay dra inage basin with a total surface arearnand perimeter of 838km2 and 171 km respectively. Two other regional drainage basinsrnborder the catchment: the Awash drainage basin to the east and southeast, and thernGhibe- Orno drainage basin to the south.rnThe main rock outcrops in the area are the basaJlic lava now of Tertiary andrnQuaternary age, trachytic lava flow and pyroclastic deposits of Quaternary age, and thernMesozoic sandstone. Application of di ITerent modeling techniques shows that thernsandstone is part of the Upper sandstone sequence.rnOn annual basis, the area has 1143.3mm, 809mm and 162.8mm of mean totalrnrainfall , Actu al Evapotranspiration (AET) , and grou ndwater recharge respectively.rnThere exist various water bodies in the area: two crater lakes (Lake Dandi and LakernWanchi with a total surface area of 7.4 and 4km2 respect ively), river, streams, andrnspri ngs (both thermal and cold springs).rnHydrogeological descriptions of the lithological units in the area is mainly based onrnqualitative interpretation employing various approaches such as degree of weath eringrnand fracturing, existence/ or absence of springs, vegetation cover, soil cover, e tc.rnDespite the presence of many wells there is only a limited well data. Howeve r, there arernsome wells with well data from which some hydraulic parameters have beenrndetermined. The existing well data as well as the qualitative interpretation s how thatrnthe main aquifer in the area are the fractu red and / or weathered basaltic lava flowrnVIIrnfollowed by alluvial sediments along some streams, and s81ldstone. The first two arernaquifers for cold groundwa ter whereas the sandstone is mainly 811 aquifer of thernthermal/ or mineral water. Some hydraulic parameters have been determined for thesernaqu ifers; accordingly, the basaltic lava now has a hydrau lic conductivity andrntransmissivity of 3.7 x 10 sm/s to 3.9 x 10 sm/s and 97Â· 100 m2/day respectively. Thernal luvial sediments have a transmissivity of 23.9m2/day on average. Where as, thernsand stone has a t ransmissivity of 76.5m2 / day.rnThe existence of highly permeable units (in tens ively fractured trachytic lava now andrnunwelded tuff) at the elevated lands of Dandi and Wanchi at the southern part of thernstudy area favors them to be a recharge area. If comparison is made with in the limit ofrnthe study area, good ground wa ter potential with low Total Dissolved Solids (TDS) isrnfou nd to the eastern side of Ambo town .rnThermal and/or mineral water in the a rea is stru ctu ral ly controlled ; the al ignment ofrnthe the rmal springs along the nearly eastÂ· west oriented fault line in Ambo area C81lrnjustify the situation. Currently, the thermal springs are limited to a given area, mainlyrnin Ambo town; but their past extent can be inferred from the travertine deposit in thernarea whose thermal origin can be verified from the existence of silica sinter with it.