The area is mostly a nearly level to gently rolling dissected plain. Nearly level to gently sloping uplands merge into narrow valleys that have more sloping valley walls. Large rivers that cross the area have broad but shallow valleys. Significant tracts of hilly land are along the Austin chalk escarpment near the western side of the area. Average annual precipitation -750 to 1,150 mm.
Maximum precipitation is in spring and in fall.
Average precipitation during the frost-free period is about 600 to 650 mm. In most years the moderate rainfall is adequate for crops and pastures, but summer droughts that reduce crop yields are common. The large rivers flow all years. Many large lakes and small flood-detention reservoirs provide municipal water supplies and recreation facilities and help to control flooding. Small farm ponds on individual farms are an important source of water for livestock.