The Great Plains Hydrologic Observatory (GPHO) includes the Republican River basin from its headwater drainage to the river near the Nebraska-Kansas state line (USGS gaging station near Hardy, Nebraska). The GPHO lies entirely within the Great Plains physiographic province. The area of the Republican River watershed in the GPHO is 58,000 km2. The area of the High Plains aquifer to be included in the HO is larger than that of the Republican River watershed in order to encompass the natural division of the aquifer between northwest and west-central Kansas and to include aquifer areas that interact with those within the Great Plains Observatory. The Republican River basin is the largest river system entirely enclosed within the High Plains aquifer, the largest ground-water system in North America. Although the GPHO area is several times greater than the minimum size of 10,000 km2 suggested for an observatory, the area is an appropriate size for a basin in a semi-arid region that requires a large area to produce a river of reasonable size at the downstream end. This is in contrast to a smaller watershed in a humid region that generates substantially more runoff per area, such as the Neuse River. For comparison, the mean long-term annual flow of the Neuse River near its mouth is nearly 100 m3sec, while that of the Republican River at the downstream end of the observatory is approximately 14 m3sec.
The headwater tributaries in the GPHO are distributed across the high plains of northeastern Colorado and western Kansas and Nebraska. The Arikaree River and the North Fork of the Republican River join in the southwest corner of Nebraska to form the main stem of the Republican River. Tributaries originating in northeastern Colorado and western Nebraska flow to the southeast to join the northern side of the main stem. Tributaries originating primarily in northwestern Kansas flow in a northeastern direction to join the south side of the main stem. The mean annual runoff ranges from less than 0.5 cm in the western area of the GPHO to about 7 cm at the eastern end. The minimum and maximum annual precipitation and streamflow are substantially different from the long-term average as indicated earlier.
The surface geology consists primarily of the Ogallala Formation of Tertiary age. Deposits of loess cover the Ogallala sediments in many areas and form soils with very good properties for agricultural crops. Quaternary alluvial sediments overlie the Ogallala Formation in the Republican River valley. Sand and gravel comprise a major part of these unconsolidated deposits, and form the important aquifers of the HO. Upper Cretaceous bedrock, consisting primarily of shale, limestone, chalk, and sandstone, underlies the Ogallala Formation. The regional direction of ground-water flow is generally from west to east in response to the topographic slope of the plains.
The saturated thickness of the High Plains aquifer within the GPHO ranges from zero near its boundary to a maximum of nearly 100 m in northwestern Kansas, 120 m in northeastern Colorado, and over 150 m in southwestern Nebraska near the Platte River. Yields of irrigation wells can be greater than 3 m3/min in a major part of the observatory. The total number of active irrigation wells in the Kansas and Colorado portions of the GPHO has been level since the early 1980s at a total of approximately 4000 in each state, but has been rising since that time in Nebraska to a current total of about 18,000, although the current rate of increase is much less than in the 1960s and 1970s. The annual average volume of water pumped from the High Plains aquifer within the GPHO was 3.1 x 109 m3 during the decade of the 1990s. In comparison, potential evapotranspiration associated with phreatophyte areas in the HO has been estimated as 0.6 x 109 m3. Water-level declines caused primarily by ground-water pumping have occurred in the aquifer in all three states of the GPHO; the declines are over 30 m in parts of the GPHO. The ground-water level has risen in a zone south of and paralleling the Platte River within the observatory as a result of water diverted from the river for irrigation; rises have exceeded 10 m in part of this zone. Within the GPHO, current depths to ground water range from less than a couple meters in the alluvial aquifer next to flowing streams to over 70 m in parts of the High Plains aquifer.
Recharge to the High Plains and alluvial aquifers occurs in different forms, from precipitation, from seepage through soils irrigated with ground water or surface water, and from leakage from canals and laterals in surface-water diversion areas. Estimates of aquifer recharge in the GPHO range widely, depending on the land use, land cover, and soil, from less than 0.1 cm/yr to over 10 cm/yr.
Ground water in both the High Plains and alluvial aquifers is generally very fresh, except for waters in the alluvial aquifers of some of the tributary valleys where total dissolved solids are higher as a result of concentration by evapotranspiration. The surface waters are also fresh, although the dissolved solids concentration is substantially smaller during peak flows in comparison to baseflows from ground-water discharge.
The watershed is in an area of substantial agricultural cropland that includes extensive irrigation. The percentage of irrigated land surface ranges from less to 5% to over 25% within the GPHO in all 3 states and reaches over 50% in part of the eastern portion of the GPHO in Nebraska. The primary land use in non-irrigated areas is also agricultural, and includes dryland farming, rangeland, and farmland in Conservation Reserve Programs.