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1936 ~ ~~=-=. -- .t-'<.~tIMINAFrv p''=.~n=-- . . -, '__....... r '&';';:J :....i~" ...::_-.'.0,.7 -, ~. -'.po.... . . ;-~ I~~ ~ DISCHARGE AND SPECIFIC-CONDUCTANCE TRENDS . Discharge in the Arkansas River varies spatially and temporally because of the regular timing of snowmelt runoff in the upper basin, the irregular timing of rainfall runoff in the lower basin, the large magnitude of irriga- tion diversions, and the operations of mainstem reservoirs. The general seasonal and spatial panerns of discharge in the Arkansas River are illustrated in the hydrographs for six mainstem Arkansas River gaging stations for an average runoff year (In2) (fig. 3). FIgure 3 near here. At station 07096000, which represents upper-basin inflow to the lower basin. the dominant influences on discharge are snowmelt runoff and the release of stored water from off-channel reservoirs. Discharge typically increases with snowmelt runoff in April or May. and peaks in the middle or Ianer part of June. The recession of snowmelt runoff is typically supplemented by upper-basin reservoir releases in July and August. Natural base- . flow and reservoir releases dominate the low-flow period from October through March. At station 07099400. located 0.4 mi downstream from Pueblo Dam, discharge is regulated by the operarion of Pueblo Reservoir. The hydrograph at station 07099400 is similar to that of 07096000, with the exception that discharge is considerably smaller at 07099400 during November through March. At station 07109500, located about 24 mi downstream from Pueblo Reservoir (fig. I), discharge is affected by Pueblo Reservoir operations and substantial tributary inflow from Fountain Creek and the Sl. Charles River. The shape of tbe station 07109500 hydrograph is similar to that for station 07099400, with the exceptions that tributary inflow from occasional rainfall-runoff eventS pro- duces distinctly larger peaks and discharge is substantially larger at station 07109500 during the winter months. At station 07124000, which is located 110 mi downstream from Pueblo Reservoir. the ma!mitude of discharge is . - - substantially smaller than at upstream stations. The decrease in discharge is attributable to irrigation diversions. As previously discussed. much of the discharge in the river downstream ftom La Junta (fig. 1) is comprised of irrigation rerum flow during parts of most years (Cain, 1981). Below John Martin Reservoir at stations 07130500 . and 07133000, the annual hydrograph is regulated by the storage and release operations in John Martin Reservoir. 16