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S.U SURFACE. ~YA'17;It 5.1 Sfreamllow Characteristics A Large Percentage oi' the Annual Runufl' Occurs in Spring and L'arly Summer Months as snowmelt i'rom the Higher Glevalious In perennial streams in Area 61 the typical seasonal pat tern of runoll is base flow /rom late fall to early spring, a snowmelt peak in spring and early summer months, and occasional to frequent rainfall peaks in the summer months. The typical seasonal pattern of streamflow in Area 61 is shown by a representative hydrograph of mean daily discharge (fig. 5.1-1). The hydrograph illustrates mean daily flow of the Purgatoirc River al Madrid, Colo., during water year 1979. Water year 1979, a year of greatervthan-average Clow, illustrates the phases of the annual runoff cycle. The flow ilt the river is virtually base flow from rite first of October through the winter months. llcginning in late April to mid-May, snowmelt from the higher elevations of the Culebra Mountains to the west fills the channel with spring nmoff. Typically, a large percentage of the annual runoff volume occurs in the spring and early summer months as snowuuh. lire sharp peak flows in July, August, and September arc from locally intense thunderstorms that occur in the region. As is characteristic of other streams in rite area, streamflow in the Purgatoirc River is depleted by diversions and is regulated by reservoirs. Upstream , „f, ,,,,,from the Madrid gage are about 6,000 acres of~ ~ .. ~trrigated lands and several small oncttannel reser- voirs. Average and extreme discharges for the period of record at selected streamflow gages in Area 61 are listed in table 5.1-1. Average discharge is a some- what misleading statistic for streams in Iltis area because o(the large seasonal variation iu I low. Predictive equations are available to estimate mean annual flow (Qa), in cubic feet per second for Colorado (Livingston, 1970) and for New Mexico (Iiorland, 197(1). As these cyuntions were developed on a stmcwidc basis it is to be expected that Ihcy might be ICSS applicable in some areas oC the Stale than in others. "fhc Colorado cyuation was dcvc- lopcd using dma prcdumin:uuly for the uwunrtins of central and northern Colorado. Iror Area 61, the et)ualiott.s developed for Ncw tv9c.xico, based on comparison of the methods using streamflow-gaging station records farm the area, arc more applicable. l h;u cyuation: Qa = 3.24 X 10 5 A LUI SU.JJN 1~~2.51 LA0.517 has :( standard crnrr of estimate of 51 percent. I he sieni (icanl variables arc drainage area (A) in square miles, main channel slope (S) in feet per mile, which is the average slope between points III and RS percent of the distance Imm Ilic site of interest and the drainage divide, the mean Ctctobcr through April precipitation (Pa) iu inches (front fig. 5.1-2), and the latitude at the comer of the drainage basin (LA) in degrees minus 30, lire above cyuation is for pcrcnni- al (flowing year around) sucams only. Nu cyuation exists t'or streams that are dry for a part of the year.