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1:lr::<l1 10 Base P6rlod for Runoff Comparisons In a region where runoff varies widely from wet years to dry years, comparisons of runoff from different drainage areaS should be made for the S8me period of years. Table 1 show~ that the average discharge for the water years 19~8-4~ Is r~Bsonably close to the average discharge for long periods of record. Because 8djustm~nts for diverslons and for the effect of reservoJrs on stream flow normally are small, table 1 Is an indication that the water years 1938-43 can be used satisfactorily as a base period for runoff. Table 2 lists figures of average annual precipitation that indicate about the same amounts for the years }938-43 QS for long periods of record. Also, the use of the same base period is desirable for both studies of discharge and of runoff; hence the 6-year period ending September 30, 1943, wss used 8S the base period for com- put~tions of average runoff. Computation of Runoff Average annual Runoff The fi~st step in com~uting average runoff for the base period 1938-43 was to compute ave~age annual volume of discharge for the pertod. Adjustments then were made for net diversions of water for irrigation ana other uses. Usually the computed net diversIons were based primarily on the irrigated acreages and ~ross diversions dur- ing 1939 as reported by the Bureau of the Census in its IrrigatIon of Agricultural Lands. Return flow generally was estimated, but some records of surface return flow were avaIlable. Adjustments both for net evaporation loss from large reservoirs, and for change in contents in reservoirs, were computed from available records or were partly estimated if the re~ords were In- complete. A reservoir normally reduces the volume of dIscharge immediately downstream by the difference between water losses from the water surface and water losses from the natural ground surface covered by the re- s~rvoIl'. For most or the Missouri River Basin, tbe net reduction of discharge caused by evaporation was assumed to average approximately 2 feet in depth on the re- servoir surfaces annually. ThIs assumed net red~ction in stream rlow is consistent with figures given by Follansbee.5/ The fi~ures of average annual volume of discharge, Bdj~sted for average net diverslons and stor~~e and for net evaporation losses from large reservoirs, were expressed as depth in inches and were used as average annual run- off. On each stream the average annual run- off was computed fo~ the dramage area of th~ farthe~t upstream gaging station that had adequate discharge records and for the drainage areas between successive gaging stations in downstream order along that strean!. These figures ....ere the principal data from which the map showing average annual runoff (pl. 2) was prepared. 5jFollansbeeJ Robert, Evaporation from reservoirs: Am. Soc. Civil Eng. Trans., vol. 99, pp. 708-709. '. Minimum Annual Runoff The mInimum depths of annual runoff dur- ing periods 'of stream-flow record were de- termined ror most of the gagIng stations In the Missouri River Basin. These depthS were computed from discharge records adjusted ror estImated diversions, reservoir evaporation, and change in reservoir contents whenever such adjustments seemed to be large enough and well enough defined to warrant making them. Minimum annual runoff was computed for many drainage areas between successive gaging stations along a stream as well 88 for the dralnage areas or the farthest upstream gag- ing station on each stream. The water year of lowest annual runoff for a gaged drainage area usually occurred betwAen 1930 and 1944, but for some areas it occurred In an earlier water year. Minimum depths of annual runoff were computed only withIn the period 1900-1945. Stream-flow records prior to 1900 were Insufficient to define minimum annual runoff at more than a few places in the basin, although runoff waS lOll for some of those years. (:--;..>~~ ..:..:.-.::: . Depths of observed annual runoff for drainage areas with short periods of record were compared with those. having long records or runoff. Frequently these comparisons ind~cated that the short periOds of record probably did not include the water year of minimum runoff since 1900. Depths of ob- served minimum annual runQff thRt seemed to be appreciably above the minimum for the period 1900-1945 were noted snd either were given little weight or were decreased by an estimated amount before they were used in the preparation of the _map. Runoff Maps Runoff maps like those on plates 2 and ~ are similar in many ways to isohyetal maps. Each of these two types of map is based on datR obtained at scattered places, extended and interpolated to cover a large area com- pletely. A runoff map shows approximate depths of runoff at all points of the map, and depth of runoff from an area is an aver- age of the depths of runoff at all points or that area. The volume of runoff from a drainage area can be computed from the size of the area and the areal average depth of rur.off. ,- I "'<-..:! Map of Average Annual Runoff The average annual runoff for the 6-year basa period is shown by lines on plate 2 that represent depths of runorf. In those parts of the basin where the ~off differed greatly from place to place, the lines of equal runoff are shown for depths of 0.25, 0.5, I, 2, 3, 5, 10, 15, 20, 30, snd 40 lnches. In other parts of th~ basin lines of 4, 6, 8, and 12 inches of runoff are also shown. The map Is based on many data and computations. All figures of average annual ~noff 1n inches from small areas up to areas of B few thousand square miles were noted on a map of the Missouri River Basin. Runoff on most of these areas was obviously far from uniform. The assumed distribution of aver~ge runoff within each area was based on ;. ~.~~.. ..:" :":g'. ,~. .