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<br />3 <br /> <br />11. In addition to the error introduced into streamflow computations by in- <br />accuracy of the rating curve, the frequency with which water-depth observations are <br />made is important. Observing sYf~tems employed range from staff gauges read once a <br />day to float-activated, clock-driven recording ~auges which produce an analogy <br />chart of the rise and fall in flow. Records of streamflow based on daily readings <br />will have a potential for error that will dE!pend on hO"1 rapidly the runoff over the <br />basin concentrates. For small basins with rapid concentration characteristics, a <br />major part of the flo\1 resulting from a precipita.tion event can occur during the <br />interval between daily readings. <br /> <br />12. The United states Geological Survey considers its records to be excellent <br />if the accuracy of individual streamflow measurements involves error of 2 per cent <br />or less. Errors of 5 per cent give the record a good grade. The average error of <br />their published records is considered to be less than 5 to 10 per cent for the <br />better stations. <br /> <br />13. streamflow past a gauging point may be thought of as the residual of preci- <br />pitation that has fallen over a drainage basin after certain subtractions (called <br />"losses" by hydrologists) have occured, These losses consist of water that has <br />returned to the atmosphere and ,-rater that has gone into storage above the gauging <br />point. An important form of storage, occurring in most natural basins, results from' <br />infiltration of rain or snoH-melt "rater into the soil mantle, Ivater stored in the <br />soil complicates the process of identifying the short-term relationshi.p between <br />rainfall and runoff, since not only is the stored water not immediately available <br />for gauging as streamflow at the outflow poj,nt, but some fraction of it may be <br />present to affect the amount of runoff from subsequent precipitation, As a result, <br />streamfloH observations exhibit the variability of the causative precipitation plus <br />a variability introduced by antecedent condj.tions vrhj.ch change Hi th time. <br /> <br />14. Lumb and Linsley (1971) provide some quantitative estimates of how the <br />natural ",ater budgets of hro California creeks differ, and ho"r increments of added <br />rain would be distributed among the component parts of the ",ater budget. In the <br />following table taken from their paper, the terms "13aseflo"\'I"" and "Interflow" are <br />defined by hydrologists as follows: <br /> <br />Interflow is the part of the precipitation which infiltrates the surface <br />soil and moves laterally toward streams above the main ground-water level. <br /> <br />Baseflow is the fair weather remoff composed of ground-water runoff. <br /> <br />TABLE 1: NATURA.L vlATER BUDGETS AND DISPOSITION OF INCREHENTAL RAIN <br /> <br /> DRY CREEK LA BIlE.!. CR1::r:;:: <br /> llatuxal R8.in Inc~'e1!lenta) Rain Natural Rain Ir.crC'~,ental Eai" <br /> As a per- As a per- As a per- ks a per- <br /> (2) centage centage centage .- ce:Itaee <br /> in" (3) in~ (4) mm. (5) ;_~ (6) __ (7) ins. (e) (0' <br /> f:1m h..... _J <br />Average annual <br />precipitation 51.00 1295.40 100 5.02 127.51 100 11.26 2e6.00 100 1.14 ;:'8.96 100 <br />Evapotranspiration 26.95 684.53 53 0.56 14.22 11 10.27 260.86 92 0.75 18.54 64 <br />IJaseflow 8,70 220.98 17 0.83 21.08 17 0.36 9.14 3 0.08 2.03 7 <br />Interflow 10.50 266. '{O 20 1. 71 43.43 34 0.57 9.40 3 0.14 ,3.56 12 <br />Surface runoff 4.95 125,'13 10 1.91 48.51 38 0.21 5.33 2 0,17 4.32 15 <br />Change in storage 0,10 2,54 - 0.01 0.25 - 0 0 0 0.02 0.51 2 <br /> <br />~ <br />