StreamStats in Arkansas
Arkansas StreamStats incorporates statewide regression equations for estimating instantaneous peak flows with annual exceedance probabilities of 50, 20, 10, 4, 2, 1, and 0.2 percent. These peak flows have recurrence intervals of 2, 5, 10, 25, 50, 100, and 500 years, respectively. Arkansas StreamStats also incorporates regional regression equations that are applicable to the western 2/3 of Arkansas for estimating lowflow frequency statistics, including annual 7day, 2year and 10year low flows, monthly mean flows and 7day, 10year low flows for the individual months of November through March, and 7day, 10year low flows for the combined months of November and December, November through April, January and February, and March and April. In addition, equations are available for this same area for estimating the probability of zero flow in streams at times of the annual 7day low flow, the November 7day, 10year flow, and 7day, 10year flows for November to December, and November to April. Estimates of harmonic mean flows also are available. The reports below document the regression equations, the methods used to develop them and to measure the basin characteristics used in the equations, and the errors associated with the estimates obtained from the equations. Users should familiarize themselves with these reports before using StreamStats to obtain estimates of flows for ungaged sites in drainage basins.
 Wagner, D.M., Krieger, J.D., and Veilleux, A.G., 2016, Methods for estimating annual exceedance probability discharges for streams in Arkansas, based on data through water year 2013: U.S. Geological Survey Scientific Investigations Report 2016–5081, 136 p.
 Breaker, B.K., 2015, Dry season mean monthly flow and harmonic mean flow regression equations for selected ungaged basins in Arkansas: U.S. Geological Survey Scientific Investigations Report 2015–5031, 25 p.
 Funkhouser, Jaysson E.; Eng, Ken; Moix, Matthew W., 2008, LowFlow Characteristics and Regionalization of LowFlow Characteristics for Selected Streams in Arkansas: U.S. Geological Survey Scientific Investigations Report 20085065, 162 p.
Click on this link to obtain general information on the Arkansas application, as well as specific sources and computation methods for basin characteristics.
Erroneous Drainage Area
On July 21, 2014, an error was found and corrected in the computed drainage area for the Black River in the vicinity of Pocahontas, Arkansas. The error began on the main stem of the Black River, at the outlet of HUC 11010007; latitude 36.253033, longitude 90.912041. Drainage area computations upstream from that point were not affected.
Notes on computing basin characteristics:
When using the Basin Characteristics tool, if a value for basin shape (BSHAPE) is desired, then a twostep process is required. First, use the tool to compute either the adjusted or the unadjusted 1085 stream slope, and then use it again to compute BSHAPE.
Notes on peakflow estimates:
Basin characteristics used as explanatory variables in the peakflow regression equations by Hodge and Tasker (1995) were determined by manual methods. Methods used in StreamStats to compute stream slope, basin length, and basin shape, initially resulted in values that were biased in comparison to the values for streamgages that were published by Hodge and Tasker (1995), and the biased basin characteristics were causing bias in the resulting peakflow estimates. Relations between the StreamStats and published values for these basin characteristics were determined and used to adjust the StreamStats values in an attempt to remove bias. Bias was adequately removed for regions A, B, and C, but peakflow estimates for region D still are biased high, with average bias of 4.3, 5.8, 6.7, 7.6, 8.2, 8.7, and 9.9 percent for the 50, 20, 10, 4, 2, 1, and 0.2 percent exceedance probabilities, respectively. Also, after adjusting the basin characteristics for bias, individual differences between flow estimates provided by StreamStats and those determined using the published basin characteristics were sometimes large, with some differences of as large as 40 percent. As a result, StreamStats users should expect that the errors associated with the peakflow estimates provided by StreamStats are larger than the published standard errors of prediction, which are shown in the StreamStats outputs for userselected sites.
Interpretation of zero flowprobability estimates:
At the bottom of the outputs for ungaged sites that are produced by the Estimate Flows Using Regression Equations tool are four estimates of the probabilities that minimum 7day flow will be zero. The statistic labeled PROB_7DAY is an estimate of the probability that the annual minimum 7day flow for any given climatic year (April 1 to March 31) will be zero. The likelihood that the minimum 7day low flow will be zero for a particular recurrence interval (T) can be determined by comparing the estimated probability from the StreamStats output to the reciprocal of the recurrence interval (1/T). For example, to determine if the annual 7day, 10year flow is zero, divide one (1) by the recurrence interval (10) to obtain a probability of 0.1, and then compare that probability (0.1) to the probability for PROB_7DAY in the StreamStats output. If the probability given by StreamStats exceeds 0.1, then assume the annual 7day 10year low flow is zero; otherwise use the estimate labeled as M7D10Y in the StreamStats output as the estimate. Likewise, if value of PROB_7DAY in the StreamStats output exceeds 0.5, then assume that the annual 7day, 2year flow is zero; otherwise, use the estimate labeled as M7D2Y in the StreamStats output as the estimate. For the other estimates of the probability of zero flow, labeled PROB7D11, PROB7D1104, and PROB7D1112, if the estimates of the probability of zero flow exceed 0.1, then the flow estimate for the corresponding flow statistic should be considered zero; otherwise, use the values given for the corresponding statistics, M7D10Y11, M7D10Y1104, M7D10Y1112, respectively.
Notes on lowflow and zero flowprobability estimates:
Annual, spring, or winter tau is used as an explanatory variable in all zeroflow probability equations and many of the lowflow frequency equations for Arkansas, including equations for M7D2Y, M7D10Y, M7D10Y11, M7D10Y12 in lowflow regions 2 and 3, the equation for M7D10Y0304 in lowflow region 2, and equations for M7D10Y1104 and M7D10Y1112 in lowflow region 3. Tau is the baseflow recession constant, measured in days, which characterizes the rate at which the flow in a stream recedes after a streamflow peak. Methods used by StreamStats to compute annual, spring, and winter tau for userselected sites differ from the methods that were used to determine tau values for the streamgages that were included in the regression analyses. As a result, individual differences between the estimates provided by StreamStats for the affected streamflow statistics and those determined using the published basin characteristics were sometimes large. The largest percent differences generally were associated with the smallest flow estimates, with some differences of greater than 250 percent. StreamStats users should expect that the errors associated with the estimates for the affected statistics are larger than the published standard errors of prediction, which are shown in the StreamStats outputs for userselected sites.
Arkansas StreamStats was developed in cooperation with the Arkansas State Highway and Transportation Department, the Arkansas Natural Resources Commission, the Natural Resources Conservation Service, the U.S. Forest Service, and the U.S. Army Corps of Engineers  Little Rock District.
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