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balance equation, while the second assumption involves the uniform distribution of reach gains. <br />Step 1 above describes the formulation of an equation which balances the volumetric rates <br />entering and exiting the control boundaries of a specified river reach. Simply stated, this equation <br />equates to: Qg,,„ = - (Q, - Q.,n), where Q is in cfs. However, what is not accounted for in such an <br />equation, is the volumetric changes occurring in the river channel. Including a term to account for <br />volumetric changes would result in the following expanded equation: Qgai. = - (Q. - Q.,n) + AV/At, <br />where v and t represent river channel volume and time, respectively. To include a volumetric rate <br />of change term in the water balance equation would dictate the employment of flow /river stage <br />relationships and river routing computations which in turn would require additional parameters to <br />further describe the river channel 'system'. At the present time, such river routing computations <br />are beyond the scope of this point flow software. The exclusion of ov /ot, therefore, results in a <br />portion of the calculated gain being attributable to changes in river channel storage. In a broad <br />sense, the calculated gain (Qgai.) can be thought of as having an associated error of ov /ot. The <br />magnitude and sign of this error is highly dependent on the rate of change of river channel flow <br />entering and exiting the reach. For example, if the river stage at the up river reach boundary <br />increases dramatically over a 24 hour period and the reach lag time is 24 hours, a relatively large <br />computed reach loss ( -gain) will result. This is due to a portion of the inflowing water increasing <br />the water volume within the reach. Alternatively, if the stage at the up river reach boundary <br />rapidly decreases a large river gain (+ gain) will be calculated due to the volumetric decrease <br />within the reach. The volumetric error is minimized during periods of steady - state. This occurs <br />when the flow rate of all reach fluxes remain relatively constant over a period of time. For this <br />reason it is critical to examine variations occurring in gauge flows, diversion flows, and stream <br />inflows for the period of time over which the reach gain is computed. <br />Step 2 describes the distribution of the calculated reach gains (in cfs). The gains are distributed <br />uniformly over the entire length of the river reach resulting in the units of cfs gain per river mile. <br />In actuality, gains to the river vary spatially along the river channel. Nearby land use, underlying <br />aquifer characteristics, topography, and connecting alluvial tributaries are just a few of the factors <br />contributing to river gain in the spatial dimension. As the length of the reach becomes shorter, the <br />distribution of gains in a uniform fashion becomes more justifiable. For example, if at each <br />structure the flow in the river channel could be measured, gains could be pinpointed to a single <br />subreach. The motivation of utilizing diversions which are drying the river is an attempt to make <br />the river reaches shorter, thus, distributing the gains with greater certainty. Defining and <br />janalyzing the distribution of gains can demonstrate where additional river gauges would assist in <br />understanding the underlying factors. <br />POINT FLOW COMPUTATIONS <br />River channel point flows are all calculated by evaluating gauging values, diversion values, inflow <br />values, and river gain values in a piecewise manner. The computational steps are outlined as <br />follows: <br />7 ptflguid.wpd <br />