S. Lawrence Dingman

Evaluating the potential for measuring river discharge from space

Numerous studies have demonstrated the potential usefulness of river hydraulic data obtained from satellites in developing general approaches to tracking floods and changes in river discharge from space. Few studies, however, have attempted to estimate the magnitude of discharge in rivers entirely from remotely obtained information. The present study uses multiple-regression analyses of hydraulic data from more than 1000 discharge measurements, ranging in magnitude from over 200,000 to less than 1 m3/s, to develop multi-variate river discharge estimating equations that use various combinations of potentially observable variables to estimate river discharge. Uncertainty analysis indicates that existing satellite-based sensors can measure water-surface width (or surface area), water-surface elevation, and potentially the surface velocity of rivers with accuracies sufficient to provide estimates of discharge with average uncertainty of less than 20%. Development and validation of multi-variate rating equations that are applicable to the full range of rivers that can be observed from satellite sensors, development of techniques to accurately estimate the average depth in rivers from stage measurements, and development of techniques to accurately estimate the average velocity in rivers from surface-velocity measurements will be key to successful prediction of discharge from satellite observations.

Statistical development and validation of discharge equations for natural channels

Although the Manning equation is widely accepted as the empirical flow law for rough turbulent open-channel flow, using the equation in practical situations such as slope-area computations is fraught with uncertainty because of the difficulty in specifying the value of the reach resistance, Mannings n. Riggs (1976, J. Res. US Geol. Surv., 4: 285–291) found that n was correlated with water-surface slope, and proposed a multiple-regression equation that obviates the need for estimating n in slope-area estimates of discharge. Because his relation was developed from a relatively small sample (N = 62), had potential flaws owing to multicollinearity, and was not thoroughly validated, we used an expanded data base (N = 520) and objective methods to develop a new relation for the same purpose: Q = 1.564A1.173R0.400S−0.0543logS where Q is discharge (m3 s−1), A is cross-sectional area (m2), R is hydraulic radius (m), and S is water-surface slope. We validated Riggs model and our model using 100 measurements not included in model development and found that both give similar results. Riggss model is somewhat better in terms of actual (m3 s−1) error, but ours is better in terms of relative (log Q) error. We conclude that either Riggss or our model can be used in place of Mannings equation in slope-area computations, but that our model is preferable because it has less bias, minimizes multicollinearity, and performs better when applied to discharge changes in individual reaches. We also found that our model performs better than those of Jarrett (1984, J. Hydraul. Eng., 110: 1519–1539) or Riggs in the range of applicability of Jarretts equation (0.15 m ≤ R ≤ 2.13 m; 0.002 ≤ S ≤ 0.052). Both Riggss and our models significantly overestimate Q in flows satisfying both the following conditions: Q < 3 m3s−1 and Froude number less than 0.2. For other in-bank flows in relatively straight reaches, our model can be recommended for use in slope-area computations and other applications of the Chezy or Manning equations over a wide range of channel sizes (0.41 m2 ≤ A ≤ 8520 m2) and slopes (0.00001 ≤ S ≤ 0.0418), thus obviating the difficulty of a priori determination of the resistance factor.

EFFECTS OF CLIMATIC VARIABILITY ON WINTER-SPRING RUNOFF IN NEW ENGLAND RIVER BASINS

The effects of climatic variability on the time distribution of runoff and the timing and magnitude of maximum-flow events during the winter-spring season of December-April were examined through statistical analysis of hydrologic and climatic data for thirteen river basins in Vermont, New Hampshire, and Massachusetts. Within each basin, higher temperatures and lower snowfall result in a more uniform distribution of flow throughout the season. This relationship is also observed across the regions average spatial temperature gradient. In the extreme south of the region, runoff is uniformly distributed throughout the season, while in the extreme north, runoff is more concentrated in the latter part of the season. On average, maximum flows occur earlier in basins with higher average temperature—approximately 5.4 days earlier for each 1° C—but only within basins in the extreme north of the region is there a significant relationship between temperature and the time of occurrence of the maximum daily flow. Maxi...