Stephen J. Burges

Effect of bias adjustment and rain gauge data quality control on radar rainfall estimation

Thirty major storms that passed over Goodwin Creek, a small research watershed in northern Mississippi, were analyzed to assess the bias between radar rainfall estimates at rain gauge locations and the gauge amounts. These storms, each contributing at least 10 mm of storm total rainfall, accumulated approximately 785 mm of rain, which corresponds to about half the average annual rainfall amount for the area. The focus of this study was to demonstrate the importance of (1) bias adjustment of the radar rainfall estimates and (2) the quality control of the rain gauge data used for bias adjustment. The analyses are based on Memphis Weather Surveillance Radar—1988 Doppler radar data, tipping-bucket rain gauge data, and raindrop spectra information collected within the Goodwin Creek catchment. Because of measurement and rainfall estimation uncertainties, radar observations are often combined with rain gauge data to obtain the most accurate rainfall estimates. Rain gauge data, however, are subject to characteristic error sources: for Goodwin Creek, malfunctioning of the tipping-bucket rain gauges was frequently caused by biological and mechanical fouling, and human interference. Therefore careful quality control of the rain gauge data is crucial, and only good quality rain gauge information should be used for adjusting radar rainfall estimates. By using high-quality gauge data and storm-based bias adjustment, we achieved radar rainfall estimates with root-mean-square errors (RMSE) of approximately 10% for storm total rainfall accumulations of 30 mm or more. Differences resulting from radar data processing scenarios were found to be small compared to the effect caused by bias adjustment and using high-quality rain gauge data.

Effects of urban development in the Puget Lowland, Washington, on interannual streamflow patterns: Consequences for channel form and streambed disturbance

fraction of time that streamflow exceeds the 0.5-year flood (T0.5). Urban streams had low interannual variability in annual maximum streamflow and brief duration of frequent high flows, as indicated by significant correlations between road density and both CVAMF and T0.5. The broader distribution of streamflow indicated by TQmean may be affected by urban development, but differences in TQmean between streams are also likely a result of other physiographic factors. The increase in the magnitude of frequent high flows due to urban development but not their cumulative duration has important consequences for channel form and bed stability in gravel bed streams because geomorphic equilibrium depends on moderate duration streamflow (e.g., exceeded 10% of the time). Streams with low values of TQmean and T0.5 are narrower than expected from hydraulic geometry. Dimensionless boundary shear stress (t*) for the 0.5-year flood was inversely related to T0.5 among the streams, indicating frequent and extensive bed disturbance in streams with low values of T0.5. Although stream channels expand and the size of bed material increases in response to urban streamflow patterns, these adjustments may be insufficient to reestablish the disturbance regime in urban streams because of the differential increase in the magnitude of frequent high flows causing disturbance relative to any changes in longer duration, moderate flows that establish a stable channel.

An adaptive modeling and monitoring approach to describe the hydrologic behavior of small catchments

Abstract An adaptive combination of field mapping, simple hydrologic monitoring, and hydrologic modeling was used to describe the hydrologic behavior of two zero-order catchments: one undeveloped forest, the other suburban. Field mapping was used to delineate hydrologic process zones and provide guidance for the location of monitoring stations. Hydrologic monitoring included continuous streamflow and precipitation, as well as limited piezometer information. The hydrologic model was developed in conjunction with the measurement program to assure that the level of model detail was consistent with the level of field measurements used for model input and testing. At the start of the work it was anticipated that saturation overland flow would drive peak discharge in the forested catchment and Horton overland flow from impervious surfaces would dominate storm runoff in the suburban catchment. Instead, subsurface flow was found to dominate both annual and peak discharge in the forested site, and discharge from lawns and other pervious areas accounted for roughly 60% of annual and storm runoff from the suburban catchment. The adaptive combination of modeling and measurements was essential to elucidate the various components of flow production and flow paths in these catchments.

Partial entrainment of gravel bars during floods

[1] Spatial patterns of bed material entrainment by floods were documented at seven gravel bars using arrays of metal washers (bed tags) placed in the streambed. The observed patterns were used to test a general stochastic model that bed material entrainment is a spatially independent, random process where the probability of entrainment is uniform over a gravel bar and a function of the peak dimensionless shear stress t* of the flood. The fraction of tags missing from a gravel bar during a flood, or partial entrainment, had an approximately normal distribution with respect to t* 0 with a mean value (50% of the tags entrained) of 0.085 and standard deviation of 0.022 (root-mean-square error of 0.09). Variation in partial entrainment for a given t* demonstrated the effects of flow conditioning on bed strength, with lower values of partial entrainment after intermediate magnitude floods (0.065 < t* < 0.08) than after higher magnitude floods. Although the probability of bed material entrainment was approximately uniform over a gravel bar during individual floods and independent from flood to flood, regions of preferential stability and instability emerged at some bars over the course of a wet season. Deviations from spatially uniform and independent bed material entrainment were most pronounced for reaches with varied flow and in consecutive floods with small to intermediate magnitudes. INDEX TERMS: 1815 Hydrology: Erosion and sedimentation; 1821 Hydrology: Floods; 1869 Hydrology: Stochastic processes; KEYWORDS: sediment transport, bed material entrainment, disturbance, gravel bars, stochastic model

An analysis of the influence of river channel properties on flood frequency

Abstract A one-dimensional flood routing model (DAMBRK) is used to explore the influence of some channel and floodplain hydraulic and geometric properties on the frequency distribution of peak flood flow rates as flood waves propagate downstream. Peak flood inflows for selected return periods are derived from a known flood frequency distribution and the effects of channel and floodplain properties on the associated hydrographs and peak flow rates are examined as the floods propagate down a single river reach; geometric and hydraulic properties of the reach are varied over a wide range. With no lateral inflow to the reach, a one-to-one mapping of computed peak flows and return periods of the routed hydrographs is used to determine the flood frequency distribution at downstream locations. For an upstream peak flood flow regime defined by an extreme value type I distribution (EV I) with coefficient of variation 0.6, the form of the downstream peak flow distribution appears to change from EV I to either EV II or EV III whenever inundation of the floodplain occurs, and the peak flow distribution becomes less variable with the amount of reduction in variance depending on the effective storage of the floodplain-channel reach. The findings are important when selecting data for use in regional flood frequency estimation.

Measurement of the temporal progression of scour in a pool‐riffle sequence in a gravel bed stream using an electronic scour monitor

A relatively inexpensive prototype monitor was designed and developed to record temporal variation in scour depth and was field-tested in a gravel bed stream. The device consists of plastic practice golf balls that are fitted internally with ring magnets and strung on a two-conductor cable enclosing a small reed switch. The balls are installed and oriented near-vertically in the streambed. As each ball is disturbed and released, it slides along the cable past the reed switch, and the time of circuit closure caused by passage of the magnet is recorded by a data logger. The device can be applied in arrays that span large areas of the streambed, including in wide channels that are inaccessible during a flood. Data obtained from 19 devices installed in an aggrading site described scouring processes in a pool-riffle interface during a bed load transport event. Substantial bed excavation occurred in the region of the pool edge during the rising stage, indicating existence of a local, temporally varying imbalance in bed load transport rate. Bed disturbance in the rest of the site prior to aggradation was limited to the surface and immediate subpavement layer.

Revised Coefficients for Priestley-Taylor and Makkink-Hansen Equations for Estimating Daily Reference Evapotranspiration

AbstractMany applications require estimation of reference evapotranspiration (ETo) in areas where meteorological measurements are limited. Previous studies have shown that simple evapotranspiration models based on radiation and temperature perform relatively well in humid climates but underpredict ETo in drier and windier climates. In this paper, estimates of ETo based on existing simple models were compared with ETo calculated with the more comprehensive Penman-Monteith equation using meteorological measurements at 106 locations in the contiguous United States for a range of climates. Results showed that the simpler models were closest to the more comprehensive model at sites where the annual mean relative humidity (RH) was approximately 70% and annual 2-m wind speed (U) was less than 2  m·s−1. Equations for adjusting the model coefficients were developed based on annual averages of RH [or vapor pressure deficit (VPD)] and U to improve the performance of these models for drier and windier sites. Publicly...

Use of Thermal Infrared Imagery to Complement Monitoring and Modeling of Spatial Stream Temperatures

Thermal infrared (TIR) surveys are effective methods to map surface spatial temperature patterns along a river. We used two data sets of TIR-derived longitudinal temperature profiles to analyze reach-scale spatial patterns of thermal heterogeneity of the Wenatchee River, in the Pacific Northwest region of the United States as part of a temperature total daily maximum load investigation. The TIR data indicate that the river has a general downstream heating trend; the magnitudes, reach variability, and longitudinal gradients are influenced by the headwater conditions, channel morphology, tributary locations, flow rates, and weather. Detailed TIR images facilitate identifying regions with high local thermal heterogeneity where we recommend a weighted average approach to estimate local spatial average temperature using temperatures from pixels of the thermally distinctive areas rather than using the temperature extracted from pixels sampled along the central part of the channel. TIR-derived daily maximum temp...

Sampling properties of parameter estimates for the log Pearson type 3 distribution, using moments in real space

Abstract Sample cpvariance structures of estimated parameters for the log Pearson 3 (LP3) distribution are derived from sample estimates of the mean, coefficient of variation, and skew coefficient in the natural domain. The parameters and their covariance structures are used to estimate the magnitude and variance of an event having a particular return period, assuming the population to be LP3. The procedure provides an alternative to methods currently used; limitations of procedure that use estimated skews in logarithmic space have been demonstrated by Landwehr et al. The method reported here was compared with that advocated by the U.S. Water Resources Council. Estimates of extreme flood events and their standard errors using observed annual maximum flows for three catchments are given.

Approximate estimation of the derivative of a standard gamma quantile for use in confidence interval estimates

Abstract An approximation estimation technique for computing the derivative of a standard gamma quantile, w , with respect to the distribution shape parameter, b necessary for estimating the sampling variance of a specified quantile, is developed. The modified Wilson-Hilferty transformation parameters given by Kirby are approximated with fifth-order polynomials in skew coefficient, γ, facilitating direct estimation of ∂ω/∂β for continuous quantiles. Results are compared with more precise approximations of Harters exact quantiles made by Bobee. The method presented provides excellent approximations corresponding to the probability range 0.001 ⩽ p ⩽ 0.999 for 0.6 ⩽ γ ⩽ 5 and satisfactory estimates of ∂ω/∂β for quantiles corresponding to 1 in 10- to 1 in 1000-yr. events for smaller γ.