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Clifford Raphael and John E. Hay

Abstract

The performances of three models which use satellite data to estimate solar irradiance at the Earth's surface are assessed using measured radiation data from a midlatitude location. Assessment of the models is made possible through the accurate Earth location of the satellite imagery (to within±2 pixels).

Evaluations of the models for a variety of conditions reveal the need for revised coefficients for the Hay and Hanson model and Tarpley model and demonstrate the superior performance of the physically-based Gautier et al. model on an hourly basis for partly cloudy and overcast conditions. However, compared to the clear sky case all three models give poor results under partly cloudy and overcast conditions.

An increase in the averaging period leads to marked decreases in the rms errors observed for the three models under all conditions, with the greatest improvement occurring for the Hay and Hanson model. Suggestions for improvements in the three models include 1) a more accurate and explicit treatment of cloud absorption and 2) in the Gautier et al. model and Tarpley model, the inclusion of the effects of aerosols under clear skies and the accurate and objective specification of a cloud threshold separating clear from partly cloudy and partly cloudy from overcast conditions.

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Philip W. Suckling and John E. Hay

Abstract

A synoptic approach to the analysis of solar radiation regimes is undertaken with the aim of developing a synoptic solar radiation climatology. Synoptic weather types for an area including British Columbia and the adjacent regions of the northeastern Pacific are defined using an objective correlation classification technique. These weather types are shown to determine statistically distinct solar radiation distributions.

However, further analysis shows that the distinctiveness of the solar radiation regimes is not sufficient to be used in practical applications such as interpolation between measurement stations, estimation of solar radiation inputs in the absence of observed data or in the explanation of the interannual variability of solar radiation.

As a result, attempts to base a solar radiation climatology solely on the synoptic regimes defined using the readily available data and techniques employed in this study are not justified. However, the statistical analyses do suggest that the use of more appropriate synoptic data and typing techniques may overcome many of the inadequacies in the present study.

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L. J. Bruce McArthur and John E. Hay

Abstract

A technique to map the distribution of diffuse solar radiation over the sky hemisphere is described. The method is based on an analysis of all-sky, visible photographs and concurrent actinometric measurements of diffuse solar radiance. The photographs were digitized and the resulting relative density values correlated with directly measured radiances. The resulting relationship was then used to determine the radiance for each density value, enabling a map of diffuse solar radiation for the celestial dome to be constructed.

The validity and utility of the approach are assessed by several tests. In the first test, the estimated radiances were integrated over the hemisphere and compared with measured diffuse irradiances for a horizontal surface. These were found to be within ±10% for the variety of sky conditions examined. A second test, under clear sky conditions, was performed to estimate the shortwave irradiance on several south-facing inclined surfaces. The results were found to be within ±5% of the measured irradiances. In a third test, comparisons with the normalized radiance distributions of Steven (1977) indicated good qualitative agreement.

Finally, problems and deficiencies in the technique are reviewed and possible means of surmounting them are discussed.

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Roland J. Viger, Lauren E. Hay, Steven L. Markstrom, John W. Jones, and Gary R. Buell

Abstract

The potential effects of long-term urbanization and climate change on the freshwater resources of the Flint River basin were examined by using the Precipitation-Runoff Modeling System (PRMS). PRMS is a deterministic, distributed-parameter watershed model developed to evaluate the effects of various combinations of precipitation, temperature, and land cover on streamflow and multiple intermediate hydrologic states. Precipitation and temperature output from five general circulation models (GCMs) using one current and three future climate-change scenarios were statistically downscaled for input into PRMS. Projections of urbanization through 2050 derived for the Flint River basin by the Forecasting Scenarios of Future Land-Cover (FORE-SCE) land-cover change model were also used as input to PRMS. Comparison of the central tendency of streamflow simulated based on the three climate-change scenarios showed a slight decrease in overall streamflow relative to simulations under current conditions, mostly caused by decreases in the surface-runoff and groundwater components. The addition of information about forecasted urbanization of land surfaces to the hydrologic simulation mitigated the decreases in streamflow, mainly by increasing surface runoff.

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John F. Walker, Lauren E. Hay, Steven L. Markstrom, and Michael D. Dettinger

Abstract

The U.S. Geological Survey Precipitation-Runoff Modeling System (PRMS) model was applied to basins in 14 different hydroclimatic regions to determine the sensitivity and variability of the freshwater resources of the United States in the face of current climate-change projections. Rather than attempting to choose a most likely scenario from the results of the Intergovernmental Panel on Climate Change, an ensemble of climate simulations from five models under three emissions scenarios each was used to drive the basin models.

Climate-change scenarios were generated for PRMS by modifying historical precipitation and temperature inputs; mean monthly climate change was derived by calculating changes in mean climates from current to various future decades in the ensemble of climate projections. Empirical orthogonal functions (EOFs) were fitted to the PRMS model output driven by the ensemble of climate projections and provided a basis for randomly (but representatively) generating realizations of hydrologic response to future climates. For each realization, the 1.5-yr flood was calculated to represent a flow important for sediment transport and channel geomorphology. The empirical probability density function (pdf) of the 1.5-yr flood was estimated using the results across the realizations for each basin. Of the 14 basins studied, 9 showed clear temporal shifts in the pdfs of the 1.5-yr flood projected into the twenty-first century. In the western United States, where the annual peak discharges are heavily influenced by snowmelt, three basins show at least a 10% increase in the 1.5-yr flood in the twenty-first century; the remaining two basins demonstrate increases in the 1.5-yr flood, but the temporal shifts in the pdfs and the percent changes are not as distinct. Four basins in the eastern Rockies/central United States show at least a 10% decrease in the 1.5-yr flood; the remaining two basins demonstrate decreases in the 1.5-yr flood, but the temporal shifts in the pdfs and the percent changes are not as distinct. Two basins in the eastern United States show at least a 10% decrease in the 1.5-yr flood; the remaining basin shows little or no change in the 1.5-yr flood.

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