Cool-season Precipitation Research Articles

CYCLICITY IN PRECIPITATION AND STREAM DISCHARGE IN GEORGIA AND RELATIONSHIPS WITH THE SOUTHERN OSCILLATION

Maximum entropy spectrum analysis of 10 stream discharge time series from four physiographic provinces in Georgia has revealed clear and statistically significant cycles in water-year discharge (October to September) over the period 1938 to 1994 at 15 to 17 and 5 to 6 years in all 10 series, and at 8 to 9 years in 9 series. Analysis of 13 precipitation records uncovered cycles at 15 to 18 years (10 stations), 8 to 10 years (8 stations), and 5 to 6 years (13 stations). The cycles are much more consistent and better defined in the stream discharge time series suggesting that large intra-annual variations in precipitation, possibly caused by spatial variations in summer convectional rainfall, are filtered in the stream records. Spectrum analysis of the Southern Oscillation (SO) for the 98-year period from 1897 to 1994 revealed water-year cycles at about 18, 10, and 6 to 7 years, which are not in phase with the stream-flow and precipitation cycles. However, a strong relationship is apparent between cool-season (December through March) precipitation and stream flow in the southern part of the state (the Coastal Plain) and the sum of SO values in the same four months. In addition, cycles in cool-season precipitation at about 19, 10, and 6 years agree well with those in the SO and appear in phase suggesting a causal relationship.

The relative contributions of summer and cool-season precipitation to groundwater recharge, Spring Mountains, Nevada, USA

A comparison of the stable-isotope signatures of spring waters, snow, snowmelt, summer (July thru September) rain, and cool season (October thru June) rain indicates that the high-intensity, short-duration summer convective storms, which contribute approximately a third of the annual precipitation to the Spring Mountains, provide only a small fraction (perhaps 10%) of the recharge to this major upland in southern Nevada, USA. Late spring snowmelt is the principal means of recharging the fractured Paleozoic-age carbonate rocks comprising the central and highest portion of the Spring Mountains. Daily discharge measurements at Peak Spring Canyon Creek during the period 1978–94 show that snowpacks were greatly enhanced during El Nino events.

Evaluation of Passive Microwave Precipitation Algorithms in Wintertime Midlatitude Situations

The second intercomparison project of the Global Precipitation Climatology Project (GPCP) examined the estimation of midlatitude, cool-season precipitation. As part of that effort, the authors report here on the results of two microwave techniques, the Goddard scattering algorithm and the physical retrieval algorithm of Kummerow. Results from the estimation of instantaneous rain rate for five overpasses of the Special Sensor Microwave/Imager (SSM/I) are presented in a case study mode to illustrate both the strong and weak points of each technique. These five cases represent a sampling of the various types of precipitating systems observed. Results for the complete set of 20 swaths chosen by the United Kingdom Meteorological Office (UKMO) are then categorized by scatterplots and statistics of instantaneous radar versus microwave-estimated rain rate, rain no-rain contingency tables, and scatterplots of areal coverage of rainfall. Neither algorithm produced a good statistical correlation with the radar data, yet in general, both did well at determining rainy areas. Two reasons are suggested for the low correlation coefficients between both algorithms and the radar data. Time differences between the SSM/I overpass and the radar observations can occasionally account for some of the differences. The primary reason for the low correlations, however, appears to be the predominance of very light rain in the area of interest during the winter. Both algorithms are in good spatial agreement with the radar when the radar data are restricted to rates above 1 mm/h. When all radar rain rates are included, the radar areal coverage increases by as much as a factor of 10 in some cases. Because the Kummerow algorithm does not handle such low rain rates over land very well, and because the Goddard scattering algorithm uses 1 mm/h as the minimum reliably detectable rain rate, regimes that contain large areas of very light rain present inherent difficulties for these retrieval methods. Therefore, the proliferation of low rain rates observed during the experiment is the main contributor to low correlation coefficients and high root-mean-square differences. Mididentification of cold surface (e.g., snow cover) as precipitation was also a problem in several instances.

Productivity of Long-Term Grazing Treatments in Response to Seasonal Precipitation

Estimates of forage production for long-term ungrazed, lightly, moderately, and heavily grazed treatments (0, 20, 40, 60% removal of annual forage production) established in 1939 in shortgrass steppe communities were subjected to multiple regression analyses to assess long-term temporal trends resulting from grazing and short-term sensitivities to abiotic factors. Average production based upon all data from 1939-1990 was 75, 71, 68, and 57 g m(-2)yr(-1) for ungrazed, lightly, moderately, and heavily grazed treatments, respectively. Variability in forage production was explained mostly by cool-season precipitation, and magnitude of forage production was more sensitive to annual fluctuations in precipitation than to long-term grazing treatments. Production per unit increase of precipitation was greater for cool-season than warm-season precipitation, but only when cool-season precipitation was above average. This was attributed to differences in evaporative demand of the atmosphere resulting in different utilization-efficiencies of small and large rainfall events in the 2 seasons. Based upon a regression model constructed using data from 1939 through 1962, forage production was not affected by grazing to 20 to 35% removal. For pastures of average relative productivity, grazing at 60% level of consumption for 25 years resulted in a 3% decrease in forage production in wet years and a 12% decrease in dry years. Estimates of productivity after 50 years of heavy compared to light grazing treatment were -5 and -18% for wet and average y precipitation, respectively.

Burning of Northern Mixed Prairie during Drought

Burning of Northern Mixed Prairie during Drought