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43
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84861684232
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Walter's concept is based on the premise mat soil moisture availability is a simple function of input (rainfall) minus output (evapotranspiration), ignoring runoff. Mean monthly temperature has been empirically correlated with potential evapotranspiration by J. M. Thornthwaite R. H. Shaw, Ed. AAAS, Washington, DC Walter's index uses a linear scaling of mean monthly temperature (T) in degrees Celsius and total monthly precipitation (P) in millimeters such that I = P - 2T, where I is “effective moisture” if positive and “effective drought” if negative. However, since potential evapotranspiration is defined as “… an exponential function of temperature,” Walter's index will provide an underestimate of actual drought conditions. Therefore, any statistically significant drought patterns based on this index will be conservative
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(1967)
Ground Level Climatology
, pp. 93-126
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Major, J.1
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58
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0013447060
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Freeman, New York, ed. 2 Group comparisons were made by the Wilcoxon rank-sum test and the t-test, which produced similar results. The nonparametric statistics are reported here. The significance levels refer to tests confined to the years encompassed by actual plant censuses (1915 to 1968). There are 12 independent predictor variables associated with the running Walter climadiagram (four seasons times two intensities of wet and dry time plus four seasons times one length of wet or dry time), each of which was tested separately in two-group comparisons within and between climatic regimes. Since the local precipitation record goes back to 1852, but the temperature record goes back only to 1892, two independent statistical analyses were performed for the 7 vears in which black grama seedlings were observed, (i) The 7 years were compared as a group to the two climatic regimes of this century for each of the 12 predictors calculated from the running Walter climadiagram. (ii) The seven black grama years were compared, by season, to each of the three major global climatic regimes using only the precipitation record to obtain qualitative rankings of group similarities. An analysis of precipitation alone wul resolve changes in the intensity of wet times but does not distinguish the independent intensity of dry times. Because of gaps in the record, statistical analyses of the period 1852-1900 are based on 73 percent of the years. A question remains regarding the regional extent of temporal patterns observed at a point in space. Since the northern Chihuahuan desert region is influenced by four different air masses, climatic signals can be obscured through regional averaging. Nevertheless, spatial and temporal patterns of meteorological events are interrelated (6). If temporal continuity of meteorological events is observed from a single station (for example, extended periods of rain or drought), some degree of spatial continuity of pattern is implied (from the laws of fluid dynamics). Likewise, if meteorological events at a single station appear random through time, some degree of spatial randomness of pattern is implied. These two extremes result from generally meridional, resonant flow versus generally zonal nonresonant flow as previously discussed
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(1981)
Biometry
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Sokal, R.R.1
Rohlf, F.J.2
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