Analysis of Climate Patterns Spatial Organization

Trends and patterns of climate in the NCR were examined and linked to the ecological regions classified by Bailey (1996). Bailey (1996) used a hierarchy of scales to define ecoregions, including (from larger to smaller scales) domain, division,

Domain

Division

Domain

Division

Province t

Province

Section

Figure 4.2 Bailey ecoregion classification. The lighter area in each map shows the ecore-gion represented by the KBS LTER.

province, and section. The Bailey system of classification was used in this analysis because this hierarchy is primarily based on climatic criteria. Figure 4.2 illustrates distribution of the Bailey ecoregion classification. A weather data set was geographically located at the center of each of the 1055 counties in the region (figure 4.3). Each location, therefore, comprises a 20-year set of observations of daily estimates of temperature and precipitation. These data, in addition to derived variables (see below), were organized and entered into a relational database to provide ease of manipulation and computation.

Monthly summaries of variables were computed from the daily data for each of the 1055 locations in the NCR, as described previously. Variables in the data tables consist of location name (state-county code), mean maximum monthly temperature (°C), mean minimum monthly temperature (°C), and accumulated monthly precipitation (mm) (MPP). Monthly degree-day accumulation (MDD) and a ratio of (MDD/MPP), called HPR, were derived variables. The method for computing degree-days (base 10°C) was that of Baskerville and Emin (1969). The HPR (Heat/ Precipitation Ratio) is an index of potential plant stress, assuming that a large accumulation of heat during one month associated with a small accumulation of precipitation during the same month will result in greater potential plant stress.

The interpretation of drought induced plant stress is a focus of the analysis in this chapter. Several indexes have been developed to estimate the severity of drought (Loomis and Connor 1992). Although the Palmer Drought Severity Index (Alley 1984) is widely used, a simpler Heat/Precipitation Ratio (HPR), proposed

Figure 4.3 Each weather data set is located at the center of each of the 1055 counties in the region.

by Gage and Mukerji (1977), was used in this analysis as an index to characterize potential plant stress. The HPR index is estimated by the equation HPR = MDD/ (MPP + 1), where MDD is the number of heat units accumulated during a period and MPP is the amount of precipitation accumulated during the same interval. This ratio was developed to study (1) the response of grasshoppers to combinations of heat and moisture in arid environments and (2) the subsequent crop loss caused by these insects (Gage and Mukerji 1977, 1978). Although several methods to characterize drought have been developed (Alley 1984; Harouna and Carlson 1994), using HPR to indicate potential plant stress (Gage and Mukerji 1977, 1978) avoids the need for continuous evapotranspiration observations for a large numbers of stations over a long time period—observations that are not available. High HPR values indicate that high heat accumulations are associated with low amounts of precipitation. When this occurs at a monthly timescale, high potential plant stress may occur. For example, if 80 mm of precipitation accumulate in the presence of 400 units of heat >10°C during a month, the HPR would be 400/80 = 5. However, if 40 mm of precipitation were to occur in the presence of 400 units of heat >10°C, the HPR would be 400/40 = 10. In this chapter, the analysis will focus on the patterns of the HPR ratio over time.

Table 4.1 shows the mean maximum HPR for key growing season months for the 20-year period (1972-1991) and the average monthly HPR for 1988 alone. In all cases, the average HPR in1988 is greater than the average HPR for the period of record and was 6.85-fold greater in June. The high plant stress (high HPR) in June 1988 stands out as an unusual event compared to other years in the period of record. The 1988 HPR in May, July, and August was approximately 2.5-fold above average, demonstrating that 1988 was indeed a summer of high plant stress.

Table 4.1 Mean maximum HPR (heat/precipitation ratio) during the period 1972-1991 and HPR values in 1988 in the North Central Region

Month

Mean maximum 1972-1991

Mean maximum 1988

1988/mean

May

11.89

25.42

2.14

June

17.17

117.61

6.85

July

38.68

104.78

2.70

August

26.09

73.78

2.83

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