Precipitation

Interannual variability in precipitation is a key factor that accounts for a large measure of interannual variability in crop yields. As with temperature, monthly mean values and timing are important factors controlling impact of precipitation on crop yields. Precipitation is a key input variable to the crop model that accounts for a large measure of interannual variability in crop yields. We evaluated the capability of a regional climate model (RegCM2) (Giorgi et al., 1993) to simulate precipitation for the growing season at Ames, IA. RegCM2 was driven with lateral boundary conditions over a 10-year period (1979-1988) supplied by the NNR (see Pan et al., 2001b, for more details). This 10-year period starts with the first availability of weather satellite imagery and matches the Atmospheric Model Intercomparison Project experiment (Gates, 1992), which allows comparisons of our regional model results with those of global models. Results shown in the upper plot of Figure 15.2, indicate that in some years the model is able to simulate the seasonal total quite well, but in some years (notably 1983) the model fails to capture the large seasonal total. There is a general tendency for the model to predict lower values than observed in all years. Table 15.1 gives a summary of totals for the 10-year observed and simulated periods.

The temporal distribution of rainfall intensities also influences crop development. Numerous light rainfall events that fail to provide moisture to deep roots may be far less beneficial than a single event of the same total amount. Our ability to simulate future crop development and yield by use of crop models will depend strongly on our ability to correctly simulate timing and amount of future precipitation. To examine the characteristics of model-simulated precipitation events, we plotted the distribution of daily total rainfall amounts during May to August as produced by the regional climate model RegCM2. Results (Figure 15.3) revealed that the model simulates too many low-precipitation events and not enough events in the range most usable by a crop such as corn, which develops a deep root system by the middle of the growing season. By use of the same regional model simulations, Kunkel et al. (2002) found annual and interannual extremes of precipitation to be reasonably well represented by the regional model.

Growing Season Precipitation at Ames, IA

Growing Season Precipitation at Ames, IA

79 80 81 82 83 84 85 86 87 88

20000 15000100005000 0

Simulated Corn Yield at Ames, IA

Simulated Corn Yield at Ames, IA

20000 15000100005000 0

Figure 15.2 Growing season precipitation as simulated by RegCM2 for 10 years as compared to observed amounts for Ames, IA.

Figure 15.2 Growing season precipitation as simulated by RegCM2 for 10 years as compared to observed amounts for Ames, IA.

Table 15.1 Growing Season Precipitation and Yields Observed and Simulated by RegCM2 and CERES-Maize When Supplied with 10-Year Climate Data Sets for 1979-1988 Period and 10-Year Climate Data Sets of Contemporary (contmp) and Future Scenario (futscen) Climates as Simulated by HadCM2

Normalized

Standard

Standard

Variables

Simulation Runs

Mean

Deviation

Deviation

Precipitation

Observed

449

114

0.25

(mm)

RegCM2/NNR

343

89

0.26

Yields (kg ha-1)

Observed

8,381

1,214

0.14

CERES/Observed

8,259

4,494

0.54

weather

CERES/RegCM2/NNR

5,487

3,796

0.69

CERES/RegCM2/Had

5,002

1,777

0.36

CM2 contmp

CERES/RegCM2/Had

10,610

2,721

CM2 futscen

2.5 7.5 12.5 17.5 22.5 27.5 32.5 37.5 42.5 47.5 Daily precipitation (mm)

Figure 15.3 Histogram of daily rainfall amounts as simulated by RegCM2 for May to August in 1979-1988 period.

In Figure 15.4, we plotted just the range of daily totals considered to be most effective in promoting crop development. Although the model simulates a large number of events in this range, the distribution is skewed toward lower daily totals with too few at the higher end of the range.

mrniiviWm n ,n

12.5 17.5 22.5 27.5 32.5 37.5 42.5 Daily precipitation (mm)

47.5

Figure 15.4 Histogram of precipitation events per season as simulated by RegCM2 in the range most usable by corn for May to August in 1979-1988 period.

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