To calculate inter-annual variability, I use the coefficient of variation (std. dev. divided by mean) of the residuals after the linear trend is removed. For corn, this is 0.15. Soybeans, 0.11. SRWW, 0.15.

For intra-annual variability, I use a 10-meter grid. This is scale-dependent, so your mileage may vary. Once again, I use the CV, but as this is one year there is no trend to remove. Anyhow, for corn the CV is 0.28. Soybeans, 0.20. And SRWW, 0.15.