| Year | Observ. Area (km2) | Predicted est. |
| 1978 | 0.618 | 0.53964 |
| 1988 | 0.482 | 0.428145 |
| 1998 | 0.423 | 0.336394 |
| 2008 | 0.375 | 0.50572 |
| 2013 | 0.358 | 0.464213 |