To sum the areas given in different units, we need to convert everything to the same unit. Let's convert everything to square meters (m²).

1. **Convert the areas to square meters:**
- \(2.6 \, \text{km}^2\):
\[
2.6 \, \text{km}^2 = 2.6 \times 1,000,000 \, \text{m}^2 = 2,600,000 \, \text{m}^2
\]
- \(5.87 \, \text{ha}\):
\[
5.87 \, \text{ha} = 5.87 \times 10,000 \, \text{m}^2 = 58,700 \, \text{m}^2
\]
- \(400,000 \, \text{m}^2\) is already in square meters.
- \(1300 \, \text{m}^2\) is already in square meters.

2. **Now, add the areas together:**
\[
2,600,000 \, \text{m}^2 + 58,700 \, \text{m}^2 + 400,000 \, \text{m}^2 + 1,300 \, \text{m}^2
\]

3. **Perform the addition:**
- First, add them step-by-step:
\[
2,600,000 + 58,700 = 2,658,700 \, \text{m}^2
\]
\[
2,658,700 + 400,000 = 3,058,700 \, \text{m}^2
\]
\[
3,058,700 + 1,300 = 3,060,000 \, \text{m}^2
\]

The total area is:
\[
\text{Total} = 3,060,000 \, \text{m}^2
\]

So, the final answer is:
\[
\boxed{3,060,000 \, \text{m}^2}
\]